Nanousd Renderer Scene Extraction

NanoUSD Renderer Scene Extraction

Purpose

Turn composed USD data into a renderer-owned scene representation without losing authored intent. Preserve paths, transforms, visibility, material bindings, texture identity, light units, instancing, and unsupported-feature diagnostics until the renderer deliberately consumes or rejects them.

Workflow

1. Define the extraction contract first.

   • Name the output structs and ownership rules: geometry buffers, material table, texture table, light table, camera table, prototype/instance table, path maps, warnings, and unsupported features.
   • Record coordinate convention, matrix layout, time code, up-axis policy, units policy, material model coverage, and renderer capability gaps.
   • Keep CPU scene ingestion separate from GPU upload. Upload only after counts, bounds, indices, material bindings, and light records pass invariants.

2. Open and validate the stage.

   • Use nanousd_open, nanousd_isvalid, nanousd_error, and nanousd_close; for borrowed handles, make ownership explicit and never close caller-owned stages.
   • Capture root layer, layer stack, upAxis, metersPerUnit, timeCodesPerSecond, frame range, and current evaluation time.
   • Treat load/composition warnings as scene diagnostics, not stdout noise. Include counts for prims, meshes, materials, textures, lights, curves, instances, skipped prims, and unsupported features.

3. Traverse composed prims deliberately.

   • Iterate nanousd_nprims / nanousd_prim for the composed stage, then free each prim handle with nanousd_freeprim.
   • Skip inactive, undefined, abstract, invisible, guide, and proxy data unless the renderer explicitly requests those purposes.
   • Visibility is inherited: walk ancestors before deciding a child mesh is visible. Keep USD paths in every emitted record for debugging, selection, AOVs, and stable IDs.

4. Build transforms once and document conventions.

   • Use nanousd_get_local_transform(prim, time, local, &resetXformStack) and compose parent transforms while honoring resetXformStack.
   • The Vulkan loader currently uses row-major, row-vector transforms: P_world = P_local * leaf * parent * ... * root.
   • Apply upAxis conversion and metersPerUnit exactly once at the scene boundary, and apply it consistently to mesh bounds, curve widths, light positions/extents, cameras, and instance transforms.
   • Re-evaluate transforms when the renderer time changes; do not reuse default-time matrices for animated xformOps.

5. Extract mesh geometry with interpolation intact.

   • Read points, faceVertexCounts, and faceVertexIndices; prefer NanoUSD zero-copy array readers for speed, but copy data the renderer must retain because later NanoUSD calls can invalidate cached array data.
   • Validate index ranges before triangulation. Triangulate polygons deterministically and preserve the source path and face/primitive mapping if picking or material subsets need it.
   • Handle normals, primvars:normals, primvars:st, alternate UV names, displayColor, displayOpacity, orientation, double-sidedness, subdivision scheme, and interpolation metadata.
   • Distinguish authored opinions from USD schema fallbacks. For visibility-critical fields such as Mesh doubleSided, record whether the value was authored or defaulted, and validate primary, shadow, and instance traversal semantics with a reduced fixture before tuning light intensity or denoising.
   • Face-varying UVs or normals require vertex expansion. Generate smooth or flat normals only when absent, and label generated data as generated.
   • Curves, points, volumes, subdivision surfaces, and unsupported prim types must produce either a converted renderer record or a structured unsupported-feature record; do not silently drop them.

6. Preserve instancing and prototypes.

   • Use NanoUSD instance/prototype APIs (nanousd_isinstance, nanousd_prototype, nanousd_ninstances, nanousd_instance) and PointInstancer attributes (prototypes, protoIndices, positions, orientations, scales) to build prototype and instance tables.
   • Store prototype geometry once, then store per-instance transforms, visibility, material overrides, and stable IDs.
   • USD quatf/quath arrays read into NanoUSD float arrays as (x, y, z, w) in the current Vulkan loader pattern; verify ordering before composing instance transforms.
   • Keep prototype-only prims hidden from normal draw lists while still available for instance expansion.

7. Resolve materials and textures from USD graphs.

   • Resolve material:binding relationships on the prim and ancestors. If material subsets are supported, preserve face ranges and subset binding strength.
   • Follow shader connections with nanousd_nconnections / nanousd_connection; strip output suffixes to locate upstream shader or texture prims with nanousd_primpath.
   • Support renderer-known models explicitly, such as UsdPreviewSurface, OmniPBR, OmniGlass, OmniSurface, and sidecar MaterialX. Unknown nodegraphs should become unsupported material diagnostics, not valid white defaults.
   • Map base color, opacity, opacity threshold, metallic, roughness, IOR, clearcoat, normal, normal scale, emissive, occlusion/ORM, specular workflow, subsurface, transmission, texture transforms, and fallback values.
   • Deduplicate textures by resolved identity plus color-space/sampler semantics. Mark base color and emissive textures as sRGB; mark normal, roughness, metallic, AO, opacity, and ORM data textures as linear data.
   • Resolve asset paths relative to the authored layer or stage context, not process cwd. For sidecar scans, bound the search to the scene directory and make expensive scans opt-in or cached.

8. Extract lights and cameras as first-class scene data.

   • Handle DomeLight, RectLight, DistantLight, SphereLight, and any renderer-supported Disk/Cylinder variants. Count known unsupported light types.
   • Preserve inputs:color, inputs:intensity, inputs:exposure, inputs:normalize, dimensions, radius, angle, shaping controls, visibility, texture/HDR paths, and dome rotation.
   • Derive light world frames from the same transform pipeline as geometry. For RectLight, store center, normal, and half-extent axes; for DistantLight, store normalized direction and angle; for SphereLight, store radius and world position.
   • Extract authored cameras when needed: transform, projection, focal length, apertures, clipping range, focus distance, and active-camera selection. If auto-framing is a fallback, report it as a fallback.

9. Emit a renderer-ready scene with diagnostics.

   • Return geometry buffers, material/light/texture arrays, curves, camera records, bounds, path-to-index maps, prototype/instance maps, and unsupported-feature diagnostics.
   • Include stable path hashes or IDs for meshes, lights, materials, emitters, and instances so renderer AOVs and logs can tie pixels back to USD.
   • Track load timing and memory by stage open, material load, mesh extraction, instancing, curves, lights, texture decode, GPU upload, and acceleration-structure build.

10. Validate with focused fixtures before full assets.

• Add reduced scenes for resetXformStack, up-axis plus metersPerUnit, face-varying UV seams, inherited visibility, material binding, texture connections, point instancing, DomeLight HDR, direct lights, emissive materials, and unsupported prims.
• Compare against OpenUSD/usdview/Hydra or a known reference renderer when parity matters. Match camera, exposure, tone mapping, color space, light/environment, sample count, and seed.
• Use AOVs to isolate ingest bugs: geometry IDs, normals, UVs, base color, material ID, roughness, metallic, opacity, emissive, direct light, shadow visibility, indirect, and depth.

Reference Contract and Cross-Cutting Conventions

These are the parts an agent cannot infer and that have silently broken existing backends. Treat the renderers' scene.h as the reference shape; do not freeze a field list here — match the canonical struct. The durable content is the invariant and the failure it prevents.

Scene struct shape (point at scene.h, preserve these groups). Every renderer needs, and must keep until the backend consumes them: geometry (flat positions plus optional normals/displayColor/UVs and triangulated indices — copy what the renderer retains, since later NanoUSD calls can invalidate zero-copy arrays); identity (the USD path on every record, so AOVs, picking, and logs tie pixels back to USD); transform (one world matrix per record in a single documented convention); bounds (world-space and object-space — object-space lets you bound expanded instances without re-transforming every shared prototype vertex); material binding (an index into a material table, not inlined material data); instancing (a prototype index plus a "prototype-only" flag, so PointInstancer prototype subtrees are materialized via instances and never drawn standalone); and scene-level data (up-axis, dome/IBL metadata, light table, structured unsupported-feature list). Do not flatten instancing or drop paths at ingest "to simplify" — that information cannot be recovered later.

Vertex layout (example — verify against the header). An interleaved per-vertex layout such as 48-byte / 12-float [pos.xyz, normal.xyz, pad.xyz, uv.xy, matID] is one workable choice. The durable rule: the host writer and every shader/pipeline that reads the buffer derive stride and offsets from ONE definition, and any later pass that touches uv/matID must use the same stride. A mismatch yields garbage attributes with no error.

Transform convention and the silent translation-drop bug (invariant + signature). Pick one convention and apply one named conversion at the GPU boundary. The NanoUSD loaders use row-vector USD (translation in the matrix's last row). A flat element copy across a row-vector/column-vector boundary silently drops translation: the object renders at the origin and per-frame transform updates look like no-ops. Verify with a translate-and-re-render test — a static identity-vs-identity test passes tautologically and hides exactly this. This bug shipped in more than one backend.

Host/shader struct sync. Any struct shared between host and shader (per-segment geometry, push constants, material records) has two definitions in two languages and they must change together. Keep them adjacent or generated, and note the duplication at each site; a silent mismatch mis-decodes every record. See renderer-backend-port for the cross-language alignment/padding trap.

NanoUSD Entry Points

• Stage: nanousd_open, nanousd_close, nanousd_isvalid, nanousd_error, nanousd_stage_get_root_layer_path, nanousd_stage_n_layers, nanousd_stage_layer_path.
• Metadata: nanousd_metadatas, nanousd_metadatad, nanousd_timecodes_per_second, nanousd_frames_per_second, nanousd_start_time, nanousd_end_time.
• Traversal: nanousd_nprims, nanousd_prim, nanousd_primpath, nanousd_defaultprim, nanousd_parent, nanousd_nchildren, nanousd_child.
• Prim state: nanousd_path, nanousd_name, nanousd_typename, nanousd_kind, nanousd_isactive, nanousd_isdefined, nanousd_isabstract, nanousd_isa, nanousd_hasapi.
• Attributes: nanousd_hasattrib, nanousd_attribtype, scalar/vector readers, nanousd_attribarraylen, nanousd_attribarrayf, nanousd_attribarrayi, nanousd_arraydataf, nanousd_arraydatai, nanousd_attrib_token, nanousd_attribasset, nanousd_attrib_interpolation, nanousd_attrib_authored.
• Time samples: nanousd_hassamples, nanousd_nsamplekeys, nanousd_samplekey, nanousd_sample*.
• Relationships/connections: nanousd_hasrel, nanousd_nreltargets, nanousd_reltarget, nanousd_hasconnections, nanousd_nconnections, nanousd_connection.
• Transforms: nanousd_get_local_transform, nanousd_mul_m4d.
• Instancing: nanousd_isinstance, nanousd_isprototype, nanousd_isinprototype, nanousd_prototype, nanousd_ninstances, nanousd_instance.

Failure Signatures

• Exploded or mirrored scene: transform order, reset stack, quaternion order, up-axis, or units applied twice.
• White or default-gray scene: material binding not inherited, shader connections not followed, unknown material treated as valid, or texture path resolved from cwd.
• Texture looks washed out or normal map is wrong: sRGB applied to data textures, missing scale/bias, channel swizzle mismatch, or ORM channels mapped incorrectly.
• Missing shelves, chess pieces, or repeated assets: prototype-only prims drawn or hidden incorrectly, PointInstancer expansion skipped, or native instances flattened without prototype lookup.
• Lights visible but no shadows or no direct light: light transform, intensity/exposure/normalize, visibility, unsupported light type, alpha cutout, or TLAS membership mismatch.
• Black glass, opaque cutouts, dark roof openings, or missing shadow bands: opacity, opacity threshold, transmission, IOR, authored/schema-fallback sidedness, or shadow opacity policy diverges between raster, primary RT, instance RT, and shadow rays.
• Correct first frame but wrong animation: default-time values cached despite time-varying transforms, geometry, materials, or visibility.

Handoff

When finishing loader work, report the scene path, time code, root layer, counts, bounds, material/light/texture totals, instancing totals, unsupported features, key diagnostics, validation command, and any renderer-degraded semantics that remain.