Question

Is this the correct way of accessing all pixels in cv::Mat:

for( row = 0; row < mat.rows; ++row) 
    {
            for ( col = 0; col < mat.cols; ++col) 
            {



            }
    }

Or is there a formula method similar to this formula for an IplImage *:

temp_ptr = &((uchar*)(img->imageData + (img->widthStep*pt.x)))[pt.y*3];

Answer 1

In the best case, where all the pixels are stored contiguously you should be able to do:

uchar* pixel = mat.data;
for(int i = 0; i < mat.rows * mat.cols; ++i) 
{
    // access pixel[0],pixel[1],pixel[2] here
    pixel += 3; // move to next pixel
}

To be a bit more generic, but still fast, have a look at the sample code mentioned with Mat::isContinuous(). The general formula for calculating the address of an element can be seen here (Reproduced below).