CV Reference Manual



Image Processing

Note:
The chapter describes functions for image processing and analysis. Most of the functions work with 2d arrays of pixels. We refer the arrays as "images" however they do not neccesserily have to be IplImage’s, they may be CvMat’s or CvMatND’s as well.


Gradients, Edges and Corners


Sobel

Calculates first, second, third or mixed image derivatives using extended Sobel operator

void cvSobel( const CvArr* src, CvArr* dst, int xorder, int yorder, int aperture_size=3 );

src
Source image.
dst
Destination image.
xorder
Order of the derivative x .
yorder
Order of the derivative y .
aperture_size
Size of the extended Sobel kernel, must be 1, 3, 5 or 7. In all cases except 1, aperture_size ×aperture_size separable kernel will be used to calculate the derivative. For aperture_size=1 3x1 or 1x3 kernel is used (Gaussian smoothing is not done). There is also special value CV_SCHARR (=-1) that corresponds to 3x3 Scharr filter that may give more accurate results than 3x3 Sobel. Scharr aperture is:
| -3 0  3|
|-10 0 10|
| -3 0  3|
for x-derivative or transposed for y-derivative.

The function cvSobel calculates the image derivative by convolving the image with the appropriate kernel:

dst(x,y) = dxorder+yodersrc/dxxorder•dyyorder |(x,y)
The Sobel operators combine Gaussian smoothing and differentiation so the result is more or less robust to the noise. Most often, the function is called with (xorder=1, yorder=0, aperture_size=3) or (xorder=0, yorder=1, aperture_size=3) to calculate first x- or y- image derivative. The first case corresponds to

  |-1  0  1|
  |-2  0  2|
  |-1  0  1|

kernel and the second one corresponds to

  |-1 -2 -1|
  | 0  0  0|
  | 1  2  1|
or
  | 1  2  1|
  | 0  0  0|
  |-1 -2 -1|
kernel, depending on the image origin (origin field of IplImage structure). No scaling is done, so the destination image usually has larger by absolute value numbers than the source image. To avoid overflow, the function requires 16-bit destination image if the source image is 8-bit. The result can be converted back to 8-bit using cvConvertScale or cvConvertScaleAbs functions. Besides 8-bit images the function can process 32-bit floating-point images. Both source and destination must be single-channel images of equal size or ROI size.


Laplace

Calculates Laplacian of the image

void cvLaplace( const CvArr* src, CvArr* dst, int aperture_size=3 );

src
Source image.
dst
Destination image.
aperture_size
Aperture size (it has the same meaning as in cvSobel).

The function cvLaplace calculates Laplacian of the source image by summing second x- and y- derivatives calculated using Sobel operator:

dst(x,y) = d2src/dx2 + d2src/dy2

Specifying aperture_size=1 gives the fastest variant that is equal to convolving the image with the following kernel:

|0  1  0|
|1 -4  1|
|0  1  0|

Similar to cvSobel function, no scaling is done and the same combinations of input and output formats are supported.


Canny

Implements Canny algorithm for edge detection

void cvCanny( const CvArr* image, CvArr* edges, double threshold1,
              double threshold2, int aperture_size=3 );

image
Input image.
edges
Image to store the edges found by the function.
threshold1
The first threshold.
threshold2
The second threshold.
aperture_size
Aperture parameter for Sobel operator (see cvSobel).

The function cvCanny finds the edges on the input image image and marks them in the output image edges using the Canny algorithm. The smallest of threshold1 and threshold2 is used for edge linking, the largest - to find initial segments of strong edges.


PreCornerDetect

Calculates feature map for corner detection

void cvPreCornerDetect( const CvArr* image, CvArr* corners, int aperture_size=3 );

image
Input image.
corners
Image to store the corner candidates.
aperture_size
Aperture parameter for Sobel operator (see cvSobel).

The function cvPreCornerDetect calculates the function Dx2Dyy+Dy2Dxx - 2DxDyDxy where D? denotes one of the first image derivatives and D?? denotes a second image derivative. The corners can be found as local maximums of the function:

// assume that the image is floating-point
IplImage* corners = cvCloneImage(image);
IplImage* dilated_corners = cvCloneImage(image);
IplImage* corner_mask = cvCreateImage( cvGetSize(image), 8, 1 );
cvPreCornerDetect( image, corners, 3 );
cvDilate( corners, dilated_corners, 0, 1 );
cvSubS( corners, dilated_corners, corners );
cvCmpS( corners, 0, corner_mask, CV_CMP_GE );
cvReleaseImage( &corners );
cvReleaseImage( &dilated_corners );

CornerEigenValsAndVecs

Calculates eigenvalues and eigenvectors of image blocks for corner detection

void cvCornerEigenValsAndVecs( const CvArr* image, CvArr* eigenvv,
                               int block_size, int aperture_size=3 );

image
Input image.
eigenvv
Image to store the results. It must be 6 times wider than the input image.
block_size
Neighborhood size (see discussion).
aperture_size
Aperture parameter for Sobel operator (see cvSobel).

For every pixel The function cvCornerEigenValsAndVecs considers block_size × block_size neigborhood S(p). It calcualtes covariation matrix of derivatives over the neigborhood as:

    | sumS(p)(dI/dx)2   sumS(p)(dI/dx•dI/dy)|
M = |                                 |
    | sumS(p)(dI/dx•dI/dy)  sumS(p)(dI/dy)2 |

After that it finds eigenvectors and eigenvalues of the matrix and stores them into destination image in form (λ1, λ2, x1, y1, x2, y2), where
λ1, λ2 - eigenvalues of M; not sorted
(x1, y1) - eigenvector corresponding to λ1
(x2, y2) - eigenvector corresponding to λ2


CornerMinEigenVal

Calculates minimal eigenvalue of gradient matrices for corner detection

void cvCornerMinEigenVal( const CvArr* image, CvArr* eigenval, int block_size, int aperture_size=3 );

image
Input image.
eigenval
Image to store the minimal eigen values. Should have the same size as image
block_size
Neighborhood size (see discussion of cvCornerEigenValsAndVecs).
aperture_size
Aperture parameter for Sobel operator (see cvSobel). format. In the case of floating-point input format this parameter is the number of the fixed float filter used for differencing.

The function cvCornerMinEigenVal is similar to cvCornerEigenValsAndVecs but it calculates and stores only the minimal eigen value of derivative covariation matrix for every pixel, i.e. min(λ1, λ2) in terms of the previous function.


CornerHarris

Harris edge detector

void cvCornerHarris( const CvArr* image, CvArr* harris_responce,
                     int block_size, int aperture_size=3, double k=0.04 );

image
Input image.
harris_responce
Image to store the Harris detector responces. Should have the same size as image
block_size
Neighborhood size (see discussion of cvCornerEigenValsAndVecs).
aperture_size
Aperture parameter for Sobel operator (see cvSobel). format. In the case of floating-point input format this parameter is the number of the fixed float filter used for differencing.
k
Harris detector free parameter. See the formula below.

The function cvCornerHarris runs the Harris edge detector on image. Similarly to cvCornerMinEigenVal and cvCornerEigenValsAndVecs, for each pixel it calculates 2x2 gradient covariation matrix M over block_size×block_size neighborhood. Then, it stores

det(M) - k*trace(M)2
to the destination image. Corners in the image can be found as local maxima of the destination image.


FindCornerSubPix

Refines corner locations

void cvFindCornerSubPix( const CvArr* image, CvPoint2D32f* corners,
                         int count, CvSize win, CvSize zero_zone,
                         CvTermCriteria criteria );

image
Input image.
corners
Initial coordinates of the input corners and refined coordinates on output.
count
Number of corners.
win
Half sizes of the search window. For example, if win=(5,5) then 5*2+1 × 5*2+1 = 11 × 11 search window is used.
zero_zone
Half size of the dead region in the middle of the search zone over which the summation in formulae below is not done. It is used sometimes to avoid possible singularities of the autocorrelation matrix. The value of (-1,-1) indicates that there is no such size.
criteria
Criteria for termination of the iterative process of corner refinement. That is, the process of corner position refinement stops either after certain number of iteration or when a required accuracy is achieved. The criteria may specify either of or both the maximum number of iteration and the required accuracy.

The function cvFindCornerSubPix iterates to find the sub-pixel accurate location of corners, or radial saddle points, as shown in on the picture below.

Sub-pixel accurate corner locator is based on the observation that every vector from the center q to a point p located within a neighborhood of q is orthogonal to the image gradient at p subject to image and measurement noise. Consider the expression:

εi=DIpiT•(q-pi)
where DIpi is the image gradient at the one of the points pi in a neighborhood of q. The value of q is to be found such that εi is minimized. A system of equations may be set up with εi' set to zero:

sumi(DIpi•DIpiT)•q - sumi(DIpi•DIpiT•pi) = 0

where the gradients are summed within a neighborhood ("search window") of q. Calling the first gradient term G and the second gradient term b gives:

q=G-1•b

The algorithm sets the center of the neighborhood window at this new center q and then iterates until the center keeps within a set threshold.


GoodFeaturesToTrack

Determines strong corners on image

void cvGoodFeaturesToTrack( const CvArr* image, CvArr* eig_image, CvArr* temp_image,
                            CvPoint2D32f* corners, int* corner_count,
                            double quality_level, double min_distance,
                            const CvArr* mask=NULL, int block_size=3,
                            int use_harris=0, double k=0.04 );

image
The source 8-bit or floating-point 32-bit, single-channel image.
eig_image
Temporary floating-point 32-bit image of the same size as image.
temp_image
Another temporary image of the same size and same format as eig_image.
corners
Output parameter. Detected corners.
corner_count
Output parameter. Number of detected corners.
quality_level
Multiplier for the maxmin eigenvalue; specifies minimal accepted quality of image corners.
min_distance
Limit, specifying minimum possible distance between returned corners; Euclidian distance is used.
mask
Region of interest. The function selects points either in the specified region or in the whole image if the mask is NULL.
block_size
Size of the averaging block, passed to underlying cvCornerMinEigenVal or cvCornerHarris used by the function.
use_harris
If nonzero, Harris operator (cvCornerHarris) is used instead of default cvCornerMinEigenVal.
k
Free parameter of Harris detector; used only if use_harris≠0

The function cvGoodFeaturesToTrack finds corners with big eigenvalues in the image. The function first calculates the minimal eigenvalue for every source image pixel using cvCornerMinEigenVal function and stores them in eig_image. Then it performs non-maxima suppression (only local maxima in 3x3 neighborhood remain). The next step is rejecting the corners with the minimal eigenvalue less than quality_level•max(eig_image(x,y)). Finally, the function ensures that all the corners found are distanced enough one from another by considering the corners (the most strongest corners are considered first) and checking that the distance between the newly considered feature and the features considered earlier is larger than min_distance. So, the function removes the features than are too close to the stronger features.


Sampling, Interpolation and Geometrical Transforms


SampleLine

Reads raster line to buffer

int cvSampleLine( const CvArr* image, CvPoint pt1, CvPoint pt2,
                  void* buffer, int connectivity=8 );

image
Image to sample the line from.
pt1
Starting the line point.
pt2
Ending the line point.
buffer
Buffer to store the line points; must have enough size to store max( |pt2.x-pt1.x|+1, |pt2.y-pt1.y|+1 ) points in case of 8-connected line and |pt2.x-pt1.x|+|pt2.y-pt1.y|+1 in case of 4-connected line.
connectivity
The line connectivity, 4 or 8.

The function cvSampleLine implements a particular case of application of line iterators. The function reads all the image points lying on the line between pt1 and pt2, including the ending points, and stores them into the buffer.


GetRectSubPix

Retrieves pixel rectangle from image with sub-pixel accuracy

void cvGetRectSubPix( const CvArr* src, CvArr* dst, CvPoint2D32f center );

src
Source image.
dst
Extracted rectangle.
center
Floating point coordinates of the extracted rectangle center within the source image. The center must be inside the image.

The function cvGetRectSubPix extracts pixels from src:

dst(x, y) = src(x + center.x - (width(dst)-1)*0.5, y + center.y - (height(dst)-1)*0.5)

where the values of pixels at non-integer coordinates are retrieved using bilinear interpolation. Every channel of multiple-channel images is processed independently. Whereas the rectangle center must be inside the image, the whole rectangle may be partially occluded. In this case, the replication border mode is used to get pixel values beyond the image boundaries.


GetQuadrangleSubPix

Retrieves pixel quadrangle from image with sub-pixel accuracy

void cvGetQuadrangleSubPix( const CvArr* src, CvArr* dst, const CvMat* map_matrix );

src
Source image.
dst
Extracted quadrangle.
map_matrix
The transformation 2 × 3 matrix [A|b] (see the discussion).

The function cvGetQuadrangleSubPix extracts pixels from src at sub-pixel accuracy and stores them to dst as follows:

dst(x, y)= src( A11x'+A12y'+b1, A21x'+A22y'+b2),

where A and b are taken from map_matrix
             | A11 A12  b1 |
map_matrix = |            |
             | A21 A22  b2 |,

x'=x-(width(dst)-1)*0.5, y'=y-(height(dst)-1)*0.5

where the values of pixels at non-integer coordinates A•(x,y)T+b are retrieved using bilinear interpolation. When the function needs pixels outside of the image, it uses replication border mode to reconstruct the values. Every channel of multiple-channel images is processed independently.


Resize

Resizes image

void cvResize( const CvArr* src, CvArr* dst, int interpolation=CV_INTER_LINEAR );

src
Source image.
dst
Destination image.
interpolation
Interpolation method:

The function cvResize resizes image src so that it fits exactly to dst. If ROI is set, the function consideres the ROI as supported as usual.


WarpAffine

Applies affine transformation to the image

void cvWarpAffine( const CvArr* src, CvArr* dst, const CvMat* map_matrix,
                   int flags=CV_INTER_LINEAR+CV_WARP_FILL_OUTLIERS,
                   CvScalar fillval=cvScalarAll(0) );

src
Source image.
dst
Destination image.
map_matrix
2×3 transformation matrix.
flags
A combination of interpolation method and the following optional flags:
fillval
A value used to fill outliers.

The function cvWarpAffine transforms source image using the specified matrix:

dst(x’,y’)<-src(x,y)
(x’,y’)T=map_matrix•(x,y,1)T+b if CV_WARP_INVERSE_MAP is not set,
(x, y)T=map_matrix•(x’,y&apos,1)T+b otherwise

The function is similar to cvGetQuadrangleSubPix but they are not exactly the same. cvWarpAffine requires input and output image have the same data type, has larger overhead (so it is not quite suitable for small images) and can leave part of destination image unchanged. While cvGetQuadrangleSubPix may extract quadrangles from 8-bit images into floating-point buffer, has smaller overhead and always changes the whole destination image content.

To transform a sparse set of points, use cvTransform function from cxcore.


2DRotationMatrix

Calculates affine matrix of 2d rotation

CvMat* cv2DRotationMatrix( CvPoint2D32f center, double angle,
                           double scale, CvMat* map_matrix );

center
Center of the rotation in the source image.
angle
The rotation angle in degrees. Positive values mean couter-clockwise rotation (the coordiate origin is assumed at top-left corner).
scale
Isotropic scale factor.
map_matrix
Pointer to the destination 2×3 matrix.

The function cv2DRotationMatrix calculates matrix:

[  α  β  |  (1-α)*center.x - β*center.y ]
[ -β  α  |  β*center.x + (1-α)*center.y ]

where α=scale*cos(angle), β=scale*sin(angle)

The transformation maps the rotation center to itself. If this is not the purpose, the shift should be adjusted.


WarpPerspective

Applies perspective transformation to the image

void cvWarpPerspective( const CvArr* src, CvArr* dst, const CvMat* map_matrix,
                        int flags=CV_INTER_LINEAR+CV_WARP_FILL_OUTLIERS,
                        CvScalar fillval=cvScalarAll(0) );

src
Source image.
dst
Destination image.
map_matrix
3×3 transformation matrix.
flags
A combination of interpolation method and the following optional flags:
fillval
A value used to fill outliers.

The function cvWarpPerspective transforms source image using the specified matrix:

dst(x’,y’)<-src(x,y)
(t•x’,t•y’,t)T=map_matrix•(x,y,1)T+b if CV_WARP_INVERSE_MAP is not set,
(t•x, t•y, t)T=map_matrix•(x’,y&apos,1)T+b otherwise

For a sparse set of points use cvPerspectiveTransform function from cxcore.


WarpPerspectiveQMatrix

Calculates perspective transform from 4 corresponding points

CvMat* cvWarpPerspectiveQMatrix( const CvPoint2D32f* src,
                                 const CvPoint2D32f* dst,
                                 CvMat* map_matrix );

src
Coordinates of 4 quadrangle vertices in the source image.
dst
Coordinates of the 4 corresponding quadrangle vertices in the destination image.
map_matrix
Pointer to the destination 3×3 matrix.

The function cvWarpPerspectiveQMatrix calculates matrix of perspective transform such that:

(ti•x'i,ti•y'i,ti)T=map_matrix•(xi,yi,1)T

where dst(i)=(x'i,y'i), src(i)=(xi,yi), i=0..3.


Remap

Applies generic geometrical transformation to the image

void cvRemap( const CvArr* src, CvArr* dst,
              const CvArr* mapx, const CvArr* mapy,
              int flags=CV_INTER_LINEAR+CV_WARP_FILL_OUTLIERS,
              CvScalar fillval=cvScalarAll(0) );

src
Source image.
dst
Destination image.
mapx
The map of x-coordinates (32fC1 image).
mapy
The map of y-coordinates (32fC1 image).
flags
A combination of interpolation method and the following optional flag(s):
fillval
A value used to fill outliers.

The function cvRemap transforms source image using the specified map:

dst(x,y)<-src(mapx(x,y),mapy(x,y))

Similar to other geometrical transformations, some interpolation method (specified by user) is used to extract pixels with non-integer coordinates.


LogPolar

Remaps image to log-polar space

void cvLogPolar( const CvArr* src, CvArr* dst,
                 CvPoint2D32f center, double M,
                 int flags=CV_INTER_LINEAR+CV_WARP_FILL_OUTLIERS );

src
Source image.
dst
Destination image.
center
The transformation center, where the output precision is maximal.
M
Magnitude scale parameter. See below.
flags
A combination of interpolation method and the following optional flags:
fillval
A value used to fill outliers.

The function cvLogPolar transforms source image using the following transformation:

Forward transformation (CV_WARP_INVERSE_MAP is not set):
dst(phi,rho)<-src(x,y)

Inverse transformation (CV_WARP_INVERSE_MAP is set):
dst(x,y)<-src(phi,rho),

where rho=M*log(sqrt(x2+y2))
      phi=atan(y/x)

The function emulates the human "foveal" vision and can be used for fast scale and rotation-invariant template matching, for object tracking etc.

Example. Log-polar transformation.

#include <cv.h>
#include <highgui.h>

int main(int argc, char** argv)
{
    IplImage* src;

    if( argc == 2 && (src=cvLoadImage(argv[1],1) != 0 )
    {
        IplImage* dst = cvCreateImage( cvSize(256,256), 8, 3 );
        IplImage* src2 = cvCreateImage( cvGetSize(src), 8, 3 );
        cvLogPolar( src, dst, cvPoint2D32f(src->width/2,src->height/2), 40, CV_INTER_LINEAR+CV_WARP_FILL_OUTLIERS );
        cvLogPolar( dst, src2, cvPoint2D32f(src->width/2,src->height/2), 40, CV_INTER_LINEAR+CV_WARP_FILL_OUTLIERS+CV_WARP_INVERSE_MAP );
        cvNamedWindow( "log-polar", 1 );
        cvShowImage( "log-polar", dst );
        cvNamedWindow( "inverse log-polar", 1 );
        cvShowImage( "inverse log-polar", src2 );
        cvWaitKey();
    }
    return 0;
}

And this is what the program displays when opencv/samples/c/fruits.jpg is passed to it


Morphological Operations


CreateStructuringElementEx

Creates structuring element

IplConvKernel* cvCreateStructuringElementEx( int cols, int rows, int anchor_x, int anchor_y,
                                             int shape, int* values=NULL );

cols
Number of columns in the structuring element.
rows
Number of rows in the structuring element.
anchor_x
Relative horizontal offset of the anchor point.
anchor_y
Relative vertical offset of the anchor point.
shape
Shape of the structuring element; may have the following values:
values
Pointer to the structuring element data, a plane array, representing row-by-row scanning of the element matrix. Non-zero values indicate points that belong to the element. If the pointer is NULL, then all values are considered non-zero, that is, the element is of a rectangular shape. This parameter is considered only if the shape is CV_SHAPE_CUSTOM .

The function cv CreateStructuringElementEx allocates and fills the structure IplConvKernel, which can be used as a structuring element in the morphological operations.


ReleaseStructuringElement

Deletes structuring element

void cvReleaseStructuringElement( IplConvKernel** element );

element
Pointer to the deleted structuring element.

The function cvReleaseStructuringElement releases the structure IplConvKernel that is no longer needed. If *element is NULL, the function has no effect.


Erode

Erodes image by using arbitrary structuring element

void cvErode( const CvArr* src, CvArr* dst, IplConvKernel* element=NULL, int iterations=1 );

src
Source image.
dst
Destination image.
element
Structuring element used for erosion. If it is NULL, a 3×3 rectangular structuring element is used.
iterations
Number of times erosion is applied.

The function cvErode erodes the source image using the specified structuring element that determines the shape of a pixel neighborhood over which the minimum is taken:

dst=erode(src,element):  dst(x,y)=min((x',y') in element))src(x+x',y+y')

The function supports the in-place mode. Erosion can be applied several (iterations) times. In case of color image each channel is processed independently.


Dilate

Dilates image by using arbitrary structuring element

void cvDilate( const CvArr* src, CvArr* dst, IplConvKernel* element=NULL, int iterations=1 );

src
Source image.
dst
Destination image.
element
Structuring element used for erosion. If it is NULL, a 3×3 rectangular structuring element is used.
iterations
Number of times erosion is applied.

The function cvDilate dilates the source image using the specified structuring element that determines the shape of a pixel neighborhood over which the maximum is taken:

dst=dilate(src,element):  dst(x,y)=max((x',y') in element))src(x+x',y+y')

The function supports the in-place mode. Dilation can be applied several (iterations) times. In case of color image each channel is processed independently.


MorphologyEx

Performs advanced morphological transformations

void cvMorphologyEx( const CvArr* src, CvArr* dst, CvArr* temp,
                     IplConvKernel* element, int operation, int iterations=1 );

src
Source image.
dst
Destination image.
temp
Temporary image, required in some cases.
element
Structuring element.
operation
Type of morphological operation, one of:
CV_MOP_OPEN - opening
CV_MOP_CLOSE - closing
CV_MOP_GRADIENT - morphological gradient
CV_MOP_TOPHAT - "top hat"
CV_MOP_BLACKHAT - "black hat"
iterations
Number of times erosion and dilation are applied.

The function cvMorphologyEx can perform advanced morphological transformations using erosion and dilation as basic operations.

Opening:
dst=open(src,element)=dilate(erode(src,element),element)

Closing:
dst=close(src,element)=erode(dilate(src,element),element)

Morphological gradient:
dst=morph_grad(src,element)=dilate(src,element)-erode(src,element)

"Top hat":
dst=tophat(src,element)=src-open(src,element)

"Black hat":
dst=blackhat(src,element)=close(src,element)-src

The temporary image temp is required for morphological gradient and, in case of in-place operation, for "top hat" and "black hat".


Filters and Color Conversion


Smooth

Smooths the image in one of several ways

void cvSmooth( const CvArr* src, CvArr* dst,
               int smoothtype=CV_GAUSSIAN,
               int param1=3, int param2=0, double param3=0 );

src
The source image.
dst
The destination image.
smoothtype
Type of the smoothing:
param1
The first parameter of smoothing operation.
param2
The second parameter of smoothing operation. In case of simple scaled/non-scaled and Gaussian blur if param2 is zero, it is set to param1.
param3
In case of Gaussian parameter this parameter may specify Gaussian sigma (standard deviation). If it is zero, it is calculated from the kernel size:
              sigma = (n/2 - 1)*0.3 + 0.8, where n=param1 for horizontal kernel,
                                                 n=param2 for vertical kernel.
              
Using standard sigma for small kernels (3×3 to 7×7) gives better speed. If param3 is not zero, while param1 and param2 are zeros, the kernel size is calculated from the sigma (to provide accurate enough operation).

The function cvSmooth smooths image using one of several methods. Every of the methods has some features and restrictions listed below

Blur with no scaling works with single-channel images only and supports accumulation of 8-bit to 16-bit format (similar to cvSobel and cvLaplace) and 32-bit floating point to 32-bit floating-point format.

Simple blur and Gaussian blur support 1- or 3-channel, 8-bit and 32-bit floating point images. These two methods can process images in-place.

Median and bilateral filters work with 1- or 3-channel 8-bit images and can not process images in-place.


Filter2D

Convolves image with the kernel

void cvFilter2D( const CvArr* src, CvArr* dst,
                 const CvMat* kernel,
                 CvPoint anchor=cvPoint(-1,-1));

src
The source image.
dst
The destination image.
kernel
Convolution kernel, single-channel floating point matrix. If you want to apply different kernels to different channels, split the image using cvSplit into separate color planes and process them individually.
anchor
The anchor of the kernel that indicates the relative position of a filtered point within the kernel. The anchor shoud lie within the kernel. The special default value (-1,-1) means that it is at the kernel center.

The function cvFilter2D applies arbitrary linear filter to the image. In-place operation is supported. When the aperture is partially outside the image, the function interpolates outlier pixel values from the nearest pixels that is inside the image.


CopyMakeBorder

Copies image and makes border around it

void cvCopyMakeBorder( const CvArr* src, CvArr* dst, CvPoint offset,
                       int bordertype, CvScalar value=cvScalarAll(0) );

src
The source image.
dst
The destination image.
offset
Coordinates of the top-left corner (or bottom-left in case of images with bottom-left origin) of the destination image rectangle where the source image (or its ROI) is copied. Size of the rectanlge matches the source image size/ROI size.
bordertype
Type of the border to create around the copied source image rectangle:
IPL_BORDER_CONSTANT - border is filled with the fixed value, passed as last parameter of the function.
IPL_BORDER_REPLICATE - the pixels from the top and bottom rows, the left-most and right-most columns are replicated to fill the border.
(The other two border types from IPL, IPL_BORDER_REFLECT and IPL_BORDER_WRAP, are currently unsupported).
value
Value of the border pixels if bordertype=IPL_BORDER_CONSTANT.

The function cvCopyMakeBorder copies the source 2D array into interior of destination array and makes a border of the specified type around the copied area. The function is useful when one needs to emulate border type that is different from the one embedded into a specific algorithm implementation. For example, morphological functions, as well as most of other filtering functions in OpenCV, internally use replication border type, while the user may need zero border or a border, filled with 1's or 255's.


Integral

Calculates integral images

void cvIntegral( const CvArr* image, CvArr* sum, CvArr* sqsum=NULL, CvArr* tilted_sum=NULL );

image
The source image, W×H, 8-bit or floating-point (32f or 64f) image.
sum
The integral image, W+1×H+1, 32-bit integer or double precision floating-point (64f).
sqsum
The integral image for squared pixel values, W+1×H+1, double precision floating-point (64f).
tilted_sum
The integral for the image rotated by 45 degrees, W+1×H+1, the same data type as sum.

The function cvIntegral calculates one or more integral images for the source image as following:

sum(X,Y)=sumx<X,y<Yimage(x,y)

sqsum(X,Y)=sumx<X,y<Yimage(x,y)2

tilted_sum(X,Y)=sumy<Y,abs(x-X)<yimage(x,y)

Using these integral images, one may calculate sum, mean, standard deviation over arbitrary up-right or rotated rectangular region of the image in a constant time, for example:

sumx1<=x<x2,y1<=y<y2image(x,y)=sum(x2,y2)-sum(x1,y2)-sum(x2,y1)+sum(x1,x1)

It makes possible to do a fast blurring or fast block correlation with variable window size etc. In case of multi-channel images sums for each channel are accumulated independently.


CvtColor

Converts image from one color space to another

void cvCvtColor( const CvArr* src, CvArr* dst, int code );

src
The source 8-bit (8u), 16-bit (16u) or single-precision floating-point (32f) image.
dst
The destination image of the same data type as the source one. The number of channels may be different.
code
Color conversion operation that can be specifed using CV_<src_color_space>2<dst_color_space> constants (see below).

The function cvCvtColor converts input image from one color space to another. The function ignores colorModel and channelSeq fields of IplImage header, so the source image color space should be specified correctly (including order of the channels in case of RGB space, e.g. BGR means 24-bit format with B0 G0 R0 B1 G1 R1 ... layout, whereas RGB means 24-format with R0 G0 B0 R1 G1 B1 ... layout).

The conventional range for R,G,B channel values is:

Of course, in case of linear transformations the range can be arbitrary, but in order to get correct results in case of non-linear transformations, the input image should be scaled if necessary.

The function can do the following transformations:


Threshold

Applies fixed-level threshold to array elements

void cvThreshold( const CvArr* src, CvArr* dst, double threshold,
                  double max_value, int threshold_type );

src
Source array (single-channel, 8-bit of 32-bit floating point).
dst
Destination array; must be either the same type as src or 8-bit.
threshold
Threshold value.
max_value
Maximum value to use with CV_THRESH_BINARY and CV_THRESH_BINARY_INV thresholding types.
threshold_type
Thresholding type (see the discussion)

The function cvThreshold applies fixed-level thresholding to single-channel array. The function is typically used to get bi-level (binary) image out of grayscale image (cvCmpS could be also used for this purpose) or for removing a noise, i.e. filtering out pixels with too small or too large values. There are several types of thresholding the function supports that are determined by threshold_type:

threshold_type=CV_THRESH_BINARY:
dst(x,y) = max_value, if src(x,y)>threshold
           0, otherwise

threshold_type=CV_THRESH_BINARY_INV:
dst(x,y) = 0, if src(x,y)>threshold
           max_value, otherwise

threshold_type=CV_THRESH_TRUNC:
dst(x,y) = threshold, if src(x,y)>threshold
           src(x,y), otherwise

threshold_type=CV_THRESH_TOZERO:
dst(x,y) = src(x,y), if src(x,y)>threshold
           0, otherwise

threshold_type=CV_THRESH_TOZERO_INV:
dst(x,y) = 0, if src(x,y)>threshold
           src(x,y), otherwise

And this is the visual description of thresholding types:


AdaptiveThreshold

Applies adaptive threshold to array

void cvAdaptiveThreshold( const CvArr* src, CvArr* dst, double max_value,
                          int adaptive_method=CV_ADAPTIVE_THRESH_MEAN_C,
                          int threshold_type=CV_THRESH_BINARY,
                          int block_size=3, double param1=5 );

src
Source image.
dst
Destination image.
max_value
Maximum value that is used with CV_THRESH_BINARY and CV_THRESH_BINARY_INV.
adaptive_method
Adaptive thresholding algorithm to use: CV_ADAPTIVE_THRESH_MEAN_C or CV_ADAPTIVE_THRESH_GAUSSIAN_C (see the discussion).
threshold_type
Thresholding type; must be one of
block_size
The size of a pixel neighborhood that is used to calculate a threshold value for the pixel: 3, 5, 7, ...
param1
The method-dependent parameter. For the methods CV_ADAPTIVE_THRESH_MEAN_C and CV_ADAPTIVE_THRESH_GAUSSIAN_C it is a constant subtracted from mean or weighted mean (see the discussion), though it may be negative.

The function cvAdaptiveThreshold transforms grayscale image to binary image according to the formulae:

threshold_type=CV_THRESH_BINARY:
dst(x,y) = max_value, if src(x,y)>T(x,y)
           0, otherwise

threshold_type=CV_THRESH_BINARY_INV:
dst(x,y) = 0, if src(x,y)>T(x,y)
           max_value, otherwise

where TI is a threshold calculated individually for each pixel.

For the method CV_ADAPTIVE_THRESH_MEAN_C it is a mean of block_size × block_size pixel neighborhood, subtracted by param1.

For the method CV_ADAPTIVE_THRESH_GAUSSIAN_C it is a weighted sum (gaussian) of block_size × block_size pixel neighborhood, subtracted by param1.


Pyramids and the Applications


PyrDown

Downsamples image

void cvPyrDown( const CvArr* src, CvArr* dst, int filter=CV_GAUSSIAN_5x5 );

src
The source image.
dst
The destination image, should have 2x smaller width and height than the source.
filter
Type of the filter used for convolution; only CV_GAUSSIAN_5x5 is currently supported.

The function cvPyrDown performs downsampling step of Gaussian pyramid decomposition. First it convolves source image with the specified filter and then downsamples the image by rejecting even rows and columns.


PyrUp

Upsamples image

void cvPyrUp( const CvArr* src, CvArr* dst, int filter=CV_GAUSSIAN_5x5 );

src
The source image.
dst
The destination image, should have 2x smaller width and height than the source.
filter
Type of the filter used for convolution; only CV_GAUSSIAN_5x5 is currently supported.

The function cvPyrUp performs up-sampling step of Gaussian pyramid decomposition. First it upsamples the source image by injecting even zero rows and columns and then convolves result with the specified filter multiplied by 4 for interpolation. So the destination image is four times larger than the source image.


PyrSegmentation

Implements image segmentation by pyramids

void cvPyrSegmentation( IplImage* src, IplImage* dst,
                        CvMemStorage* storage, CvSeq** comp,
                        int level, double threshold1, double threshold2 );

src
The source image.
dst
The destination image.
storage
Storage; stores the resulting sequence of connected components.
comp
Pointer to the output sequence of the segmented components.
level
Maximum level of the pyramid for the segmentation.
threshold1
Error threshold for establishing the links.
threshold2
Error threshold for the segments clustering.

The function cvPyrSegmentation implements image segmentation by pyramids. The pyramid builds up to the level level. The links between any pixel a on level i and its candidate father pixel b on the adjacent level are established if

p(c(a),c(b))<threshold1. After the connected components are defined, they are joined into several clusters. Any two segments A and B belong to the same cluster, if
p(c(A),c(B))<threshold2. The input image has only one channel, then
p(c¹,c²)=|c¹-c²|. If the input image has three channels (red, green and blue), then
p(c¹,c²)=0,3·(c¹r-c²r)+0,59·(c¹g-c²g)+0,11·(c¹b-c²b) . There may be more than one connected component per a cluster.
The images src and dst should be 8-bit single-channel or 3-channel images or equal size


Connected Components


CvConnectedComp

Connected component

    typedef struct CvConnectedComp
    {
        double area; /* area of the segmented component */
        float value; /* gray scale value of the segmented component */
        CvRect rect; /* ROI of the segmented component */
    } CvConnectedComp;

FloodFill

Fills a connected component with given color

void cvFloodFill( CvArr* image, CvPoint seed_point, CvScalar new_val,
                  CvScalar lo_diff=cvScalarAll(0), CvScalar up_diff=cvScalarAll(0),
                  CvConnectedComp* comp=NULL, int flags=4, CvArr* mask=NULL );
#define CV_FLOODFILL_FIXED_RANGE (1 << 16)
#define CV_FLOODFILL_MASK_ONLY   (1 << 17)

image
Input 1- or 3-channel, 8-bit or floating-point image. It is modified by the function unless CV_FLOODFILL_MASK_ONLY flag is set (see below).
seed_point
The starting point.
new_val
New value of repainted domain pixels.
lo_diff
Maximal lower brightness/color difference between the currently observed pixel and one of its neighbor belong to the component or seed pixel to add the pixel to component. In case of 8-bit color images it is packed value.
up_diff
Maximal upper brightness/color difference between the currently observed pixel and one of its neighbor belong to the component or seed pixel to add the pixel to component. In case of 8-bit color images it is packed value.
comp
Pointer to structure the function fills with the information about the repainted domain.
flags
The operation flags. Lower bits contain connectivity value, 4 (by default) or 8, used within the function. Connectivity determines which neighbors of a pixel are considered. Upper bits can be 0 or combination of the following flags:
  • CV_FLOODFILL_FIXED_RANGE - if set the difference between the current pixel and seed pixel is considered, otherwise difference between neighbor pixels is considered (the range is floating).
  • CV_FLOODFILL_MASK_ONLY - if set, the function does not fill the image (new_val is ignored), but the fills mask (that must be non-NULL in this case).
mask
Operation mask, should be singe-channel 8-bit image, 2 pixels wider and 2 pixels taller than image. If not NULL, the function uses and updates the mask, so user takes responsibility of initializing mask content. Floodfilling can't go across non-zero pixels in the mask, for example, an edge detector output can be used as a mask to stop filling at edges. Or it is possible to use the same mask in multiple calls to the function to make sure the filled area do not overlap. Note: because mask is larger than the filled image, pixel in mask that corresponds to (x,y) pixel in image will have coordinates (x+1,y+1).

The function cvFloodFill fills a connected component starting from the seed point with the specified color. The connectivity is determined by the closeness of pixel values. The pixel at (x, y) is considered to belong to the repainted domain if:

src(x',y')-lo_diff<=src(x,y)<=src(x',y')+up_diff,     grayscale image, floating range
src(seed.x,seed.y)-lo<=src(x,y)<=src(seed.x,seed.y)+up_diff, grayscale image, fixed range

src(x',y')r-lo_diffr<=src(x,y)r<=src(x',y')r+up_diffr and
src(x',y')g-lo_diffg<=src(x,y)g<=src(x',y')g+up_diffg and
src(x',y')b-lo_diffb<=src(x,y)b<=src(x',y')b+up_diffb, color image, floating range

src(seed.x,seed.y)r-lo_diffr<=src(x,y)r<=src(seed.x,seed.y)r+up_diffr and
src(seed.x,seed.y)g-lo_diffg<=src(x,y)g<=src(seed.x,seed.y)g+up_diffg and
src(seed.x,seed.y)b-lo_diffb<=src(x,y)b<=src(seed.x,seed.y)b+up_diffb, color image, fixed range
where src(x',y') is value of one of pixel neighbors. That is, to be added to the connected component, a pixel’s color/brightness should be close enough to:
  • color/brightness of one of its neighbors that are already referred to the connected component in case of floating range
  • color/brightness of the seed point in case of fixed range.


FindContours

Finds contours in binary image

int cvFindContours( CvArr* image, CvMemStorage* storage, CvSeq** first_contour,
                    int header_size=sizeof(CvContour), int mode=CV_RETR_LIST,
                    int method=CV_CHAIN_APPROX_SIMPLE, CvPoint offset=cvPoint(0,0) );

image
The source 8-bit single channel image. Non-zero pixels are treated as 1’s, zero pixels remain 0’s - that is image treated as binary. To get such a binary image from grayscale, one may use cvThreshold, cvAdaptiveThreshold or cvCanny. The function modifies the source image content.
storage
Container of the retrieved contours.
first_contour
Output parameter, will contain the pointer to the first outer contour.
header_size
Size of the sequence header, >=sizeof(CvChain) if method=CV_CHAIN_CODE, and >=sizeof(CvContour) otherwise.
mode
Retrieval mode.
  • CV_RETR_EXTERNAL - retrive only the extreme outer contours
  • CV_RETR_LIST - retrieve all the contours and puts them in the list
  • CV_RETR_CCOMP - retrieve all the contours and organizes them into two-level hierarchy: top level are external boundaries of the components, second level are bounda boundaries of the holes
  • CV_RETR_TREE - retrieve all the contours and reconstructs the full hierarchy of nested contours
method
Approximation method (for all the modes, except CV_RETR_RUNS, which uses built-in approximation).
  • CV_CHAIN_CODE - output contours in the Freeman chain code. All other methods output polygons (sequences of vertices).
  • CV_CHAIN_APPROX_NONE - translate all the points from the chain code into points;
  • CV_CHAIN_APPROX_SIMPLE - compress horizontal, vertical, and diagonal segments, that is, the function leaves only their ending points;
  • CV_CHAIN_APPROX_TC89_L1,
    CV_CHAIN_APPROX_TC89_KCOS - apply one of the flavors of Teh-Chin chain approximation algorithm.
  • CV_LINK_RUNS - use completely different contour retrieval algorithm via linking of horizontal segments of 1’s. Only CV_RETR_LIST retrieval mode can be used with this method.
offset
Offset, by which every contour point is shifted. This is useful if the contours are extracted from the image ROI and then they should be analyzed in the whole image context.

The function cvFindContours retrieves contours from the binary image and returns the number of retrieved contours. The pointer first_contour is filled by the function. It will contain pointer to the first most outer contour or NULL if no contours is detected (if the image is completely black). Other contours may be reached from first_contour using h_next and v_next links. The sample in cvDrawContours discussion shows how to use contours for connected component detection. Contours can be also used for shape analysis and object recognition - see squares sample in CVPR 2001 tutorial course located at SourceForge site.


StartFindContours

Initializes contour scanning process

CvContourScanner cvStartFindContours( CvArr* image, CvMemStorage* storage,
                                      int header_size=sizeof(CvContour),
                                      int mode=CV_RETR_LIST,
                                      int method=CV_CHAIN_APPROX_SIMPLE,
                                      CvPoint offset=cvPoint(0,0) );

image
The source 8-bit single channel binary image.
storage
Container of the retrieved contours.
header_size
Size of the sequence header, >=sizeof(CvChain) if method=CV_CHAIN_CODE, and >=sizeof(CvContour) otherwise.
mode
Retrieval mode; see cvFindContours.
method
Approximation method. It has the same meaning as in cvFindContours, but CV_LINK_RUNS can not be used here.
offset
ROI offset; see cvFindContours.

The function cvStartFindContours initializes and returns pointer to the contour scanner. The scanner is used further in cvFindNextContour to retrieve the rest of contours.


FindNextContour

Finds next contour in the image

CvSeq* cvFindNextContour( CvContourScanner scanner );

scanner
Contour scanner initialized by The function cvStartFindContours .

The function cvFindNextContour locates and retrieves the next contour in the image and returns pointer to it. The function returns NULL, if there is no more contours.


SubstituteContour

Replaces retrieved contour

void cvSubstituteContour( CvContourScanner scanner, CvSeq* new_contour );

scanner
Contour scanner initialized by the function cvStartFindContours .
new_contour
Substituting contour.

The function cvSubstituteContour replaces the retrieved contour, that was returned from the preceding call of The function cvFindNextContour and stored inside the contour scanner state, with the user-specified contour. The contour is inserted into the resulting structure, list, two-level hierarchy, or tree, depending on the retrieval mode. If the parameter new_contour=NULL, the retrieved contour is not included into the resulting structure, nor all of its children that might be added to this structure later.


EndFindContours

Finishes scanning process

CvSeq* cvEndFindContours( CvContourScanner* scanner );

scanner
Pointer to the contour scanner.

The function cvEndFindContours finishes the scanning process and returns the pointer to the first contour on the highest level.


Image and Contour moments


Moments

Calculates all moments up to third order of a polygon or rasterized shape

void cvMoments( const CvArr* arr, CvMoments* moments, int binary=0 );

arr
Image (1-channel or 3-channel with COI set) or polygon (CvSeq of points or a vector of points).
moments
Pointer to returned moment state structure.
binary
(For images only) If the flag is non-zero, all the zero pixel values are treated as zeroes, all the others are treated as 1’s.

The function cvMoments calculates spatial and central moments up to the third order and writes them to moments. The moments may be used then to calculate gravity center of the shape, its area, main axises and various shape characeteristics including 7 Hu invariants.


GetSpatialMoment

Retrieves spatial moment from moment state structure

double cvGetSpatialMoment( CvMoments* moments, int x_order, int y_order );

moments
The moment state, calculated by cvMoments.
x_order
x order of the retrieved moment, x_order >= 0.
y_order
y order of the retrieved moment, y_order >= 0 and x_order + y_order <= 3.

The function cvGetSpatialMoment retrieves the spatial moment, which in case of image moments is defined as:

Mx_order,y_order=sumx,y(I(x,y)•xx_order•yy_order)

where I(x,y) is the intensity of the pixel (x, y).


GetCentralMoment

Retrieves central moment from moment state structure

double cvGetCentralMoment( CvMoments* moments, int x_order, int y_order );

moments
Pointer to the moment state structure.
x_order
x order of the retrieved moment, x_order >= 0.
y_order
y order of the retrieved moment, y_order >= 0 and x_order + y_order <= 3.

The function cvGetCentralMoment retrieves the central moment, which in case of image moments is defined as:

μx_order,y_order=sumx,y(I(x,y)•(x-xc)x_order•(y-yc)y_order),

where xc=M10/M00, yc=M01/M00 - coordinates of the gravity center


GetNormalizedCentralMoment

Retrieves normalized central moment from moment state structure

double cvGetNormalizedCentralMoment( CvMoments* moments, int x_order, int y_order );

moments
Pointer to the moment state structure.
x_order
x order of the retrieved moment, x_order >= 0.
y_order
y order of the retrieved moment, y_order >= 0 and x_order + y_order <= 3.

The function cvGetNormalizedCentralMoment retrieves the normalized central moment:

ηx_order,y_order= μx_order,y_order/M00((y_order+x_order)/2+1)

GetHuMoments

Calculates seven Hu invariants

void cvGetHuMoments( CvMoments* moments, CvHuMoments* hu_moments );

moments
Pointer to the moment state structure.
hu_moments
Pointer to Hu moments structure.

The function cvGetHuMoments calculates seven Hu invariants that are defined as:

 h12002

 h2=(η2002)²+4η11²

 h3=(η30-3η12)²+ (3η2103)²

 h4=(η3012)²+ (η2103)²

 h5=(η30-3η12)(η3012)[(η3012)²-3(η2103)²]+(3η2103)(η2103)[3(η3012)²-(η2103)²]

 h6=(η2002)[(η3012)²- (η2103)²]+4η113012)(η2103)

 h7=(3η2103)(η2103)[3(η3012)²-(η2103)²]-(η30-3η12)(η2103)[3(η3012)²-(η2103)²]

These values are proved to be invariants to the image scale, rotation, and reflection except the seventh one, whose sign is changed by reflection.


Special Image Transforms


HoughLines2

Finds lines in binary image using Hough transform

CvSeq* cvHoughLines2( CvArr* image, void* line_storage, int method,
                      double rho, double theta, int threshold,
                      double param1=0, double param2=0 );

image
The input 8-bit single-channel binary image. In case of probabilistic method the image is modified by the function.
line_storage
The storage for the lines detected. It can be a memory storage (in this case a sequence of lines is created in the storage and returned by the function) or single row/single column matrix (CvMat*) of a particular type (see below) to which the lines' parameters are written. The matrix header is modified by the function so its cols or rows will contain a number of lines detected. If line_storage is a matrix and the actual number of lines exceeds the matrix size, the maximum possible number of lines is returned (in case of standard hough transform the lines are sorted by the accumulator value).
method
The Hough transform variant, one of:
  • CV_HOUGH_STANDARD - classical or standard Hough transform. Every line is represented by two floating-point numbers (ρ, θ), where ρ is a distance between (0,0) point and the line, and θ is the angle between x-axis and the normal to the line. Thus, the matrix must be (the created sequence will be) of CV_32FC2 type.
  • CV_HOUGH_PROBABILISTIC - probabilistic Hough transform (more efficient in case if picture contains a few long linear segments). It returns line segments rather than the whole lines. Every segment is represented by starting and ending points, and the matrix must be (the created sequence will be) of CV_32SC4 type.
  • CV_HOUGH_MULTI_SCALE - multi-scale variant of classical Hough transform. The lines are encoded the same way as in CV_HOUGH_STANDARD.
rho
Distance resolution in pixel-related units.
theta
Angle resolution measured in radians.
threshold
Threshold parameter. A line is returned by the function if the corresponding accumulator value is greater than threshold.
param1
The first method-dependent parameter:
  • For classical Hough transform it is not used (0).
  • For probabilistic Hough transform it is the minimum line length.
  • For multi-scale Hough transform it is divisor for distance resolution rho. (The coarse distance resolution will be rho and the accurate resolution will be (rho / param1)).
param2
The second method-dependent parameter:
  • For classical Hough transform it is not used (0).
  • For probabilistic Hough transform it is the maximum gap between line segments lieing on the same line to treat them as the single line segment (i.e. to join them).
  • For multi-scale Hough transform it is divisor for angle resolution theta. (The coarse angle resolution will be theta and the accurate resolution will be (theta / param2)).

The function cvHoughLines2 implements a few variants of Hough transform for line detection.

Example. Detecting lines with Hough transform.

/* This is a standalone program. Pass an image name as a first parameter of the program.
   Switch between standard and probabilistic Hough transform by changing "#if 1" to "#if 0" and back */
#include <cv.h>
#include <highgui.h>
#include <math.h>

int main(int argc, char** argv)
{
    IplImage* src;
    if( argc == 2 && (src=cvLoadImage(argv[1], 0))!= 0)
    {
        IplImage* dst = cvCreateImage( cvGetSize(src), 8, 1 );
        IplImage* color_dst = cvCreateImage( cvGetSize(src), 8, 3 );
        CvMemStorage* storage = cvCreateMemStorage(0);
        CvSeq* lines = 0;
        int i;
        cvCanny( src, dst, 50, 200, 3 );
        cvCvtColor( dst, color_dst, CV_GRAY2BGR );
#if 1
        lines = cvHoughLines2( dst, storage, CV_HOUGH_STANDARD, 1, CV_PI/180, 100, 0, 0 );

        for( i = 0; i < MIN(lines->total,100); i++ )
        {
            float* line = (float*)cvGetSeqElem(lines,i);
            float rho = line[0];
            float theta = line[1];
            CvPoint pt1, pt2;
            double a = cos(theta), b = sin(theta);
            double x0 = a*rho, y0 = b*rho;
            pt1.x = cvRound(x0 + 1000*(-b));
            pt1.y = cvRound(y0 + 1000*(a));
            pt2.x = cvRound(x0 - 1000*(-b));
            pt2.y = cvRound(y0 - 1000*(a));
            cvLine( color_dst, pt1, pt2, CV_RGB(255,0,0), 3, 8 );
        }
#else
        lines = cvHoughLines2( dst, storage, CV_HOUGH_PROBABILISTIC, 1, CV_PI/180, 80, 30, 10 );
        for( i = 0; i < lines->total; i++ )
        {
            CvPoint* line = (CvPoint*)cvGetSeqElem(lines,i);
            cvLine( color_dst, line[0], line[1], CV_RGB(255,0,0), 3, 8 );
        }
#endif
        cvNamedWindow( "Source", 1 );
        cvShowImage( "Source", src );

        cvNamedWindow( "Hough", 1 );
        cvShowImage( "Hough", color_dst );

        cvWaitKey(0);
    }
}

This is the sample picture the function parameters have been tuned for:

And this is the output of the above program in case of probabilistic Hough transform ("#if 0" case):


HoughCircles

Finds circles in grayscale image using Hough transform

CvSeq* cvHoughCircles( CvArr* image, void* circle_storage,
                       int method, double dp, double min_dist,
                       double param1=100, double param2=100 );

image
The input 8-bit single-channel grayscale image.
circle_storage
The storage for the circles detected. It can be a memory storage (in this case a sequence of circles is created in the storage and returned by the function) or single row/single column matrix (CvMat*) of type CV_32FC3, to which the circles' parameters are written. The matrix header is modified by the function so its cols or rows will contain a number of lines detected. If circle_storage is a matrix and the actual number of lines exceeds the matrix size, the maximum possible number of circles is returned. Every circle is encoded as 3 floating-point numbers: center coordinates (x,y) and the radius.
method
Currently, the only implemented method is CV_HOUGH_GRADIENT, which is basically 21HT, described in [Yuen03].
dp
Resolution of the accumulator used to detect centers of the circles. For example, if it is 1, the accumulator will have the same resolution as the input image, if it is 2 - accumulator will have twice smaller width and height, etc.
min_dist
Minimum distance between centers of the detected circles. If the parameter is too small, multiple neighbor circles may be falsely detected in addition to a true one. If it is too large, some circles may be missed.
param1
The first method-specific parameter. In case of CV_HOUGH_GRADIENT it is the higher threshold of the two passed to Canny edge detector (the lower one will be twice smaller).
param2
The second method-specific parameter. In case of CV_HOUGH_GRADIENT it is accumulator threshold at the center detection stage. The smaller it is, the more false circles may be detected. Still, circles, corresponding to the larger accumulator values, will be returned first.

The function cvHoughCircles finds circles in grayscale image using some modification of Hough transform.

Example. Detecting circles with Hough transform.

#include <cv.h>
#include <highgui.h>
#include <math.h>

int main(int argc, char** argv)
{
    IplImage* img;
    if( argc == 2 && (img=cvLoadImage(argv[1], 1))!= 0)
    {
        IplImage* gray = cvCreateImage( cvGetSize(img), 8, 1 );
        CvMemStorage* storage = cvCreateMemStorage(0);
        cvCvtColor( img, gray, CV_BGR2GRAY );
        cvSmooth( gray, gray, CV_GAUSSIAN, 9, 9 ); // smooth it, otherwise a lot of false circles may be detected
        CvSeq* circles = cvHoughCircles( gray, storage, CV_HOUGH_GRADIENT, 2, gray->height/4, 200, 100 );
        int i;
        for( i = 0; i < c->total; i++ )
        {
             float* p = (float*)cvGetSeqElem( c, i );
             cvCircle( img, cvPoint(cvRound(p[0]),cvRound(p[1])), cvRound(p[2]), CV_RGB(255,0,0), 3, 8, 0 );
        }
        cvNamedWindow( "cicles", 1 );
        cvShowImage( "circles", img );
    }
    return 0;
}

DistTransform

Calculates distance to closest zero pixel for all non-zero pixels of source image

void cvDistTransform( const CvArr* src, CvArr* dst, int distance_type=CV_DIST_L2,
                      int mask_size=3, const float* mask=NULL, CvArr* labels=NULL );

src
Source 8-bit single-channel (binary) image.
dst
Output image with calculated distances (32-bit floating-point, single-channel).
distance_type
Type of distance; can be CV_DIST_L1, CV_DIST_L2, CV_DIST_C or CV_DIST_USER.
mask_size
Size of distance transform mask; can be 3 or 5. In case of CV_DIST_L1 or CV_DIST_C the parameter is forced to 3, because 3×3 mask gives the same result as 5×5 yet it is faster.
mask
User-defined mask in case of user-defined distance, it consists of 2 numbers (horizontal/vertical shift cost, diagonal shift cost) in case of 3×3 mask and 3 numbers (horizontal/vertical shift cost, diagonal shift cost, knight’s move cost) in case of 5×5 mask.
labels
The optional output 2d array of labels of integer type and the same size as src and dst.

The function cvDistTransform calculates the approximated distance from every binary image pixel to the nearest zero pixel. For zero pixels the function sets the zero distance, for others it finds the shortest path consisting of basic shifts: horizontal, vertical, diagonal or knight’s move (the latest is available for 5×5 mask). The overal distance is calculated as a sum of these basic distances. Because the distance function should be symmetric, all the horizontal and vertical shifts must have the same cost (that is denoted as a), all the diagonal shifts must have the same cost (denoted b), and all knight’s moves must have the same cost (denoted c). For CV_DIST_C and CV_DIST_L1 types the distance is calculated precisely, whereas for CV_DIST_L2 (Euclidian distance) the distance can be calculated only with some relative error (5×5 mask gives more accurate results), OpenCV uses the values suggested in [Borgefors86]:

CV_DIST_C (3×3):
a=1, b=1

CV_DIST_L1 (3×3):
a=1, b=2

CV_DIST_L2 (3×3):
a=0.955, b=1.3693

CV_DIST_L2 (5×5):
a=1, b=1.4, c=2.1969

And below are samples of distance field (black (0) pixel is in the middle of white square) in case of user-defined distance:

User-defined 3×3 mask (a=1, b=1.5)

4.543.533.544.5
432.522.534
3.52.51.511.52.53.5
3210123
3.52.51.511.52.53.5
432.522.534
4.543.533.544.5

User-defined 5×5 mask (a=1, b=1.5, c=2)

4.53.53333.54.5
3.5322233.5
321.511.523
3210123
321.511.523
3.5322233.5
43.53333.54

Typically, for fast coarse distance estimation CV_DIST_L2, 3×3 mask is used, and for more accurate distance estimation CV_DIST_L2, 5×5 mask is used.

When the output parameter labels is not NULL, for every non-zero pixel the function also finds the nearest connected component consisting of zero pixels. The connected components themselves are found as contours in the beginning of the function.

In this mode the processing time is still O(N), where N is the number of pixels. Thus, the function provides a very fast way to compute approximate Voronoi diagram for the binary image.


Histograms


CvHistogram

Muti-dimensional histogram

typedef struct CvHistogram
{
    int     type;
    CvArr*  bins;
    float   thresh[CV_MAX_DIM][2]; /* for uniform histograms */
    float** thresh2; /* for non-uniform histograms */
    CvMatND mat; /* embedded matrix header for array histograms */
}
CvHistogram;

CreateHist

Creates histogram

CvHistogram* cvCreateHist( int dims, int* sizes, int type,
                           float** ranges=NULL, int uniform=1 );

dims
Number of histogram dimensions.
sizes
Array of histogram dimension sizes.
type
Histogram representation format: CV_HIST_ARRAY means that histogram data is represented as an multi-dimensional dense array CvMatND; CV_HIST_SPARSE means that histogram data is represented as a multi-dimensional sparse array CvSparseMat.
ranges
Array of ranges for histogram bins. Its meaning depends on the uniform parameter value. The ranges are used for when histogram is calculated or backprojected to determine, which histogram bin corresponds to which value/tuple of values from the input image[s].
uniform
Uniformity flag; if not 0, the histogram has evenly spaced bins and for every 0<=i<cDims ranges[i] is array of two numbers: lower and upper boundaries for the i-th histogram dimension. The whole range [lower,upper] is split then into dims[i] equal parts to determine i-th input tuple value ranges for every histogram bin. And if uniform=0, then i-th element of ranges array contains dims[i]+1 elements: lower0, upper0, lower1, upper1 == lower2, ..., upperdims[i]-1, where lowerj and upperj are lower and upper boundaries of i-th input tuple value for j-th bin, respectively. In either case, the input values that are beyond the specified range for a histogram bin, are not counted by cvCalcHist and filled with 0 by cvCalcBackProject.

The function cvCreateHist creates a histogram of the specified size and returns the pointer to the created histogram. If the array ranges is 0, the histogram bin ranges must be specified later via The function cvSetHistBinRanges, though cvCalcHist and cvCalcBackProject may process 8-bit images without setting bin ranges, they assume equally spaced in 0..255 bins.


SetHistBinRanges

Sets bounds of histogram bins

void cvSetHistBinRanges( CvHistogram* hist, float** ranges, int uniform=1 );

hist
Histogram.
ranges
Array of bin ranges arrays, see cvCreateHist.
uniform
Uniformity flag, see cvCreateHist.

The function cvSetHistBinRanges is a stand-alone function for setting bin ranges in the histogram. For more detailed description of the parameters ranges and uniform see cvCalcHist function, that can initialize the ranges as well. Ranges for histogram bins must be set before the histogram is calculated or backproject of the histogram is calculated.


ReleaseHist

Releases histogram

void cvReleaseHist( CvHistogram** hist );

hist
Double pointer to the released histogram.

The function cvReleaseHist releases the histogram (header and the data). The pointer to histogram is cleared by the function. If *hist pointer is already NULL, the function does nothing.


ClearHist

Clears histogram

void cvClearHist( CvHistogram* hist );

hist
Histogram.

The function cvClearHist sets all histogram bins to 0 in case of dense histogram and removes all histogram bins in case of sparse array.


MakeHistHeaderForArray

Makes a histogram out of array

CvHistogram*  cvMakeHistHeaderForArray( int dims, int* sizes, CvHistogram* hist,
                                        float* data, float** ranges=NULL, int uniform=1 );

dims
Number of histogram dimensions.
sizes
Array of histogram dimension sizes.
hist
The histogram header initialized by the function.
data
Array that will be used to store histogram bins.
ranges
Histogram bin ranges, see cvCreateHist.
uniform
Uniformity flag, see cvCreateHist.

The function cvMakeHistHeaderForArray initializes the histogram, which header and bins are allocated by user. No cvReleaseHist need to be called afterwards. Only dense histograms can be initialized this way. The function returns hist.


QueryHistValue_*D

Queries value of histogram bin

#define cvQueryHistValue_1D( hist, idx0 ) \
    cvGetReal1D( (hist)->bins, (idx0) )
#define cvQueryHistValue_2D( hist, idx0, idx1 ) \
    cvGetReal2D( (hist)->bins, (idx0), (idx1) )
#define cvQueryHistValue_3D( hist, idx0, idx1, idx2 ) \
    cvGetReal3D( (hist)->bins, (idx0), (idx1), (idx2) )
#define cvQueryHistValue_nD( hist, idx ) \
    cvGetRealND( (hist)->bins, (idx) )

hist
Histogram.
idx0, idx1, idx2, idx3
Indices of the bin.
idx
Array of indices

The macros cvQueryHistValue_*D return the value of the specified bin of 1D, 2D, 3D or N-D histogram. In case of sparse histogram the function returns 0, if the bin is not present in the histogram, and no new bin is created.


GetHistValue_*D

Returns pointer to histogram bin

#define cvGetHistValue_1D( hist, idx0 ) \
    ((float*)(cvPtr1D( (hist)->bins, (idx0), 0 ))
#define cvGetHistValue_2D( hist, idx0, idx1 ) \
    ((float*)(cvPtr2D( (hist)->bins, (idx0), (idx1), 0 ))
#define cvGetHistValue_3D( hist, idx0, idx1, idx2 ) \
    ((float*)(cvPtr3D( (hist)->bins, (idx0), (idx1), (idx2), 0 ))
#define cvGetHistValue_nD( hist, idx ) \
    ((float*)(cvPtrND( (hist)->bins, (idx), 0 ))

hist
Histogram.
idx0, idx1, idx2, idx3
Indices of the bin.
idx
Array of indices

The macros cvGetHistValue_*D return pointer to the specified bin of 1D, 2D, 3D or N-D histogram. In case of sparse histogram the function creates a new bin and sets it to 0, unless it exists already.


GetMinMaxHistValue

Finds minimum and maximum histogram bins

void cvGetMinMaxHistValue( const CvHistogram* hist,
                           float* min_value, float* max_value,
                           int* min_idx=NULL, int* max_idx=NULL );

hist
Histogram.
min_value
Pointer to the minimum value of the histogram
max_value
Pointer to the maximum value of the histogram
min_idx
Pointer to the array of coordinates for minimum
max_idx
Pointer to the array of coordinates for maximum

The function cvGetMinMaxHistValue finds the minimum and maximum histogram bins and their positions. Any of output arguments is optional. Among several extremums with the same value the ones with minimum index (in lexicographical order) In case of several maximums or minimums the earliest in lexicographical order extrema locations are returned.


NormalizeHist

Normalizes histogram

void cvNormalizeHist( CvHistogram* hist, double factor );

hist
Pointer to the histogram.
factor
Normalization factor.

The function cvNormalizeHist normalizes the histogram bins by scaling them, such that the sum of the bins becomes equal to factor.


ThreshHist

Thresholds histogram

void cvThreshHist( CvHistogram* hist, double threshold );

hist
Pointer to the histogram.
threshold
Threshold level.

The function cvThreshHist clears histogram bins that are below the specified threshold.


CompareHist

Compares two dense histograms

double cvCompareHist( const CvHistogram* hist1, const CvHistogram* hist2, int method );

hist1
The first dense histogram.
hist2
The second dense histogram.
method
Comparison method, one of:
  • CV_COMP_CORREL
  • CV_COMP_CHISQR
  • CV_COMP_INTERSECT
  • CV_COMP_BHATTACHARYYA

The function cvCompareHist compares two dense histograms using the specified method as following (H1 denotes the first histogram, H2 - the second):

Correlation (method=CV_COMP_CORREL):
d(H1,H2)=sumI(H'1(I)•H'2(I))/sqrt(sumI[H'1(I)2]•sumI[H'2(I)2])
where
H'k(I)=Hk(I)-1/N•sumJHk(J) (N=number of histogram bins)

Chi-Square (method=CV_COMP_CHISQR):
d(H1,H2)=sumI[(H1(I)-H2(I))/(H1(I)+H2(I))]

Intersection (method=CV_COMP_INTERSECT):
d(H1,H2)=sumImax(H1(I),H2(I))

Bhattacharyya distance (method=CV_COMP_BHATTACHARYYA):
d(H1,H2)=sqrt(1-sumI(sqrt(H1(I)•H2(I))))

The function returns d(H1,H2) value.

Note: the method CV_COMP_BHATTACHARYYA only works with normalized histograms.

To compare sparse histogram or more general sparse configurations of weighted points, consider using cvCalcEMD2 function.


CopyHist

Copies histogram

void cvCopyHist( const CvHistogram* src, CvHistogram** dst );

src
Source histogram.
dst
Pointer to destination histogram.

The function cvCopyHist makes a copy of the histogram. If the second histogram pointer *dst is NULL, a new histogram of the same size as src is created. Otherwise, both histograms must have equal types and sizes. Then the function copies the source histogram bins values to destination histogram and sets the same bin values ranges as in src.


CalcHist

Calculates histogram of image(s)

void cvCalcHist( IplImage** image, CvHistogram* hist,
                 int accumulate=0, const CvArr* mask=NULL );

image
Source images (though, you may pass CvMat** as well).
hist
Pointer to the histogram.
accumulate
Accumulation flag. If it is set, the histogram is not cleared in the beginning. This feature allows user to compute a single histogram from several images, or to update the histogram online.
mask
The operation mask, determines what pixels of the source images are counted.

The function cvCalcHist calculates the histogram of one or more single-channel images. The elements of a tuple that is used to increment a histogram bin are taken at the same location from the corresponding input images.

Sample. Calculating and displaying 2D Hue-Saturation histogram of a color image

#include <cv.h>
#include <highgui.h>

int main( int argc, char** argv )
{
    IplImage* src;
    if( argc == 2 && (src=cvLoadImage(argv[1], 1))!= 0)
    {
        IplImage* h_plane = cvCreateImage( cvGetSize(src), 8, 1 );
        IplImage* s_plane = cvCreateImage( cvGetSize(src), 8, 1 );
        IplImage* v_plane = cvCreateImage( cvGetSize(src), 8, 1 );
        IplImage* planes[] = { h_plane, s_plane };
        IplImage* hsv = cvCreateImage( cvGetSize(src), 8, 3 );
        int h_bins = 30, s_bins = 32;
        int hist_size[] = {h_bins, s_bins};
        float h_ranges[] = { 0, 180 }; /* hue varies from 0 (~0°red) to 180 (~360°red again) */
        float s_ranges[] = { 0, 255 }; /* saturation varies from 0 (black-gray-white) to 255 (pure spectrum color) */
        float* ranges[] = { h_ranges, s_ranges };
        int scale = 10;
        IplImage* hist_img = cvCreateImage( cvSize(h_bins*scale,s_bins*scale), 8, 3 );
        CvHistogram* hist;
        float max_value = 0;
        int h, s;

        cvCvtColor( src, hsv, CV_BGR2HSV );
        cvCvtPixToPlane( hsv, h_plane, s_plane, v_plane, 0 );
        hist = cvCreateHist( 2, hist_size, CV_HIST_ARRAY, ranges, 1 );
        cvCalcHist( planes, hist, 0, 0 );
        cvGetMinMaxHistValue( hist, 0, &max_value, 0, 0 );
        cvZero( hist_img );

        for( h = 0; h < h_bins; h++ )
        {
            for( s = 0; s < s_bins; s++ )
            {
                float bin_val = cvQueryHistValue_2D( hist, h, s );
                int intensity = cvRound(bin_val*255/max_value);
                cvRectangle( hist_img, cvPoint( h*scale, s*scale ),
                             cvPoint( (h+1)*scale - 1, (s+1)*scale - 1),
                             CV_RGB(intensity,intensity,intensity), /* graw a grayscale histogram.
                                                                       if you have idea how to do it
                                                                       nicer let us know */
                             CV_FILLED );
            }
        }

        cvNamedWindow( "Source", 1 );
        cvShowImage( "Source", src );

        cvNamedWindow( "H-S Histogram", 1 );
        cvShowImage( "H-S Histogram", hist_img );

        cvWaitKey(0);
    }
}

CalcBackProject

Calculates back projection

void cvCalcBackProject( IplImage** image, CvArr* back_project, const CvHistogram* hist );

image
Source images (though you may pass CvMat** as well).
back_project
Destination back projection image of the same type as the source images.
hist
Histogram.

The function cvCalcBackProject calculates the back project of the histogram. For each tuple of pixels at the same position of all input single-channel images the function puts the value of the histogram bin, corresponding to the tuple, to the destination image. In terms of statistics, the value of each output image pixel is probability of the observed tuple given the distribution (histogram). For example, to find a red object in the picture, one may do the following:

  1. Calculate a hue histogram for the red object assuming the image contains only this object. The histogram is likely to have a strong maximum, corresponding to red color.
  2. Calculate back projection of a hue plane of input image where the object is searched, using the histogram. Threshold the image.
  3. Find connected components in the resulting picture and choose the right component using some additional criteria, for example, the largest connected component.
That is the approximate algorithm of Camshift color object tracker, except for the 3rd step, instead of which CAMSHIFT algorithm is used to locate the object on the back projection given the previous object position.


CalcBackProjectPatch

Locates a template within image by histogram comparison

void cvCalcBackProjectPatch( IplImage** image, CvArr* dst,
                             CvSize patch_size, CvHistogram* hist,
                             int method, float factor );

image
Source images (though, you may pass CvMat** as well)
dst
Destination image.
patch_size
Size of patch slid though the source image.
hist
Histogram
method
Compasion method, passed to cvCompareHist (see description of that function).
factor
Normalization factor for histograms, will affect normalization scale of destination image, pass 1. if unsure.

The function cvCalcBackProjectPatch calculates back projection by comparing histograms of the source image patches with the given histogram. Taking measurement results from some image at each location over ROI creates an array image. These results might be one or more of hue, x derivative, y derivative, Laplacian filter, oriented Gabor filter, etc. Each measurement output is collected into its own separate image. The image image array is a collection of these measurement images. A multi-dimensional histogram hist is constructed by sampling from the image image array. The final histogram is normalized. The hist histogram has as many dimensions as the number of elements in image array.

Each new image is measured and then converted into an image image array over a chosen ROI. Histograms are taken from this image image in an area covered by a "patch" with anchor at center as shown in the picture below. The histogram is normalized using the parameter norm_factor so that it may be compared with hist. The calculated histogram is compared to the model histogram; hist uses The function cvCompareHist with the comparison method=method). The resulting output is placed at the location corresponding to the patch anchor in the probability image dst. This process is repeated as the patch is slid over the ROI. Iterative histogram update by subtracting trailing pixels covered by the patch and adding newly covered pixels to the histogram can save a lot of operations, though it is not implemented yet.

Back Project Calculation by Patches


CalcProbDensity

Divides one histogram by another

void  cvCalcProbDensity( const CvHistogram* hist1, const CvHistogram* hist2,
                         CvHistogram* dst_hist, double scale=255 );

hist1
first histogram (the divisor).
hist2
second histogram.
dst_hist
destination histogram.
scale
scale factor for the destination histogram.

The function cvCalcProbDensity calculates the object probability density from the two histograms as:

dist_hist(I)=0      if hist1(I)==0
             scale  if hist1(I)!=0 && hist2(I)>hist1(I)
             hist2(I)*scale/hist1(I) if hist1(I)!=0 && hist2(I)<=hist1(I)

So the destination histogram bins are within less than scale.


EqualizeHist

Equalizes histogram of grayscale image

void  cvEqualizeHist( const CvArr* src, CvArr* dst );

src
The input 8-bit single-channel image.
dst
The output image of the same size and the same data type as src.

The function cvEqualizeHist equalizes histogram of the input image using the following algorithm:

1. calculate histogram H for src.
2. normalize histogram, so that the sum of histogram bins is 255.
3. compute integral of the histogram:
   H’(i) = sum0≤j≤iH(j)
4. transform the image using H’ as a look-up table: dst(x,y)=H’(src(x,y))

The algorithm normalizes brightness and increases contrast of the image.


Matching


MatchTemplate

Compares template against overlapped image regions

void cvMatchTemplate( const CvArr* image, const CvArr* templ,
                      CvArr* result, int method );

image
Image where the search is running. It should be 8-bit or 32-bit floating-point.
templ
Searched template; must be not greater than the source image and the same data type as the image.
result
A map of comparison results; single-channel 32-bit floating-point. If image is W×H and templ is w×h then result must be W-w+1×H-h+1.
method
Specifies the way the template must be compared with image regions (see below).

The function cvMatchTemplate is similiar to cvCalcBackProjectPatch. It slids through image, compares overlapped patches of size w×h against templ using the specified method and stores the comparison results to result. Here are the formulae for the different comparison methods one may use (I denotes image, T - template, R - result. The summation is done over template and/or the image patch: x'=0..w-1, y'=0..h-1):

method=CV_TM_SQDIFF:
R(x,y)=sumx',y'[T(x',y')-I(x+x',y+y')]2

method=CV_TM_SQDIFF_NORMED:
R(x,y)=sumx',y'[T(x',y')-I(x+x',y+y')]2/sqrt[sumx',y'T(x',y')2•sumx',y'I(x+x',y+y')2]

method=CV_TM_CCORR:
R(x,y)=sumx',y'[T(x',y')•I(x+x',y+y')]

method=CV_TM_CCORR_NORMED:
R(x,y)=sumx',y'[T(x',y')•I(x+x',y+y')]/sqrt[sumx',y'T(x',y')2•sumx',y'I(x+x',y+y')2]

method=CV_TM_CCOEFF:
R(x,y)=sumx',y'[T'(x',y')•I'(x+x',y+y')],

where T'(x',y')=T(x',y') - 1/(w•h)•sumx",y"T(x",y")
      I'(x+x',y+y')=I(x+x',y+y') - 1/(w•h)•sumx",y"I(x+x",y+y")

method=CV_TM_CCOEFF_NORMED:
R(x,y)=sumx',y'[T'(x',y')•I'(x+x',y+y')]/sqrt[sumx',y'T'(x',y')2•sumx',y'I'(x+x',y+y')2]

After the function finishes comparison, the best matches can be found as global minimums (CV_TM_SQDIFF*) or maximums (CV_TM_CCORR* and CV_TM_CCOEFF*) using cvMinMaxLoc function. In case of color image and template summation in both numerator and each sum in denominator is done over all the channels (and separate mean values are used for each channel).


MatchShapes

Compares two shapes

double cvMatchShapes( const void* object1, const void* object2,
                      int method, double parameter=0 );

object1
First contour or grayscale image
object2
Second contour or grayscale image
method
Comparison method, one of CV_CONTOUR_MATCH_I1, CV_CONTOURS_MATCH_I2 or CV_CONTOURS_MATCH_I3.
parameter
Method-specific parameter (is not used now).

The function cvMatchShapes compares two shapes. The 3 implemented methods all use Hu moments (see cvGetHuMoments) (A ~ object1, B - object2):

method=CV_CONTOUR_MATCH_I1:
I1(A,B)=sumi=1..7abs(1/mAi - 1/mBi)

method=CV_CONTOUR_MATCH_I2:
I2(A,B)=sumi=1..7abs(mAi - mBi)

method=CV_CONTOUR_MATCH_I3:
I3(A,B)=sumi=1..7abs(mAi - mBi)/abs(mAi)

where
mAi=sign(hAi)•log(hAi),
mBi=sign(hBi)•log(hBi),
hAi, hBi - Hu moments of A and B, respectively.

CalcEMD2

Computes "minimal work" distance between two weighted point configurations

float cvCalcEMD2( const CvArr* signature1, const CvArr* signature2, int distance_type,
                  CvDistanceFunction distance_func=NULL, const CvArr* cost_matrix=NULL,
                  CvArr* flow=NULL, float* lower_bound=NULL, void* userdata=NULL );
typedef float (*CvDistanceFunction)(const float* f1, const float* f2, void* userdata);

signature1
First signature, size1×dims+1 floating-point matrix. Each row stores the point weight followed by the point coordinates. The matrix is allowed to have a single column (weights only) if the user-defined cost matrix is used.
signature2
Second signature of the same format as signature1, though the number of rows may be different. The total weights may be different, in this case an extra "dummy" point is added to either signature1 or signature2.
distance_type
Metrics used; CV_DIST_L1, CV_DIST_L2, and CV_DIST_C stand for one of the standard metrics; CV_DIST_USER means that a user-defined function distance_func or pre-calculated cost_matrix is used.
distance_func
The user-defined distance function. It takes coordinates of two points and returns the distance between the points.
cost_matrix
The user-defined size1×size2 cost matrix. At least one of cost_matrix and distance_func must be NULL. Also, if a cost matrix is used, lower boundary (see below) can not be calculated, because it needs a metric function.
flow
The resultant size1×size2 flow matrix: flowij is a flow from i-th point of signature1 to j-th point of signature2
lower_bound
Optional input/output parameter: lower boundary of distance between the two signatures that is a distance between mass centers. The lower boundary may not be calculated if the user-defined cost matrix is used, the total weights of point configurations are not equal, or there is the signatures consist of weights only (i.e. the signature matrices have a single column). User must initialize *lower_bound. If the calculated distance between mass centers is greater or equal to *lower_bound (it means that the signatures are far enough) the function does not calculate EMD. In any case *lower_bound is set to the calculated distance between mass centers on return. Thus, if user wants to calculate both distance between mass centers and EMD, *lower_bound should be set to 0.
userdata
Pointer to optional data that is passed into the user-defined distance function.

The function cvCalcEMD2 computes earth mover distance and/or a lower boundary of the distance between the two weighted point configurations. One of the application desctibed in [RubnerSept98] is multi-dimensional histogram comparison for image retrieval. EMD is a transportation problem that is solved using some modification of simplex algorithm, thus the complexity is exponential in the worst case, though, it is much faster in average. In case of a real metric the lower boundary can be calculated even faster (using linear-time algorithm) and it can be used to determine roughly whether the two signatures are far enough so that they cannot relate to the same object.


Structural Analysis


Contour Processing Functions


ApproxChains

Approximates Freeman chain(s) with polygonal curve

CvSeq* cvApproxChains( CvSeq* src_seq, CvMemStorage* storage,
                       int method=CV_CHAIN_APPROX_SIMPLE,
                       double parameter=0, int minimal_perimeter=0, int recursive=0 );

src_seq
Pointer to the chain that can refer to other chains.
storage
Storage location for the resulting polylines.
method
Approximation method (see the description of the function cvFindContours).
parameter
Method parameter (not used now).
minimal_perimeter
Approximates only those contours whose perimeters are not less than minimal_perimeter. Other chains are removed from the resulting structure.
recursive
If not 0, the function approximates all chains that access can be obtained to from src_seq by h_next or v_next links. If 0, the single chain is approximated.

This is a stand-alone approximation routine. The function cvApproxChains works exactly in the same way as cvFindContours with the corresponding approximation flag. The function returns pointer to the first resultant contour. Other approximated contours, if any, can be accessed via v_next or h_next fields of the returned structure.


StartReadChainPoints

Initializes chain reader

void cvStartReadChainPoints( CvChain* chain, CvChainPtReader* reader );

chain Pointer to chain. reader Chain reader state.

The function cvStartReadChainPoints initializes a special reader (see Dynamic Data Structures for more information on sets and sequences).


ReadChainPoint

Gets next chain point

CvPoint cvReadChainPoint( CvChainPtReader* reader );

reader
Chain reader state.

The function cvReadChainPoint returns the current chain point and updates the reader position.


ApproxPoly

Approximates polygonal curve(s) with desired precision

CvSeq* cvApproxPoly( const void* src_seq, int header_size, CvMemStorage* storage,
                     int method, double parameter, int parameter2=0 );

src_seq
Sequence of array of points.
header_size
Header size of approximated curve[s].
storage
Container for approximated contours. If it is NULL, the input sequences' storage is used.
method
Approximation method; only CV_POLY_APPROX_DP is supported, that corresponds to Douglas-Peucker algorithm.
parameter
Method-specific parameter; in case of CV_POLY_APPROX_DP it is a desired approximation accuracy.
parameter2
If case if src_seq is sequence it means whether the single sequence should be approximated or all sequences on the same level or below src_seq (see cvFindContours for description of hierarchical contour structures). And if src_seq is array (CvMat*) of points, the parameter specifies whether the curve is closed (parameter2!=0) or not (parameter2=0).

The function cvApproxPoly approximates one or more curves and returns the approximation result[s]. In case of multiple curves approximation the resultant tree will have the same structure as the input one (1:1 correspondence).


BoundingRect

Calculates up-right bounding rectangle of point set

CvRect cvBoundingRect( CvArr* points, int update=0 );

points
2D point set, either a sequence or vector (CvMat) of points.
update
The update flag. Here is list of possible combination of the flag values and type of contour:
  • update=0, contour ~ CvContour*: the bounding rectangle is not calculated, but it is taken from rect field of the contour header.
  • update=1, contour ~ CvContour*: the bounding rectangle is calculated and written to rect field of the contour header.
  • update=0, contour ~ CvSeq* or CvMat*: the bounding rectangle is calculated and returned.
  • update=1, contour ~ CvSeq* or CvMat*: runtime error is raised.

The function cvBoundingRect returns the up-right bounding rectangle for 2d point set.


ContourArea

Calculates area of the whole contour or contour section

double cvContourArea( const CvArr* contour, CvSlice slice=CV_WHOLE_SEQ );

contour
Contour (sequence or array of vertices).
slice
Starting and ending points of the contour section of interest, by default area of the whole contour is calculated.

The function cvContourArea calculates area of the whole contour or contour section. In the latter case the total area bounded by the contour arc and the chord connecting the 2 selected points is calculated as shown on the picture below:

NOTE: Orientation of the contour affects the area sign, thus the function may return negative result. Use fabs() function from C runtime to get the absolute value of area.


ArcLength

Calculates contour perimeter or curve length

double cvArcLength( const void* curve, CvSlice slice=CV_WHOLE_SEQ, int is_closed=-1 );

curve
Sequence or array of the curve points.
slice
Starting and ending points of the curve, by default the whole curve length is calculated.
is_closed
Indicates whether the curve is closed or not. There are 3 cases:
  • is_closed=0 - the curve is assumed to be unclosed.
  • is_closed>0 - the curve is assumed to be closed.
  • is_closed<0 - if curve is sequence, the flag CV_SEQ_FLAG_CLOSED of ((CvSeq*)curve)->flags is checked to determine if the curve is closed or not, otherwise (curve is represented by array (CvMat*) of points) it is assumed to be unclosed.

The function cvArcLength calculates length or curve as sum of lengths of segments between subsequent points


CreateContourTree

Creates hierarchical representation of contour

CvContourTree* cvCreateContourTree( const CvSeq* contour, CvMemStorage* storage, double threshold );

contour
Input contour.
storage
Container for output tree.
threshold
Approximation accuracy.

The function cvCreateContourTree creates binary tree representation for the input contour and returns the pointer to its root. If the parameter threshold is less than or equal to 0, the function creates full binary tree representation. If the threshold is greater than 0, the function creates representation with the precision threshold: if the vertices with the interceptive area of its base line are less than threshold, the tree should not be built any further. The function returns the created tree.


ContourFromContourTree

Restores contour from tree

CvSeq* cvContourFromContourTree( const CvContourTree* tree, CvMemStorage* storage,
                                 CvTermCriteria criteria );

tree
Contour tree.
storage
Container for the reconstructed contour.
criteria
Criteria, where to stop reconstruction.

The function cvContourFromContourTree restores the contour from its binary tree representation. The parameter criteria determines the accuracy and/or the number of tree levels used for reconstruction, so it is possible to build approximated contour. The function returns reconstructed contour.


MatchContourTrees

Compares two contours using their tree representations

double cvMatchContourTrees( const CvContourTree* tree1, const CvContourTree* tree2,
                            int method, double threshold );

tree1
First contour tree.
tree2
Second contour tree.
method
Similarity measure, only CV_CONTOUR_TREES_MATCH_I1 is supported.
threshold
Similarity threshold.

The function cvMatchContourTrees calculates the value of the matching measure for two contour trees. The similarity measure is calculated level by level from the binary tree roots. If at the certain level difference between contours becomes less than threshold, the reconstruction process is interrupted and the current difference is returned.


Computational Geometry


MaxRect

Finds bounding rectangle for two given rectangles

CvRect cvMaxRect( const CvRect* rect1, const CvRect* rect2 );

rect1
First rectangle
rect2
Second rectangle

The function cvMaxRect finds minimum area rectangle that contains both input rectangles inside:


CvBox2D

Rotated 2D box

typedef struct CvBox2D
{
    CvPoint2D32f center;  /* center of the box */
    CvSize2D32f  size;    /* box width and length */
    float angle;          /* angle between the horizontal axis
                             and the first side (i.e. length) in radians */
}
CvBox2D;

PointSeqFromMat

Initializes point sequence header from a point vector

CvSeq* cvPointSeqFromMat( int seq_kind, const CvArr* mat,
                          CvContour* contour_header,
                          CvSeqBlock* block );

seq_kind
Type of the point sequence: point set (0), a curve (CV_SEQ_KIND_CURVE), closed curve (CV_SEQ_KIND_CURVE+CV_SEQ_FLAG_CLOSED) etc.
mat
Input matrix. It should be continuous 1-dimensional vector of points, that is, it should have type CV_32SC2 or CV_32FC2.
contour_header
Contour header, initialized by the function.
block
Sequence block header, initialized by the function.

The function cvPointSeqFromMat initializes sequence header to create a "virtual" sequence which elements reside in the specified matrix. No data is copied. The initialized sequence header may be passed to any function that takes a point sequence on input. No extra elements could be added to the sequence, but some may be removed. The function is a specialized variant of cvMakeSeqHeaderForArray and uses the latter internally. It returns pointer to the initialized contour header. Note that the bounding rectangle (field rect of CvContour strucuture is not initialized by the function. If you need one, use cvBoundingRect.

Here is the simple usage example.

CvContour header;
CvSeqBlock block;
CvMat* vector = cvCreateMat( 1, 3, CV_32SC2 );

CV_MAT_ELEM( *vector, CvPoint, 0, 0 ) = cvPoint(100,100);
CV_MAT_ELEM( *vector, CvPoint, 0, 1 ) = cvPoint(100,200);
CV_MAT_ELEM( *vector, CvPoint, 0, 2 ) = cvPoint(200,100);

IplImage* img = cvCreateImage( cvSize(300,300), 8, 3 );
cvZero(img);

cvDrawContours( img, cvPointSeqFromMat(CV_SEQ_KIND_CURVE+CV_SEQ_FLAG_CLOSED,
   vector, &header, &block), CV_RGB(255,0,0), CV_RGB(255,0,0), 0, 3, 8, cvPoint(0,0));

CvBox2D

Rotated 2D box

typedef struct CvBox2D
{
    CvPoint2D32f center;  /* center of the box */
    CvSize2D32f  size;    /* box width and length */
    float angle;          /* angle between the horizontal axis
                             and the first side (i.e. length) in radians */
}
CvBox2D;

BoxPoints

Finds box vertices

void cvBoxPoints( CvBox2D box, CvPoint2D32f pt[4] );

box
Box
pt
Array of vertices

The function cvBoxPoints calculates vertices of the input 2d box. Here is the function code:

void cvBoxPoints( CvBox2D box, CvPoint2D32f pt[4] )
{
    float a = (float)cos(box.angle)*0.5f;
    float b = (float)sin(box.angle)*0.5f;

    pt[0].x = box.center.x - a*box.size.height - b*box.size.width;
    pt[0].y = box.center.y + b*box.size.height - a*box.size.width;
    pt[1].x = box.center.x + a*box.size.height - b*box.size.width;
    pt[1].y = box.center.y - b*box.size.height - a*box.size.width;
    pt[2].x = 2*box.center.x - pt[0].x;
    pt[2].y = 2*box.center.y - pt[0].y;
    pt[3].x = 2*box.center.x - pt[1].x;
    pt[3].y = 2*box.center.y - pt[1].y;
}

FitEllipse

Fits ellipse to set of 2D points

CvBox2D cvFitEllipse2( const CvArr* points );

points
Sequence or array of points.

The function cvFitEllipse calculates ellipse that fits best (in least-squares sense) to a set of 2D points. The meaning of the returned structure fields is similar to those in cvEllipse except that size stores the full lengths of the ellipse axises, not half-lengths


FitLine

Fits line to 2D or 3D point set

void  cvFitLine( const CvArr* points, int dist_type, double param,
                 double reps, double aeps, float* line );

points
Sequence or array of 2D or 3D points with 32-bit integer or floating-point coordinates.
dist_type
The distance used for fitting (see the discussion).
param
Numerical parameter (C) for some types of distances, if 0 then some optimal value is chosen.
reps, aeps
Sufficient accuracy for radius (distance between the coordinate origin and the line) and angle, respectively, 0.01 would be a good defaults for both. is used.
line
The output line parameters. In case of 2d fitting it is array of 4 floats (vx, vy, x0, y0) where (vx, vy) is a normalized vector collinear to the line and (x0, y0) is some point on the line. In case of 3D fitting it is array of 6 floats (vx, vy, vz, x0, y0, z0) where (vx, vy, vz) is a normalized vector collinear to the line and (x0, y0, z0) is some point on the line.

The function cvFitLine fits line to 2D or 3D point set by minimizing sumiρ(ri), where ri is distance between i-th point and the line and ρ(r) is a distance function, one of:

dist_type=CV_DIST_L2 (L2):
ρ(r)=r2/2 (the simplest and the fastest least-squares method)

dist_type=CV_DIST_L1 (L1):
ρ(r)=r

dist_type=CV_DIST_L12 (L1-L2):
ρ(r)=2•[sqrt(1+r2/2) - 1]

dist_type=CV_DIST_FAIR (Fair):
ρ(r)=C2•[r/C - log(1 + r/C)],  C=1.3998

dist_type=CV_DIST_WELSCH (Welsch):
ρ(r)=C2/2•[1 - exp(-(r/C)2)],  C=2.9846

dist_type=CV_DIST_HUBER (Huber):
ρ(r)= r2/2,   if r < C
      C•(r-C/2),   otherwise;   C=1.345


ConvexHull2

Finds convex hull of point set

CvSeq* cvConvexHull2( const CvArr* input, void* hull_storage=NULL,
                      int orientation=CV_CLOCKWISE, int return_points=0 );

points
Sequence or array of 2D points with 32-bit integer or floating-point coordinates.
hull_storage
The destination array (CvMat*) or memory storage (CvMemStorage*) that will store the convex hull. If it is array, it should be 1d and have the same number of elements as the input array/sequence. On output the header is modified so to truncate the array downto the hull size.
orientation
Desired orientation of convex hull: CV_CLOCKWISE or CV_COUNTER_CLOCKWISE.
return_points
If non-zero, the points themselves will be stored in the hull instead of indices if hull_storage is array, or pointers if hull_storage is memory storage.

The function cvConvexHull2 finds convex hull of 2D point set using Sklansky’s algorithm. If hull_storage is memory storage, the function creates a sequence containing the hull points or pointers to them, depending on return_points value and returns the sequence on output.

Example. Building convex hull for a sequence or array of points

#include "cv.h"
#include "highgui.h"
#include <stdlib.h>

#define ARRAY  0 /* switch between array/sequence method by replacing 0<=>1 */

void main( int argc, char** argv )
{
    IplImage* img = cvCreateImage( cvSize( 500, 500 ), 8, 3 );
    cvNamedWindow( "hull", 1 );

#if !ARRAY
        CvMemStorage* storage = cvCreateMemStorage();
#endif

    for(;;)
    {
        int i, count = rand()%100 + 1, hullcount;
        CvPoint pt0;
#if !ARRAY
        CvSeq* ptseq = cvCreateSeq( CV_SEQ_KIND_GENERIC|CV_32SC2, sizeof(CvContour),
                                     sizeof(CvPoint), storage );
        CvSeq* hull;

        for( i = 0; i < count; i++ )
        {
            pt0.x = rand() % (img->width/2) + img->width/4;
            pt0.y = rand() % (img->height/2) + img->height/4;
            cvSeqPush( ptseq, &pt0 );
        }
        hull = cvConvexHull2( ptseq, 0, CV_CLOCKWISE, 0 );
        hullcount = hull->total;
#else
        CvPoint* points = (CvPoint*)malloc( count * sizeof(points[0]));
        int* hull = (int*)malloc( count * sizeof(hull[0]));
        CvMat point_mat = cvMat( 1, count, CV_32SC2, points );
        CvMat hull_mat = cvMat( 1, count, CV_32SC1, hull );

        for( i = 0; i < count; i++ )
        {
            pt0.x = rand() % (img->width/2) + img->width/4;
            pt0.y = rand() % (img->height/2) + img->height/4;
            points[i] = pt0;
        }
        cvConvexHull2( &point_mat, &hull_mat, CV_CLOCKWISE, 0 );
        hullcount = hull_mat.cols;
#endif
        cvZero( img );
        for( i = 0; i < count; i++ )
        {
#if !ARRAY
            pt0 = *CV_GET_SEQ_ELEM( CvPoint, ptseq, i );
#else
            pt0 = points[i];
#endif
            cvCircle( img, pt0, 2, CV_RGB( 255, 0, 0 ), CV_FILLED );
        }

#if !ARRAY
        pt0 = **CV_GET_SEQ_ELEM( CvPoint*, hull, hullcount - 1 );
#else
        pt0 = points[hull[hullcount-1]];
#endif

        for( i = 0; i < hullcount; i++ )
        {
#if !ARRAY
            CvPoint pt = **CV_GET_SEQ_ELEM( CvPoint*, hull, i );
#else
            CvPoint pt = points[hull[i]];
#endif
            cvLine( img, pt0, pt, CV_RGB( 0, 255, 0 ));
            pt0 = pt;
        }

        cvShowImage( "hull", img );

        int key = cvWaitKey(0);
        if( key == 27 ) // 'ESC'
            break;

#if !ARRAY
        cvClearMemStorage( storage );
#else
        free( points );
        free( hull );
#endif
    }
}

CheckContourConvexity

Tests contour convex

int cvCheckContourConvexity( const CvArr* contour );

contour
Tested contour (sequence or array of points).

The function cvCheckContourConvexity tests whether the input contour is convex or not. The contour must be simple, i.e. without self-intersections.


CvConvexityDefect

Structure describing a single contour convexity detect

typedef struct CvConvexityDefect
{
    CvPoint* start; /* point of the contour where the defect begins */
    CvPoint* end; /* point of the contour where the defect ends */
    CvPoint* depth_point; /* the farthest from the convex hull point within the defect */
    float depth; /* distance between the farthest point and the convex hull */
} CvConvexityDefect;

Picture. Convexity defects of hand contour.


ConvexityDefects

Finds convexity defects of contour

CvSeq* cvConvexityDefects( const CvArr* contour, const CvArr* convexhull,
                           CvMemStorage* storage=NULL );

contour
Input contour.
convexhull
Convex hull obtained using cvConvexHull2 that should contain pointers or indices to the contour points, not the hull points themselves, i.e. return_points parameter in cvConvexHull2 should be 0.
storage
Container for output sequence of convexity defects. If it is NULL, contour or hull (in that order) storage is used.

The function cvConvexityDefects finds all convexity defects of the input contour and returns a sequence of the CvConvexityDefect structures.


PointPolygonTest

Point in contour test

double cvPointPolygonTest( const CvArr* contour,
                           CvPoint2D32f pt, int measure_dist );

contour
Input contour.
pt
The point tested against the contour.
measure_dist
If it is non-zero, the function estimates distance from the point to the nearest contour edge.

The function cvPointPolygonTest determines whether the point is inside contour, outside, or lies on an edge (or coinsides with a vertex). It returns positive, negative or zero value, correspondingly. When measure_dist=0, the return value is +1, -1 and 0, respectively. When measure_dist≠0, it is a signed distance between the point and the nearest contour edge.

Here is the sample output of the function, where each image pixel is tested against the contour.


MinAreaRect2

Finds circumscribed rectangle of minimal area for given 2D point set

CvBox2D  cvMinAreaRect2( const CvArr* points, CvMemStorage* storage=NULL );

points
Sequence or array of points.
storage
Optional temporary memory storage.

The function cvMinAreaRect2 finds a circumscribed rectangle of the minimal area for 2D point set by building convex hull for the set and applying rotating calipers technique to the hull.

Picture. Minimal-area bounding rectangle for contour


MinEnclosingCircle

Finds circumscribed circle of minimal area for given 2D point set

int cvMinEnclosingCircle( const CvArr* points, CvPoint2D32f* center, float* radius );

points
Sequence or array of 2D points.
center
Output parameter. The center of the enclosing circle.
radius
Output parameter. The radius of the enclosing circle.

The function cvMinEnclosingCircle finds the minimal circumscribed circle for 2D point set using iterative algorithm. It returns nonzero if the resultant circle contains all the input points and zero otherwise (i.e. algorithm failed).


CalcPGH

Calculates pair-wise geometrical histogram for contour

void cvCalcPGH( const CvSeq* contour, CvHistogram* hist );

contour
Input contour. Currently, only integer point coordinates are allowed.
hist
Calculated histogram; must be two-dimensional.

The function cvCalcPGH calculates 2D pair-wise geometrical histogram (PGH), described in [Iivarinen97], for the contour. The algorithm considers every pair of the contour edges. The angle between the edges and the minimum/maximum distances are determined for every pair. To do this each of the edges in turn is taken as the base, while the function loops through all the other edges. When the base edge and any other edge are considered, the minimum and maximum distances from the points on the non-base edge and line of the base edge are selected. The angle between the edges defines the row of the histogram in which all the bins that correspond to the distance between the calculated minimum and maximum distances are incremented (that is, the histogram is transposed relatively to [Iivarninen97] definition). The histogram can be used for contour matching.


Planar Subdivisions


CvSubdiv2D

Planar subdivision

#define CV_SUBDIV2D_FIELDS()    \
    CV_GRAPH_FIELDS()           \
    int  quad_edges;            \
    int  is_geometry_valid;     \
    CvSubdiv2DEdge recent_edge; \
    CvPoint2D32f  topleft;      \
    CvPoint2D32f  bottomright;

typedef struct CvSubdiv2D
{
    CV_SUBDIV2D_FIELDS()
}
CvSubdiv2D;

Planar subdivision is a subdivision of a plane into a set of non-overlapped regions (facets) that cover the whole plane. The above structure describes a subdivision built on 2d point set, where the points are linked together and form a planar graph, which, together with a few edges connecting exterior subdivision points (namely, convex hull points) with infinity, subdivides a plane into facets by its edges.

For every subdivision there exists dual subdivision there facets and points (subdivision vertices) swap their roles, that is, a facet is treated as a vertex (called virtual point below) of dual subdivision and the original subdivision vertices become facets. On the picture below original subdivision is marked with solid lines and dual subdivision with dot lines

OpenCV subdivides plane into triangles using Delaunay’s algorithm. Subdivision is built iteratively starting from a dummy triangle that includes all the subdivision points for sure. In this case the dual subdivision is Voronoi diagram of input 2d point set. The subdivisions can be used for 3d piece-wise transformation of a plane, morphing, fast location of points on the plane, building special graphs (such as NNG,RNG) etc.


CvQuadEdge2D

Quad-edge of planar subdivision

/* one of edges within quad-edge, lower 2 bits is index (0..3)
   and upper bits are quad-edge pointer */
typedef long CvSubdiv2DEdge;

/* quad-edge structure fields */
#define CV_QUADEDGE2D_FIELDS()     \
    int flags;                     \
    struct CvSubdiv2DPoint* pt[4]; \
    CvSubdiv2DEdge  next[4];

typedef struct CvQuadEdge2D
{
    CV_QUADEDGE2D_FIELDS()
}
CvQuadEdge2D;

Quad-edge is a basic element of subdivision, it contains four edges (e, eRot and reversed e & eRot):


CvSubdiv2DPoint

Point of original or dual subdivision

#define CV_SUBDIV2D_POINT_FIELDS()\
    int            flags;      \
    CvSubdiv2DEdge first;      \
    CvPoint2D32f   pt;

#define CV_SUBDIV2D_VIRTUAL_POINT_FLAG (1 << 30)

typedef struct CvSubdiv2DPoint
{
    CV_SUBDIV2D_POINT_FIELDS()
}
CvSubdiv2DPoint;

Subdiv2DGetEdge

Returns one of edges related to given

CvSubdiv2DEdge  cvSubdiv2DGetEdge( CvSubdiv2DEdge edge, CvNextEdgeType type );
#define cvSubdiv2DNextEdge( edge ) cvSubdiv2DGetEdge( edge, CV_NEXT_AROUND_ORG )

edge
Subdivision edge (not a quad-edge)
type
Specifies, which of related edges to return, one of:
  • CV_NEXT_AROUND_ORG - next around the edge origin (eOnext on the picture above if e is the input edge)
  • CV_NEXT_AROUND_DST - next around the edge vertex (eDnext)
  • CV_PREV_AROUND_ORG - previous around the edge origin (reversed eRnext)
  • CV_PREV_AROUND_DST - previous around the edge destination (reversed eLnext)
  • CV_NEXT_AROUND_LEFT - next around the left facet (eLnext)
  • CV_NEXT_AROUND_RIGHT - next around the right facet (eRnext)
  • CV_PREV_AROUND_LEFT - previous around the left facet (reversed eOnext)
  • CV_PREV_AROUND_RIGHT - previous around the right facet (reversed eDnext)

The function cvSubdiv2DGetEdge returns one the edges related to the input edge.


Subdiv2DRotateEdge

Returns another edge of the same quad-edge

CvSubdiv2DEdge  cvSubdiv2DRotateEdge( CvSubdiv2DEdge edge, int rotate );

edge
Subdivision edge (not a quad-edge)
type
Specifies, which of edges of the same quad-edge as the input one to return, one of:
  • 0 - the input edge (e on the picture above if e is the input edge)
  • 1 - the rotated edge (eRot)
  • 2 - the reversed edge (reversed e (in green))
  • 3 - the reversed rotated edge (reversed eRot (in green))

The function cvSubdiv2DRotateEdge returns one the edges of the same quad-edge as the input edge.


Subdiv2DEdgeOrg

Returns edge origin

CvSubdiv2DPoint* cvSubdiv2DEdgeOrg( CvSubdiv2DEdge edge );

edge
Subdivision edge (not a quad-edge)

The function cvSubdiv2DEdgeOrg returns the edge origin. The returned pointer may be NULL if the edge is from dual subdivision and the virtual point coordinates are not calculated yet. The virtual points can be calculated using function cvCalcSubdivVoronoi2D.


Subdiv2DEdgeDst

Returns edge destination

CvSubdiv2DPoint* cvSubdiv2DEdgeDst( CvSubdiv2DEdge edge );

edge
Subdivision edge (not a quad-edge)

The function cvSubdiv2DEdgeDst returns the edge destination. The returned pointer may be NULL if the edge is from dual subdivision and the virtual point coordinates are not calculated yet. The virtual points can be calculated using function cvCalcSubdivVoronoi2D.


CreateSubdivDelaunay2D

Creates empty Delaunay triangulation

CvSubdiv2D* cvCreateSubdivDelaunay2D( CvRect rect, CvMemStorage* storage );

rect
Rectangle that includes all the 2d points that are to be added to subdivision.
storage
Container for subdivision.

The function cvCreateSubdivDelaunay2D creates an empty Delaunay subdivision, where 2d points can be added further using function cvSubdivDelaunay2DInsert. All the points to be added must be within the specified rectangle, otherwise a runtime error will be raised.


SubdivDelaunay2DInsert

Inserts a single point to Delaunay triangulation

CvSubdiv2DPoint*  cvSubdivDelaunay2DInsert( CvSubdiv2D* subdiv, CvPoint2D32f pt);

subdiv
Delaunay subdivision created by function cvCreateSubdivDelaunay2D.
pt
Inserted point.

The function cvSubdivDelaunay2DInsert inserts a single point to subdivision and modifies the subdivision topology appropriately. If a points with same coordinates exists already, no new points is added. The function returns pointer to the allocated point. No virtual points coordinates is calculated at this stage.


Subdiv2DLocate

Inserts a single point to Delaunay triangulation

CvSubdiv2DPointLocation  cvSubdiv2DLocate( CvSubdiv2D* subdiv, CvPoint2D32f pt,
                                           CvSubdiv2DEdge* edge,
                                           CvSubdiv2DPoint** vertex=NULL );

subdiv
Delaunay or another subdivision.
pt
The point to locate.
edge
The output edge the point falls onto or right to.
vertex
Optional output vertex double pointer the input point coinsides with.

The function cvSubdiv2DLocate locates input point within subdivision. There are 5 cases:

  • point falls into some facet. The function returns CV_PTLOC_INSIDE and *edge will contain one of edges of the facet.
  • point falls onto the edge. The function returns CV_PTLOC_ON_EDGE and *edge will contain this edge.
  • point coinsides with one of subdivision vertices. The function returns CV_PTLOC_VERTEX and *vertex will contain pointer to the vertex.
  • point is outside the subdivsion reference rectangle. The function returns CV_PTLOC_OUTSIDE_RECT and no pointers is filled.
  • one of input arguments is invalid. Runtime error is raised or, if silent or "parent" error processing mode is selected, CV_PTLOC_ERROR is returnd.


FindNearestPoint2D

Finds the closest subdivision vertex to given point

CvSubdiv2DPoint* cvFindNearestPoint2D( CvSubdiv2D* subdiv, CvPoint2D32f pt );

subdiv
Delaunay or another subdivision.
pt
Input point.

The function cvFindNearestPoint2D is another function that locates input point within subdivision. It finds subdivision vertex that is the closest to the input point. It is not necessarily one of vertices of the facet containing the input point, though the facet (located using cvSubdiv2DLocate) is used as a starting point. The function returns pointer to the found subdivision vertex


CalcSubdivVoronoi2D

Calculates coordinates of Voronoi diagram cells

void cvCalcSubdivVoronoi2D( CvSubdiv2D* subdiv );

subdiv
Delaunay subdivision, where all the points are added already.

The function cvCalcSubdivVoronoi2D calculates coordinates of virtual points. All virtual points corresponding to some vertex of original subdivision form (when connected together) a boundary of Voronoi cell of that point.


ClearSubdivVoronoi2D

Removes all virtual points

void cvClearSubdivVoronoi2D( CvSubdiv2D* subdiv );

subdiv
Delaunay subdivision.

The function cvClearSubdivVoronoi2D removes all virtual points. It is called internally in cvCalcSubdivVoronoi2D if the subdivision was modified after previous call to the function.


There are a few other lower-level functions that work with planar subdivisions, see cv.h and the sources. Demo script delaunay.c that builds Delaunay triangulation and Voronoi diagram of random 2d point set can be found at opencv/samples/c.


Motion Analysis and Object Tracking Reference


Accumulation of Background Statistics


Acc

Adds frame to accumulator

void cvAcc( const CvArr* image, CvArr* sum, const CvArr* mask=NULL );

image
Input image, 1- or 3-channel, 8-bit or 32-bit floating point. (each channel of multi-channel image is processed independently).
sum
Accumulator of the same number of channels as input image, 32-bit or 64-bit floating-point.
mask
Optional operation mask.

The function cvAcc adds the whole image image or its selected region to accumulator sum:

sum(x,y)=sum(x,y)+image(x,y) if mask(x,y)!=0

SquareAcc

Adds the square of source image to accumulator

void cvSquareAcc( const CvArr* image, CvArr* sqsum, const CvArr* mask=NULL );

image
Input image, 1- or 3-channel, 8-bit or 32-bit floating point (each channel of multi-channel image is processed independently).
sqsum
Accumulator of the same number of channels as input image, 32-bit or 64-bit floating-point.
mask
Optional operation mask.

The function cvSquareAcc adds the input image image or its selected region, raised to power 2, to the accumulator sqsum:

sqsum(x,y)=sqsum(x,y)+image(x,y)2 if mask(x,y)!=0

MultiplyAcc

Adds product of two input images to accumulator

void cvMultiplyAcc( const CvArr* image1, const CvArr* image2, CvArr* acc, const CvArr* mask=NULL );

image1
First input image, 1- or 3-channel, 8-bit or 32-bit floating point (each channel of multi-channel image is processed independently).
image2
Second input image, the same format as the first one.
acc
Accumulator of the same number of channels as input images, 32-bit or 64-bit floating-point.
mask
Optional operation mask.

The function cvMultiplyAcc adds product of 2 images or thier selected regions to accumulator acc:

acc(x,y)=acc(x,y) + image1(x,y)•image2(x,y) if mask(x,y)!=0

RunningAvg

Updates running average

void cvRunningAvg( const CvArr* image, CvArr* acc, double alpha, const CvArr* mask=NULL );

image
Input image, 1- or 3-channel, 8-bit or 32-bit floating point (each channel of multi-channel image is processed independently).
acc
Accumulator of the same number of channels as input image, 32-bit or 64-bit floating-point.
alpha
Weight of input image.
mask
Optional operation mask.

The function cvRunningAvg calculates weighted sum of input image image and the accumulator acc so that acc becomes a running average of frame sequence:

acc(x,y)=(1-α)•acc(x,y) + α•image(x,y) if mask(x,y)!=0

where α (alpha) regulates update speed (how fast accumulator forgets about previous frames).


Motion Templates


UpdateMotionHistory

Updates motion history image by moving silhouette

void cvUpdateMotionHistory( const CvArr* silhouette, CvArr* mhi,
                            double timestamp, double duration );

silhouette
Silhouette mask that has non-zero pixels where the motion occurs.
mhi
Motion history image, that is updated by the function (single-channel, 32-bit floating-point)
timestamp
Current time in milliseconds or other units.
duration
Maximal duration of motion track in the same units as timestamp.

The function cvUpdateMotionHistory updates the motion history image as following:

mhi(x,y)=timestamp  if silhouette(x,y)!=0
         0          if silhouette(x,y)=0 and mhi(x,y)<timestamp-duration
         mhi(x,y)   otherwise

That is, MHI pixels where motion occurs are set to the current timestamp, while the pixels where motion happened far ago are cleared.


CalcMotionGradient

Calculates gradient orientation of motion history image

void cvCalcMotionGradient( const CvArr* mhi, CvArr* mask, CvArr* orientation,
                           double delta1, double delta2, int aperture_size=3 );

mhi
Motion history image.
mask
Mask image; marks pixels where motion gradient data is correct. Output parameter.
orientation
Motion gradient orientation image; contains angles from 0 to ~360°.
delta1, delta2
The function finds minimum (m(x,y)) and maximum (M(x,y)) mhi values over each pixel (x,y) neihborhood and assumes the gradient is valid only if
min(delta1,delta2) <= M(x,y)-m(x,y) <= max(delta1,delta2).
aperture_size
Aperture size of derivative operators used by the function: CV_SCHARR, 1, 3, 5 or 7 (see cvSobel).

The function cvCalcMotionGradient calculates the derivatives Dx and Dy of mhi and then calculates gradient orientation as:

orientation(x,y)=arctan(Dy(x,y)/Dx(x,y))

where both Dx(x,y)' and Dy(x,y)' signs are taken into account (as in cvCartToPolar function). After that mask is filled to indicate where the orientation is valid (see delta1 and delta2 description).


CalcGlobalOrientation

Calculates global motion orientation of some selected region

double cvCalcGlobalOrientation( const CvArr* orientation, const CvArr* mask, const CvArr* mhi,
                                double timestamp, double duration );

orientation
Motion gradient orientation image; calculated by the function cvCalcMotionGradient.
mask
Mask image. It may be a conjunction of valid gradient mask, obtained with cvCalcMotionGradient and mask of the region, whose direction needs to be calculated.
mhi
Motion history image.
timestamp
Current time in milliseconds or other units, it is better to store time passed to cvUpdateMotionHistory before and reuse it here, because running cvUpdateMotionHistory and cvCalcMotionGradient on large images may take some time.
duration
Maximal duration of motion track in milliseconds, the same as in cvUpdateMotionHistory.

The function cvCalcGlobalOrientation calculates the general motion direction in the selected region and returns the angle between 0° and 360°. At first the function builds the orientation histogram and finds the basic orientation as a coordinate of the histogram maximum. After that the function calculates the shift relative to the basic orientation as a weighted sum of all orientation vectors: the more recent is the motion, the greater is the weight. The resultant angle is a circular sum of the basic orientation and the shift.


SegmentMotion

Segments whole motion into separate moving parts

CvSeq* cvSegmentMotion( const CvArr* mhi, CvArr* seg_mask, CvMemStorage* storage,
                        double timestamp, double seg_thresh );

mhi
Motion history image.
seg_mask
Image where the mask found should be stored, single-channel, 32-bit floating-point.
storage
Memory storage that will contain a sequence of motion connected components.
timestamp
Current time in milliseconds or other units.
seg_thresh
Segmentation threshold; recommended to be equal to the interval between motion history "steps" or greater.

The function cvSegmentMotion finds all the motion segments and marks them in seg_mask with individual values each (1,2,...). It also returns a sequence of CvConnectedComp structures, one per each motion components. After than the motion direction for every component can be calculated with cvCalcGlobalOrientation using extracted mask of the particular component (using cvCmp)


Object Tracking


MeanShift

Finds object center on back projection

int cvMeanShift( const CvArr* prob_image, CvRect window,
                 CvTermCriteria criteria, CvConnectedComp* comp );

prob_image
Back projection of object histogram (see cvCalcBackProject).
window
Initial search window.
criteria
Criteria applied to determine when the window search should be finished.
comp
Resultant structure that contains converged search window coordinates (comp->rect field) and sum of all pixels inside the window (comp->area field).

The function cvMeanShift iterates to find the object center given its back projection and initial position of search window. The iterations are made until the search window center moves by less than the given value and/or until the function has done the maximum number of iterations. The function returns the number of iterations made.


CamShift

Finds object center, size, and orientation

int cvCamShift( const CvArr* prob_image, CvRect window, CvTermCriteria criteria,
                CvConnectedComp* comp, CvBox2D* box=NULL );

prob_image
Back projection of object histogram (see cvCalcBackProject).
window
Initial search window.
criteria
Criteria applied to determine when the window search should be finished.
comp
Resultant structure that contains converged search window coordinates (comp->rect field) and sum of all pixels inside the window (comp->area field).
box
Circumscribed box for the object. If not NULL, contains object size and orientation.

The function cvCamShift implements CAMSHIFT object tracking algrorithm ([Bradski98]). First, it finds an object center using cvMeanShift and, after that, calculates the object size and orientation. The function returns number of iterations made within cvMeanShift.

CvCamShiftTracker class declared in cv.hpp implements color object tracker that uses the function.


SnakeImage

Changes contour position to minimize its energy

void cvSnakeImage( const IplImage* image, CvPoint* points, int length,
                   float* alpha, float* beta, float* gamma, int coeff_usage,
                   CvSize win, CvTermCriteria criteria, int calc_gradient=1 );

image
The source image or external energy field.
points
Contour points (snake).
length
Number of points in the contour.
alpha
Weight[s] of continuity energy, single float or array of length floats, one per each contour point.
beta
Weight[s] of curvature energy, similar to alpha.
gamma
Weight[s] of image energy, similar to alpha.
coeff_usage
Variant of usage of the previous three parameters:
  • CV_VALUE indicates that each of alpha, beta, gamma is a pointer to a single value to be used for all points;
  • CV_ARRAY indicates that each of alpha, beta, gamma is a pointer to an array of coefficients different for all the points of the snake. All the arrays must have the size equal to the contour size.
win
Size of neighborhood of every point used to search the minimum, both win.width and win.height must be odd.
criteria
Termination criteria.
calc_gradient
Gradient flag. If not 0, the function calculates gradient magnitude for every image pixel and consideres it as the energy field, otherwise the input image itself is considered.

The function cvSnakeImage updates snake in order to minimize its total energy that is a sum of internal energy that depends on contour shape (the smoother contour is, the smaller internal energy is) and external energy that depends on the energy field and reaches minimum at the local energy extremums that correspond to the image edges in case of image gradient.

The parameter criteria.epsilon is used to define the minimal number of points that must be moved during any iteration to keep the iteration process running.

If at some iteration the number of moved points is less than criteria.epsilon or the function performed criteria.max_iter iterations, the function terminates.


Optical Flow


CalcOpticalFlowHS

Calculates optical flow for two images

void cvCalcOpticalFlowHS( const CvArr* prev, const CvArr* curr, int use_previous,
                          CvArr* velx, CvArr* vely, double lambda,
                          CvTermCriteria criteria );

prev
First image, 8-bit, single-channel.
curr
Second image, 8-bit, single-channel.
use_previous
Uses previous (input) velocity field.
velx
Horizontal component of the optical flow of the same size as input images, 32-bit floating-point, single-channel.
vely
Vertical component of the optical flow of the same size as input images, 32-bit floating-point, single-channel.
lambda
Lagrangian multiplier.
criteria
Criteria of termination of velocity computing.

The function cvCalcOpticalFlowHS computes flow for every pixel of the first input image using Horn & Schunck algorithm [Horn81].


CalcOpticalFlowLK

Calculates optical flow for two images

void cvCalcOpticalFlowLK( const CvArr* prev, const CvArr* curr, CvSize win_size,
                          CvArr* velx, CvArr* vely );

prev
First image, 8-bit, single-channel.
curr
Second image, 8-bit, single-channel.
win_size
Size of the averaging window used for grouping pixels.
velx
Horizontal component of the optical flow of the same size as input images, 32-bit floating-point, single-channel.
vely
Vertical component of the optical flow of the same size as input images, 32-bit floating-point, single-channel.

The function cvCalcOpticalFlowLK computes flow for every pixel of the first input image using Lucas & Kanade algorithm [Lucas81].


CalcOpticalFlowBM

Calculates optical flow for two images by block matching method

void cvCalcOpticalFlowBM( const CvArr* prev, const CvArr* curr, CvSize block_size,
                          CvSize shift_size, CvSize max_range, int use_previous,
                          CvArr* velx, CvArr* vely );

prev
First image, 8-bit, single-channel.
curr
Second image, 8-bit, single-channel.
block_size
Size of basic blocks that are compared.
shift_size
Block coordinate increments.
max_range
Size of the scanned neighborhood in pixels around block.
use_previous
Uses previous (input) velocity field.
velx
Horizontal component of the optical flow of
floor((prev->width - block_size.width)/shiftSize.width) × floor((prev->height - block_size.height)/shiftSize.height) size, 32-bit floating-point, single-channel.
vely
Vertical component of the optical flow of the same size velx, 32-bit floating-point, single-channel.

The function cvCalcOpticalFlowBM calculates optical flow for overlapped blocks block_size.width×block_size.height pixels each, thus the velocity fields are smaller than the original images. For every block in prev the functions tries to find a similar block in curr in some neighborhood of the original block or shifted by (velx(x0,y0),vely(x0,y0)) block as has been calculated by previous function call (if use_previous=1)


CalcOpticalFlowPyrLK

Calculates optical flow for a sparse feature set using iterative Lucas-Kanade method in pyramids

void cvCalcOpticalFlowPyrLK( const CvArr* prev, const CvArr* curr, CvArr* prev_pyr, CvArr* curr_pyr,
                             const CvPoint2D32f* prev_features, CvPoint2D32f* curr_features,
                             int count, CvSize win_size, int level, char* status,
                             float* track_error, CvTermCriteria criteria, int flags );

prev
First frame, at time t.
curr
Second frame, at time t + dt .
prev_pyr
Buffer for the pyramid for the first frame. If the pointer is not NULL , the buffer must have a sufficient size to store the pyramid from level 1 to level #level ; the total size of (image_width+8)*image_height/3 bytes is sufficient.
curr_pyr
Similar to prev_pyr, used for the second frame.
prev_features
Array of points for which the flow needs to be found.
curr_features
Array of 2D points containing calculated new positions of input
features
in the second image.
count
Number of feature points.
win_size
Size of the search window of each pyramid level.
level
Maximal pyramid level number. If 0 , pyramids are not used (single level), if 1 , two levels are used, etc.
status
Array. Every element of the array is set to 1 if the flow for the corresponding feature has been found, 0 otherwise.
error
Array of double numbers containing difference between patches around the original and moved points. Optional parameter; can be NULL .
criteria
Specifies when the iteration process of finding the flow for each point on each pyramid level should be stopped.
flags
Miscellaneous flags:
  • CV_LKFLOW_PYR_A_READY , pyramid for the first frame is precalculated before the call;
  • CV_LKFLOW_PYR_B_READY , pyramid for the second frame is precalculated before the call;
  • CV_LKFLOW_INITIAL_GUESSES , array B contains initial coordinates of features before the function call.

The function cvCalcOpticalFlowPyrLK implements sparse iterative version of Lucas-Kanade optical flow in pyramids ([Bouguet00]). It calculates coordinates of the feature points on the current video frame given their coordinates on the previous frame. The function finds the coordinates with sub-pixel accuracy.

Both parameters prev_pyr and curr_pyr comply with the following rules: if the image pointer is 0, the function allocates the buffer internally, calculates the pyramid, and releases the buffer after processing. Otherwise, the function calculates the pyramid and stores it in the buffer unless the flag CV_LKFLOW_PYR_A[B]_READY is set. The image should be large enough to fit the Gaussian pyramid data. After the function call both pyramids are calculated and the readiness flag for the corresponding image can be set in the next call (i.e., typically, for all the image pairs except the very first one CV_LKFLOW_PYR_A_READY is set).


Estimators


CvKalman

Kalman filter state

typedef struct CvKalman
{
    int MP;                     /* number of measurement vector dimensions */
    int DP;                     /* number of state vector dimensions */
    int CP;                     /* number of control vector dimensions */

    /* backward compatibility fields */
#if 1
    float* PosterState;         /* =state_pre->data.fl */
    float* PriorState;          /* =state_post->data.fl */
    float* DynamMatr;           /* =transition_matrix->data.fl */
    float* MeasurementMatr;     /* =measurement_matrix->data.fl */
    float* MNCovariance;        /* =measurement_noise_cov->data.fl */
    float* PNCovariance;        /* =process_noise_cov->data.fl */
    float* KalmGainMatr;        /* =gain->data.fl */
    float* PriorErrorCovariance;/* =error_cov_pre->data.fl */
    float* PosterErrorCovariance;/* =error_cov_post->data.fl */
    float* Temp1;               /* temp1->data.fl */
    float* Temp2;               /* temp2->data.fl */
#endif

    CvMat* state_pre;           /* predicted state (x'(k)):
                                    x(k)=A*x(k-1)+B*u(k) */
    CvMat* state_post;          /* corrected state (x(k)):
                                    x(k)=x'(k)+K(k)*(z(k)-H*x'(k)) */
    CvMat* transition_matrix;   /* state transition matrix (A) */
    CvMat* control_matrix;      /* control matrix (B)
                                   (it is not used if there is no control)*/
    CvMat* measurement_matrix;  /* measurement matrix (H) */
    CvMat* process_noise_cov;   /* process noise covariance matrix (Q) */
    CvMat* measurement_noise_cov; /* measurement noise covariance matrix (R) */
    CvMat* error_cov_pre;       /* priori error estimate covariance matrix (P'(k)):
                                    P'(k)=A*P(k-1)*At + Q)*/
    CvMat* gain;                /* Kalman gain matrix (K(k)):
                                    K(k)=P'(k)*Ht*inv(H*P'(k)*Ht+R)*/
    CvMat* error_cov_post;      /* posteriori error estimate covariance matrix (P(k)):
                                    P(k)=(I-K(k)*H)*P'(k) */
    CvMat* temp1;               /* temporary matrices */
    CvMat* temp2;
    CvMat* temp3;
    CvMat* temp4;
    CvMat* temp5;
}
CvKalman;

The structure CvKalman is used to keep Kalman filter state. It is created by cvCreateKalman function, updated by cvKalmanPredict and cvKalmanCorrect functions and released by cvReleaseKalman functions. Normally, the structure is used for standard Kalman filter (notation and the formulae below are borrowed from the excellent Kalman tutorial [Welch95]):

xk=A•xk-1+B•uk+wk
zk=H•xk+vk,

where:

xk (xk-1) - state of the system at the moment k (k-1)
zk - measurement of the system state at the moment k
uk - external control applied at the moment k

wk and vk are normally-distributed process and measurement noise, respectively:
p(w) ~ N(0,Q)
p(v) ~ N(0,R),

that is,
Q - process noise covariance matrix, constant or variable,
R - measurement noise covariance matrix, constant or variable

In case of standard Kalman filter, all the matrices: A, B, H, Q and R are initialized once after CvKalman structure is allocated via cvCreateKalman. However, the same structure and the same functions may be used to simulate extended Kalman filter by linearizing extended Kalman filter equation in the current system state neighborhood, in this case A, B, H (and, probably, Q and R) should be updated on every step.


CreateKalman

Allocates Kalman filter structure

CvKalman* cvCreateKalman( int dynam_params, int measure_params, int control_params=0 );

dynam_params
dimensionality of the state vector
measure_params
dimensionality of the measurement vector
control_params
dimensionality of the control vector

The function cvCreateKalman allocates CvKalman and all its matrices and initializes them somehow.


ReleaseKalman

Deallocates Kalman filter structure

void cvReleaseKalman( CvKalman** kalman );

kalman
double pointer to the Kalman filter structure.

The function cvReleaseKalman releases the structure CvKalman and all underlying matrices.


KalmanPredict

Estimates subsequent model state

const CvMat* cvKalmanPredict( CvKalman* kalman, const CvMat* control=NULL );
#define cvKalmanUpdateByTime cvKalmanPredict

kalman
Kalman filter state.
control
Control vector (uk), should be NULL iff there is no external control (control_params=0).

The function cvKalmanPredict estimates the subsequent stochastic model state by its current state and stores it at kalman->state_pre:

    x'k=A•xk+B•uk
    P'k=A•Pk-1*AT + Q,
where
x'k is predicted state (kalman->state_pre),
xk-1 is corrected state on the previous step (kalman->state_post)
                (should be initialized somehow in the beginning, zero vector by default),
uk is external control (control parameter),
P'k is priori error covariance matrix (kalman->error_cov_pre)
Pk-1 is posteriori error covariance matrix on the previous step (kalman->error_cov_post)
                (should be initialized somehow in the beginning, identity matrix by default),
The function returns the estimated state.


KalmanCorrect

Adjusts model state

const CvMat* cvKalmanCorrect( CvKalman* kalman, const CvMat* measurement );
#define cvKalmanUpdateByMeasurement cvKalmanCorrect

kalman
Pointer to the structure to be updated.
measurement
Pointer to the structure CvMat containing the measurement vector.

The function cvKalmanCorrect adjusts stochastic model state on the basis of the given measurement of the model state:

Kk=P'k•HT•(H•P'k•HT+R)-1
xk=x'k+Kk•(zk-H•x'k)
Pk=(I-Kk•H)•P'k
where
zk - given measurement (mesurement parameter)
Kk - Kalman "gain" matrix.

The function stores adjusted state at kalman->state_post and returns it on output.

Example. Using Kalman filter to track a rotating point

#include "cv.h"
#include "highgui.h"
#include <math.h>

int main(int argc, char** argv)
{
    /* A matrix data */
    const float A[] = { 1, 1, 0, 1 };

    IplImage* img = cvCreateImage( cvSize(500,500), 8, 3 );
    CvKalman* kalman = cvCreateKalman( 2, 1, 0 );
    /* state is (phi, delta_phi) - angle and angle increment */
    CvMat* state = cvCreateMat( 2, 1, CV_32FC1 );
    CvMat* process_noise = cvCreateMat( 2, 1, CV_32FC1 );
    /* only phi (angle) is measured */
    CvMat* measurement = cvCreateMat( 1, 1, CV_32FC1 );
    CvRandState rng;
    int code = -1;

    cvRandInit( &rng, 0, 1, -1, CV_RAND_UNI );

    cvZero( measurement );
    cvNamedWindow( "Kalman", 1 );

    for(;;)
    {
        cvRandSetRange( &rng, 0, 0.1, 0 );
        rng.disttype = CV_RAND_NORMAL;

        cvRand( &rng, state );

        memcpy( kalman->transition_matrix->data.fl, A, sizeof(A));
        cvSetIdentity( kalman->measurement_matrix, cvRealScalar(1) );
        cvSetIdentity( kalman->process_noise_cov, cvRealScalar(1e-5) );
        cvSetIdentity( kalman->measurement_noise_cov, cvRealScalar(1e-1) );
        cvSetIdentity( kalman->error_cov_post, cvRealScalar(1));
        /* choose random initial state */
        cvRand( &rng, kalman->state_post );

        rng.disttype = CV_RAND_NORMAL;

        for(;;)
        {
            #define calc_point(angle)                                      \
                cvPoint( cvRound(img->width/2 + img->width/3*cos(angle)),  \
                         cvRound(img->height/2 - img->width/3*sin(angle)))

            float state_angle = state->data.fl[0];
            CvPoint state_pt = calc_point(state_angle);

            /* predict point position */
            const CvMat* prediction = cvKalmanPredict( kalman, 0 );
            float predict_angle = prediction->data.fl[0];
            CvPoint predict_pt = calc_point(predict_angle);
            float measurement_angle;
            CvPoint measurement_pt;

            cvRandSetRange( &rng, 0, sqrt(kalman->measurement_noise_cov->data.fl[0]), 0 );
            cvRand( &rng, measurement );

            /* generate measurement */
            cvMatMulAdd( kalman->measurement_matrix, state, measurement, measurement );

            measurement_angle = measurement->data.fl[0];
            measurement_pt = calc_point(measurement_angle);

            /* plot points */
            #define draw_cross( center, color, d )                                 \
                cvLine( img, cvPoint( center.x - d, center.y - d ),                \
                             cvPoint( center.x + d, center.y + d ), color, 1, 0 ); \
                cvLine( img, cvPoint( center.x + d, center.y - d ),                \
                             cvPoint( center.x - d, center.y + d ), color, 1, 0 )

            cvZero( img );
            draw_cross( state_pt, CV_RGB(255,255,255), 3 );
            draw_cross( measurement_pt, CV_RGB(255,0,0), 3 );
            draw_cross( predict_pt, CV_RGB(0,255,0), 3 );
            cvLine( img, state_pt, predict_pt, CV_RGB(255,255,0), 3, 0 );

            /* adjust Kalman filter state */
            cvKalmanCorrect( kalman, measurement );

            cvRandSetRange( &rng, 0, sqrt(kalman->process_noise_cov->data.fl[0]), 0 );
            cvRand( &rng, process_noise );
            cvMatMulAdd( kalman->transition_matrix, state, process_noise, state );

            cvShowImage( "Kalman", img );
            code = cvWaitKey( 100 );

            if( code > 0 ) /* break current simulation by pressing a key */
                break;
        }
        if( code == 27 ) /* exit by ESCAPE */
            break;
    }

    return 0;
}

CvConDensation

ConDenstation state

    typedef struct CvConDensation
    {
        int MP;     //Dimension of measurement vector
        int DP;     // Dimension of state vector
        float* DynamMatr;       // Matrix of the linear Dynamics system
        float* State;           // Vector of State
        int SamplesNum;         // Number of the Samples
        float** flSamples;      // array of the Sample Vectors
        float** flNewSamples;   // temporary array of the Sample Vectors
        float* flConfidence;    // Confidence for each Sample
        float* flCumulative;    // Cumulative confidence
        float* Temp;            // Temporary vector
        float* RandomSample;    // RandomVector to update sample set
        CvRandState* RandS;     // Array of structures to generate random vectors
    } CvConDensation;

The structure CvConDensation stores CONditional DENSity propagATION tracker state. The information about the algorithm can be found at http://www.dai.ed.ac.uk/CVonline/LOCAL_COPIES/ISARD1/condensation.html


CreateConDensation

Allocates ConDensation filter structure

CvConDensation* cvCreateConDensation( int dynam_params, int measure_params, int sample_count );

dynam_params
Dimension of the state vector.
measure_params
Dimension of the measurement vector.
sample_count
Number of samples.

The function cvCreateConDensation creates CvConDensation structure and returns pointer to the structure.


ReleaseConDensation

Deallocates ConDensation filter structure

void cvReleaseConDensation( CvConDensation** condens );

condens
Pointer to the pointer to the structure to be released.

The function cvReleaseConDensation releases the structure CvConDensation (see cvConDensation) and frees all memory previously allocated for the structure.


ConDensInitSampleSet

Initializes sample set for ConDensation algorithm

void cvConDensInitSampleSet( CvConDensation* condens, CvMat* lower_bound, CvMat* upper_bound );

condens
Pointer to a structure to be initialized.
lower_bound
Vector of the lower boundary for each dimension.
upper_bound
Vector of the upper boundary for each dimension.

The function cvConDensInitSampleSet fills the samples arrays in the structure CvConDensation with values within specified ranges.


ConDensUpdateByTime

Estimates subsequent model state

void cvConDensUpdateByTime( CvConDensation* condens );

condens
Pointer to the structure to be updated.

The function cvConDensUpdateByTime estimates the subsequent stochastic model state from its current state.


Pattern Recognition


Object Detection

The object detector described below has been initially proposed by Paul Viola [Viola01] and improved by Rainer Lienhart [Lienhart02]. First, a classifier (namely a cascade of boosted classifiers working with haar-like features) is trained with a few hundreds of sample views of a particular object (i.e., a face or a car), called positive examples, that are scaled to the same size (say, 20x20), and negative examples - arbitrary images of the same size.

After a classifier is trained, it can be applied to a region of interest (of the same size as used during the training) in an input image. The classifier outputs a "1" if the region is likely to show the object (i.e., face/car), and "0" otherwise. To search for the object in the whole image one can move the search window across the image and check every location using the classifier. The classifier is designed so that it can be easily "resized" in order to be able to find the objects of interest at different sizes, which is more efficient than resizing the image itself. So, to find an object of an unknown size in the image the scan procedure should be done several times at different scales.

The word "cascade" in the classifier name means that the resultant classifier consists of several simpler classifiers (stages) that are applied subsequently to a region of interest until at some stage the candidate is rejected or all the stages are passed. The word "boosted" means that the classifiers at every stage of the cascade are complex themselves and they are built out of basic classifiers using one of four different boosting techniques (weighted voting). Currently Discrete Adaboost, Real Adaboost, Gentle Adaboost and Logitboost are supported. The basic classifiers are decision-tree classifiers with at least 2 leaves. Haar-like features are the input to the basic classifers, and are calculated as described below. The current algorithm uses the following Haar-like features:

The feature used in a particular classifier is specified by its shape (1a, 2b etc.), position within the region of interest and the scale (this scale is not the same as the scale used at the detection stage, though these two scales are multiplied). For example, in case of the third line feature (2c) the response is calculated as the difference between the sum of image pixels under the rectangle covering the whole feature (including the two white stripes and the black stripe in the middle) and the sum of the image pixels under the black stripe multiplied by 3 in order to compensate for the differences in the size of areas. The sums of pixel values over a rectangular regions are calculated rapidly using integral images (see below and cvIntegral description).

To see the object detector at work, have a look at HaarFaceDetect demo.

The following reference is for the detection part only. There is a separate application called haartraining that can train a cascade of boosted classifiers from a set of samples. See opencv/apps/haartraining for details.


CvHaarFeature, CvHaarClassifier, CvHaarStageClassifier, CvHaarClassifierCascade

Boosted Haar classifier structures

#define CV_HAAR_FEATURE_MAX  3

/* a haar feature consists of 2-3 rectangles with appropriate weights */
typedef struct CvHaarFeature
{
    int  tilted;  /* 0 means up-right feature, 1 means 45--rotated feature */
    
    /* 2-3 rectangles with weights of opposite signs and
       with absolute values inversely proportional to the areas of the rectangles.
       if rect[2].weight !=0, then
       the feature consists of 3 rectangles, otherwise it consists of 2 */
    struct
    {
        CvRect r;
        float weight;
    } rect[CV_HAAR_FEATURE_MAX];
}
CvHaarFeature;

/* a single tree classifier (stump in the simplest case) that returns the response for the feature
   at the particular image location (i.e. pixel sum over subrectangles of the window) and gives out
   a value depending on the responce */
typedef struct CvHaarClassifier
{
    int count;  /* number of nodes in the decision tree */

    /* these are "parallel" arrays. Every index i
       corresponds to a node of the decision tree (root has 0-th index).

       left[i] - index of the left child (or negated index if the left child is a leaf)
       right[i] - index of the right child (or negated index if the right child is a leaf)
       threshold[i] - branch threshold. if feature responce is <= threshold, left branch
                      is chosen, otherwise right branch is chosed.
       alpha[i] - output value correponding to the leaf. */
    CvHaarFeature* haar_feature;
    float* threshold;
    int* left;
    int* right;
    float* alpha;
}
CvHaarClassifier;

/* a boosted battery of classifiers(=stage classifier):
   the stage classifier returns 1
   if the sum of the classifiers' responces
   is greater than threshold and 0 otherwise */
typedef struct CvHaarStageClassifier
{
    int  count;  /* number of classifiers in the battery */
    float threshold; /* threshold for the boosted classifier */
    CvHaarClassifier* classifier; /* array of classifiers */

    /* these fields are used for organizing trees of stage classifiers,
       rather than just stright cascades */
    int next;
    int child;
    int parent;
}
CvHaarStageClassifier;

typedef struct CvHidHaarClassifierCascade CvHidHaarClassifierCascade;

/* cascade or tree of stage classifiers */
typedef struct CvHaarClassifierCascade
{
    int  flags; /* signature */
    int  count; /* number of stages */
    CvSize orig_window_size; /* original object size (the cascade is trained for) */

    /* these two parameters are set by cvSetImagesForHaarClassifierCascade */
    CvSize real_window_size; /* current object size */
    double scale; /* current scale */
    CvHaarStageClassifier* stage_classifier; /* array of stage classifiers */
    CvHidHaarClassifierCascade* hid_cascade; /* hidden optimized representation of the cascade,
                                                created by cvSetImagesForHaarClassifierCascade */
}
CvHaarClassifierCascade;

All the structures are used for representing a cascaded of boosted Haar classifiers. The cascade has the following hierarchical structure:

    Cascade:
        Stage1:
            Classifier11:
                Feature11
            Classifier12:
                Feature12
            ...
        Stage2:
            Classifier21:
                Feature21
            ...
        ...

The whole hierarchy can be constructed manually or loaded from a file or an embedded base using function cvLoadHaarClassifierCascade.


cvLoadHaarClassifierCascade

Loads a trained cascade classifier from file or the classifier database embedded in OpenCV

CvHaarClassifierCascade* cvLoadHaarClassifierCascade(
                         const char* directory,
                         CvSize orig_window_size );

directory
Name of directory containing the description of a trained cascade classifier.
orig_window_size
Original size of objects the cascade has been trained on. Note that it is not stored in the cascade and therefore must be specified separately.

The function cvLoadHaarClassifierCascade loads a trained cascade of haar classifiers from a file or the classifier database embedded in OpenCV. The base can be trained using haartraining application (see opencv/apps/haartraining for details).

The function is obsolete. Nowadays object detection classifiers are stored in XML or YAML files, rather than in directories. To load cascade from a file, use cvLoad function.


cvReleaseHaarClassifierCascade

Releases haar classifier cascade

void cvReleaseHaarClassifierCascade( CvHaarClassifierCascade** cascade );

cascade
Double pointer to the released cascade. The pointer is cleared by the function.

The function cvReleaseHaarClassifierCascade deallocates the cascade that has been created manually or loaded using cvLoadHaarClassifierCascade or cvLoad.


cvHaarDetectObjects

Detects objects in the image

typedef struct CvAvgComp
{
    CvRect rect; /* bounding rectangle for the object (average rectangle of a group) */
    int neighbors; /* number of neighbor rectangles in the group */
}
CvAvgComp;

CvSeq* cvHaarDetectObjects( const CvArr* image, CvHaarClassifierCascade* cascade,
                            CvMemStorage* storage, double scale_factor=1.1,
                            int min_neighbors=3, int flags=0,
                            CvSize min_size=cvSize(0,0) );

image
Image to detect objects in.
cascade
Haar classifier cascade in internal representation.
storage
Memory storage to store the resultant sequence of the object candidate rectangles.
scale_factor
The factor by which the search window is scaled between the subsequent scans, for example, 1.1 means increasing window by 10%.
min_neighbors
Minimum number (minus 1) of neighbor rectangles that makes up an object. All the groups of a smaller number of rectangles than min_neighbors-1 are rejected. If min_neighbors is 0, the function does not any grouping at all and returns all the detected candidate rectangles, which may be useful if the user wants to apply a customized grouping procedure.
flags
Mode of operation. Currently the only flag that may be specified is CV_HAAR_DO_CANNY_PRUNING. If it is set, the function uses Canny edge detector to reject some image regions that contain too few or too much edges and thus can not contain the searched object. The particular threshold values are tuned for face detection and in this case the pruning speeds up the processing.
min_size
Minimum window size. By default, it is set to the size of samples the classifier has been trained on (~20×20 for face detection).

The function cvHaarDetectObjects finds rectangular regions in the given image that are likely to contain objects the cascade has been trained for and returns those regions as a sequence of rectangles. The function scans the image several times at different scales (see cvSetImagesForHaarClassifierCascade). Each time it considers overlapping regions in the image and applies the classifiers to the regions using cvRunHaarClassifierCascade. It may also apply some heuristics to reduce number of analyzed regions, such as Canny prunning. After it has proceeded and collected the candidate rectangles (regions that passed the classifier cascade), it groups them and returns a sequence of average rectangles for each large enough group. The default parameters (scale_factor=1.1, min_neighbors=3, flags=0) are tuned for accurate yet slow object detection. For a faster operation on real video images the settings are: scale_factor=1.2, min_neighbors=2, flags=CV_HAAR_DO_CANNY_PRUNING, min_size=<minimum possible face size> (for example, ~1/4 to 1/16 of the image area in case of video conferencing).

Example. Using cascade of Haar classifiers to find objects (e.g. faces).

#include "cv.h"
#include "highgui.h"

CvHaarClassifierCascade* load_object_detector( const char* cascade_path )
{
    return (CvHaarClassifierCascade*)cvLoad( cascade_path );
}

void detect_and_draw_objects( IplImage* image,
                              CvHaarClassifierCascade* cascade,
                              int do_pyramids )
{
    IplImage* small_image = image;
    CvMemStorage* storage = cvCreateMemStorage(0);
    CvSeq* faces;
    int i, scale = 1;

    /* if the flag is specified, down-scale the input image to get a
       performance boost w/o loosing quality (perhaps) */
    if( do_pyramids )
    {
        small_image = cvCreateImage( cvSize(image->width/2,image->height/2), IPL_DEPTH_8U, 3 );
        cvPyrDown( image, small_image, CV_GAUSSIAN_5x5 );
        scale = 2;
    }

    /* use the fastest variant */
    faces = cvHaarDetectObjects( small_image, cascade, storage, 1.2, 2, CV_HAAR_DO_CANNY_PRUNING );

    /* draw all the rectangles */
    for( i = 0; i < faces->total; i++ )
    {
        /* extract the rectanlges only */
        CvRect face_rect = *(CvRect*)cvGetSeqElem( faces, i, 0 );
        cvRectangle( image, cvPoint(face_rect.x*scale,face_rect.y*scale),
                     cvPoint((face_rect.x+face_rect.width)*scale,
                             (face_rect.y+face_rect.height)*scale),
                     CV_RGB(255,0,0), 3 );
    }

    if( small_image != image )
        cvReleaseImage( &small_image );
    cvReleaseMemStorage( &storage );
}

/* takes image filename and cascade path from the command line */
int main( int argc, char** argv )
{
    IplImage* image;
    if( argc==3 && (image = cvLoadImage( argv[1], 1 )) != 0 )
    {
        CvHaarClassifierCascade* cascade = load_object_detector(argv[2]);
        detect_and_draw_objects( image, cascade, 1 );
        cvNamedWindow( "test", 0 );
        cvShowImage( "test", image );
        cvWaitKey(0);
        cvReleaseHaarClassifierCascade( &cascade );
        cvReleaseImage( &image );
    }

    return 0;
}

cvSetImagesForHaarClassifierCascade

Assigns images to the hidden cascade

void cvSetImagesForHaarClassifierCascade( CvHaarClassifierCascade* cascade,
                                          const CvArr* sum, const CvArr* sqsum,
                                          const CvArr* tilted_sum, double scale );

cascade
Hidden Haar classifier cascade, created by cvCreateHidHaarClassifierCascade.
sum
Integral (sum) single-channel image of 32-bit integer format. This image as well as the two subsequent images are used for fast feature evaluation and brightness/contrast normalization. They all can be retrieved from input 8-bit or floating point single-channel image using The function cvIntegral.
sqsum
Square sum single-channel image of 64-bit floating-point format.
tilted_sum
Tilted sum single-channel image of 32-bit integer format.
scale
Window scale for the cascade. If scale=1, original window size is used (objects of that size are searched) - the same size as specified in cvLoadHaarClassifierCascade (24x24 in case of "<default_face_cascade>"), if scale=2, a two times larger window is used (48x48 in case of default face cascade). While this will speed-up search about four times, faces smaller than 48x48 cannot be detected.

The function cvSetImagesForHaarClassifierCascade assigns images and/or window scale to the hidden classifier cascade. If image pointers are NULL, the previously set images are used further (i.e. NULLs mean "do not change images"). Scale parameter has no such a "protection" value, but the previous value can be retrieved by cvGetHaarClassifierCascadeScale function and reused again. The function is used to prepare cascade for detecting object of the particular size in the particular image. The function is called internally by cvHaarDetectObjects, but it can be called by user if there is a need in using lower-level function cvRunHaarClassifierCascade.


cvRunHaarClassifierCascade

Runs cascade of boosted classifier at given image location

int cvRunHaarClassifierCascade( CvHaarClassifierCascade* cascade,
                                CvPoint pt, int start_stage=0 );

cascade
Haar classifier cascade.
pt
Top-left corner of the analyzed region. Size of the region is a original window size scaled by the currenly set scale. The current window size may be retrieved using cvGetHaarClassifierCascadeWindowSize function.
start_stage
Initial zero-based index of the cascade stage to start from. The function assumes that all the previous stages are passed. This feature is used internally by cvHaarDetectObjects for better processor cache utilization.

The function cvRunHaarHaarClassifierCascade runs Haar classifier cascade at a single image location. Before using this function the integral images and the appropriate scale (=> window size) should be set using cvSetImagesForHaarClassifierCascade. The function returns positive value if the analyzed rectangle passed all the classifier stages (it is a candidate) and zero or negative value otherwise.


Camera Calibration and 3D Reconstruction


Pinhole Camera Model, Distortion

The functions in this section use so-called pinhole camera model. That is, a scene view is formed by projecting 3D points into the image plane using perspective transformation.


s*m' = A*[R|t]*M', or

 [u]   [fx 0 cx] [r11 r12 r13 t1] [X]
s[v] = [0 fy cy]*[r21 r22 r23 t2]*[Y]
 [1]   [0  0  1] [r31 r32 r33 t2] [Z]
                                 [1]
Where (X, Y, Z) are coordinates of a 3D point in the world coordinate space, (u, v) are coordinates of point projection in pixels. A is called a camera matrix, or matrix of intrinsic parameters. (cx, cy) is a principal point (that is usually at the image center), and fx, fy are focal lengths expressed in pixel-related units. Thus, if an image from camera is up-sampled/down-sampled by some factor, all these parameters (fx, fy, cx and cy) should be scaled (multiplied/divided, respectively) by the same factor. The matrix of intrinsic parameters does not depend on the scene viewed and, once estimated, can be re-used (as long as the focal length is fixed (in case of zoom lens)). The joint rotation-translation matrix [R|t] is called a matrix of extrinsic parameters. It is used to describe the camera motion around a static scene, or vice versa, rigid motion of an object in front of still camera. That is, [R|t] translates coordinates of a point (X, Y, Z) to some coordinate system, fixed with respect to the camera. The transformation above is equivalent to the following (when z≠0):
[x]     [X]
[y] = R*[Y] + t
[z]     [Z]

x' = x/z
y' = y/z

u = fx*x' + cx
v = fy*y' + cy
Real lens usually have some distortion, which major components are radial distorion and slight tangential distortion. So, the above model is extended as:
[x]     [X]
[y] = R*[Y] + t
[z]     [Z]

x' = x/z
y' = y/z

x" = x'*(1 + k1r2 + k2r4) + 2*p1x'*y' + p2(r2+2*x'2)
y" = y'*(1 + k1r2 + k2r4) + p1(r2+2*y'2) + 2*p2*x'*y'
where r2 = x'2+y'2

u = fx*x" + cx
v = fy*y" + cy
k1, k2 are radial distortion coefficients, p1, p2 are tangential distortion coefficients. Higher-order coefficients are not considered in OpenCV. The distortion coefficients also do not depend on the scene viewed, thus they are intrinsic camera parameters. And they remain the same regardless of the captured image resolution.

The functions below use the above model to

  • Project 3D points to the image plane given intrinsic and extrinsic parameters
  • Compute extrinsic parameters given intrinsic parameters, a few 3D points and their projections.
  • Estimate intrinsic and extrinsic camera parameters from several views of a known calibration pattern (i.e. every view is described by several 3D-2D point correspodences).

Camera Calibration


ProjectPoints2

Projects 3D points to image plane

void cvProjectPoints2( const CvMat* object_points, const CvMat* rotation_vector,
                       const CvMat* translation_vector, const CvMat* intrinsic_matrix,
                       const CvMat* distortion_coeffs, CvMat* image_points,
                       CvMat* dpdrot=NULL, CvMat* dpdt=NULL, CvMat* dpdf=NULL,
                       CvMat* dpdc=NULL, CvMat* dpddist=NULL );

object_points
The array of object points, 3xN or Nx3, where N is the number of points in the view.
rotation_vector
The rotation vector, 1x3 or 3x1.
translation_vector
The translation vector, 1x3 or 3x1.
intrinsic_matrix
The camera matrix (A) [fx 0 cx; 0 fy cy; 0 0 1].
distortion_coeffs
The vector of distortion coefficients, 4x1 or 1x4 [k1, k2, p1, p2]. If it is NULL, all distortion coefficients are considered 0's.
image_points
The output array of image points, 2xN or Nx2, where N is the total number of points in the view.
dpdrot
Optional Nx3 matrix of derivatives of image points with respect to components of the rotation vector.
dpdt
Optional Nx3 matrix of derivatives of image points w.r.t. components of the translation vector.
dpdf
Optional Nx2 matrix of derivatives of image points w.r.t. fx and fy.
dpdc
Optional Nx2 matrix of derivatives of image points w.r.t. cx and cy.
dpddist
Optional Nx4 matrix of derivatives of image points w.r.t. distortion coefficients.

The function cvProjectPoints2 computes projections of 3D points to the image plane given intrinsic and extrinsic camera parameters. Optionally, the function computes jacobians - matrices of partial derivatives of image points as functions of all the input parameters w.r.t. the particular parameters, intrinsic and/or extrinsic. The jacobians are used during the global optimization in cvCalibrateCamera2 and cvFindExtrinsicCameraParams2. The function itself is also used to compute back-projection error for with current intrinsic and extrinsic parameters.

Note, that with intrinsic and/or extrinsic parameters set to special values, the function can be used to compute just extrinsic transformation or just intrinsic transformation (i.e. distortion of a sparse set of points).


FindHomography

Finds perspective transformation between two planes

void cvFindHomography( const CvMat* src_points,
                       const CvMat* dst_points,
                       CvMat* homography );

src_points
Point coordinates in the original plane, 2xN, Nx2, 3xN or Nx3 array (the latter two are for representation in homogenious coordinates), where N is the number of points.
dst_points
Point coordinates in the destination plane, 2xN, Nx2, 3xN or Nx3 array (the latter two are for representation in homogenious coordinates)
homography
Output 3x3 homography matrix.

The function cvFindHomography finds perspective transformation H=||hij|| between the source and the destination planes:

  [x'i]   [xi]
si[y'i]~H*[yi]
  [1  ]  [ 1]
So that the back-projection error is minimized:
sum_i((x'i-(h11*xi + h12*yi + h13)/(h31*xi + h32*yi + h33))2+
      (y'i-(h21*xi + h22*yi + h23)/(h31*xi + h32*yi + h33))2) -> min
The function is used to find initial intrinsic and extrinsic matrices. Homography matrix is determined up to a scale, thus it is normalized to make h33=1.


CalibrateCamera2

Finds intrinsic and extrinsic camera parameters using calibration pattern

void cvCalibrateCamera2( const CvMat* object_points, const CvMat* image_points,
                         const CvMat* point_counts, CvSize image_size,
                         CvMat* intrinsic_matrix, CvMat* distortion_coeffs,
                         CvMat* rotation_vectors=NULL, CvMat* translation_vectors=NULL,
                         int flags=0 );

object_points
The joint matrix of object points, 3xN or Nx3, where N is the total number of points in all views.
image_points
The joint matrix of corresponding image points, 2xN or Nx2, where N is the total number of points in all views.
point_counts
Vector containing numbers of points in each particular view, 1xM or Mx1, where M is the number of a scene views.
image_size
Size of the image, used only to initialize intrinsic camera matrix.
intrinsic_matrix
The output camera matrix (A) [fx 0 cx; 0 fy cy; 0 0 1]. If CV_CALIB_USE_INTRINSIC_GUESS and/or CV_CALIB_FIX_ASPECT_RATION are specified, some or all of fx, fy, cx, cy must be initialized.
distortion_coeffs
The output 4x1 or 1x4 vector of distortion coefficients [k1, k2, p1, p2].
rotation_vectors
The output 3xM or Mx3 array of rotation vectors (compact representation of rotation matrices, see cvRodrigues2).
translation_vectors
The output 3xM or Mx3 array of translation vectors.
flags
Different flags, may be 0 or combination of the following values:
CV_CALIB_USE_INTRINSIC_GUESS - intrinsic_matrix contains valid initial values of fx, fy, cx, cy that are optimized further. Otherwise, (cx, cy) is initially set to the image center (image_size is used here), and focal distances are computed in some least-squares fashion. Note, that if intrinsic parameters are known, there is no need to use this function. Use cvFindExtrinsicCameraParams2 instead.
CV_CALIB_FIX_PRINCIPAL_POINT - The principal point is not changed during the global optimization, it stays at the center and at the other location specified (when CV_CALIB_USE_INTRINSIC_GUESS is set as well).
CV_CALIB_FIX_ASPECT_RATIO - The optimization procedure consider only one of fx and fy as independent variable and keeps the aspect ratio fx/fy the same as it was set initially in intrinsic_matrix. In this case the actual initial values of (fx, fy) are either taken from the matrix (when CV_CALIB_USE_INTRINSIC_GUESS is set) or estimated somehow (in the latter case fx, fy may be set to arbitrary values, only their ratio is used).
CV_CALIB_ZERO_TANGENT_DIST - Tangential distortion coefficients are set to zeros and do not change during the optimization.

The function cvCalibrateCamera2 estimates intrinsic camera parameters and extrinsic parameters for each of the views. The coordinates of 3D object points and their correspondent 2D projections in each view must be specified. That may be achieved by using an object with known geometry and easily detectable feature points. Such an object is called calibration rig or calibration pattern, and OpenCV has built-in support for a chessboard as a calibration rig (see cvFindChessboardCornerGuesses). Currently, initialization of inrtrinsic parameters (when CV_CALIB_USE_INTRINSIC_GUESS is not set) is only implemented for planar calibration rigs (z-coordinates of object points must be all 0's or all 1's). 3D rigs can still be used as long as initial intrinsic_matrix is provided. After the initial values of intrinsic and extrinsic parameters are computed, they are optimized to minimize the total back-projection error - the sum of squared differences between the actual coordinates of image points and the ones computed using cvProjectPoints2.


FindExtrinsicCameraParams2

Finds extrinsic camera parameters for particular view

void cvFindExtrinsicCameraParams2( const CvMat* object_points,
                                   const CvMat* image_points,
                                   const CvMat* intrinsic_matrix,
                                   const CvMat* distortion_coeffs,
                                   CvMat* rotation_vector,
                                   CvMat* translation_vector );

object_points
The array of object points, 3xN or Nx3, where N is the number of points in the view.
image_points
The array of corresponding image points, 2xN or Nx2, where N is the number of points in the view.
intrinsic_matrix
The camera matrix (A) [fx 0 cx; 0 fy cy; 0 0 1].
distortion_coeffs
The vector of distortion coefficients, 4x1 or 1x4 [k1, k2, p1, p2]. If it is NULL, all distortion coefficients are considered 0's.
rotation_vector
The output 3x1 or 1x3 rotation vector (compact representation of a rotation matrix, see cvRodrigues2).
translation_vector
The output 3x1 or 1x3 translation vector.

The function cvFindExtrinsicCameraParams2 estimates extrinsic camera parameters using known intrinsic parameters and and extrinsic parameters for each view. The coordinates of 3D object points and their correspondent 2D projections must be specified. This function also minimizes back-projection error.


Rodrigues2

Converts rotation matrix to rotation vector or vice versa

int  cvRodrigues2( const CvMat* src, CvMat* dst, CvMat* jacobian=0 );

src
The input rotation vector (3x1 or 1x3) or rotation matrix (3x3).
dst
The output rotation matrix (3x3) or rotation vector (3x1 or 1x3), respectively.
jacobian
Optional output Jacobian matrix, 3x9 or 9x3 - partial derivatives of the output array components w.r.t the input array components.

The function cvRodrigues2 converts a rotation vector to rotation matrix or vice versa. Rotation vector is a compact representation of rotation matrix. Direction of the rotation vector is the rotation axis and the length of the vector is the rotation angle around the axis. The rotation matrix R, corresponding to the rotation vector r, is computed as following:

theta <- norm(r)
r <- r/theta
                                                   [0 -rz ry]
R = cos(theta)*I + (1-cos(theta))*rrT + sin(theta)*[rz 0 -rx]
                                                   [ry rx 0]
Inverse transformation can also be done easily as

[0 -rz ry]
sin(theta)*[rz 0 -rx] = (R - RT)/2
[ry rx 0]

Rotation vector is a convenient representation of a rotation matrix as a matrix with only 3 degrees of freedom. The representation is used in the global optimization procedures inside cvFindExtrinsicCameraParams2 and cvCalibrateCamera2.


Undistort2

Transforms image to compensate lens distortion

void cvUndistort2( const CvArr* src, CvArr* dst,
                   const CvMat* intrinsic_matrix,
                   const CvMat* distortion_coeffs );

src
The input (distorted) image.
dst
The output (corrected) image.
intrinsic_matrix
The camera matrix (A) [fx 0 cx; 0 fy cy; 0 0 1].
distortion_coeffs
The vector of distortion coefficients, 4x1 or 1x4 [k1, k2, p1, p2].

The function cvUndistort2 transforms the image to compensate radial and tangential lens distortion. The camera matrix and distortion parameters can be determined using cvCalibrateCamera2. For every pixel in the output image the function computes coordinates of the corresponding location in the input image using the formulae in the section beginning. Then, the pixel value is computed using bilinear interpolation. If the resolution of images is different from what was used at the calibration stage, fx, fy, cx and cy need to be adjusted appropriately, while the distortion coefficients remain the same.


InitUndistortMap

Computes undistorion map

void cvInitUndistortMap( const CvMat* intrinsic_matrix,
                         const CvMat* distortion_coeffs,
                         CvArr* mapx, CvArr* mapy );

intrinsic_matrix
The camera matrix (A) [fx 0 cx; 0 fy cy; 0 0 1].
distortion_coeffs
The vector of distortion coefficients, 4x1 or 1x4 [k1, k2, p1, p2].
mapx
The output array of x-coordinates of the map.
mapy
The output array of y-coordinates of the map.

The function cvInitUndistortMap pre-computes the undistortion map - coordinates of the corresponding pixel in the distorted image for every pixel in the corrected image. Then, the map (together with input and output images) can be passed to cvRemap function.


FindChessboardCorners

Finds positions of internal corners of the chessboard

int cvFindChessboardCorners( const void* image, CvSize pattern_size,
                             CvPoint2D32f* corners, int* corner_count=NULL,
                             int flags=CV_CALIB_CB_ADAPTIVE_THRESH );

image
Source chessboard view; it must be 8-bit grayscale or color image.
pattern_size
The number of inner corners per chessboard row and column.
corners
The output array of corners detected.
corner_count
The output corner counter. If it is not NULL, the function stores there the number of corners found.
flags
Various operation flags, can be 0 or a combination of the following values:
CV_CALIB_CB_ADAPTIVE_THRESH - use adaptive thresholding to convert the image to black-n-white, rather than a fixed threshold level (computed from the average image brightness).
CV_CALIB_CB_NORMALIZE_IMAGE - normalize the image using cvNormalizeHist before applying fixed or adaptive thresholding.
CV_CALIB_CB_FILTER_QUADS - use additional criteria (like contour area, perimeter, square-like shape) to filter out false quads that are extracted at the contour retrieval stage.

The function cvFindChessboardCorners attempts to determine whether the input image is a view of the chessboard pattern and locate internal chessboard corners. The function returns non-zero value if all the corners have been found and they have been placed in a certain order (row by row, left to right in every row), otherwise, if the function fails to find all the corners or reorder them, it returns 0. For example, a regular chessboard has 8 x 8 squares and 7 x 7 internal corners, that is, points, where the black squares touch each other. The coordinates detected are approximate, and to determine their position more accurately, the user may use the function cvFindCornerSubPix.


DrawChessBoardCorners

Renders the detected chessboard corners

void cvDrawChessboardCorners( CvArr* image, CvSize pattern_size,
                              CvPoint2D32f* corners, int count,
                              int pattern_was_found );

image
The destination image; it must be 8-bit color image.
pattern_size
The number of inner corners per chessboard row and column.
corners
The array of corners detected.
count
The number of corners.
pattern_was_found
Indicates whether the complete board was found (≠0) or not (=0). One may just pass the return value cvFindChessboardCorners here.

The function cvDrawChessboardCorners draws the individual chessboard corners detected (as red circles) in case if the board was not found (pattern_was_found=0) or the colored corners connected with lines when the board was found (pattern_was_found≠0).


Pose Estimation


CreatePOSITObject

Initializes structure containing object information

CvPOSITObject* cvCreatePOSITObject( CvPoint3D32f* points, int point_count );

points
Pointer to the points of the 3D object model.
point_count
Number of object points.

The function cvCreatePOSITObject allocates memory for the object structure and computes the object inverse matrix.

The preprocessed object data is stored in the structure CvPOSITObject, internal for OpenCV, which means that the user cannot directly access the structure data. The user may only create this structure and pass its pointer to the function.

Object is defined as a set of points given in a coordinate system. The function cvPOSIT computes a vector that begins at a camera-related coordinate system center and ends at the points[0] of the object.

Once the work with a given object is finished, the function cvReleasePOSITObject must be called to free memory.


POSIT

Implements POSIT algorithm

void cvPOSIT( CvPOSITObject* posit_object, CvPoint2D32f* image_points, double focal_length,
              CvTermCriteria criteria, CvMatr32f rotation_matrix, CvVect32f translation_vector );

posit_object
Pointer to the object structure.
image_points
Pointer to the object points projections on the 2D image plane.
focal_length
Focal length of the camera used.
criteria
Termination criteria of the iterative POSIT algorithm.
rotation_matrix
Matrix of rotations.
translation_vector
Translation vector.

The function cvPOSIT implements POSIT algorithm. Image coordinates are given in a camera-related coordinate system. The focal length may be retrieved using camera calibration functions. At every iteration of the algorithm new perspective projection of estimated pose is computed.

Difference norm between two projections is the maximal distance between corresponding points. The parameter criteria.epsilon serves to stop the algorithm if the difference is small.


ReleasePOSITObject

Deallocates 3D object structure

void cvReleasePOSITObject( CvPOSITObject** posit_object );

posit_object
Double pointer to CvPOSIT structure.

The function cvReleasePOSITObject releases memory previously allocated by the function cvCreatePOSITObject.


CalcImageHomography

Calculates homography matrix for oblong planar object (e.g. arm)

void cvCalcImageHomography( float* line, CvPoint3D32f* center,
                            float* intrinsic, float* homography );

line
the main object axis direction (vector (dx,dy,dz)).
center
object center ((cx,cy,cz)).
intrinsic
intrinsic camera parameters (3x3 matrix).
homography
output homography matrix (3x3).

The function cvCalcImageHomography calculates the homography matrix for the initial image transformation from image plane to the plane, defined by 3D oblong object line (See Figure 6-10 in OpenCV Guide 3D Reconstruction Chapter).


Epipolar Geometry


FindFundamentalMat

Calculates fundamental matrix from corresponding points in two images

int cvFindFundamentalMat( const CvMat* points1,
                          const CvMat* points2,
                          CvMat* fundamental_matrix,
                          int    method=CV_FM_RANSAC,
                          double param1=1.,
                          double param2=0.99,
                          CvMat* status=NULL);

points1
Array of the first image points of 2xN, Nx2, 3xN or Nx3 size (where N is number of points). Multi-channel 1xN or Nx1 array is also acceptable. The point coordinates should be floating-point (single or double precision)
points2
Array of the second image points of the same size and format as points1
fundamental_matrix
The output fundamental matrix or matrices. The size should be 3x3 or 9x3 (7-point method may return up to 3 matrices).
method
Method for computing the fundamental matrix
CV_FM_7POINT - for 7-point algorithm. N == 7
CV_FM_8POINT - for 8-point algorithm. N >= 8
CV_FM_RANSAC - for RANSAC algorithm. N >= 8
CV_FM_LMEDS - for LMedS algorithm. N >= 8
param1
The parameter is used for RANSAC or LMedS methods only. It is the maximum distance from point to epipolar line in pixels, beyond which the point is considered an outlier and is not used for computing the final fundamental matrix. Usually it is set to 0.5 or 1.0.
param2
The parameter is used for RANSAC or LMedS methods only. It denotes the desirable level of confidence that the matrix is correct.
status
The optional output array of N elements, every element of which is set to 0 for outliers and to 1 for the other points. The array is computed only in RANSAC and LMedS methods. For other methods it is set to all 1’s.

The epipolar geometry is described by the following equation:

p2T*F*p1=0,

where F is fundamental matrix, p1 and p2 are corresponding points in the first and the second images, respectively.

The function cvFindFundamentalMat calculates fundamental matrix using one of four methods listed above and returns the number of fundamental matrices found (1 or 3) and 0, if no matrix is found.

The calculated fundamental matrix may be passed further to cvComputeCorrespondEpilines that finds epipolar lines corresponding to the specified points.

Example. Estimation of fundamental matrix using RANSAC algorithm

int point_count = 100;
CvMat* points1;
CvMat* points2;
CvMat* status;
CvMat* fundamental_matrix;

points1 = cvCreateMat(1,point_count,CV_32FC2);
points2 = cvCreateMat(1,point_count,CV_32FC2);
status = cvCreateMat(1,point_count,CV_8UC1);

/* Fill the points here ... */
for( i = 0; i < point_count; i++ )
{
    points1->data.db[i*2] = <x1,i>;
    points1->data.db[i*2+1] = <y1,i>;
    points2->data.db[i*2] = <x2,i>;
    points2->data.db[i*2+1] = <y2,i>;
}

fundamental_matrix = cvCreateMat(3,3,CV_32FC1);
int fm_count = cvFindFundamentalMat( points1,points2,fundamental_matrix,
                                     CV_FM_RANSAC,1.0,0.99,status );

ComputeCorrespondEpilines

For points in one image of stereo pair computes the corresponding epilines in the other image

void cvComputeCorrespondEpilines( const CvMat* points,
                                  int which_image,
                                  const CvMat* fundamental_matrix,
                                  CvMat* correspondent_lines);

points
The input points. 2xN, Nx2, 3xN or Nx3 array (where N number of points). Multi-channel 1xN or Nx1 array is also acceptable.
which_image
Index of the image (1 or 2) that contains the points
fundamental_matrix
Fundamental matrix
correspondent_lines
Computed epilines, 3xN or Nx3 array

For every point in one of the two images of stereo-pair the function cvComputeCorrespondEpilines finds equation of a line that contains the corresponding point (i.e. projection of the same 3D point) in the other image. Each line is encoded by a vector of 3 elements l=[a,b,c]T, so that:


lT*[x, y, 1]T=0, or
a*x + b*y + c = 0

From the fundamental matrix definition (see cvFindFundamentalMatrix discussion), line l2 for a point p1 in the first image (which_image=1) can be computed as:
l2=F*p1
and the line l1 for a point p2 in the second image (which_image=1) can be computed as:
l1=FT*p2

Line coefficients are defined up to a scale. They are normalized (a2+b2=1) are stored into correspondent_lines.


ConvertPointsHomogenious

Convert points to/from homogenious coordinates

void cvConvertPointsHomogenious( const CvMat* src, CvMat* dst );

src
The input point array, 2xN, Nx2, 3xN, Nx3, 4xN or Nx4 (where N is the number of points). Multi-channel 1xN or Nx1 array is also acceptable.
dst
The output point array, must contain the same number of points as the input; The dimensionality must be the same, 1 less or 1 more than the input, and also within 2..4.

The function cvConvertPointsHomogenious converts 2D or 3D points from/to homogenious coordinates, or simply copies or transposes the array. In case if the input array dimensionality is larger than the output, each point coordinates are divided by the last coordinate:

(x,y[,z],w) -> (x',y'[,z']):

x' = x/w
y' = y/w
z' = z/w (if output is 3D)
If the output array dimensionality is larger, an extra 1 is appended to each point.
(x,y[,z]) -> (x,y[,z],1)
Otherwise, the input array is simply copied (with optional tranposition) to the output. Note that, because the function accepts a large variety of array layouts, it may report an error when input/output array dimensionality is ambiguous. It is always safe to use the function with number of points N>=5, or to use multi-channel Nx1 or 1xN arrays.

Alphabetical List of Functions


2

2DRotationMatrix

A

Acc ApproxChains ArcLength
AdaptiveThreshold ApproxPoly

B

BoundingRect BoxPoints

C

CalcBackProject CalibrateCamera2 ConvexityDefects
CalcBackProjectPatch CamShift CopyHist
CalcEMD2 Canny CopyMakeBorder
CalcGlobalOrientation CheckContourConvexity CornerEigenValsAndVecs
CalcHist ClearHist CornerHarris
CalcImageHomography ClearSubdivVoronoi2D CornerMinEigenVal
CalcMotionGradient CompareHist CreateConDensation
CalcOpticalFlowBM ComputeCorrespondEpilines CreateContourTree
CalcOpticalFlowHS ConDensInitSampleSet CreateHist
CalcOpticalFlowLK ConDensUpdateByTime CreateKalman
CalcOpticalFlowPyrLK ContourArea CreatePOSITObject
CalcPGH ContourFromContourTree CreateStructuringElementEx
CalcProbDensity ConvertPointsHomogenious CreateSubdivDelaunay2D
CalcSubdivVoronoi2D ConvexHull2 CvtColor

D

Dilate DistTransform DrawChessBoardCorners

E

EndFindContours EqualizeHist Erode

F

Filter2D FindExtrinsicCameraParams2 FindNextContour
FindChessboardCorners FindFundamentalMat FitEllipse
FindContours FindHomography FitLine2D
FindCornerSubPix FindNearestPoint2D FloodFill

G

GetCentralMoment GetMinMaxHistValue GetRectSubPix
GetHistValue_*D GetNormalizedCentralMoment GetSpatialMoment
GetHuMoments GetQuadrangleSubPix GoodFeaturesToTrack

H

HaarDetectObjects HoughCircles HoughLines2

I

InitUndistortMap Integral

K

KalmanCorrect KalmanPredict

L

Laplace LoadHaarClassifierCascade LogPolar

M

MakeHistHeaderForArray MaxRect Moments
MatchContourTrees MeanShift MorphologyEx
MatchShapes MinAreaRect2 MultiplyAcc
MatchTemplate MinEnclosingCircle

N

NormalizeHist

P

POSIT PreCornerDetect PyrSegmentation
PointPolygonTest ProjectPoints2 PyrUp
PointSeqFromMat PyrDown

Q

QueryHistValue_*D

R

ReadChainPoint ReleaseKalman Resize
ReleaseConDensation ReleasePOSITObject Rodrigues2
ReleaseHaarClassifierCascade ReleaseStructuringElement RunHaarClassifierCascade
ReleaseHist Remap RunningAvg

S

SampleLine Sobel Subdiv2DGetEdge
SegmentMotion SquareAcc Subdiv2DLocate
SetHistBinRanges StartFindContours Subdiv2DRotateEdge
SetImagesForHaarClassifierCascade StartReadChainPoints SubdivDelaunay2DInsert
Smooth Subdiv2DEdgeDst SubstituteContour
SnakeImage Subdiv2DEdgeOrg

T

ThreshHist Threshold

U

Undistort2 UpdateMotionHistory

W

WarpAffine WarpPerspective WarpPerspectiveQMatrix

Bibliography

This bibliography provides a list of publications that were might be useful to the OpenCV users. This list is not complete; it serves only as a starting point.

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