Implement minimal reflection probes.

[pcwalton]

Objective

This pull request implements reflection probes, which generalize environment maps to allow for multiple environment maps in the same scene, each of which has an axis-aligned bounding box. This is a standard feature of physically-based renderers and was inspired by the corresponding feature in Blender's Eevee renderer.

Solution

This is a minimal implementation of reflection probes that allows artists to define cuboid bounding regions associated with environment maps. For every view, on every frame, a system builds up a list of the nearest 4 reflection probes that are within the view's frustum and supplies that list to the shader. The PBR fragment shader searches through the list, finds the first containing reflection probe, and uses it for indirect lighting, falling back to the view's environment map if none is found. Both forward and deferred renderers are fully supported.

A reflection probe is an entity with a pair of components, LightProbe and EnvironmentMapLight (as well as the standard SpatialBundle, to position it in the world). The LightProbe component (along with the Transform) defines the bounding region, while the EnvironmentMapLight component specifies the associated diffuse and specular cubemaps.

A frequent question is "why two components instead of just one?" The advantages of this setup are:

1. It's readily extensible to other types of light probes, in particular irradiance volumes (also known as ambient cubes or voxel global illumination), which use the same approach of bounding cuboids. With a single component that applies to both reflection probes and irradiance volumes, we can share the logic that implements falloff and blending between multiple light probes between both of those features.

2. It reduces duplication between the existing EnvironmentMapLight and these new reflection probes. Systems can treat environment maps attached to cameras the same way they treat environment maps applied to reflection probes if they wish.

Internally, we gather up all environment maps in the scene and place them in a cubemap array. At present, this means that all environment maps must have the same size, mipmap count, and texture format. A warning is emitted if this restriction is violated. We could potentially relax this in the future as part of the automatic mipmap generation work, which could easily do texture format conversion as part of its preprocessing.

An easy way to generate reflection probe cubemaps is to bake them in Blender and use the export-blender-gi tool that's part of the bevy-baked-gi project. This tool takes a .blend file containing baked cubemaps as input and exports cubemap images, pre-filtered with an embedded fork of the glTF IBL Sampler, alongside a corresponding .scn.ron file that the scene spawner can use to recreate the reflection probes.

Note that this is intentionally a minimal implementation, to aid reviewability. Known issues are:

• Reflection probes are basically unsupported on WebGL 2, because WebGL 2 has no cubemap arrays. (Strictly speaking, you can have precisely one reflection probe in the scene if you have no other cubemaps anywhere, but this isn't very useful.)

• Reflection probes have no falloff, so reflections will abruptly change when objects move from one bounding region to another.

• As mentioned before, all cubemaps in the world of a given type (diffuse or specular) must have the same size, format, and mipmap count.

Future work includes:

• Blending between multiple reflection probes.

• A falloff/fade-out region so that reflected objects disappear gradually instead of vanishing all at once.

• Irradiance volumes for voxel-based global illumination. This should reuse much of the reflection probe logic, as they're both GI techniques based on cuboid bounding regions.

• Support for WebGL 2, by breaking batches when reflection probes are used.

These issues notwithstanding, I think it's best to land this with roughly the current set of functionality, because this patch is useful as is and adding everything above would make the pull request significantly larger and harder to review.

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Changelog

Added

• A new LightProbe component is available that specifies a bounding region that an EnvironmentMapLight applies to. The combination of a LightProbe and an EnvironmentMapLight offers reflection probe functionality similar to that available in other engines.

[pcwalton]

Both issues have been fixed. I want to improve the example before submitting this for review. This requires messing with the glTF IBL Sampler some more.

[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.

[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.

[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.

[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.

[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.

[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.

[pcwalton]

This is ready for review.

[pcwalton]

For both the view cubemap and any light probes, if the mesh has a lightmap, we should probably ignore the diffuse lighting from cubemaps. Otherwise, we'd double count it.

Yep, now that lightmaps have landed I made that change.

[JMS55]

Nvm, I missed that you were using textureNumLevels for the light probes.

[mockersf]

This broke rendering on macOS. Example load_gltf panics with

thread 'Compute Task Pool (2)' panicked at .cargo/registry/src/index.crates.io-6f17d22bba15001f/naga-0.14.2/src/back/msl/writer.rs:229:21:
internal error: entered unreachable code: metal requires all arrays be constant sized

so https://github.com/gfx-rs/wgpu/blob/4400a5847080d1164bdca93a90622414963ed9f3/naga/src/back/msl/writer.rs#L228-L228

[mockersf]

it also broke iOS:

wgpu error: Validation Error

Caused by:
    In Device::create_bind_group_layout
      note: label = `mesh_view_layout`
    Binding 14 entry is invalid
    Features Features(TEXTURE_BINDING_ARRAY) are required but not enabled on the device

[mockersf]

and Android:

wgpu error: Validation Error
Caused by:
    In Device::create_bind_group_layout
      note: label = `mesh_view_layout`
    Too many bindings of type SampledTextures in Stage ShaderStages(FRAGMENT), limit is 16, count was 21