Add basic support for voxel rendering and styling

[j9liu]

Description

Depends on CesiumGS/cesium-native#1188.
Depends on #1694, so merge that first.
Depends on #1778, so merge that first.

This PR adds support for loading and rendering tilesets with 3DTILES_content_voxels, parsing glTFs with EXT_primitive_voxels payloads.

Data courtesy of Swisstopo

This is a meaty set of changes, so I'll add a written walkthrough of the changes in a follow-up comment.

Author checklist

• I have submitted a Contributor License Agreement (only needed once).
• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
  - [ ] I have added or updated unit tests to ensure consistent code coverage as necessary.
• I have updated the documentation as necessary.

Remaining Tasks

• Improve memory / lifetime management
• Revisit voxel traversal mechanics. -- See changes in Add support for ellipsoid-based voxels #1711
  • I'm not happy with the way voxels are currently selected for rendering in VoxelResources. It's a weird compromise between CesiumJS's VoxelTraversal and the cesium-native traversal, and it falls apart for larger datasets.

Testing plan

Reach out to me offline for the pictured test data.

• Load in a voxel tileset (same as any other Cesium3DTileset).
• Use UCesiumVoxelMetadataComponent to gather its properties.
• Test out applying custom shaders to the data.

Compatibility checks:

Unreal Version	DX 11/12	Vulkan	Metal	Editor	Built
5.4	✅	✅		✅
5.5			✅
5.6

[j9liu]

Finally getting around to @kring's comments...

I noticed that the tileset disappears when I get too close to it (inside its bounding volume, presumably). How realistic is it to avoid that? I was hoping to be able to fly through the voxel field. 😀

This should be possible if you check Two Sided on the resulting material! Here's a closeup from being inside the cube but still seeing the voxels:

This also allows you to fly into the voxel volume, but you just end up seeing a solid color of whatever voxel you're in. Maybe that's expected for this particular dataset??

I can get this weird lighting effect just by rotating the camera. I say it's lighting related because it goes away in the "Unlit" view. Any idea what this is?

I was still seeing this even after I switched to the Unlit lighting mode. It appears to be the result of the atmosphere's interaction with translucent materials. Here's the difference after I toggle CesiumSunSky:

Before	After

The sharp line across the back is the result of material being two-sided, but the light is still there when it's toggled off:

So I'm not sure how to get rid of this interaction. Maybe this is an artifact of using a cube mesh for rendering, rather than doing a separate post process / draw pass. I would be willing to explore the latter, but that will probably be another week or so of effort. Is it bad enough to hold up this PR?

[j9liu]

This restores the loadPrimitive code so it only deals with typical 3D mesh data; the splats route has been moved to loadGaussianSplats

[j9liu]

Same story here; splats / voxels have their own load___GameThread function

[j9liu]

Context: This approach tries to match CesiumJS, which uses a custom traversal algorithm. Their algorithm uses a priority queue based on screen space error to retain parent detail more aggressively (avoiding LOD skips and holes).

I'm happy to take other suggestions or experiment with this, esp. because I feel like it's somewhat wasteful to do sort based on SSE every frame when Native is already handling tiles to us. At the same time, there needs to be a heuristic to determine when it's acceptable to kick a tile from its megatexture slot, since that visibility info is not sync'd 1-to-1 with Native's.

[j9liu]

Happy to mark this as ready for another review! I left a few comments that explain some of the decisions made, but please let me know if there's anything else I can clear up. I'm also happy to do a "pair programming" session to walk through some of the larger chunks of code.

@azrogers, would love to have your eyes on this, especially since you've dabbled in non-traditional rendering techniques (i.e. splats) and I figure you'll bring a fresh perspective
@kring, if you get the time to do another pass, I would greatly appreciate that too!