Question

I am processing video frames in real time. I loop through each pixel one at a time, do some math, and then set the pixel to a specific color (if necessary).

In a nutshell, my algorithm looks like this:

     //videoFrame is IplImage, 8 bit, 4 channel
     for(int i=0;i<videoFrame.width;i++){
        for(int j=0;j<videoFrame.height;j++){

            CvScalar pixel = cvGet2D(&videoFrame, i, j);

            double red = pixel.val[0];
            double green = pixel.val[1];
            double blue = pixel.val[2];

            //do some math


            if (someCondition) {
                cvSet2D(&videoFrame, i, j, white);
            }
        }       
    }

but i find that calls cvGet2D(&videoFrame, i, j); and cvSet2D(&videoFrame, i, j, white); are really expensive(slow). Is there an alternative faster way I can access red/green/blue value of each pixel?

Thanks!

Answer 1

Try accessing the data directly with pointer as following.

for(int i=0;i<videoFrame.width;i++){
  for(int j=0;j<videoFrame.height;j++){
    uchar B =((uchar *)(videoFrame.imageData + j*videoFrame.widthStep))[i*videoFrame.nChannels + 0];                      
    uchar G =((uchar *)(videoFrame.imageData + j*videoFrame.widthStep))[i*videoFrame.nChannels + 1];       
    uchar R =((uchar *)(videoFrame.imageData + j*videoFrame.widthStep))[i*videoFrame.nChannels + 2];       


    //do some math

    ((uchar *)(videoFrame.imageData + j*videoFrame.widthStep))[i*videoFrame.nChannels + 0] = B;
    ((uchar *)(videoFrame.imageData + j*videoFrame.widthStep))[i*videoFrame.nChannels + 1] = G;
    ((uchar *)(videoFrame.imageData + j*videoFrame.widthStep))[i*videoFrame.nChannels + 2] = R;

  }
}