Rihamark: Perceptual image hash benchmarking

We identify which hash function has the best characteristics for various applications. In some of those the computation speed may be the most important, in others the ability to distinguish similar images, and sometimes the robustness of the hash against attacks is the primary goal. We compare the hash functions and provide test results. The block mean value based image hash function outperforms the other hash functions in terms of speed. The discrete cosine transform (DCT) based image hash function is the slowest. Although the Marr- Hildreth operator based image hash function is neither the fastest nor the most robust, it offers by far the best discriminative abilities. Interestingly enough, the performance in terms of discriminative ability does not depend on the content of the images. That is, no matter whether the visual appearance of the images compared was very similar or not, the performance of the particular hash function did not change significantly. Different image operations, like horizontal flipping, rotating or resizing, were used to test the robustness of the image hash functions. An interesting result is that none of the tested image hash function is robust against flipping an image horizontally.