perf(audio): stretch speed regions through libavfilter atempo instead of WSOLA — fixes exports frozen at ~80% - #371
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August 14, 2026 17:36
… of WSOLA WSOLA is O(grain x search-radius) per rendered sample. On a long clip with speed regions (measured: 65.4M samples after speed-segment quantization) it runs for many minutes at 100% of one core, and the export appears frozen at ~80% progress — audio stretching is the pipeline's last big job. Users kill the export; nothing fails, it is just unreachably slow. Route stretch_pcm_to_length through an in-process abuffer -> atempo -> abuffersink graph instead. atempo is the same pitch-preserving time-stretch, but O(n) with ffmpeg's SIMD routines: the same input takes seconds. avfilter already ships in the app — fetch-ffmpeg.mjs vendors every av*.dll of the BtbN LGPL-shared build, and the addon sits beside those DLLs — so this only links a library that was already in the box. - build.rs: link avfilter (bindgen already allowlists avfilter_*/ via the existing "av.*" filter, and the Linux osff_ symbol-rename table derives from the soname list) - build-linux-compositor-addon.mjs: stage libavfilter.so.11 alongside the other renamed libs - wrappers: include libavfilter headers - audio.rs: avfilter_atempo_stretch() mounts the graph, feeds planar f32 chunks, drains, and pads/truncates to the exact target length; speeds outside atempo's [0.5, 100] window chain multiple stages (0.2 -> atempo=0.5,atempo=0.5,atempo=0.8). Any failure returns None and falls back to the existing WSOLA path unchanged. - sink negotiation may yield flt (interleaved) or fltp (planar); both are deinterleaved into PlanarPcm Verified with cargo test: a 10 s 440 Hz stereo sine at speed 1.25 returns exactly 8 s and measures 440 Hz +/- 2 Hz by zero crossings (pitch preserved — a plain resample would shift it).
Two hardening guards found while diagnosing the slow-export hang: - decode_clip_audio: a container whose audio track is truncated or corrupt at the end can keep av_read_frame from ever returning AVERROR_EOF, so decoder_eof never propagates and the demux loop spins at 100% CPU forever. Cap it with a 60 s time budget — time, not iterations, because av_read_frame can be slow on a corrupt stream and an iteration cap would either never trigger or cut healthy long clips short. - WsolaTimeStretcher::process: if find_best_delta keeps returning a delta that puts grain_pos back where it was, the buf_end break is never reached and the loop spins forever. Detect the stagnation (100 consecutive non-advancing grains) and force the exit — the fallback path after the previous commit's atempo change, so this only protects the unlikely case where WSOLA still runs.
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Included review availability: Your plan includes up to 4 reviews per rolling hour; 3 remain after this review. 📝 WalkthroughWalkthroughThe compositor links FFmpeg ChangesAudio time-stretching
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: ⚪ Minimal · up to The change replaces the slow audio-stretching path with a faster filter-based implementation while retaining a fallback path and adding hang protections. No actionable merge-blocking risk remains in the supplied evidence. Sequence Diagram(s)sequenceDiagram
participant stretch_pcm_to_length
participant FFmpegFilterGraph
participant WSOLA
stretch_pcm_to_length->>FFmpegFilterGraph: Process PCM with chained atempo filters
FFmpegFilterGraph-->>stretch_pcm_to_length: Return exact-length audio or failure
stretch_pcm_to_length->>WSOLA: Use fallback when FFmpeg processing fails
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Inline comments:
In `@crates/compositor/src/audio.rs`:
- Around line 284-305: Update the decode loop budget near loop_start and
loop_budget so it scales with the requested window duration while retaining a
minimum floor, rather than using a fixed 60-second limit. Derive the duration
from the existing window or source timing symbols, preserve the timeout’s
guaranteed termination and forced decoder_eof behavior, and keep the existing
timeout logging and loop flow intact.
- Around line 986-1044: Update the atempo drain logic around
av_buffersrc_add_frame and av_buffersink_get_frame to check and propagate
non-AVERROR_EOF/AVERROR_EAGAIN failures as None instead of padding them with
silence. Track the flush result, classify sink returns correctly, and reject
implausibly short stretched output so stretch_pcm_to_length uses the WSOLA
fallback; preserve normal EOF/EAGAIN completion and exact resize behavior for
valid output.
In `@crates/compositor/wrapper_macos.h`:
- Around line 20-22: Separate the concatenated libswscale and libavfilter
include directives in the macOS wrapper so each `#include` occupies its own line,
preserving the existing buffersrc and buffersink includes.
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crates/compositor/build.rscrates/compositor/src/audio.rscrates/compositor/wrapper_linux.hcrates/compositor/wrapper_macos.hcrates/compositor/wrapper_windows.hscripts/build-linux-compositor-addon.mjs
- wrapper_macos.h: the appended avfilter include landed on the same line as the trailing swscale include (the file had no final newline), so the preprocessor never saw it — split them onto separate lines. macOS builds would have produced no avfilter bindings at all. - decode budget: scale with the requested window (x8, floor 60 s) instead of a flat 60 s, so slow storage / heavy codecs decoding a long window are not cut off into trailing silence. - atempo drain: only AVERROR_EOF / AVERROR_EAGAIN are benign; any other negative return is a real filter failure — return None so the WSOLA fallback runs instead of exporting partial audio padded with silence. The buffersrc flush return is checked for the same reason.
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Problem
Exports with speed regions appear to freeze at ~80% progress and never finish. Nothing fails — the process just spins at 100% of one core, effectively forever, on long clips.
Root cause
stretch_pcm_to_lengthuses WSOLA, which isO(grain x search_radius)per rendered sample. On a 22-minute clip with a 1.25x speed region, speed-segment quantization produces ~65.4M samples of audio to stretch; the WSOLA pass measured >10 minutes without completing. Audio stretching is the pipeline's last big job, so the progress bar sits at ~80% while it runs, and users kill the export.Fix
Route
stretch_pcm_to_lengththrough an in-process libavfilter graph (abuffer -> atempo -> abuffersink):atempoperforms the same pitch-preserving time-stretch, but is O(n) with ffmpeg's SIMD routines — the same input finishes in seconds.avfilteralready ships in the app:fetch-ffmpeg.mjsvendors everyav*.dllof the BtbN LGPL-shared build and the addon sits beside those DLLs. This PR only links a library that was already in the box — no new dependency, no packaging changes on Windows.build.rs: linkavfilter(bindgen already allowlistsavfilter_*via the existing"av.*"pattern)build-linux-compositor-addon.mjs: stagelibavfilter.so.11alongside the other renamed libs (theosff_symbol-rename table derives from this list); macOS picks dylibs up automaticallyaudio.rs:avfilter_atempo_stretch()mounts the graph, feeds planar f32 chunks, drains, and pads/truncates to the exact target length. Speeds outside atempo's[0.5, 100]window chain multiple stages (e.g. 0.2 ->atempo=0.5,atempo=0.5,atempo=0.8). Any failure returnsNoneand falls back to the existing WSOLA path unchanged.flt(interleaved) orfltp(planar); both are deinterleaved intoPlanarPcmFollow-up commit adds two guards found while diagnosing:
decode_clip_audio: 60s time budget — a truncated/corrupt audio track can keepav_read_framefrom ever returningAVERROR_EOF, spinning the demux loop forever.WsolaTimeStretcher::process: stagnation detection — iffind_best_deltakeeps returning deltas that don't advancegrain_pos, the loop spins forever (only protects the WSOLA fallback now).Testing
cargo test -p openscreen-compositor audio::— 9 tests pass, including new ones: a 10s 440 Hz stereo sine at speed 1.25 returns exactly 8s and measures 440 Hz +/- 2 Hz by zero-crossing count (pitch preserved; a plain resample would shift it), plus length-exactness and multi-stage (out-of-range speed) cases.Notes
Summary by CodeRabbit
New Features
Bug Fixes