How to crop a CvMat in OpenCV

Question

I have an image converted in a CvMat Matrix say CVMat source  Once I get a region of interest from source I want the rest of the algorithm to be applied to that region of interest only  For that I think I will have to somehow crop the source matrix which I am unable to do so  Is there a method or a function that could crop a CvMat Matrix and return another cropped CvMat matrix  thanks

User · Accepted Answer

OpenCV has region of interest functions which you may find useful. If you are using the cv::Mat then you could use something like the following.

// You mention that you start with a CVMat* imagesource
CVMat * imagesource;

// Transform it into the C++ cv::Mat format
cv::Mat image(imagesource); 

// Setup a rectangle to define your region of interest
cv::Rect myROI(10, 10, 100, 100);

// Crop the full image to that image contained by the rectangle myROI
// Note that this doesn't copy the data
cv::Mat croppedImage = image(myROI);

Documentation for extracting sub image

User · Answer

To create a copy of the crop we want  we can do the following      Read img cv  Mat img   cv  imread  imgFileName    cv  Mat croppedImg      This line picks out the rectangle from the image    and copies to a new Mat img cv  Rect xMin yMin xMax-xMin yMax-yMin   copyTo croppedImg       Display diff cv  imshow   Original Image    img    cv  imshow   Cropped Image    croppedImg   cv  waitKey

User · Answer

I know this question is already solved   but there is a very easy way to crop  you can just do it in one line-  Mat cropedImage   fullImage Rect X Y Width Height

User · Answer

To get better results and robustness against differents types of matrices  you can do this in addition to the first answer  that copy the data    cv  Mat source   getYourSource        Setup a rectangle to define your region of interest cv  Rect myROI 10  10  100  100       Crop the full image to that image contained by the rectangle myROI    Note that this doesn t copy the data cv  Mat croppedRef source  myROI    cv  Mat cropped     Copy the data into new matrix croppedRef copyTo cropped

User · Answer

I understand this question has been answered but perhaps this might be useful to someone     If you wish to copy the data into a separate cv  Mat object you could use a function similar to this   void ExtractROI Mat amp  inImage  Mat amp  outImage  Rect roi          Create the image        outImage   Mat roi height  roi width  inImage type    Scalar 0            Populate the image        for  int i   roi y  i  lt   roi y roi height   i             uchar  inP   inImage ptr lt uchar gt  i           uchar  outP   outImage ptr lt uchar gt  i-roi y           for  int j   roi x  j  lt   roi x roi width   j                 outP j-roi x    inP j                       It would be important to note that this would only function properly on single channel images

User · Answer

You can easily crop a Mat using opencv funtions   setMouseCallback  Original  mouse call     The mouse callis given below    void mouse call int event int x int y int void                 if event  EVENT LBUTTONDOWN                        leftDown true              cor1 x x              cor1 y y             cout  lt  lt  Corner 1    lt  lt cor1 lt  lt endl                     if event  EVENT LBUTTONUP                        if abs x-cor1 x  gt 20 amp  amp abs y-cor1 y  gt 20    checking whether the region is too small                               leftup true                  cor2 x x                  cor2 y y                  cout lt  lt  Corner 2    lt  lt cor2 lt  lt endl                            else                               cout lt  lt  Select a region more than 20 pixels  lt  lt endl                                   if leftDown  true amp  amp leftup  false    when the left button is down                       Point pt              pt x x              pt y y              Mat temp img img clone                rectangle temp img cor1 pt Scalar 0 0 255      drawing a rectangle continuously             imshow  Original  temp img                      if leftDown  true amp  amp leftup  true    when the selection is done                        box width abs cor1 x-cor2 x               box height abs cor1 y-cor2 y               box x min cor1 x cor2 x               box y min cor1 y cor2 y               Mat crop img box       Selecting a ROI region of interest  from the original pic             namedWindow  Cropped Image                imshow  Cropped Image  crop     showing the cropped image             leftDown false              leftup false                     For details you can visit the link Cropping the Image using Mouse

[opencv] How to crop a CvMat in OpenCV?

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