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I'd say accessing it's data pointer.

Mat img = imread("filename.jpg",CV_LOAD_IMAGE_COLOR);
unsigned char *input = (unsigned char*)(img.data);

int i,j,r,g,b;
for(int i = 0;i < img.cols;i++){
    for(int j = 0;j < img.rows;j++){
        b = input[img.cols * j + i ] ;
        g = input[img.cols * j + i + 1];
        r = input[img.cols * j + i + 2];
        }
  }