C++ How to split video into 16 by 16 pixel for the whole frame?

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This is the solution to do 16x16 and then 8x8.

for (int i = 0; i < height-16; i += 16) 
{ 

   for (int j = 0; j < width-16; j+= 16) 
   {
      Mat block = dctImage(Rect(j, i, 8, 8));

      vector<Mat> planes; 

      split(block, planes);
    }
}