CNA-GAP-038: Mirrored glTF placements keep the shared winding, so under the default cull mode they render back-facing

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

A placement whose world transform has a negative determinant is detected and reported (mirrored-winding-unapplied) but CNA neither reverses its winding nor changes cull state, and exposes no per-mesh flag for the application to do it.

Identifier

CNA-GAP-038

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> on glTF files with negatively scaled or mirroring node transforms; Model::Draw

Expected behaviour

glTF §3.7.4 requires the front-face winding to be reversed when a node's global transform has a negative determinant, so mirrored geometry stays front-facing.

Actual behaviour at TARGET

ContentManager.cpp logs, per mirrored placement, that CNA "does not reverse the triangle winding for it, so this placement's front faces are back-facing under the default cull mode", and GltfImportCore.cpp (AppendGltfInstanceReportEXT) records mirrored-winding-unapplied: "the application must reverse culling for this draw". The reason is architectural: the index and vertex buffers are shared with unmirrored placements of the same mesh, and cull state is per-draw device state. No public Model or ModelMesh property carries a per-mesh mirroring flag. The only public route is the Model's import report (Model::getGltfImportReportEXTProperty), which lists each mirrored placement by node name in a diagnostic whose Code is mirrored-winding-unapplied, so an application has to map that node name to the ModelMesh it draws, or recompute the determinant from the bone transforms.

Source locations

• modules/content/src/Xna/ContentManager.cpp — ReadGltfModel: GLTF-116 warning per mirrored placement
• modules/content/src/GltfImport/GltfImportCore.cpp — AppendGltfInstanceReportEXT: mirrored-winding-unapplied
• modules/content/tests/CNA/Internal/GltfImport/GltfMirroringTests.cpp — detection and untouched shared buffers

Evidence

Checked by reading at 009d40f5; not executed. GltfMirroringTests.cpp pins the detection and that the buffers stay untouched; nothing renders a mirrored placement with a reversed cull mode.

Focused reproduction

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

Current tests

GltfMirroringTests.cpp covers detection through hierarchies, 180° rotations, shared buffers and tangent handedness.

Regression test

If a per-draw reversal is added, an L7 fixture (xf-negative-scale exists in the corpus) expecting the mirrored placement to be visible under default culling.

Blast radius

glTF assets with negative scale or mirroring hierarchies drawn with Model::Draw under CullCounterClockwise.

Workaround

Find the mirrored placements in the Model's import report (getGltfImportReportEXTProperty(), diagnostic mirrored-winding-unapplied, subject = node name), then draw such meshes with RasterizerState::CullNone, or detect a negative determinant of the absolute bone transform and switch to the opposite cull mode for that draw.