static ImageSurface showImageWithGrayToBlackSlider(S name, final BufferedImage img) {
  final ImageSurface is = showImage(name, img);
  final SimpleLiveValue<Float> lv = floatLiveValue();
  
  Runnable update = r {
    float darkening = min(lv!, 0.99f);
    float factor = 1/(1-darkening);
    float offset = (-darkening*255)*factor;
    //print("factor=" + factor + ", offset=" + offset);
    is.setImage(rgbContrastAndBrightness(factor, offset, img));
  };
  
  lv.onChange(update);
  addToWindowAndPack(is,
    withLabel("Darkening factor:", jLiveValueSliderZeroToOne(lv)));
  ret is;
}

Began life as a copy of #1018736