Fast and simple image hashing algorithm

Question

I need a (preferably simple and fast) image hashing algorithm. The hash value is used in a lookup table, not for cryptography.

Some of the images are \"computer grap

Answer 1

If you want to make it very fast, you should consider taking a random subset of the pixels to avoid reading the entire image. Next, compute a hash function on the sequence of values at those pixels. The random subset should be selected by a deterministic pseudo-random number generator with fixed seed so that identical images produce identical subsets and consequently identical hash values.

This should work reasonably well even for artificial images. However, if you have images which differ from each other by a small number of pixels, this is going to give hash collisions. More iterations give better reliability. If that is the case, for instance, if your images set is likely to have pairs with one different pixel, you must read every pixel to compute the hash value. Taking a simple linear combination with pseudo-random coefficients would be good enough even for artificial images.

pseudo-code of a simple algorithm

Random generator = new generator(2847)  // Initialized with fixed seed
int num_iterations = 100

int hash(Image image) {
    generator.reset()   //To ensure consistency on each evaluation
    int value = 0
    for num_iteration steps {
        int nextValue = image.getPixel(generator.nextInt()%image.getSize()).getValue()
        value = value + nextValue*generator.nextInt()
    }
    return value
}

Answer 2

Have a look at this tutorial on the phash algorithm http://www.hackerfactor.com/blog/index.php?/archives/432-Looks-Like-It.html which is used to find closely matching images.