feat(core): transparent ARGB compositing with retained raster layer passes - #248
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feat(core): transparent ARGB compositing with retained raster layer passes#248odex21 wants to merge 2 commits into
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ARGB targets now clear to fully transparent and write straight color + coverage into transparent destination pixels, so hosts can composite the scene over their own background layers. When the destination already holds content, blends use true straight-alpha src-over: out_a = a + dst_a * (1 - a) out_rgb = (src * a + dst_rgb * dst_a * (1 - a)) / out_a instead of the previous "blend against opaque black and force alpha 255", which flattened semi-transparent destination pixels (glyph edges over another layer, translucent backgrounds) on layered surfaces. Opaque destinations reduce to the previous mix (same rgb, alpha 255), so RGBA/RGB565 targets and existing hosts are unchanged. The ARGB memory layout test now seeds a fullscreen opaque rect so both targets carry content in every pixel under the transparent-clear contract; a reference-math test pins the semi-transparent src-over result.
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Opt-in rasterCache property (id 143) plus Ui::draw_retained(), returning ordered Draw/Layer passes. Translation-only layer roots keep stable layer-local DrawLists, so capable hosts can retain pixels and only move composition coordinates; the flat Ui::draw() ABI stays byte-compatible. Layers require overflow:hidden and no shadow to avoid clipping out-of-bounds children or shadow pixels; both fall back to regular Draw passes.
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Summary
Two commits on top of upstream main (f658cd0):
rasterCacheproperty (id 143) plusUi::draw_retained(), returning ordered Draw/Layer passes. Translation-only layer roots keep stable layer-local DrawLists, so capable hosts can retain pixels and only move composition coordinates; the flatUi::draw()ABI stays byte-compatible.Motivation
The ESP32-P4 product host composites a transparent UI layer over a fluid background and scrolls a retained lyric layer by translation only. The previous PSP-era "blend against opaque black and force alpha 255" flattened semi-transparent destination pixels on layered surfaces; the retained passes let hosts skip re-rasterization when only the layer position changes.
Layer constraints
A node becomes a retained layer only when all of these hold:
rasterCache = Retained(opt-in hint)overflow: hidden(visible would clip out-of-bounds children)shadow(shadow draws outside node bounds and would be clipped)Nodes that do not meet these constraints paint into the surrounding regular Draw pass, preserving flat rendering equivalence.
Verification
cargo test: 115/115 (each commit individually green)spec.ts<-> generatedspec.rs)Ui::draw()output unchanged (existing layout/golden tests)Performance (measured on the ESP32-P4 product host)
On-device A/B with a minimal scrolling-list app (no audio/WS/cover/fluid), identical code except
rasterCache0 vs 1, sampled via the host serial perf probe (1 s windows, medians):Retained layers cut per-frame raster cost by ~95% (13.1 ms -> 0.63 ms) and raise fps by ~15%; the remaining wall is the host-side retained composite chain (blend + pipeline wait), not core raster. Numbers are host-specific; the core-level benefit is the stable layer-local DrawList verified by tests.
Notes
draw.rs/lib.rs/raster.rs/tests.rs) but with unrelated mechanisms; any conflicts would be mechanical.docs/RETAINED_LAYERS.md; property id 143 is append-only (next free after 142).DamageTracker; hosts own cache invalidation and composition scheduling.