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Lip sync cannot be judged silently. extract.sh now pulls the audio track and writes manifest.json alongside the frames; the page reads the true extraction rate from it rather than assuming one, since a guessed fps desynchronises picture from sound - the one thing this view exists to show. Audio is the clock: frame = floor(currentTime * fps). A slow render loop drops frames instead of drifting, and half/quarter speed work via playbackRate with the picture following for free. Scrubbing, stepping and clicking a thumbnail all seek the audio too. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
38 lines
1.6 KiB
Bash
Executable file
38 lines
1.6 KiB
Bash
Executable file
#!/usr/bin/env bash
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# Extract a clip to a PNG sequence + audio + manifest for the take builder.
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#
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# Frames are pre-extracted rather than decoded in the page on purpose: browser
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# video seeking by currentTime is approximate and requestVideoFrameCallback only
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# delivers frames at playback speed, so neither gives a deterministic per-frame
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# pass. A PNG sequence is exact, instantly seekable, and reproducible.
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#
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# Audio comes out alongside because the page uses it as the PLAYBACK CLOCK -
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# frame = floor(audio.currentTime * fps) - so picture and sound cannot drift
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# apart no matter how long the shot is or how slow the render loop runs.
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set -euo pipefail
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src="${1:?usage: ./extract.sh CLIP [FPS] [OUTDIR]}"
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fps="${2:-12}"
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out="${3:-frames}"
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rm -rf "$out"; mkdir -p "$out"
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ffmpeg -hide_banner -loglevel warning -i "$src" -vf "fps=$fps" "$out/%04d.png"
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count=$(ls -1 "$out" | wc -l)
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# Mono is enough for judging sync and halves the file. Absent audio is not fatal.
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if ffprobe -v error -select_streams a:0 -show_entries stream=codec_type \
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-of csv=p=0 "$src" 2>/dev/null | grep -q audio; then
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ffmpeg -hide_banner -loglevel warning -y -i "$src" -vn -ac 1 -ar 44100 audio.wav
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audio='"audio.wav"'
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echo "audio -> audio.wav"
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else
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audio='null'
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echo "no audio stream"
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fi
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# The page must know the true extraction rate: if it guessed, audio and picture
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# would drift. Source of truth lives here, next to the frames it describes.
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printf '{"fps":%s,"frames":%s,"dir":"%s","audio":%s,"source":"%s"}\n' \
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"$fps" "$count" "$out" "$audio" "$(basename "$src")" > manifest.json
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echo "$count frames at ${fps}fps -> $out/ (manifest.json written)"
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