What is Tydra?

September 3, 2026 · View on GitHub

Tydra

LightUSD does not support Hydra interface in pxrUSD at the moment. We think Hydra(multi-purpose sceneDelegate/renderDelegate interface) is too much for LightUSD usecases(AR, lightweight 3D viewer/runtime, DCC exchange, etc).

Instead, we'd like to propose Tydra(Tiny Hydra), something like a three-headed monster(Please imagine Gidorah: https://en.wikipedia.org/wiki/King_Ghidorah), which directly converts(publishes) USD scene graph(Stage. Prim hierarchy) to a renderer-friendly data structure or published data format(imagine glTF). API design of Tydra is completely different from Hydra.

Currently Tydra is considering following three usecases in mind:

Runtime publishment(e.g. to glTF), DCC conversion and exchange for realtime graphics(AR, VR, MR, games, etc).
Scene conversion to GL/Vulkan renderer(e.g. WebGL rendering) See ../../examples/tydra_to_renderscene
Scene conversion to Ray tracing renderer(e.g. Vulkan/OptiX ray tracing) See ../../examples/sdlviewer/ for SW raytracing example.

OpenPBR Surface shader conversion
Autodesk Standard Surface shader conversion (via OpenPBR)
UsdPreviewSurface shader conversion
NodeGraph traversal with texture/normal/tangent extraction
MaterialXConfigAPI metadata support
See doc/materialx.md for full details

Tangent Computation and Quantization

Multiple tangent-space computation methods:

Method	Speed	Quality
Lengyel	~16 MTri/s	Good (1-5 deg from reference)
MikkTSpace	~0.7 MTri/s	Reference
FastMikkTSpace	~2.0 MTri/s	Identical to MikkTSpace
Hybrid	~3.5 MTri/s	Identical to MikkTSpace

GPU-friendly packed formats:

Format	Bytes/vertex	Max Error
INT_2_10_10_10_REV	4	0.08 deg
SNorm8x4	4	0.34 deg
FP16x4	8	0.03 deg

Normal quantization (INT_2_10_10_10_REV) also supported for 67% storage savings.

See doc/tydra-tangent.md for details.

Memory Estimation

RenderMesh::estimate_memory_usage() - points, indices, normals, tangents, texcoords, joint data, blend shapes
RenderScene::estimate_memory_usage() - meshes, textures, nodes, materials, animations, skeletons

Configuration (`MeshConverterConfig`)

Option	Default	Effect
`tangent_method`	FastMikkTSpace	Tangent computation algorithm
`tangent_storage`	Packed1010102 (WASM) / PackedFp16 (native)	Tangent quantization format
`normal_storage`	Packed1010102 (WASM) / Float3 (native)	Normal quantization format
`defer_tangent_computation`	false (true in WASM)	Defer tangent work to reduce initial load
`lowmem`	false (true in WASM)	Free source GeomMesh after conversion
`preserve_texel_bitdepth`	false (true in WASM)	Keep uint8 textures, avoid float32 bloat

Key Source Files

File	Purpose
`render-data.hh`	RenderScene, RenderMesh, RenderMaterial, MeshConverterConfig
`render-data.cc`	ConvertToRenderScene, tangent computation, memory estimation
`scene-access.hh`	Scene traversal and query APIs
`texture-util.hh`	Texture loading and colorspace conversion
`fast-mikktspace.hh`	FastMikkTSpace and Hybrid tangent implementations
`tangent-quantize.hh`	Packed tangent/normal formats for GPU
`materialx-to-json.hh`	MaterialX to JSON conversion

Documentation

doc/materialx.md - MaterialX pipeline details
doc/skinning.md - UsdSkel skinning equations and Tydra export
doc/tydra-tangent.md - Tangent computation algorithms and quantization
doc/threejs.md - Three.js animation and material integration
doc/memory-and-performance.md - Memory profiling and optimization

Remaining work

Data structure suited for realtime DCC.
Data structure suited for Ray tracing
ValueClip support (clip scheduling, time remapping)
Vertex animation (points timeSamples)
Dedicated typed material/camera/light animation targets and exporter mappings beyond the current generic CustomProperty tracks.
Keep current Tydra and tydra/next render extraction behavior aligned for material, texture, skeleton, and animation data.

The maintained next-core and Tydra-next API documentation is in doc/lightusd-next.md. This README describes the established legacy RenderScene API; it is not a release checklist.