Non-deterministic UVAtlas parameterization

[sterlinglaird]

I'm trying to generate repeatable UV atlases but the parameterization seems to be different every time.
Specifically, calling UVAtlasCreate() with the same inputs produces different output vertex, index, and remap buffers.

Is this expected? Or is there some other way I can ensure reproducible results?

[walbourn]

The library makes use of rand for some decisions.

[sterlinglaird]

@walbourn Thanks for the quick reply.
To confirm then, there is no way to force reproducible results via providing a fixed seed or otherwise?

[walbourn]

I modified the library from the original use of the C rand function in favor of the C++ <random> header. I currently make use of std::random_device in both isochartengine and UVAtlasRepacker.

It sounds like this is a request to add the ability to provide a specific seed as part of the API.

[sterlinglaird]

Not being able to reproduce a parameterization is a bit problematic in some cases, so having an API to provide a specific seed would be pretty useful for me and likely others. Thanks.

[nh2]

I would also enjoy deterministic results.

Side note:

The previous version also called srand(), and I think that was incorrect when compiled with _OPENMP because ParameterizeChartsInHeapParallelized() called into pChart->Partition() from inside #pragma omp for, and as per srand() docs, it is "not guaranteed to be thread-safe".

Probably irrelevant since the change to <random>, but maybe still worth pointing out.

I found this because i noticed nondeterministic results in the version from before the switch to <random>, with _OPENMP nbot defined, and I cannot explain why because the only calls to srand() are the srand(2) ones, which should give us deterministic results when run single-threaded.

Would be cool to find out how that can be.

Edit: Ah, I got it, my application does multiple UVAtlasCreate() in parallel. The srand(2)s of those affect each other nondeterministically (and are illegal because srand() is not thread-safe as mentioned).