dHash generates a “difference hash” for a given image - a perceptual hash based on Neal Krawetz’s dHash algorithm in this “Hacker Factor” blog entry. http://www.hackerfactor.com/blog/index.php?/archives/529-Kind-of-Like-That.html 1. Reduce size 2. Reduce color 3. Compute the difference 4. Assign bits The resulting hash value represents the relative change in brightness intensity. To compare two hashes, just count the number of bits that are different (called Hamming distance