VulkanViewportMapping has inverted the front face since V6c, on the standard
argument that rendering with a negative viewport height mirrors framebuffer
space and therefore reverses triangle orientation. The world arm is the first
consumer that culls anything, and it falsified the inversion twice over on one
frame.
Nothing exercised it before now. Every Vulkan consumer through V6i - TextRenderer,
DebugLineRenderer and the bring-up scene - declares Cull = GpuCullMode.None, so
the mapping had never decided a single fragment. That is why a wrong answer
survived four slices and a validation-clean run: an unexercised path.
What the world arm measured, on the same offline scene the GL pixel gate captures.
Terrain is the one single-sided surface acdream draws - FrontFace(Ccw) plus
Cull(Back), matching ACRender::landPolysDraw's per-triangle eye-side predicate -
and under the inversion it vanished completely, 190 multi-draw commands issuing
against 625 loaded landblocks with nothing on screen. Every closed building shell
rendered inside-out in the same frame: the front wall culled and the interior
beams visible through the gap, which is what a back-face-front cull looks like on
geometry that is only nearly convex. Declaring the GL winding verbatim restores
both at once - terrain draws single-sided from above, and the shells close.
Two independent surfaces, one change, and the correction is the identity mapping.
Recorded here rather than worked around in the renderers, because a renderer that
compensates for its backend is exactly the shape this file exists to prevent: the
contract says renderers speak GL and the backend translates, and the backend was
translating wrongly.
The viewport flip itself is untouched and still correct - it is what puts
GL-authored geometry the right way up with no shader or matrix change. What goes
is the claim that a winding inversion has to travel with it. The scissor's
explicit flip is a separate correction with a separate justification and is
likewise untouched.
The test suite says so now rather than describing the old behaviour: the
pass-through is asserted directly, and the exact-inverses test becomes a
travels-alone test, so a later change that reintroduces the inversion fails here
first and on any single-sided surface second.
Gates. Release build green. App tests 4,112 passed / 3 skipped, the unchanged
baseline. GL offline pixel gate unaffected by construction - this file has no GL
arm - and measured with the world arm in commit 2.
No divergence-register row: this corrects a backend translation error rather than
introducing a deviation from retail.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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