# Background Music Generation - Implementation Progress

## Task List Reference
- **Source**: `docs/BACKGROUND_MUSIC_PLAN.md`
- **Feature**: Add consistent background music using Google Vertex AI Lyria 2

## Progress Summary

| Stage | Description | Status |
|-------|-------------|--------|
| 1 | Music Data Models | ✅ Complete |
| 2 | Music Director Agent | ✅ Complete |
| 3 | Lyria 2 Music Generator | ✅ Complete |
| 4 | VEO Prompt Engineering | ✅ Complete |
| 5 | FFmpeg Audio Mixing | ✅ Complete |
| 6 | CLI Integration | ✅ Complete |

## Completed Tasks

### Stage 1: Music Data Models ✅
**Commit**: `1f402ea`

**Files Created**:
- `src/sip_videogen/models/music.py` - Music data models
- `tests/test_music_models.py` - Model tests (14 tests, all passing)

**Files Modified**:
- `src/sip_videogen/models/__init__.py` - Export new models

**Models Added**:
- `MusicMood` - Enum for emotional quality (upbeat, calm, dramatic, etc.)
- `MusicGenre` - Enum for music style (orchestral, electronic, ambient, etc.)
- `MusicBrief` - Output from Music Director agent with prompt, mood, genre, instruments
- `GeneratedMusic` - Metadata for generated music track

### Stage 2: Music Director Agent ✅
**Commit**: `80c6883`

**Files Created**:
- `src/sip_videogen/agents/music_director.py` - Agent implementation
- `src/sip_videogen/agents/prompts/music_director.md` - Agent prompt (detailed instructions for music design)
- `tests/test_music_director.py` - Agent tests (11 tests, all passing)

**Files Modified**:
- `src/sip_videogen/agents/showrunner.py` - Added `music_director` as tool
- `src/sip_videogen/agents/prompts/showrunner.md` - Added Step 5 for Music Director coordination

**Agent Features**:
- Analyzes script tone, pacing, and emotional arc
- Outputs structured `MusicBrief` with prompt, mood, genre, tempo, instruments
- Integrated into Showrunner workflow (called after continuity validation)
- Follows existing agent patterns (prompt file, output type, helper function)

### Stage 3: Lyria 2 Music Generator ✅
**Commit**: `aefc2c5`

**Files Created**:
- `src/sip_videogen/generators/music_generator.py` - Lyria 2 integration
- `tests/test_music_generator.py` - Generator tests (14 tests, all passing)

**Files Modified**:
- `src/sip_videogen/config/settings.py` - Added music settings
- `src/sip_videogen/generators/__init__.py` - Export MusicGenerator

**Generator Features**:
- `MusicGenerator` class using Google Vertex AI Lyria 2 API
- Uses Application Default Credentials (ADC) for authentication
- Generates ~30-second WAV clips at 48kHz stereo
- Supports negative prompts to exclude unwanted elements
- Optional seed parameter for reproducible generation
- Retry logic with exponential backoff (3 attempts)
- Comprehensive error handling (HTTP errors, network errors, auth failures)

**Settings Added**:
- `sip_enable_background_music` (bool, default: True) - Enable/disable music generation
- `sip_music_volume` (float, default: 0.2) - Background music volume (0.0-1.0)

### Stage 4: VEO Prompt Engineering ✅
**Commit**: `3f09c03`

**Files Created**:
- `tests/test_video_generator_audio.py` - Audio prompt tests (43 tests, all passing)

**Files Modified**:
- `src/sip_videogen/generators/video_generator.py` - Added audio instruction generation

**Key Features**:
- `_build_audio_instruction()` - Generates audio prompts for VEO
- `_infer_ambient_sounds()` - Detects setting-based sounds (beach, forest, city, etc.)
- `_infer_action_sounds()` - Detects action-based sounds (footsteps, typing, etc.)
- Modified `_build_prompt()` to include audio instructions by default
- Added `exclude_background_music` parameter for backward compatibility
- Audio instructions preserve dialogue/SFX while excluding background music

**Audio Instruction Format**:
```
Audio: [inferred sounds]. No background music, no soundtrack, no musical score
```

**Supported Environments**:
- Beach/ocean: waves crashing, seagulls
- Forest/nature: birds chirping, rustling leaves, wind
- City/urban: city traffic, distant sirens, urban ambience
- Indoor: room tone
- Sports/gym: sneakers squeaking, crowd noise
- Restaurant/cafe: clinking dishes, ambient chatter
- And more...

**Supported Actions**:
- Walking/running: footsteps
- Doors: door sounds
- Vehicles: car engine, bicycle sounds
- Typing: keyboard typing
- And more...

### Stage 5: FFmpeg Audio Mixing ✅
**Commit**: `2feaaae`

**Files Created**:
- `tests/test_ffmpeg_music.py` - FFmpeg music mixing tests (22 tests, all passing)

**Files Modified**:
- `src/sip_videogen/assembler/ffmpeg.py` - Added music mixing method

**Key Features**:
- `assemble_with_music()` method for mixing background music with video
- Concatenates video clips, then overlays looped background music
- Applies fade in/out effects for smooth music transitions
- Configurable music volume (default: 20% to not overpower dialogue)
- Slightly reduces video audio volume to make room for music
- Uses `-stream_loop -1` to loop music indefinitely
- Uses `-shortest` to stop when video ends
- Video stream is copied (no re-encode), audio is encoded as AAC
- Proper temp file cleanup on success and error

**FFmpeg Filter Complex**:
```
[1:a]afade=t=in:st=0:d={fade_duration},afade=t=out:st={fade_out_start}:d={fade_duration},volume={music_volume}[music];
[0:a]volume={video_audio_volume}[video_audio];
[video_audio][music]amix=inputs=2:duration=first:dropout_transition=2[audio_out]
```

### Stage 6: CLI Integration ✅
**Commit**: `b3d338a`

**Files Modified**:
- `src/sip_videogen/cli.py` - Add music flags and generation step
- `src/sip_videogen/models/script.py` - Add `music_brief` field to VideoScript
- `src/sip_videogen/models/agent_outputs.py` - Add `music_brief` field to ShowrunnerOutput
- `src/sip_videogen/agents/showrunner.py` - Transfer music_brief to VideoScript

**Key Features**:
- `--no-music` flag to disable background music generation
- `--music-volume` option (0.0-1.0) to control music volume
- `music_brief` field added to `VideoScript` model for music metadata
- `music_brief` field added to `ShowrunnerOutput` for agent output
- MusicGenerator integrated into CLI pipeline as Stage 6
- Dynamic stage numbering (7 stages with music, 6 without)
- Uses FFmpeg's `assemble_with_music()` for audio overlay
- Music info displayed in final summary
- Graceful fallback if music generation fails (continues without music)

**CLI Usage**:
```bash
sip-videogen generate "your idea"              # With background music (default)
sip-videogen generate "your idea" --no-music   # Without background music
sip-videogen generate "your idea" --music-volume 0.3  # Custom volume
```

## Feature Complete

All 6 stages of the Background Music Generation feature are now complete:

1. **Music Data Models** - Pydantic models for music metadata
2. **Music Director Agent** - AI agent to design music style
3. **Lyria 2 Music Generator** - Google Vertex AI integration
4. **VEO Prompt Engineering** - Exclude BG music from video generation
5. **FFmpeg Audio Mixing** - Overlay music onto final video
6. **CLI Integration** - User-facing commands and flags

## Testing Summary
- All 202 tests pass (excluding 3 pre-existing ImageGenerator failures)
- Music-specific tests: 61 tests across 4 test files
- Comprehensive coverage for models, agent, generator, and FFmpeg mixing

## Cost
- Music generation: $0.06 per ~30-second clip
- Single clip is looped for any video length

## Next Steps (Manual Testing)
1. Generate video with `--dry-run` to verify music brief in script
2. Generate short test video with music
3. Compare video quality with/without music
4. Test `--no-music` flag
5. Test different `--music-volume` levels
