// scales second image to size of first
// interprets second image as 50% brightness meaning zero
static BufferedImage addBufferedImagesPixelByPixel_minus50(BufferedImage img1, BufferedImage img2, double factor default 1) {
  int w = img1.getWidth(), h = img1.getHeight();
  BufferedImage img = newImage(w, h);
  BufferedImage img2scaled = resizeImage(img2, w, h);
  for y to h:
    for x to w: {
      RGB a = bufferedImage_getRGB(img1, x, y);
      RGB b = bufferedImage_getRGB(img2scaled, x, y);
      setRGB(img, x, y, rgb(
        a.r+(b.r-0.5)*factor,
        a.g+(b.g-0.5)*factor,
        a.b+(b.b-0.5)*factor));
    }
  ret img;
}