Score deviates from upstream

[ghtm2]

Hi.

I've just compiled both upstream and this and was a bit surprised to find quite a bit of deviation in the score on the same set of images.

Here is a sample.

With upstream ssimulacra2 I get a score of 77.94549165, with ssimulacra2_bin one of 78.74256984.

Is this known/expected?

[FreezyLemon]

This is known and pretty much unavoidable. Both the C++ and the Rust versions have some optimizations in (maybe different) places to increase speed in certain places, and unless there is extra effort invested into matching these optimized functions 1:1, there will always be a difference.

That said, if the deviation is too large it is actually a problem, so it might be worth investigating this more.