http://www.hackerfactor.com/blog/?/archives/529-Kind-of-Like-That.html aHash (average hash, mean hash) と 同程度に高速 pHash (perceptive hash/DCTを利用)に 近い精度 dHash. Absolutely amazing... Very few false positives. For example, the image with two known matches ended up matching 6 pictures total (4 false positives). The scores were: 10, 0, 8, 10, 0, and 10. The two zeros were the correct matches; all of the false- positive matches had higher scores. As speed goes, dHash is as fast as aHash. Well, technically it is faster since it doesn't need to compute the mean color value. The dHash algorithm has all the speed of aHash with very few false-positives.