Proposal: add an ability to Y-flip images during transcode

[mosra]

While the encoder-provided option to flip the image during encoding is everything needed when one controls the whole pipeline, it's of no use for general image viewers that have no power over how the input was generated. Having the chance to configure Y flipping during the transcode makes it easier to integrate, as the application doesn't need to worry about patching texture coordinates or doing texture coordinate transform in a shader.

While flipping the compressed image post-transcode is possible, it's a non-trivial amount of work, considering the broad range of target formats supported. On the other hand, when done directly in Basis, flipping can be done on the internal ETC1S representation:

1. patching the output pointer and making row stride negative, which has very little impact on existing code (apart from making pointer arithmetic work with signed types);
2. and bit-flipping the selector bits using a modified 3-operation bit reverse trick.

Assuming the encoder's output is reasonably invariant to image flip, this new code path could be tested by comparing these two outputs for equality:

• flipped PNG -> basis -> target format -> RGBA
• PNG -> basis -> flipped target format -> RGBA

This was tested on ETC1/2, BC1-7, ASTC and RGBA8 output, however it currently doesn't handle flipping for PVRTC textures, as there's cross-block modulation and I don't have deep enough understanding of the format to be able to code that. Similarly for FXT1 due to a different block size. Treat the code more like a proof-of-concept (and feel free to do anything with it, of course).

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At first I thought about just opening a feature request, but then went on to try how feasible this would be, to avoid asking for the impossible :) It's of course entirely possible that I missed some important aspect of the format and it can't work this way -- however if it can, I dare to say the code changes needed won't be so invasive and the additional maintenance/testing effort for this feature is in reasonable bounds.

[Squareys]

This would be amazing, since it would remove the need to flip UVs on WebGL/OpenGL! 🎉

[richgel999]

Yes, this is very interesting. I like this idea of being able to flip the output, it fits within the transcoder's design & goals. I think flipping PVRTC should be relatively easy too.

This is on my TODO list. Hold on. :-)

[richgel999]

This is an older PR, but I like this idea. The challenge is making sure it works correctly on all the formats. Once this feature is in I'll close this PR.