# Fix Audio Playback + Dynamic Waveform Resolution

## Task Description
1. **Fix missing audio on preview playback** — Video plays but with no sound in WKWebView.
2. **Dynamic waveform resolution** — Waveform should increase in detail as the user zooms in.

## Root Cause Analysis (Audio)

The preview file is correctly muxed (H.264 640x360 + AAC 128kbps, moov atom at offset 24 — fast-start). WKWebView config analysis:
- wry 0.55.1 defaults `autoplay: true` → sets `mediaTypesRequiringUserActionForPlayback = None`
- tauri-runtime-wry uses `WebViewBuilder::new_with_web_context()` which inherits this default
- Tauri v2.11.6 doesn't expose or override `autoplay`
- So WKWebView SHOULD be configured for audio playback

Despite correct configuration, WKWebView's Tauri asset protocol (`https://asset.localhost/...`) appears to silently drop audio tracks when streaming local files — possibly due to range request handling or MIME type issues in the custom protocol handler.

**Fix**: Load the video file as a `blob:` URL instead of using the asset protocol. This bypasses the asset protocol entirely and uses WKWebView's native blob URL handling, which reliably plays both audio and video tracks.

## Changes Made

### Audio Fix: Blob URL Video Loading (`src/lib/components/VideoPlayer.svelte`)
- Added a `$effect` that fetches the preview file via `convertFileSrc` URL, converts it to a Blob, then creates a `blob:` URL
- Video element now uses the blob URL instead of the asset protocol URL
- Added `playsinline` attribute to the video element
- Added explicit `volume = 1` and `muted = false` on `loadeddata`
- Added `.play()` Promise error handling (catches and logs rejections)
- Shows "Preparing video…" state while blob is loading
- Falls back to asset protocol URL if blob creation fails
- Properly revokes old blob URLs on session change

### Dynamic Waveform: Backend (`src-tauri/src/services/waveform_generator.rs`)
- Added `extract_waveform_range(file_path, start_time, end_time, peak_count)` function
- Uses `ffmpeg -ss {start} -t {duration}` with 44100 Hz sample rate for high-resolution extraction
- Computes peaks for just the specified time range

### Dynamic Waveform: Command (`src-tauri/src/commands/media_analysis.rs`)
- Added `extract_waveform_range` Tauri command
- Registered in `lib.rs`

### Dynamic Waveform: Frontend Binding (`src/lib/bindings/mediaAnalysis.ts`)
- Added `extractWaveformRange(filePath, startTime, endTime, peakCount)` binding

### Dynamic Waveform: Session State (`src/lib/stores/videoSession.svelte.ts`)
- Added `waveformDetailPeaks`, `waveformDetailStart`, `waveformDetailEnd` to session state
- Increased initial waveform extraction from 8,000 to 50,000 peaks (good for most zoom levels)
- Detail fields cleared on session reset

### Dynamic Waveform: Renderer (`src/lib/timeline/waveformRenderer.ts`)
- Refactored `drawWaveform` to accept a `WaveformData` object with both overview and detail peaks
- Renderer automatically uses detail peaks when they cover the visible viewport
- Falls back to overview peaks when detail is not available

### Dynamic Waveform: Timeline Component (`src/lib/components/Timeline.svelte`)
- Added auto-fetch `$effect` that monitors zoom level and viewport
- When overview peaks per pixel drops below 2, triggers a 300ms-debounced detail extraction
- Detail extraction pads visible range by 50% on each side to avoid re-fetching on small pans
- Requests 4 peaks per pixel for crisp detail
- Passes `WaveformData` to both main timeline and minimap renderers

### Renderer Types Updated (`src/lib/timeline/renderer.ts`)
- `drawTimeline` now accepts `WaveformData` instead of `number[]`

## Lessons Learned
- WKWebView's custom protocol handlers (like Tauri's `https://asset.localhost/`) can have subtle audio issues even when video plays fine. Blob URLs are a reliable workaround.
- For waveform LOD (level of detail), a two-tier approach (high-count overview + on-demand range extraction) provides the best UX: fast initial display with detail on demand.
- Debouncing the detail waveform fetch prevents spamming ffmpeg during rapid zoom/pan.
- Padding the extraction range by 50% on each side significantly reduces re-fetch frequency during small viewport adjustments.

## Follow-Up Items
- Investigate if Tauri's asset protocol can be configured to properly serve audio (may be a wry bug)
- Consider pre-computing multiple LOD levels for the waveform instead of on-demand extraction
- The blob approach loads the entire preview file into memory (~97MB for a 54-min video at 360p) — acceptable for desktop but may need optimization for very long videos