static RGBImage renderColorPredicate(F1<Int, Bool> predicate, RGBImage img) {
  int w = img.getWidth(), h = img.getHeight();
  RGBImage out = new(w, h);
  for y to h:
    for x to w:
      if (callF(predicate, img.getInt(x, y)))
        out.setPixel(x, y, Color.white);
  ret out;
}

Began life as a copy of #1022943