Image-based lighting

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Closes #4.

We need a way to transcode BC6H formats on mobile. https://github.com/KhronosGroup/BC6H-Decoder-WASM seems decent.

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A few pics:

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The harder but nicer (for.me) thing to do might be to write my own bc6h decompressor. The format is atleast bit-exact. Links:

https://docs.microsoft.com/en-us/windows/win32/direct3d11/bc6h-format#decoding-the-bc6h-format

https://github.com/hglm/detex/blob/master/decompress-bptc-float.c

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Fun fact: ASTC also supports HDR data according to https://themaister.net/blog/2021/08/29/compressed-gpu-texture-formats-a-review-and-compute-shader-decoders-part-3-3/.

We could theoretically use that as the format for IBL images and decompress to fp16 on desktop. It'd ideally use a compute shader though.

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Re: decoding bc6h textures for mobile GPUs, here's what I've tried so far:

1. Converting https://github.com/KhronosGroup/BC6H-Decoder-WASM/blob/main/assembly/decoder.ts to rust. Hard because it's written in an asm flavour of js with some wasm built-ins
2. Converting https://github.com/Themaister/Granite/blob/master/assets/shaders/decode/bc6.comp to rust. Got pretty far, but ran into a wall (I think?) because of uint->int casting differences between glsl and rust.
3. Using https://github.com/ifeherva/bcndecode. Ran into an issue where the blue channel wasn't decoded correctly: DDS BC6 decoding has a broken blue channel python-pillow/Pillow#6344.

What I might try is converting https://github.com/Themaister/Granite/blob/master/assets/shaders/decode/bc6.comp to a full-screen fragment shader. This might be the best thing to do anyway because it'd be using gpu multithreading.

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What I might try is converting https://github.com/Themaister/Granite/blob/master/assets/shaders/decode/bc6.comp to a full-screen fragment shader. This might be the best thing to do anyway because it'd be using gpu multithreading.

This worked!