The article directly describes the phash algorithm presented in the Hacker Factor post, while your second link is an implementation of the ahash algorithm in th

The article directly describes the phash algorithm presented in the Hacker Factor post, while your second link is an implementation of the ahash algorithm in the same post. libpHash's implementation is slightly more sophisticated (and slower) as it adds a box filter over the image before downscaling and uses the median of the AC coefficients rather than mean for computing the hash bits. It also offers a few alternative hashing methods.