DZone

Perceptual image hashing is a relatively new process used primarily in the multimedia industry for content identification and authentication. The process itself uses an algorithm to extract specific features from an image and calculate a hash value based on that information. The hash value that is generated acts as a kind of ‘fingerprint’ for the image; it is a distinct identifier that is unique to its parent image. 

As you may have guessed by the fingerprint comparison, perceptual image hashing is particularly useful for digital forensics, but it has become an important player in prohibiting online copyright infringement as well. By comparing the hash value of an original/authentic image with the hash value of a similar image, you can identify and match various images and calculate the Hamming Distance between them. For reference, Hamming Distance measures the minimum number of substitutions it takes to change one image to the other, so hash values that are closer together are more similar. 

Source: DZone