static BWImage img_distanceFromColor_withGain(RGBImage img, RGB color, double gain) {
  int w = img.getWidth(), h = img.getHeight();
  int col = color.getInt();
  BWImage bw = new(w, h);
  for y to h:
    for x to w:
      bw.setPixel(x, y, rgbDiff(img.getInt(x, y), col)*(float) gain);
  ret bw;
}

static BWImage img_distanceFromColor_withGain(BufferedImage img, RGB color, double gain) {
  ret img_distanceFromColor_withGain(RGBImage(img), color, gain);
}