IntroductionThis is an implementation of a perceptual image hash,
Difference From I found the phash package to be a little error prone with respect to file I/O. This package, while the API is very similar, is different in some key ways.
phashbinds directly to the pHash library.
canvas-phashis a direct implementation, written in coffeescript.
phashis callback-based while
canvas-phashis promise-based (specifically, it uses bluebird for promise management).
phashgenerally takes longer to compute the hash of an image but is faster at finding the hamming distance between two hashes.
- The hash output by
phashis an integer, expressed as a string. The hash output by
canvas-phashis a 128-byte
- Comparing the two libraries on the basis of the correlation between hamming distance and "perceived difference" had mixed results.
phashwas better at some things,
canvas-phashwas better at others.
PerformanceI ran some preliminary tests to check the performance against
phashand found it's fairly comparable.
Computing A HashThe time taken ranged from just under 75ms to 150ms. For my tests, it generally took
phashabout 1-2 times longer to compute a hash as it took
Finding the Hamming DistanceTypical time taken ranged from 0.2ms to 0.3ms. For my tests, it generally took
canvas-phashabout 2-3 times longer to find the hamming distance of two hashes. When comparing against a large collection of images, this is potentially significant. That being said, this library has not been optimized. Also, the actual hash created is 128 bytes long and takes up about 2-3 times more space.
getImageHash- Accepts 1 parameter, the path of the image. Returns a promise with eventual value equal to the "Block Mean Value Based" pHash.
getHammingDistance- Accepts 2 parameters, two instances of
Bufferof length 128 (this is what is returned from
getSHA256- This computes the SHA256 hash of the pixel data. The only parameter is setup like that of
getImageHash. This is useful for fast checks of exact matches. Ignores metadata.
readImage- Reads an image at the specified path and returns an object with properties:
data, the byte array,
width, the width of the image, and
height, the height of the image.
Example Usage(Another example exists in the repo)
In the previous example, Promise.all is used to make the code readable.
phash = require 'canvas-phash' Promise = require 'bluebird' Promise.all([ phash.getImageHash 'image.jpg' phash.getImageHash 'otherImage.jpg' ]) .spread (hash1, hash2)-> dist = phash.getHammingDistance hash1, hash2
bluebirdis not necessary to use this package. The typical use-case would be to compute the hash of a single image via
phash.getImageHash('image.jpg').then (hash)->and compare that against a list of pre-existing hashes for close matches.