static float rgbLocalSimilarity(RGBImage img, Rect r1, Rect r2) {
  int w = r1.w, h = r1.h;
  if (w != r2.w || h != r2.h) ret 0;
  float diff = 0;
  for y to h: for x to w:
    diff += rgbDiff(img.getInt(r1.x+x, r1.y+y), img.getInt(r2.x+x, r2.y+y));
  ret 1f-diff/(w*h);
}