CNA-GAP-033: CNA has no counterpart of XNA's MissingVertexShaderInput draw check, and renderers resolve a stock-effect draw that lacks an input differently

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

XNA refuses a draw whose vertex declaration lacks a semantic the shader reads; CNA has no such check, so EasyGL/OPENGL4 feed neutral values, WEBGPU substitutes, SDL_GPU throws and VULKAN aliases TEXCOORD1.

Identifier

CNA-GAP-033

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

GraphicsDevice::DrawPrimitives / DrawIndexedPrimitives / DrawUserPrimitives with a stock effect whose required vertex semantic is missing from the VertexDeclaration

Expected behaviour

The genuine XNA 4.0 GraphicsDevice refuses such a draw with InvalidOperationException (FrameworkResources.MissingVertexShaderInput, reached from its draw-time verification), identically on every device.

Actual behaviour at TARGET

GraphicsDevice::validateDrawState in GraphicsDevice.cpp implements XNA's draw-time float-filtering and blending restrictions but no input check; no source under modules/ implements MissingVertexShaderInput. StockEffectUsesVertexSemantic in IGraphicsRenderer.hpp encodes the per-variant table but decides only which streams are consumed. EasyGL and OPENGL4 leave a missing attribute disabled and read the (0, 0, 0, 1) neutral record; WEBGPU's SelectStockVertexShapeEXT falls back from EnvironmentMapEffect without NORMAL0/TEXCOORD0 to a BasicEffect shape and sends alpha-test or dual-texture draws without TEXCOORD0 to the stride-16 coloured path; SDL_GPU throws NotSupportedException for those cases (std::invalid_argument for SkinnedEffect); VULKAN aliases TEXCOORD1 onto TEXCOORD0 for DualTextureEffect where the others read (0, 0).

Source locations

• modules/graphics/src/Xna/GraphicsDevice.cpp — GraphicsDevice::validateDrawState: no input check
• modules/graphics/include/CNA/Internal/Renderers/Common/IGraphicsRenderer.hpp — StockEffectUsesVertexSemantic
• modules/renderers/easygl/src/EasyGLRenderer.cpp — ConfigureDeclarationForStockProgramEXT: missing element left disabled
• modules/renderers/opengl4/src/OpenGL4StockDraw.cpp — ConfigureDeclarationForStockProgram
• modules/renderers/webgpu/src/WebGPURenderer.cpp — SelectStockVertexShapeEXT fallbacks
• modules/renderers/sdl-gpu/src/SdlGpuRenderer.cpp — stock dispatch: NotSupportedException / invalid_argument
• modules/renderers/vulkan/src/VulkanRenderer.cpp — BuildDualTextureVertexLayoutEXT TEXCOORD1 alias

Evidence

Checked by reading at 009d40f5 and the XNA 4.0 Graphics IL in the reference cache (MissingVertexShaderInput and its details string); not executed. The per-renderer outcomes are spot-checked at the named functions, not traced for every effect variant.

Focused reproduction

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

Current tests

Stock-effect tests draw with complete declarations; none asserts a refusal for a missing input.

Regression test

A renderer-independent test per stock effect variant that draws with a declaration missing one required semantic and expects InvalidOperationException before the renderer is reached.

Blast radius

Code that XNA would reject: it runs with renderer-dependent results in CNA. Correct XNA code is unaffected.

Workaround

Test ports on a reference renderer and supply every semantic the stock effect reads.