OpenCV Image Overlay w/Transparency

gistfile1.txt

void OverlayImage(Mat* src, Mat* overlay, const Point& location)
{
  for (int y = max(location.y, 0); y < src->rows; ++y)
  {
    int fY = y - location.y;
  
    if (fY >= overlay->rows)
      break;
  
    for (int x = max(location.x, 0); x < src->cols; ++x)
    {
      int fX = x - location.x;
      
      if (fX >= overlay->cols)
        break;
      
      double opacity = ((double)overlay->data[fY * overlay->step + fX * overlay->channels() + 3]) / 255;
      
      for (int c = 0; opacity > 0 && c < src->channels(); ++c)
      {
        unsigned char overlayPx = overlay->data[fY * overlay->step + fX * overlay->channels() + c];
        unsigned char srcPx = src->data[y * src->step + x * src->channels() + c];
        src->data[y * src->step + src->channels() * x + c] = srcPx * (1. - opacity) + overlayPx * opacity;
      }
    }
  }
}