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

Simple algorithm:

void SetChannel(Mat mat, uint channel, uchar value)
{
    const uint channels = mat.channels();
    if (channel > channels - 1)
        return;

    uchar * data = mat.data;
    uint N = mat.rows * mat.step / mat.elemSize1();

    for (uint i = channel; i < N; i += channels)
        data[i] = value;
}