C++ Cheatsheet
==============

.. highlight:: cpp


The section is just a summary "cheatsheet" of common things you may want to do with cv::Mat:. The code snippets below all assume the correct 
 namespace is used:
 



::


    
    using namespace cv;
    using namespace std; 
     
    

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Convert an IplImage or CvMat to an cv::Mat and a cv::Mat to an IplImage or CvMat:
 



::


    
     // Assuming somewhere IplImage *iplimg; exists 
     // and has been allocated and cv::Mat Mimg has been defined
     Mat imgMat(iplimg);  //Construct an Mat image "img" out of an IplImage
     Mimg = iplimg;       //Or just set the header of pre existing cv::Mat 
                          //Ming to iplimg's data (no copying is done)
     
     //Convert to IplImage or CvMat, no data copying
     IplImage ipl_img = img;
     CvMat cvmat = img; // convert cv::Mat -> CvMat
     
    

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A very simple way to operate on a rectanglular sub-region of an image (ROI -- "Region of Interest"):



::


    
     //Make a rectangle 
     Rect roi(10, 20, 100, 50);
     //Point a cv::Mat header at it (no allocation is done)
     Mat image_roi = image(roi);
    

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A bit advanced, but should you want efficiently to sample from a circular region in an image  
(below, instead of sampling, we just draw into a BGR image) :




::


    
    // the function returns x boundary coordinates of 
    // the circle for each y. RxV[y1] = x1 means that 
    // when y=y1, -x1 <=x<=x1 is inside the circle
    void getCircularROI(int R, vector < int > & RxV)
    {
        RxV.resize(R+1);
        for( int y = 0; y <= R; y++ )
            RxV[y] = cvRound(sqrt((double)R*R - y*y));
    }
    
    // This draws a circle in the green channel
    // (note the "[1]" for a BGR" image,
    // blue and red channels are not modified),
    // but is really an example of how to *sample* from a circular region. 
    void drawCircle(Mat &image, int R, Point center)
    {
        vector<int> RxV;
        getCircularROI(R, RxV);
            
        Mat_<Vec3b>& img = (Mat_<Vec3b>&)image; //3 channel pointer to image
        for( int dy = -R; dy <= R; dy++ )
        {
            int Rx = RxV[abs(dy)];
            for( int dx = -Rx; dx <= Rx; dx++ )
                img(center.y+dy, center.x+dx)[1] = 255;
        }
    }
    

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