Add EXT_meshopt_compression support on Editor / Standalone / Android / WSA (libmeshoptimizer is currently bundled only for iOS and WebGL)

[cedrickamermans]

Feature

Summary

Cesium for Unity silently fails to render glTF tiles that require EXT_meshopt_compression on every platform except
iOS and WebGL. The tile hierarchy populates and metadata extensions (EXT_structural_metadata, EXT_mesh_features)
are honored, but the geometry buffers are dropped, leaving placeholder GameObjects with CesiumModelMetadata and no
MeshFilter / MeshRenderer children. The same tilesets render fine in CesiumJS.

Reproduction

• Package: com.cesium.unity 1.23.2 (registry), commit hash AA71215DE0C4 in Library/PackageCache.
• Unity: 6000.x, Windows 11 Editor, HDRP project.
• Steps:
  1. Add a Cesium3DTileset under a CesiumGeoreference with origin in Amsterdam.
  2. Set Source = From Url, URL = https://data.3dbag.nl/v20250903/cesium3dtiles/lod22/tileset.json.
  3. Place a properly georeferenced camera over the dataset.
• Expected: Buildings render, as they do in CesiumJS.
• Actual: ~259 tile-placeholder GameObjects are added under the tileset, each with only Transform +
  CesiumModelMetadata. Zero MeshFilter / MeshRenderer are created anywhere in the subtree. No errors are logged;
  ComputeLoadProgress() returns 100.

Evidence the cause is missing meshoptimizer on desktop

Inspecting an individual tile GLB header (https://data.3dbag.nl/v20250903/cesium3dtiles/lod22/t/4/384/896.glb):

"extensionsRequired": ["EXT_meshopt_compression", "KHR_mesh_quantization"]
"extensionsUsed":     ["EXT_mesh_features", "EXT_meshopt_compression",
                       "KHR_mesh_quantization", "EXT_structural_metadata"]

All vertex/index buffer views are stored as EXT_meshopt_compression blobs.

Searching the installed package for the meshopt decoder library:

./Plugins/iOS/lib/libmeshoptimizer.a
./Plugins/WebGL/lib/libmeshoptimizer.a

libmeshoptimizer is present only for iOS and WebGL. There is no equivalent binary under Editor/x86_64, Editor/arm64,
Plugins/Standalone, Plugins/Android, or Plugins/WSA. Because EXT_meshopt_compression is declared in
extensionsRequired, cesium-native must skip the geometry on builds where the decoder isn't linked, which matches the
observed behavior exactly (metadata processes, geometry doesn't).

Cross-check with CesiumJS

The same tileset URL renders correctly in CesiumJS 1.133 in a browser, because CesiumJS ships its own meshopt decoder.
 So the dataset is conformant; only the Cesium for Unity desktop/Android/WSA path is affected.

Impact

3DBAG (Dutch national 3D buildings) and other open 3D Tiles publishers increasingly emit meshopt-compressed GLBs
because it dramatically reduces transfer size. Users who can't or don't want to round-trip through Cesium Ion's
transcoding pipeline currently have no working option on the Unity Editor or any desktop/Android target — they only
discover this after the tileset appears to "load" (because metadata processes) but renders nothing.

Suggested fix

Link libmeshoptimizer into the cesium-native builds for the remaining platforms (Editor/x86_64, Editor/arm64,
Plugins/Standalone/*, Plugins/Android/*, Plugins/WSA/*) and ship the binaries with the package, the same way the iOS
and WebGL builds already do. The cesium-native side already knows how to call the decoder — it's purely a
build/packaging gap, not a code-change in the loader.

[j9liu]

Thanks for the bug report @cedrickamermans! I confirmed that the tileset is not showing up in the Editor, and it appears that no tiles are being made. However I have not confirmed whether meshopt is the culprit.

The Unity Editor version does not seem to have an impact, since I was able to reproduce in Unity 2022.3 as well.

[j9liu]

For the record, this tileset doesn't seem to show up in Cesium for Unreal either...