Implement filmic color grading.

[pcwalton]

This commit expands Bevy's existing tonemapping feature to a complete set of filmic color grading tools, matching those of engines like Unity, Unreal, and Godot. The following features are supported:

• White point adjustment. This is inspired by Unity's implementation of the feature, but simplified and optimized. Temperature and tint control the adjustments to the x and y chromaticity values of CIE 1931. Following Unity, the adjustments are made relative to the D65 standard illuminant in the LMS color space.

• Hue rotation. This simply converts the RGB value to HSV, alters the hue, and converts back.

• Color correction. This allows the gamma, gain, and lift values to be adjusted according to the standard ASC CDL combined function.

• Separate color correction for shadows, midtones, and highlights. Blender's source code was used as a reference for the implementation of this. The midtone ranges can be adjusted by the user. To avoid abrupt color changes, a small crossfade is used between the different sections of the image, again following Blender's formulas.

A new example, color_grading, has been added, offering a GUI to change all the color grading settings. It uses the same test scene as the existing tonemapping example, which has been factored out into a shared glTF scene.

Changelog

Added

• Many new filmic color grading options have been added to the ColorGrading component.

Migration Guide

• ColorGrading::gamma and ColorGrading::pre_saturation are now set separately for the shadows, midtones, and highlights sections. You can migrate code with the ColorGrading::all_sections and ColorGrading::all_sections_mut functions, which access and/or update all sections at once.
• ColorGrading::post_saturation and ColorGrading::exposure are now fields of ColorGrading::global.

Screenshots

[github-actions]

The generated examples/README.md is out of sync with the example metadata in Cargo.toml or the example readme template. Please run cargo run -p build-templated-pages -- update examples to update it, and commit the file change.

[torsteingrindvik]

Out of interest and learning, how/why is this most efficient?
I'm thinking it has to do with the fact that the fields of the struct are of decreasing size?

[pcwalton]

Because:

(1) Shadows, midtones, and highlights are split out into struct-of-vectors instead of array-of-structs, turning the math into a series of dot products;

(2) White balance is baked into a 3x3 matrix ahead of time so the work doesn't have to be redone for every pixel.

[NthTensor]

This is some of the highest quality and best documented code I've had the pleasure of reviewing. Color math looks right, the method seems solid.

I really appreciate the technical precision in the comments. Would that the entire engine was this exact with its color terminology.

I haven't played with this locally, but I plan to later today. Will report back if I find anything.

[alice-i-cecile]

@Kurble @fintelia if either of you have the time, I'd be interested in a review from you based on your work with auto-exposure.

[alice-i-cecile]

I'm actually quite comfortable with this PR's contents! I've had a solid chance to get acquainted with the color math and basic concepts of color manipulation in the past, and the shaders are straightforward enough.

I'm very pleased with the end user API as well: clearly documented, intuitive parameters.

As expected, the code quality and documentation are both top notch.