§5.5.7 asked for "a Vulkan composition host — a slice the plan has never
scoped." This scopes it and stops there, for the same reason V6f stopped short
of the world fork: the honest measurement is worth more than a half-built second
path, and §3.1 and §7.1 rule 3 exist to prevent exactly the unexercised arm this
would otherwise have landed.
Three findings, all verified against source rather than inferred.
The fork seam mostly already exists. GameWindowPlatformResult<TGraphics, TInput>
is fully generic — only its call sites pin GL — and the host phase already has
IHostInputCameraCompositionFactory with a single Retail implementation, so the
Vulkan device arm is a new file rather than a modification. What does not exist
is a Vulkan frame root: FrameRootComposition assembles the clear phase, both
pass executors and the GPU profiler measurement from a GL handle plus six raw-GL
world renderers, none of which exist on Vulkan. That second assembly is the
slice's centre of gravity, and it is most of V4h.
The retained UI is NOT blocked on V4t. V6f wrote that the retail widget tree's
chrome comes from a GL-only TextureCache, which is true of the type but not of
the path: V4a and V6d already moved UploadUiTexture onto IGpuDevice, and the
public UploadRgba8 that IconComposer composes retail icons with routes into it.
The raw-_gl uploads that remain are the world's Texture2D/Texture2DArray paths,
which no UI draw reaches. Three small things stand in the way — a non-null GL in
TextureCache's constructor, one ((GlGpuTexture)texture).GlName cast for VRAM
accounting, and the UI-probe screenshot controller — and none of them is V4t.
So V4t is a hard prerequisite for the WORLD arm, not for the host, which moves
the host ahead of it in the sequence rather than behind.
That reordering is the point of the commit. The corrected sequence puts the
composition host at step 2, before V4t, because its acceptance criterion — the
real UI renders — makes it the first frame acdream draws on Vulkan that is the
client's own frame rather than a scene written to prove the backend.
Also records the argument for keeping VulkanBringUpHost as a capability-probe
harness: its window/instance/surface/device/swapchain sequence is what the
composition arm needs, and deleting it would mean writing that twice.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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