CNA-BUG-110: Stock-effect state is captured at draw time, so a property changed after EffectPass::Apply() affects the next draw, unlike XNA

CNA snapshot 009d40f5  ·  Known Issues › Current bugs  ·  source links pinned to 009d40f5

GraphicsDevice calls currentEffect_->FillGpuDrawParams() inside every draw, reading the stock effect's current World, colours and so on; XNA commits effect state at Apply() and a later property change only marks the effect dirty for the next Apply.

Identifier

CNA-BUG-110

Category

Bug

Subsystem

Graphics & renderers

Status

Open

Verified against

CNA 009d40f5 (009d40f5dd085c4e674d3479675fac84b12b3e0a)

Severity

Low (a triage suggestion, not a project priority)

Evidence basis

Source-verified: read at TARGET, not executed

Tests touching this area

None

Affected contract

BasicEffect and the other stock effects' properties between EffectPass::Apply() and GraphicsDevice::Draw*Primitives

Expected behaviour

In XNA 4.0 (IL) a stock-effect setter such as BasicEffect.set_World stores the value and sets dirty flags; EffectPass.Apply runs OnApply and commits the parameters, so the draw uses the values current at Apply. Compiled effects in CNA already synchronise at Apply.

Actual behaviour at TARGET

Every draw overload in GraphicsDevice.cpp builds a GpuDrawParams with currentEffect_->FillGpuDrawParams(p) at the draw call. The sequence effect.World = A; pass.Apply(); effect.World = B; Draw(); draws with B in CNA and with A in XNA.

Source locations

• modules/graphics/src/Xna/GraphicsDevice.cpp — GraphicsDevice::DrawPrimitives and DrawIndexedPrimitives (FillGpuDrawParams at draw time)
• modules/graphics/src/Xna/Effect.cpp — Effect::ApplyPassInternal / SyncCompiledParameters for compiled effects

Evidence

Checked by reading the named sources at 009d40f5; nothing was built or executed for this entry. XNA behaviour from the genuine XNA 4.0 Graphics IL (setter stores + dirty flags; Apply commits). The site's effect object model page already describes the draw-time capture.

Focused reproduction

// Illustrative; not compiled or run for this entry.
effect.setWorldProperty(worldA);
effect.getCurrentTechniqueProperty()->getPassesProperty()[0]->Apply();
effect.setWorldProperty(worldB);          // e.g. preparing the next object too early
device.DrawIndexedPrimitives(...);        // XNA draws with worldA, CNA with worldB

Current tests

No test changes a stock-effect property between Apply and a draw.

Regression test

A HEADLESS test that records the GpuDrawParams world matrix of a draw after such a change and expects the Apply-time value.

Blast radius

Code that mutates stock effects between Apply and Draw (batching helpers, shared effects across objects); the usual set-then-Apply-then-Draw order is unaffected.

Workaround

Set all properties before Apply and do not touch the effect until the draw.