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It depends on how much you want to sacrifice for time performance. The code will be faster but dirtier. If time is really really important you can do:

1. Avoid using 'for' loops. Just write 'suma' as sum of 9 values.

2. Don't use .at<>. Instead use pointers to access memory directly.

   uchar* p = grayImage.ptr(ay2) + ax2;

   uchar* q = grayImage.ptr(by2) + bx2;

   // difference at top left pixel of 3x3 patch

   difa = p[-step-1] - q[-step-1];

   // and so on...