led2/.planning/phases/02-audio-engine/02-03-SUMMARY.md

phase, plan, subsystem, tags, requires, provides, affects, tech-stack, key-files, key-decisions, patterns-established, requirements-completed, duration, completed

phase	plan	subsystem	tags	requires	provides	affects	tech-stack	key-files	key-decisions	patterns-established	requirements-completed	duration	completed
02-audio-engine	03	audio	waveform soundfile numpy ffmpeg transport-ui canvas websocket	phase provides 02-audio-engine/02-01 MPVEngine with get_state(), play/pause/seek via WebSocket phase provides 02-audio-engine/02-02 WebSocket broadcast loop (tick messages), REST audio endpoints in lightsync/api/audio.py	extract_peaks() function in lightsync/audio/waveform.py (AUD-05) GET /api/audio/waveform endpoint returning downsampled peak array Transport bar UI with play/pause/stop/seek/load controls Waveform canvas in timeline panel with position cursor Real-time position cursor via WebSocket tick dispatch in handleMessage	03-timeline-editor 04-show-builder	added patterns soundfile numpy (waveform extraction) ffmpeg (mp3 via subprocess pipe) MP3 via ffmpeg pipe: subprocess.run(['ffmpeg', '-f', 'f32le', 'pipe:1']) + np.frombuffer asyncio.to_thread for waveform extraction (seconds-long operation) Transport button visibility toggle: btnPlay/btnPause display swap on tick paused state Seek bar _dragging flag prevents position jumps while user scrubs Waveform click-to-seek: clientX/rect.left normalized to audioDuration	created modified lightsync/audio/waveform.py lightsync/api/audio.py lightsync/frontend/index.html lightsync/frontend/app.js lightsync/frontend/style.css	ffmpeg subprocess pipe for MP3 (libsndfile/soundfile does not support MP3 natively) peaks param clamped 100-5000 server-side; client requests 2000 for waveform display CSS variables from existing palette used unchanged — --accent, --bg-primary, --bg-panel, --border-dim Playback cursor uses CSS transition: left 0.1s linear for smooth 10Hz tick interpolation	Pattern: waveform extraction returns list[float] 0.0-1.0, normalized to peak amplitude per chunk Pattern: transport state driven by WebSocket tick — no polling from frontend	AUD-05	8min	2026-04-06

Phase 02 Plan 03: Waveform Display and Transport UI Summary

soundfile/ffmpeg waveform extractor, GET /api/audio/waveform endpoint, and full transport bar with play/pause/seek controls and real-time position cursor on canvas

Performance

• Duration: ~8 min
• Started: 2026-04-06T13:10:00Z
• Completed: 2026-04-06T13:18:00Z
• Tasks: 2 auto + 1 checkpoint (human-verify)
• Files modified: 5 (1 created)

Accomplishments

• Waveform extraction module handles MP3 via ffmpeg pipe and WAV/FLAC/OGG via soundfile+numpy (AUD-05)
• GET /api/audio/waveform endpoint wraps extraction in asyncio.to_thread, returns {peaks, count, file}
• Transport bar replaces placeholder: play/pause/stop buttons, seek slider, time display (M:SS.s), audio path input + LOAD button
• Timeline panel waveform canvas draws peak bars using --accent color at device pixel ratio
• Position cursor moves at 10Hz from WebSocket tick dispatch, play/pause button visibility toggles with state
• Click-to-seek on waveform canvas, seek bar dragging, window resize redraws waveform

Task Commits

Each task was committed atomically:

1. Task 1: Waveform extraction module and HTTP endpoint - da474d9 (feat)
2. Task 2: Transport UI controls and waveform canvas - 148f16b (feat)
3. Task 3: Human verify - awaiting checkpoint approval

Files Created/Modified

• lightsync/audio/waveform.py — extract_peaks(), _extract_peaks_mp3(), _extract_peaks_soundfile(), _downsample_peaks()
• lightsync/api/audio.py — Added GET /waveform endpoint with asyncio.to_thread + peaks clamp
• lightsync/frontend/index.html — Timeline panel with waveform canvas + cursor; transport bar with all controls
• lightsync/frontend/app.js — drawWaveform(), loadWaveform(), formatTime(), updatePosition(), transport event handlers, handleMessage tick dispatch
• lightsync/frontend/style.css — .transport-btn, .transport-time, .seek-slider, .transport-file, .waveform-container, .playback-cursor

Decisions Made

• Used ffmpeg subprocess pipe for MP3 extraction (soundfile/libsndfile doesn't support MP3 natively)
• peaks query param server-side clamped to 100-5000; frontend requests 2000 for display
• CSS used existing variable names unchanged (--accent, --bg-primary, --bg-panel, --border-dim) — no new color variables added
• CSS transition on playback-cursor (left 0.1s linear) interpolates between 10Hz ticks for smooth movement

Deviations from Plan

Auto-fixed Issues

None — plan executed exactly as written, with CSS variable name adaptation (plan used --bg/--surface/--border, existing CSS uses --bg-primary/--bg-panel/--border-dim — adapted to match actual variable names).

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Total deviations: 0 Impact on plan: CSS variable name adaptation was minor and necessary for correctness. No scope creep.

Issues Encountered

None during auto-tasks. Human verification checkpoint (Task 3) is pending.

Known Stubs

None — waveform extraction and transport UI are fully wired. The position cursor and transport controls depend on WebSocket tick messages which are broadcast by Phase 02-02's broadcast loop.

Next Phase Readiness

• GET /api/audio/waveform usable by any future phase needing waveform data
• Transport controls drive audio via WebSocket commands (play/pause/seek/load)
• Position cursor provides real-time visual feedback for timeline editor (Phase 3+)
• All Phase 1 UI (devices panel, device form) preserved

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Phase: 02-audio-engine Completed: 2026-04-06