OpenCV: Matrix Iteration

Question

I am new to OpenCV. I am trying to use iterator instead of \"for\" loop, which is too slow for my case. I tried some codes like this:

MatIterator_

        

Answer 1

It's not the for loop which is slow it is the exampleMat.at(i) which is doing range checking.

To efficently loop over all the pixels you can get a pointer to the data at the start of each row with .ptr()

for(int row = 0; row < img.rows; ++row) {
    uchar* p = img.ptr(row);
    for(int col = 0; col < img.cols; ++col) {
         *p++  //points to each pixel value in turn assuming a CV_8UC1 greyscale image 
    }

    or 
    for(int col = 0; col < img.cols*3; ++col) {
         *p++  //points to each pixel B,G,R value in turn assuming a CV_8UC3 color image 
    }

}