CNA-GAP-043: glTF skins with more than 72 joints are refused at import

CNA snapshot 009d40f5  ·  Known Issues › Functional gaps  ·  source links pinned to 009d40f5

BuildSkeleton throws for a skin above the 72-matrix GPU palette (SkinnedEffect::MaxBones), naming the joint count, instead of truncating it.

Identifier

CNA-GAP-043

Category

Functional gap

Subsystem

Models & glTF

Status

Open

Verified against

CNA 009d40f5 (009d40f5dd085c4e674d3479675fac84b12b3e0a)

Evidence basis

Source-verified: read at TARGET, not executed

Tests touching this area

Yes: see Current tests

Affected contract

ContentManager::Load<Model> and the offline converters on glTF skins with more than 72 joints

Expected behaviour

Many current glTF rigs exceed 72 joints (facial and finger bones); a glTF importer is expected to load them.

Actual behaviour at TARGET

BuildSkeleton in GltfImportCore.cpp (GLTF-261) throws when a skin has more than kMaxPaletteBones (72) joints. The limit is XNA's own SkinnedEffect.MaxBones, which sizes every renderer's bone uniform array, and refusing is a deliberate choice over truncation, which would collapse the extra joints toward the origin.

Source locations

• modules/content/src/GltfImport/GltfImportCore.cpp — BuildSkeleton: GLTF-261 refusal above 72 joints

Evidence

Checked by reading at 009d40f5; not executed. The limit matches XNA 4.0's SkinnedEffect.

Focused reproduction

No focused reproduction is known. Nothing has been invented here; the evidence above is what exists.

Current tests

The refusal lives in BuildSkeleton; the extraction probe in GltfContainerValidationTests.cpp builds every skin so that such a refusal is observed rather than reported as a clean import.

Regression test

If the palette grows or skins are split per mesh, an over-72-joint fixture expecting a correct pose.

Blast radius

glTF characters with large rigs; they cannot be loaded at all.

Workaround

Reduce the rig to at most 72 joints, or split the mesh so each part is skinned by at most 72 joints.