CNA-GAP-032: Engine-layer PBR limits: one punctual light per PbrEffect draw, 8-bit environment cubes, no back-face normal flip for double-sided materials, no alpha-tested shadow casters

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

PbrEffect takes one punctual light, EnvironmentProcessor builds its cubes as SurfaceFormat::Color, no stock or engine-layer shader reads a front-facing input to flip normals, and the shadow-caster shaders write depth without an alpha test.

Identifier

CNA-GAP-032

Category

Functional gap

Subsystem

Graphics & renderers

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

PbrEffect::setPunctualLightEXT; CNA::Graphics::EnvironmentProcessor; PbrEffect::getDoubleSidedEXTProperty; the engine layer's shadow-caster passes

Expected behaviour

glTF 2.0 reverses the normal of a back face of a double-sided material before lighting and lets alpha-mask materials cut out their shadows; HDR image-based lighting keeps radiance above 1.0.

Actual behaviour at TARGET

PbrEffect.hpp holds a single PunctualLightEXT (documented as a deliberate ceiling in cnaext-engine-layer.md). EnvironmentProcessor.cpp creates every generated cube as SurfaceFormat::Color. The doubleSided mapping in gltf-limitations.md selects CullNone; no shader under modules/ used by the stock PBR or engine-layer paths reads gl_FrontFacing, SV_IsFrontFace or a WGSL/MSL front-facing builtin, so back faces are lit with the front normal. The shadow-caster shaders in modules/graphics-ext/src/shaders/shadow_caster sample no texture and never discard, so alpha-tested geometry casts solid shadows.

Source locations

• modules/graphics/include/Microsoft/Xna/Framework/Graphics/PbrEffect.hpp — setPunctualLightEXT: one light
• docs/cnaext-engine-layer.md — One punctual light per draw, a deliberate ceiling
• modules/graphics-ext/src/EnvironmentProcessor.cpp — TextureCube created with SurfaceFormat::Color
• docs/gltf-limitations.md — doubleSided row: CullNone
• modules/graphics-ext/src/shaders/shadow_caster/directional.desktop.frag.glsl — depth-only caster fragment shader (no texture, no discard)

Evidence

Checked by reading at 009d40f5 (the front-facing search was a grep of all of modules/); not executed. The four limits were reported by the living-room-simulator sample @ 2bf3f56b against an earlier CNA revision and re-established individually here.

Independent re-verification: Checked by reading at 009d40f5 (the front-facing search was a grep of all of modules/, docs and tools: front-facing inputs appear only in compiled-effect translation, the Software compiled-effect runtime and PortableGL's rasteriser, not in the stock PBR or engine-layer shaders; doubleSided never reaches GpuDrawParams); not executed. The four limits were reported by the living-room-simulator sample @ 2bf3f56b against an earlier CNA revision and re-established individually here. Tests: engine-layer shadow, IBL and PBR tests exist; the EasyGL_Gltf_AlphaBlend example draws a double-sided back face under CullNone but with ambient-only lighting and no directional light, so no test lights a back face, casts an alpha-masked shadow, or carries HDR radiance through an environment cube.

Focused reproduction

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

Current tests

Engine-layer shadow, IBL and PBR tests exist; none draws a double-sided back face, an alpha-masked shadow caster or HDR environment radiance.

Regression test

Per limit, once lifted: a back-face lighting pixel test, an alpha-mask shadow test, an IBL test with radiance above 1.0, and a two-punctual-light draw.

Blast radius

glTF scenes with double-sided foliage or cloth, cut-out shadow casters, bright HDR environments, or several shadowed punctual lights per object.

Workaround

Duplicate double-sided geometry with flipped normals; split draws per punctual light.