How to set given channel of a cv::Mat to a given value efficiently without changing other channels?

Question

How to set given channel of a cv::Mat to a given value efficiently without changing other channels? For example, I want to set its fourth channel (alpha channel

Answer 1

Mat img;
[...]
const int cols = img.cols;
const int step = img.channels();
const int rows = img.rows;
for (int y = 0; y < rows; y++) {
    unsigned char* p_row = img.ptr(y) + SELECTED_CHANNEL_NUMBER; //gets pointer to the first byte to be changed in this row, SELECTED_CHANNEL_NUMBER is 3 for alpha
    for (int x = 0; x < cols; x++) {
         *p_row = value;
         p_row += step; //Goes to the next byte to be changed
    }
}

Note: This works both for continuous and uncontinuous matrices, according to the use of the term for opencv: http://docs.opencv.org/modules/core/doc/basic_structures.html#bool%20Mat::isContinuous%28%29%20const