CNA-GAP-032: PbrEffect limits: one punctual light per draw, and no back-face normal flip for double-sided materials outside EasyGL

CNA snapshot c1c316b9  ·  Known Issues › Functional gaps  ·  source links pinned to c1c316b9

PbrEffect takes one punctual light, and only EasyGL's PBR programs reverse the normal of a double-sided material's back face (GSC-0010); every other PBR renderer lights the back face with the front normal.

Identifier

CNA-GAP-032

Category

Functional gap

Subsystem

Graphics & renderers

Status

Narrowed (partially fixed; describes only what survives)

Verified against

CNA c1c316b9 (c1c316b9c7a846ce8002809c151fcd1af14942c9)

Evidence basis

Source-verified: read at TARGET, not executed

Tests touching this area

Yes: see Current tests

Affected contract

PbrEffect::setPunctualLightEXT; PbrEffect/SkinnedPbrEffect doubleSided on renderers other than EasyGL

Expected behaviour

glTF 2.0 reverses the normal of a back face of a double-sided material before lighting.

Actual behaviour at TARGET

PbrEffect.hpp holds a single PunctualLightEXT (“Supplies one punctual light and its shadow”). The doubleSided mapping in gltf-limitations.md selects CullNone; PbrEffect and SkinnedPbrEffect pass the material's flag as GpuDrawParams::pbrDoubleSided (IGraphicsRenderer.hpp), which only EasyGL reads: its PBR programs flip N, T and B on a back face. VULKAN, SDL_GPU, WEBGPU, DIRECTX9/11 and METAL ignore it, so their back faces are lit with the front normal.

Fixed since first recorded: the back-face flip on EasyGL (GSC-0010, a1414a1b0). The entry's two limits of CNA-generated environment cubes and shadow casters do not apply at this snapshot: CNA ships no environment-cube or shadow-map generator, and an application supplies those textures to the PbrEffect receivers itself.

Source locations

• modules/graphics/include/Microsoft/Xna/Framework/Graphics/PbrEffect.hpp — setPunctualLightEXT: one light
• docs/gltf-limitations.md — doubleSided row: CullNone
• modules/graphics/include/CNA/Internal/Renderers/Common/IGraphicsRenderer.hpp — GpuDrawParams::pbrDoubleSided
• modules/renderers/easygl/examples/easygl_pbr_double_sided_test.cpp — EasyGL only: a double-sided back face lit from the side that shows it

Evidence

Checked by reading at c1c316b9 (the front-facing search was a grep of all of modules/); not executed. The original limits were reported by the living-room-simulator sample @ 2bf3f56b against an earlier CNA revision and re-established individually.

Re-verified at b0e97bb1: PbrEffect.hpp still holds one PunctualLightEXT. pbrDoubleSided is read only in EasyGLRenderer.cpp (GSC-0010, a1414a1b0); the other PBR renderers bind the PBR maps but never read the flag. Not executed.

Focused reproduction

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

Current tests

EasyGL_Pbr_DoubleSided (easygl_pbr_double_sided_test.cpp) lights a double-sided back face on EasyGL; no other renderer has such a test, and no test draws two punctual lights.

Regression test

A back-face lighting pixel test per PBR renderer; a two-punctual-light draw once that limit is lifted.

Blast radius

glTF scenes with double-sided foliage or cloth on renderers other than EasyGL, or several punctual lights per object.

Workaround

Duplicate double-sided geometry with flipped normals on renderers other than EasyGL; split draws per punctual light.