CNA-BUG-180: Win32GlContext ignores multisample and per-channel format requests and reports the requested description as if it were granted

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

DescribeFormat ignores GlContextDescription's per-channel and multisample fields, and GetContextAttributes returns the requested description verbatim, so a legacy fallback context is reported as the requested version.

Identifier

CNA-BUG-180

Category

Bug

Subsystem

Platforms

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

Yes: see Current tests

Affected contract

CNA::Platform::IPlatformGlContext::CreateContext and GetContextAttributes on the Win32 backend

Expected behaviour

GetContextAttributes is documented as returning what the driver actually granted: “A driver may grant more than was requested (a higher version, more samples) or less ... the granted values are queryable rather than assumed.” A renderer reads it to learn the real GL version and framebuffer format.

Actual behaviour at TARGET

DescribeFormat builds a PIXELFORMATDESCRIPTOR with a fixed cColorBits = 32 and cAlphaBits = 8, honouring only depth and stencil (clamped) and the double-buffer flag; it ignores GlContextDescription::redBits/greenBits/blueBits and multisampleBuffers/multisampleSamples, and Win32Window reads neither WindowDescription::renderIntent nor openGlFramebuffer. CreateContext stores granted = description (the requested values) and GetContextAttributes returns that record, so it reports the request, not the result. When wglCreateContextAttribsARB is absent or fails, the legacy bootstrap context (created with no version or profile attributes) is returned while GetContextAttributes still reports the requested version and profile.

Source locations

• modules/platform/src/Win32/Win32GraphicsServices.cpp — DescribeFormat (fixed format, ignores multisample/per-channel); CreateContext (granted = description; legacy fallback); GetContextAttributes (returns the requested description)
• modules/platform/src/Win32/Win32Window.cpp — does not read renderIntent or openGlFramebuffer
• modules/platform/include/CNA/Platform/IPlatformGlContext.hpp — GetContextAttributes documents granted-not-requested values; GlContextDescription's multisample and per-channel fields

Evidence

Checked by reading DescribeFormat, CreateContext and GetContextAttributes in Win32GraphicsServices.cpp, the IPlatformGlContext contract and Win32Window's window creation at 009d40f5. Not executed (no Windows host). The Win32 conformance test only checks that the granted major version is positive, which the verbatim copy satisfies. The Development Win32 page records these WGL gaps.

Focused reproduction

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

Current tests

Win32GraphicsServicesTests.cpp's AContextEitherIsCreatedAndUsableOrFailsExplicitly requests 3.3 Core and asserts only granted.majorVersion > 0; no test requests multisampling or a specific channel layout, or checks the reported version against the real context.

Regression test

Request multisampling and a non-default channel layout and assert either that they are honoured or that GetContextAttributes reports the real granted values (query the created context with glGetIntegerv/wglGetPixelFormatAttribivARB), not the request.

Blast radius

The direct effect at 009d40f5 is narrow. No in-tree GL renderer requests multisampling or a channel layout through GlContextDescription: EasyGL's RequestedGlContext and OpenGL4's RequestedContext ask only for a version and profile, a 24-bit depth buffer, an 8-bit stencil buffer and double buffering, which DescribeFormat honours. Nothing in the tree reads GetContextAttributes outside tests (PlatformGlContextOwner::GetAttributes has no caller). The in-tree consequence is the legacy fallback: on a driver without, or refusing, wglCreateContextAttribsARB, an EasyGL or OpenGL4 renderer that asked for a 3.3 Core context silently receives the legacy bootstrap context and nothing at context creation reports the downgrade. Out-of-tree renderers or tools built on the platform service get the requested description back instead of the granted one.

Workaround

Do not rely on Win32 GetContextAttributes for the true context version or format; query the live GL context directly.