Crust Render

Limitations

Crust Render is a single-author project. Its gaps are documented rather than hidden.

Each capability's design record in the repository ends with a "Known gaps" section, with the full detail and workarounds. This page summarizes the gaps that affect what you can render.

Platform

  • CPU only. Rendering runs on the CPU, in parallel with rayon. There is no GPU or wavefront renderer, and no coherent ray-packet traversal.
  • SIMD stops at 128 bits. BVH traversal and triangle tests use SSE2/NEON-width vectors. Going wider would need nightly Rust or unsafe code (why).

Geometry

  • Motion blur is transform-only. It is set with crust:motion:translate, and the transform is interpolated linearly. There is no deformation blur, no rotation blur, and animated transforms aren't blurred.

  • No OpenVDB or UsdVolVolume. Volumes are the three crust:volume:type kinds: homogeneous, procedural smoke, or a voxel grid authored in USD.

  • Building the BVH of a huge scene needs extra memory for a moment. While the acceleration structure is built, its peak memory can be well above what the finished render needs, especially with subdivision. Lowering --subdiv-level is the first remedy.

  • Adaptive subdivision refines only geometry used once. --subdiv-edge-length leaves a prototype placed several times at the uniform level, whatever its distance, and leaves faces outside the camera's view at their control cage, which reflections and shadows then see. Loop subdivision meshes are refined to one level per mesh.

  • Displacement is scalar and applied at the dicing rate. Vector displacement is refused. Detail finer than the tessellation is lost, since nothing turns it into bump. The dicing rate ignores displacement: a strongly displaced region is diced as finely as its undisplaced cage asks. A displaced subdivisionScheme = "none" mesh loses its hard edges. Where a map jumps across a UV seam, one ring of triangles stretches across the step. RenderMan displacement networks other than a PxrPtexture (or a PxrBlend multiply of two) through an optional PxrDispTransform are refused. See displacement.

Materials and textures

  • Native subsurface is approximate. The crust:openpbr subsurface lobe is still a tinted diffuse. MaterialX's subsurface_bsdf uses a true random walk.
  • crust:openpbr and MaterialX's open_pbr_surface don't match exactly. The two implementations of OpenPBR still differ in places.
  • MaterialX limits. A closure tree that collapses to more than eight lobes is refused. Only the uniform_edf and generalized_schlick_edf emission nodes are supported. Zeltner sheen is evaluated as Imageworks sheen.
  • Texture filtering is isotropic. Mip levels are chosen from ray cones, but the filter has no direction. A texture seen at a grazing angle is blurred more than an anisotropic (EWA) filter would blur it.
  • Streamed Ptex can't rebuild its mip levels in linear light. A mip-mapped .ptx is loaded fully unless CRUST_PTEX_STREAM_MIPSPACE=file accepts the file's own, slightly darker, levels. Displacement Ptex is read raw, so its stored levels are already correct and it streams.
  • Colour-space conversions are a curve and a matrix. Every texture space in the ACES configs is a transfer curve followed by a change of primaries, and Crust Render converts exactly those. A conversion of any other shape, such as one through a 3D LUT, is refused with a warning, and the values are used as stored.
  • A colour that names no colour space is taken as already in the working space, even where UsdColorSpaceAPI would fall back to lin_rec709_scene. The two agree in the default working space.
  • Colour Ptex is always decoded by gamma 2.2. There is no way to declare a colour .ptx linear, so linear colour Ptex data renders too dark. Ptex read as a displacement map is read raw.
  • Some inputs are read but not used, with a warning: subsurface* and specularTint on PxrDisneyBsdf, and UsdTransform2d on UsdPreviewSurface textures.

Lights

  • Not supported: mesh lights, portal lights, light filters, and shaping on distant and dome lights. inputs:diffuse and inputs:specular are ignored with a warning.
  • Emissive surfaces aren't sampled as lights. An emissive MaterialX surface, emissive curves, instances or volumes are found only by rays that hit them. Use UsdLux lights for a scene's main light sources.
  • Light-linking gaps:
    • A collection target inside an instance prototype can't tell instances apart.
    • membershipExpression is refused.
    • A shadow-linked light is sampled only by light sampling, so it is noisier on glossy surfaces.

Path guiding

  • Surfaces only. Volumes and phase functions aren't guided.
  • Luminance only. The guide is trained on luminance, not per colour channel.

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