Question

I’m trying to get the number of difference between two pictures.
When I compare 2 images in gray scale, pixDiff <> 0 but when it come to RGB, pixDiff is always 0.

I used openCV’s compare and also a custom loop.

Mat frame, oldFrame;
cap >> oldFrame;

if(analyseMod == MONOCHROME)
    cvtColor(oldFrame, oldFrame, CV_BGR2GRAY);

nbChannels = oldFrame.channels();

while(1)
{
    pixDiff = 0;
    cap >> frame;

    //Test diff
    Mat diff;
    compare(oldFrame, frame, diff, CMP_NE);
    imshow("video 0", diff);
    imshow("video 1", frame);

    if(analyseMod == MONOCHROME)
    {
        cvtColor(frame, frame, CV_BGR2GRAY);

        for(int i=0; i<frame.rows; i++)
            for(int j=0; j<frame.cols; j++)
                if(frame.at<uchar>(i,j) < oldFrame.at<uchar>(i,j) - similarPixelTolerance || frame.at<uchar>(i,j) > oldFrame.at<uchar>(i,j) + similarPixelTolerance)
                    pixDiff++;
    }
    else if(analyseMod == RGB)
    {
        uint8_t *f = (uint8_t *)frame.data;
        uint8_t *o = (uint8_t *)oldFrame.data;

        for(int i=0; i<frame.rows; i++)
        {
            for(int j=0; j<frame.cols; j++)
            {
                if(f[nbChannels*i*frame.cols + j + RED] < o[nbChannels*i*oldFrame.cols + j + RED])
                    pixDiff++;
            }
        }
    }          

    frame.copyTo(oldFrame);

    cout << pixDiff;
    if(waitKey(30) >= 0) break;
}

Thx for help

Answer 1

I still don’t get it, why are you not using your delta in the RGB case, but here is the solution for both cases, if you want to consider color channels separately. Set CN to 1 for monochrome case and to 3 for RGB case.

const int CN = 3; // 3 for RGB, 1 for monochrome

uint8_t *f = frame.ptr<uint8_t>();
uint8_t *o = oldFrame.ptr<uint8_t>();

for(int i = 0; i < frame.rows; ++i)
{
  for(int j = 0; j < frame.cols; ++j)
  {
    for (int c = 0; c < CN; ++c)
    {
      if (abs(*f - *o) > similarPixelTolerance) ++pxDiff;
      ++f, ++o;
    }
  }
}

It is way more efficient to access pixels in this way than to call at for each pixel. The only possible problem is if you have some padding in your images, but by default OpenCV is using continuous allocation.