static float rgbLocalPrecision(RGBImage img, Rect r) {
  ret avg(
    rgbLocalSimilarity(img, r, translateRect(r, r.x2() == img.w() ? -1 : 1, 0)),
    rgbLocalSimilarity(img, r, translateRect(r, 0, r.y2() == img.h() ? -1 : 1)));
}