exr: support 16-bit (half) float export

[kmilos]

Also switched to simpler scanline format instead of tiled and added more compression options (assumes >= OpenEXR 2.2, available since Ubuntu 18.04; msys2, macports/homebrew already ship newer).

Also fixes #7965

[JLTastet]

I just tested your PR on macOS Monterrey.

All new combinations of bit depth and compression export successfully and open fine in tev (although the colors are off because tev seems to assume sRGB primaries).

16-bit uncompressed (or with compression settings other than DWA) also opens fine in macOS Preview, and the file size is reduced by ~50% vs 32-bit in the uncompressed case, as expected. Unfortunately, DWA compression leads to files which cannot be opened in Preview. But since it works in tev, this is probably just a limitation of Preview.

DWAA/DWAB compression leads to significant savings of about 5x (16-bit) or 10x (32-bit). Funnily enough, the compressed size is almost exactly the same for 16/32-bit. Is this expected?

Overall, this looks good to me.

[kmilos]

Thanks for testing and reporting the in-depth findings.

the colors are off because tev seems to assume sRGB primaries

Yeah, it might not read the primaries dt stores in the header... It's also not guaranteed dt stores them totally correctly, but that's a different topic/issue to be raised. There's also the question of what transfer curve this tev assumes in addiiton to the primaries? Does it think all EXRs are linear? You might want to set the dt output profile to a linear one as well.

DWAA/DWAB compression leads to significant savings of about 5x (16-bit) or 10x (32-bit). Funnily enough, the compressed size is almost exactly the same for 16/32-bit. Is this expected?

No idea ;) Not an expert on the EXR compression schemes. It is possible that DWA first does a lossy conversion to half and then does the compression, which would make them almost identical. The comment for PXR24 says e.g. the first step is lossy conversion to 24-bit float...

[johnny-bit]

Hey @kmilos - i just saw this PR and immediatelly remembered a weird bug... Could you take a look at #7965 and whether this PR also fixes that? Or can you check & debug a bit?

[JLTastet]

Hey @kmilos - i just saw this PR and immediatelly remembered a weird bug... Could you take a look at #7965 and whether this PR also fixes that? Or can you check & debug a bit?

Indeed this PR seems to fix #7965 (or at least improve the situation).

Without it, enabling PIZ compression reduces the size of a 32-bit EXR file from 291.8 MB to 283.3 MB. With it, the same file is compressed down to 231.1 MB.

[johnny-bit]

I hope it does. Labelling it as a bugfix! If you think it can be added - plz add in opening comment that it fixes the bug?

[kmilos]

As long as the API works, I would consider it a bug fix indeed.

As compression ratio goes, I have noticed that any of the lossless options work poorly on 32-bit floats, but rather well (~40%) on 16-bit half type...

Switching to scanlines might have also been a factor. The relatively small 100x100 tiles might have been inefficient.

[TurboGit]

Looks good, thanks!