// returns similarity (not diff)

static float rgbImageSimilarityResized(RGBImage a, RGBImage b, float similarity) {

  int aw = a.w(), ah = a.h();

  int bw = b.w(), bh = b.h();

  if (aw == bw && ah == bh) ret rgbImageSimilarity_sameSize(a, b, similarity);

  int wp = max(aw, bw), hp = max(ah, bh);

  float ratio1 = ((float) aw)/ah, ratio2 = ((float) bw)/bh;

  float mismatch = ratio1/ratio2;

  if (mismatch < 1f) mismatch = 1f/mismatch;

  float factor = wp*hp/mismatch; // ratio mismatch punishment (greater mismatch => factor smaller => returned difference bigger)

  float maxError = (1f-similarity)*factor;

  float diff = 0;

  for (int y = 0; y < hp; y++)

    for (int x = 0; x < wp; x++) {

      diff += rgbDiff(

        a.getInt(x*aw/wp, y*ah/hp),

        b.getInt(x*bw/wp, y*bh/hp));

      if (diff > maxError) ret similarity-0.001f;

    }

  ret 1f-diff/factor;

}