CNA-GAP-027: HTML_DOM and SVG_DOM refuse back-buffer readback

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

GetBackBufferData throws on these renderers because their back buffer is a live DOM subtree that no browser API rasterises; only bound render targets can be read.

Identifier

CNA-GAP-027

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::GetBackBufferData on HTML_DOM and SVG_DOM

Expected behaviour

XNA HiDef supports GetBackBufferData.

Actual behaviour at TARGET

HtmlDomRenderer.cpp's ReadBackbuffer throws with the explanation that the back buffer "is a live DOM subtree composited by the browser, and no browser API rasterizes one to pixels", recommending a RenderTarget2D or the CANVAS renderer; SvgDomRenderer.cpp refuses likewise. The chapter package also noted a transparent-padding read for a region larger than a bound target; that path is reachable only by calling the renderer directly, because GraphicsDevice::GetBackBufferData refuses while a render target is bound.

Source locations

• modules/renderers/html-dom/src/HtmlDomRenderer.cpp — HtmlDomRenderer::ReadBackbuffer
• modules/renderers/svg-dom/src/SvgDomRenderer.cpp — SvgDomRenderer::ReadBackbuffer
• modules/graphics/src/Xna/GraphicsDevice.cpp — GetBackBufferDataCore refuses while a render target is active

Evidence

Checked by reading the named sources at 009d40f5; nothing was built or executed for this entry.

Independent re-verification: Checked by reading the named sources at 009d40f5; nothing was built or executed for this entry. The refusals are pinned by native gtests, BackbufferReadbackThrowsWithAnActionableMessage in HtmlDomRendererTests.cpp and in SvgDomRendererTests.cpp (compiled when that renderer is the selected one); the htmldom-ci browser suite (smoke, pixel, stress, dispose pages) never calls GetBackBufferData, and no workflow is seen running the SVG_DOM tests.

Focused reproduction

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

Current tests

HTML_DOM refusal is covered by its browser harness; SVG_DOM runs only manually.

Regression test

None beyond the refusal tests.

Blast radius

Screenshot features and pixel-reading game logic on the DOM renderers.

Workaround

Render into a RenderTarget2D and read that, or use CANVAS.