// returns similarity (not diff)
static float rgbImageSimilarity_sameSize(RGBImage a, RGBImage b, float similarity) {
  int w = a.w(), h = a.h();
  if (w != b.w() || h != b.h()) fail("not same size");
  float factor = w*h;
  float maxError = (1f-similarity)*factor;
  float diff = 0;
  for (int y = 0; y < h; y++)
    for (int x = 0; x < w; x++) {
      diff += rgbDiff(
        a.getInt(x, y),
        b.getInt(x, y));
      if (diff > maxError) ret similarity-0.001f;
    }
  ret 1f-diff/factor;
}