Add basic support for voxel rendering and styling

[j9liu]

Description

Depends on CesiumGS/cesium-native#1188.
Depends on #1694, 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]

I could keep picking apart at this, but I think this would benefit more from a fresh pair of eyes. I'm keeping this as a draft but I would appreciate any and all reviews 🙏

[j9liu]

Turns out that UE 5.4 has drastically improved the UX of Custom HLSL nodes in materials!

It's now possible to define extra #include paths in for the shader, so I don't need to use one uber shader anymore. I've reorganized the code so that everything is separated into meaningful pieces. Hopefully this makes things easier to review, too ^^

[kring]

I was able to get a basic thing working:

There were a few things I didn't realize, and they took me awhile to figure out:

• The voxel material layer must be the bottommost layer in the material. I initially thought, for some reason, that it had to be layered on top of a ML_CesiumGltf like our other materials. And when that didn't work, I assumed it was because I misnamed the material layer. Only after trying and failing to find the expected material layer name in the code did I realize it was hard-coded to layer 0.
• The shape of this dataset isn't apparent unless your material sets the alpha to 0.0 when Temp_C == Temp_C_NO_DATA. I initially tried to just return a solid color and wondered why all I got was a rectangle.
• But even after the above, I still got a rectangle. Because my material had a Blend Mode of "Masked" instead of "Translucent".

So a lot of this wouldn't have been a problem if we had a tutorial. But if the generated material layer isn't really meant to be used in a layered material, perhaps we should just generate a material directly? That would have eliminated two out of my three errors above.

Also, I'm wondering if the "Voxel Raymarch" custom expression could have an output node for each of the CustomShaderProperties? The way the styling of other types of tiles works. Then users could implement their logic using the shader graph. Also, it seems like pressing the "Auto Generate" button again clobbers any edits to that custom expression node, so the chance of losing the user's work is pretty high with the way things are set up now.

[j9liu]

Thanks for the feedback @kring ! I agree with your points about setting up the material and generating it as a full material instead of as a layer.

Also, I'm wondering if the "Voxel Raymarch" custom expression could have an output node for each of the CustomShaderProperties? [...] pressing the "Auto Generate" button again clobbers any edits to that custom expression node, so the chance of losing the user's work is pretty high...

Yeah, I acknowledge this system isn't great. The problem is that raymarching must be done in a for loop to actually find valid voxels in the first place, and I don't see how to enable that from material nodes. I wish there was a way separate the raymarch logic from the node-based styling, I don't know how you'd re-call the raymarching logic (and not only that, to stop it once it's reached an efficiently saturated output). I can experiment with this some more, but I'm not super hopeful I'll find a better solution...

[j9liu]

Tried hacking a for-loop by using a couple custom nodes in a material, but it looks like Unreal doesn't like it:

LogMaterial: Warning: [AssetLog] /Engine/Transient.Material_0: Failed to compile Material for platform PCD3D_SM5, Default Material will be used in game.
    Expression is part of a cycle. Please make sure the material graph is acyclic.

Makes sense though... it'd probably end up nesting calls like functionA(functionB(functionC(functionA... etc.

[kring]

Works well on macOS / Metal (UE 5.5):