Why are older versions not compatible?

[fx1234]

I just did some tests with version 9.6.0 (from phash.org) and this version from github. From what I can see, the generated pHashes for images are not compatible. How can that be?? How can the main function of this library be changed without a huge notice?

[aetilius]

This is unfortunately the result of the algorithms getting fixed over time -- see #14 for the same question. The core issue is that pHash has never had a documented hash-format version or a backward-compat policy. Recent breaking changes include:

• Align DCT image hash with canonical pHash convention (breaking) #52 -- DCT image hash 8×8 block moved from (1,1)..(8,8) to canonical (0,0)..(7,7) (matches python-imagehash, Zauner thesis)
• Replace hand-rolled radon projector with bilinear line integrals (breaking) #53 -- radon projector replaced with a proper bilinear line-integral (the old one was asymmetric and produced spurious rotation drift even on identical inputs)
• Align audio hash params with Haitsma-Kalker reference (breaking) #54 -- audio hash frame size aligned with Haitsma-Kalker (was hardcoded 4096)
• Replace tumbling-window text hash with proper sliding-window winnowing (breaking) #55 -- text hash switched from tumbling window to sliding-window winnowing (matches Schleimer/Wilkerson/Aiken)

These were called out as "(breaking)" in their PR titles when they merged into 1.0.

For 2.0 onward, the right thing is probably to bake a hash-format version into the binary representation and document the migration story. Want to open a feature-request issue for that?