I realize now that I could have given the basic principles behind each of the two libraries compared here: libPuzzle: Splits the image in blocks and compute th

I realize now that I could have given the basic principles behind each of the two libraries compared here: libPuzzle: Splits the image in blocks and compute the hash based on the relationships between the adjacent blocks brightness. pHash: Computes the 8x8 DCT (http://en.wikipedia.org/wiki/Discrete_cosine_transform http://en.wikipedia.org/wiki/Discrete_cosine_transform) representation of an image (lowest frequencies of the image). It then sets the hash by comparing each of these 8×8 values to the mean DCT value (very resilient to non-structural changes in the image). I updated the post with these informations