CNA-GAP-039: The glTF vertex ABI carries two UV channels, one joint-influence set and one colour set; further sets are dropped and reported

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

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Evidence basis: source-verified at the pinned commit; tests exist (not executed for this page). Claims on this page were checked by reading the CNA source at commit 009d40f5; unless a sentence says otherwise, nothing here was built or executed. Nothing on this page was executed unless the Evidence section says so.

TEXCOORD sets beyond the first two sampled ones, JOINTS_1/WEIGHTS_1 and later, and COLOR_1 and later are not imported; each loss is named in the import report.

Identifier: CNA-GAP-039
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 primitives with extra attribute sets

Expected behaviour

A glTF primitive may carry any number of TEXCOORD_n, JOINTS_n/WEIGHTS_n and COLOR_n sets.

Actual behaviour at TARGET

CNA's packed vertex layouts carry two UV channels, four joint influences and one colour. AppendGltfMeshReportEXT in GltfImportCore.cpp reports uv-set-mismatch (maps that need a third distinct set fall back to channel 0), influence-sets-dropped (with the worst dropped weight) and color-sets-dropped. gltf-limitations.md lists the three limits (GLTF-091, GLTF-188).

Source locations

modules/content/src/GltfImport/GltfImportCore.cpp — AppendGltfMeshReportEXT: uv-set-mismatch, influence-sets-dropped, color-sets-dropped
docs/gltf-limitations.md — vertex-layout limits: COLOR_1, a third UV set, extra influence sets

Evidence

Checked by reading at 009d40f5; not executed.

Focused reproduction

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

Current tests

GltfImportReportTests.cpp and GltfAttributeCoverageTests.cpp cover the diagnostics; the dropped data is by design not rendered.

Regression test

None needed for the gap itself; a new layout would need stride-table and renderer tests.

Blast radius

Assets with more than four influences per vertex (skinning is approximated with the first set), lightmap-plus-detail materials using three UV sets, or multiple vertex colour sets.

Workaround

Re-export with at most four influences, two UV sets and one colour set.

The same subject is explained at several altitudes. These are the neighbouring pages at each one.

Deep dives: Vertex packing: hard limits · glTF import: the import report
Known issues: Functional gap index