ImageVulkan hardcoded VK_PIPELINE_STAGE_RAY_TRACING_SHADER_BIT_KHR as the
consuming stage in every layout transition. The UI font atlas is sampled by
a *compute* shader, so the upload barrier's dst scope never covered the real
consumer — a correctness gap, not just lost overlap.
Parameterize the consuming stage per image: store it once at Create
(defaulting to RT, preserving existing RT/example callers) and reuse it in
every Update / UpdateRegion / compressed-Update transition. FontAtlas passes
VK_PIPELINE_STAGE_COMPUTE_SHADER_BIT.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
FontAtlas::Update re-uploaded the entire 1024x1024 R8 atlas (1 MiB)
whenever a single glyph was rasterized, so warm-up / language-switch /
size-churn cost roughly one full-atlas copy per frame that added a
codepoint.
Accumulate a dirty bounding box (FontAtlas::DirtyRect) in MarkDirty as
glyphs are placed, and copy only that sub-rect. On Vulkan this is a new
ImageVulkan::UpdateRegion: the staging buffer keeps the full atlas row
stride, so bufferRowLength stays the atlas width and bufferOffset points
at the rect origin while imageOffset/imageExtent carve out the rect.
Layout transitions stay whole-image (cheap); only the copy extent
shrinks. WebGPU passes the rect straight to wgpuWriteAtlasRegion, which
already took x/y/w/h.
The freshly-zeroed atlas marks its whole extent dirty in Initialize so
the first Update clears the image once; thereafter every untouched texel
stays the zero it was uploaded as, keeping the image byte-identical to
the staging copy.
Tested: new FontAtlasDirtyRect unit test covers the accumulation /
clamp / lockstep-with-`dirty` math (no GPU needed); HelloUI renders text
correctly on both Vulkan (NVIDIA, validation-clean) and WebGPU.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
