Implement subpixel morphological antialiasing, or SMAA.

[pcwalton]

This commit implements a large subset of subpixel morphological antialiasing, better known as SMAA. SMAA is a 2011 antialiasing technique that detects jaggies in an aliased image and smooths them out. Despite its age, it's been a continual staple of games for over a decade. Four quality presets are available: low, medium, high, and ultra. I set the default to high, on account of modern GPUs being significantly faster than they were in 2011.

Like the already-implemented FXAA, SMAA works on an unaliased image. Unlike FXAA, it requires three passes: (1) edge detection; (2) blending weight calculation; (3) neighborhood blending. Each of the first two passes writes an intermediate texture for use by the next pass. The first pass also writes to a stencil buffer in order to dramatically reduce the number of pixels that the second pass has to examine. Also unlike FXAA, two built-in lookup textures are required; I bundle them into the library in compressed KTX2 format.

The reference implementation of SMAA is in HLSL, with abundant use of preprocessor macros to achieve GLSL compatibility. Unfortunately, the reference implementation predates WGSL by over a decade, so I had to translate the HLSL to WGSL manually. As much as was reasonably possible without sacrificing readability, I tried to translate line by line, preserving comments, both to aid reviewing and to allow patches to the HLSL to more easily apply to the WGSL. Most of SMAA's features are supported, but in the interests of making this patch somewhat less huge, I skipped a few of the more exotic ones:

• The temporal variant is currently unsupported. This is and has been used in shipping games, so supporting temporal SMAA would be useful follow-up work. It would, however, require some significant work on TAA to ensure compatibility, so I opted to skip it in this patch.

• Depth- and chroma-based edge detection are unimplemented; only luma is. Depth is lower-quality, but faster; chroma is higher-quality, but slower. Luma is the suggested default edge detection algorithm. (Note that depth-based edge detection wouldn't work on WebGL 2 anyway, because of the Naga bug whereby depth sampling is miscompiled in GLSL. This is the same bug that prevents depth of field from working on that platform.)

• Predicated thresholding is currently unsupported.

• My implementation is incompatible with SSAA and MSAA, unlike the original; MSAA must be turned off to use SMAA in Bevy. I believe this feature was rarely used in practice.

The anti_aliasing example has been updated to allow experimentation with and testing of the different SMAA quality presets. Along the way, I refactored the example's help text rendering code a bit to eliminate code repetition.

SMAA is fully supported on WebGL 2.

Fixes #9819.

Changelog

Added

• Subpixel morphological antialiasing, or SMAA, is now available. To use it, add the SmaaSettings component to your Camera.

[pcwalton]

How do I fix the typo false positives again? @mockersf

[alice-i-cecile]

How do I fix the typo false positives again? @mockersf

https://github.com/bevyengine/bevy/blob/main/typos.toml#L26

[pcwalton]

The CI failure is odd since it works here. I can try on Linux.

[pcwalton]

I can't reproduce the CI failure on Linux either. Help?

[pcwalton]

The problem is fixed and this is ready for review.

[pcwalton]

Addressed review comments.

[alice-i-cecile]

Thanks @Olle-Lukowski :) I'll take a closer look on Monday to check what I can myself.

[alice-i-cecile]

@pcwalton CI is complaining about the typos.toml file. I tried to resolve merge conflicts, but seem to have screwed up the formatting in some subtle way.

[alice-i-cecile]

Thank you to everyone involved with the authoring or reviewing of this PR! This work is relatively important and needs release notes! Head over to https://github.com/bevyengine/bevy-website/issues/1362 if you'd like to help out.