perceptual-hashes

perceptual-hashes library computes perceptual hashes for images
(supported formats include formats supported by
imago + jpeg). Perceptual hashes
are a measure of similarity between images.

Installation

You can install perceptual-hashes from Ultralisp
repository. Add Ultralisp repository to quicklisp (if you haven’t already):

(ql-dist:install-dist "http://dist.ultralisp.org/"
                      :prompt nil)

and install perceptual-hashes:

(ql:quickload :perceptual-hashes)

Supported algorithms

The following algorithms are supported:

• aHash: An algorithm based on whenever each pixel of an image is
  brighter or darker than the average luminance of all pixels.
• dHash: An algorithm based on whenever each pixel of an image is
  brighter or darker than its neighbours.

You can find more information
here.

Examples

(defconstant +threshold+ 45
    "The distance between hashes can vary from 0 to (length hash) =
1024. If it is small enough (<= ~45) the images are similar")
(defun images-similar-p (image-name-1 image-name-2)
    (let ((hash1 (perceptual-hashes:ahash image-name-1))
          (hash2 (perceptual-hashes:ahash image-name-2)))
        (< (perceptual-hashes:hamming-distance hash1 hash2)
           +threshold+)))