Adding alpha channel to a 3 channel openCV Mat

opencv-cpp-create-alpha-image.cpp

void createAlphaImage(const cv::Mat& mat, cv::Mat_<cv::Vec4b>& dst)
{
  std::vector<cv::Mat> matChannels;
  cv::split(mat, matChannels);
  
  // create alpha channel
  cv::Mat alpha = matChannels.at(0) + matChannels.at(1) + matChannels.at(2);
  matChannels.push_back(alpha);

  cv::merge(matChannels, dst);
}