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perf(gif): make GIF export ~30× faster with exact output - #1058
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The median cut sorted HashMap entries with a stable sort, so ties kept the map's per-process random order and every run picked a different palette. Two exports of the same project were never byte-identical, which also ruled out an exact A/B of any GIF optimisation.
) The brute-force scan of all 256 palette entries per pixel was 92 % of a dithered GIF export: 172 ms of the 186 ms per 864x480 frame. The arg-min never autovectorized. The palette is now sorted by red and the scan walks out from the pixel's red value, stopping once the red distance alone exceeds the best full distance. Exact: same f32 distance, ties to the lowest index. The export of a 6 s testsrc2 clip at 864x480/15 fps is byte-identical before and after, and goes from 15.4 s to 4.4 s dithered (5.8 -> 20.5 fps), 10.0 s to 3.4 s undithered.
After the pruned search, mapping and LZW were still over 90 % of a frame, all on the export thread, and they only need the frame and its palette. The export thread now decodes, composes, reads back and builds the palette, hands each frame to a pool of available_parallelism() workers, and writes the encoded frames back in order. 6 s testsrc2 clip, 864x480/15 fps, dithered, Ryzen 7 5800X: 4.4 s -> 0.49 s (20.5 -> 184 fps). 1/2/4/8/16 workers: 3.9/2.0/1.13/0.68/0.49 s. The GIF is byte-identical at every worker count. Adds the GIF counterpart of the MP4 mid-render cancellation test.
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Included review availability: This review used your included allowance. Your plan provides up to 8 included reviews per hour; 0 remain after this review. 📝 WalkthroughWalkthroughGIF export now uses a pruned palette search and a worker pool for frame mapping and LZW compression. The export thread writes completed frames in order. Palette generation is deterministic. Tests cover mapping equivalence and cancellation. ChangesGIF export pipeline
Priority: ➖ Normal Estimated code review effort: 3 (Moderate) | ~25 minutes Change: Feature · Severity of issue fixed: Medium Sequence Diagram(s)sequenceDiagram
participant Export as GIF export thread
participant Pool as Encoder worker pool
participant Encoder as FrameEncoder
participant Writer as GifWriter
Export->>Export: Read back and compose frame
Export->>Pool: Queue frame and shared palette
Pool->>Encoder: Map pixels and compress indices
Encoder-->>Export: Return encoded frame
Export->>Writer: Write completed frames in index order
Merge Risk: ⚪ Minimal · up to The ordered-write buffer remains bounded, and no actionable merge risk was identified. The change is ready for normal merge checks. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches 💡 2📝 Generate docstrings 💡
🧪 Generate unit tests (beta)
🛠️ Fix failing CI checks 💡
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Inline comments:
Review comments at @crates/compositor/src/gif_export.rs:
- Around line 326-332: Bound the number of submitted-but-unwritten frames in the
GIF export loop around FrameJob submission and write_ready. When the window
fills, receive and process worker results until it has room before sending
another job; do not rely on draining only done_rx after a blocking job_tx.send.
Keep results in index order and write them through write_ready.
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crates/compositor/src/gif_export.rscrates/compositor/tests/export_timing.rstechnical-documentation/engineering/rendering-performance.md
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Summary
GIF export spent 92 % of its time in a brute-force nearest-colour search over 256 palette entries per pixel, which never autovectorized: 172 ms per 864×480 frame. Readback, which the code comment called the dominant cost, was 2 to 3 ms.
HashMapiteration order leaked into it, so no two exports of one project were byte-identical.available_parallelism()workers, frames written back in order.rendering-performance.mdgains a "GIF export path" section with the numbers, and the readback claim is corrected.Related issue
Closes #952
Type of change
Release impact
Desktop impact
Testing
testsrc2at 864×480, 15 fps, dithered, Ryzen 7 5800X: 15.4 s → 4.4 s → 0.49 s (5.8 → 184 fps). Undithered: 10.0 s → 0.44 s. With 1/2/4/8/16 workers: 3.9/2.0/1.13/0.68/0.49 s.export_timing.cargo test --release -p openscreen-compositor --lib: 403 passed (the one failure is local: the worktree has no vendored ffmpeg).--test export_timingwith generated media: 4/4.🤖 Generated with Claude Code
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