static RGBImage rgbMarkPoint(RGBImage img, Pt p, Color color, int size) {
  if (img != null &&p != null)
    rgbFillRect(img, p.x-size/2, p.y-size/2, size, size, color);
  ret img;
}

static RGBImage rgbMarkPoint(RGBImage img, Pt p) {
  ret rgbMarkPoint(img, p, Color.red, 3);
}