blockhash values differ from other implementations' values

[chrysn]

The results using the blockhash algorithm in a simple test differed from those of other implementations.

In particular, I compared the blockhash crate (v0.2) and the Python implentation on their results for a 16bit hash of the 16x16_rgb.png image shipped with the blockhash crate (which both produced the same results):

$ ./blockhash.py ~/git/crates/blockhash/images/16x16_rgb.png --bits 16
ff00ff00ff00fe20fc3efc18f900f980f3c0f7c0ef80fe40fee07ee05e7a1804  [...]
$ cargo run ~/git/crates/blockhash/images/16x16_rgb.png
ff00ff00ff007f043f7c3f189f009f01cf03ef03f7017f007f077e077a1e1820

The img_hash program was the hash_image base program modified by #40 and with this configuration:

let hash = HasherConfig::new().hash_size(16, 16).hash_alg(img_hash::HashAlg::Blockhash).to_hasher().hash_image(&image);

Most of the difference can be explained by blockhash.py/blockhash-crate and img_hash using different bit orders -- once you flip the bits in the bytes, most of the difference goes away. By majority voting (and without looking into the spec of blockhash), I'm opening this as a bug here -- if you insist this behavior is correct, I'm happy to move it to the other projects.

The remaining two bit errors could be explained by rescaling; is there a hash configuration that can be documented and tested to be produce the exact hashes as the other implementations?