// ppt = 0.0 to 1.0
static bool rgbImageSectionsIdentical_ppt(RGBImage a, Rect r1, RGBImage b, Rect r2, float ppt) {
  assertEquals(r1.w, r2.w);
  assertEquals(r1.h, r2.h);
  int w = r1.w, h = r1.h;
  for (int yy = 0; yy < h; yy++)
    for (int xx = 0; xx < w; xx++)
      if (rgbDiff(a.getInt(r1.x+xx, r1.y+yy), b.getInt(r2.x+xx, r2.y+yy)) > ppt)
        false;
  true;
}