# SVT-AV1 Plugin Bitrate Feature Analysis

## Current State

### Existing Bitrate Control

The plugin currently implements bitrate control via the `maxrate_cap` dropdown input:

- **Type**: Dropdown with 11 preset values
- **Options**: `0` (unlimited), `2000`, `3000`, `4000`, `5000`, `6000`, `8000`, `10000`, `12000`, `15000`, `20000` kbps
- **Default**: `0*` (unlimited)
- **Implementation**: Lines 531-540 in the plugin
- **Behavior**: When set to non-zero, applies `-maxrate` and `-bufsize` (2x maxrate) to FFmpeg command

### Current Logic Flow

```mermaid
graph TD
    A[Start] --> B{maxrate_cap != 0?}
    B -->|Yes| C[Apply maxrate + bufsize<br/>Buffer = 2.0x maxrate]
    B -->|No| D[Uncapped CRF<br/>No bitrate limit]
    C --> E[Build FFmpeg Command]
    D --> E
```

## Requirements from Agent Notes

From [implementation_plan.md](file:///home/user/Public/Projects/tdarr_plugs/Local/agent_notes/implementation_plan.md):

1. **Custom Maxrate**: Allow users to manually type a specific kbps value (not limited to dropdown presets)
2. **Source-Relative Bitrate**: Allow setting bitrate cap relative to source file bitrate
   - Options: `match_source`, `75%_source`, `50%_source`, `33%_source`, `25%_source`
3. **Logic Precedence**:
   - If strategy ≠ static → Calculate from source
   - Else if custom_maxrate > 0 → Use custom value
   - Else → Use maxrate_cap dropdown

## Design Decisions

### Input Design

**1. Custom Maxrate Input**
- **Type**: Text input (string)
- **Default**: Empty string `''`
- **Validation**: Parse as integer, check > 0, handle NaN gracefully
- **Position**: After `maxrate_cap` dropdown

**2. Target Bitrate Strategy Dropdown**
- **Type**: Dropdown
- **Options**: 6 choices
  - `static*` - Use custom_maxrate or maxrate_cap (default)
  - `match_source` - Match source bitrate (100%)
  - `75%_source` - 75% of source
  - `50%_source` - 50% of source
  - `33%_source` - 33% of source
  - `25%_source` - 25% of source
- **Position**: After `custom_maxrate` input

### Bitrate Detection Strategy

```mermaid
graph TD
    A[Start] --> B[Check videoStream.bit_rate]
    B -->|Available| C[Use video stream bitrate]
    B -->|Missing| D[Check format.bit_rate]
    D -->|Available| E[Use overall file bitrate]
    D -->|Missing| F[sourceBitrateKbps = null<br/>Log warning]
    C --> G[Convert bps to kbps]
    E --> G
    G --> H[sourceBitrateKbps ready]
    F --> I[Fall back to static mode]
```

**Key Details**:
- Primary source: `file.ffProbeData.streams[videoStreamIndex].bit_rate` (bps)
- Fallback: `file.ffProbeData.format.bit_rate` (bps)
- Conversion: Divide by 1000 to get kbps
- Graceful failure: If neither available, log warning and use static mode

### Logic Precedence Implementation

```javascript
// Priority 1: target_bitrate_strategy (highest)
if (strategy !== 'static' && sourceBitrateKbps) {
  calculatedMaxrate = Math.round(sourceBitrateKbps * multiplier);
}

// Priority 2: custom_maxrate (middle)
if (!calculatedMaxrate && custom_maxrate !== '' && parseInt(custom_maxrate) > 0) {
  calculatedMaxrate = parseInt(custom_maxrate);
}

// Priority 3: maxrate_cap dropdown (lowest, existing)
if (!calculatedMaxrate && maxrate_cap !== '0') {
  calculatedMaxrate = parseInt(maxrate_cap);
}

// Priority 4: No limit (default)
if (!calculatedMaxrate) {
  // Uncapped CRF mode (existing behavior)
}
```

### Info Logging Strategy

Add clear logs to help users understand which bitrate method was used:

- **Strategy mode**: `"Using target bitrate strategy '50%_source': Source bitrate 10000k → Maxrate 5000k"`
- **Custom mode**: `"Using custom maxrate: 7500k"`
- **Dropdown mode**: `"Using maxrate cap from dropdown: 5000k"`
- **Fallback warning**: `"Warning: target_bitrate_strategy selected but source bitrate unavailable. Falling back to static mode."`

## Edge Cases to Handle

1. **Invalid custom_maxrate input**
   - Non-numeric strings → Ignore, fall through to dropdown
   - Negative numbers → Ignore, fall through to dropdown
   - Zero → Treat as empty, fall through to dropdown

2. **Missing source bitrate with strategy selected**
   - Log warning message
   - Fall back to custom_maxrate or maxrate_cap
   - Don't error/crash the plugin

3. **All inputs empty/zero**
   - Default to uncapped CRF mode (existing behavior)
   - No maxrate applied

4. **Conflicting inputs**
   - User sets both strategy and custom_maxrate
   - Strategy takes precedence (as designed)
   - Log which one was used

## Compatibility Considerations

- **Backward compatible**: Existing configurations continue to work
- **Default behavior**: `target_bitrate_strategy = 'static'` and `custom_maxrate = ''` → Original behavior
- **No breaking changes**: All new inputs have safe defaults
- **FFmpeg compatibility**: Uses existing `-maxrate` and `-bufsize` flags (no new FFmpeg requirements)