lektor-ai/docs/architecture.md

# Architecture

How `lektor_ai_comfy.py` turns subtitles into a finished video, and why each
stage works the way it does. Most of the non-obvious choices here exist because
the straightforward version was tried first and broke.

```
SRT / embedded track
   │  parse, clean, split dialogue
   ▼
subtitle blocks ──────────────► output/tts_input.srt
   │  one HTTP job per block
   ▼
ComfyUI + Higgs Audio V3
   │  per-clip atempo + fade
   ▼
clips/clip_NNNN.flac
   │  adelay + amix, batched
   ▼
tts_assembled.flac
   │  pass 1: mix with original audio
   │  pass 2: stream-copy mux
   ▼
{stem}_lektor.mkv
```

## 1. Subtitle parsing

`parse_subtitle_blocks()` reads the SRT with `utf-8-sig` (tolerating a BOM),
splits on blank lines, and keeps blocks whose start falls inside the requested
range. Times become **relative to `--start`**, which is why a later `--rebuild`
must repeat the same `--start`/`--end`.

`_clean_text()` then normalises each block, in this order:

1. **Strip formatting tags.** `{...}` ASS overrides (`{\an8}`, `{\pos(x,y)}`)
   and `<...>` HTML tags (`<i>`, `<font …>`).
2. **Detect dialogue.** Lines beginning `- ` mean two speakers and are kept
   separate; ordinary wrapped lines are joined with a space.
3. **Sentence-case ALL-CAPS blocks**, which the model would otherwise shout.
4. **Capitalise a lowercase first letter**, since a fragment continuing from the
   previous subtitle confuses the model.
5. **Add terminal punctuation** so the model produces a natural falling tail.

Step 1 must come first. `<i>ALL CAPS</i>` looks mixed-case while the tag is
still present — the `i` in `<i>` is a lowercase letter — so the caps detection
in step 3 would silently miss it.

`split_dialogue_blocks()` then divides a two-speaker block into one clip per
line, apportioning the block's duration by character count.

The result is written to `output/tts_input.srt`. That file is the source of
truth for block numbering: block *N* there is `clips/clip_NNNN.flac` and the
argument to `--regen N`.

## 2. Speech generation

For each block the script POSTs a workflow graph to `/prompt`, polls
`/history/{id}` every 3 s, and reads the resulting file directly from ComfyUI's
output directory.

The graph has five nodes:

| Node | Class | Role |
|---|---|---|
| 1 | `HiggsAudioV3EngineNode` | Model, temperature, top_p 0.95, top_k 50 |
| 2 | `LoadAudio` | Voice reference WAV from ComfyUI's input dir |
| 3 | `CharacterVoicesNode` | Pairs that audio with its transcript |
| 4 | `UnifiedTTSTextNode` | The text, the seed, `enable_audio_cache: False` |
| 5 | `SaveAudio` | Writes FLAC |

Three details matter:

- **`enable_audio_cache: False`** is essential. With caching on, ComfyUI returns
  a previous result for repeated text and every `--regen` is a no-op.
- **The seed is `base_seed + block_index`**, so a fixed `--seed` is fully
  reproducible while each clip still differs.
- **No language tag is sent.** Higgs Audio V3 detects Polish from the text
  itself; its tokenizer has no `<|lang:pl|>` token, and a `[pl:]` prefix is
  stripped by the character parser before it reaches the model. A `lang: en`
  line in the ComfyUI log is cosmetic.

Each finished clip is optionally head-trimmed (`--strip-leading-ms`), then
sped up with `atempo` and given a short fade-out — **per clip, before
assembly**. Applying `atempo` to the assembled track instead would speed up the
silences between clips and shift every subsequent timestamp.

## 3. Placement

Clips are positioned with a push-forward cursor:

```python
actual_start = max(t_start, cursor)
cursor       = actual_start + duration
```

A clip starts at its subtitle time, or when the previous clip finished if that
is later. Clips are **never trimmed** to their subtitle window — cutting speech
mid-sentence sounds far worse than running a little long. Overrun cascades
forward and then resolves by itself at the next natural gap in the subtitles,
because a gap lets the cursor fall back below the next `t_start`.

The console prints `[+1.20s behind SRT]` when drift occurs. Steady growth means
`--speed` is too low for the material.

## 4. Assembly

Each clip becomes an `adelay` filter input and everything is mixed with `amix`:

```
[0:a]adelay=1340:all=1[a0]; [1:a]adelay=4820:all=1[a1]; …
[a0][a1]…amix=inputs=N:duration=longest:normalize=0[out]
```

`normalize=0` is important — `amix` otherwise divides by the input count, which
would make every clip inaudibly quiet across hundreds of inputs.

Above 500 clips the work is **batched**, because one ffmpeg process cannot hold
thousands of open inputs. Each batch is assembled to its own temporary FLAC and
the batches are then mixed together.

Batches use **relative** delays: the first clip's start time is subtracted from
every clip in the batch, and re-applied when the batches are combined. Without
that subtraction, batch 7 would begin with 40 minutes of silence, wasting time
and making the progress bar useless.

## 5. Muxing

`merge_voiceover()` runs **two separate ffmpeg passes**. This is deliberate and
should not be collapsed back into one.

### Pass 1 — build the lektor mix

```
[0:a:0]<norm>,aformat=channel_layouts=stereo,volume=<orig>[orig_mix];
[1:a]aformat=channel_layouts=stereo,volume=<tts>[tts];
[orig_mix][tts]amix=inputs=2:duration=first:normalize=0[lektor]
```

Written to a temporary `*_mix.m4a`, AAC 192k, and deleted in a `finally` block.

**Stereo, not 5.1.** ffmpeg's AAC encoder does not signal a 5.1 channel layout —
the track comes back tagged `unknown` in every container tested, and players
that cannot route the channels play silence. Stereo is tagged correctly, plays
everywhere, and spends the 192 kbit/s on two channels instead of six. Track 0
keeps the untouched 5.1, so no surround information is lost.

**PTS normalisation.** MKV rips often start their audio at a non-zero PTS (one
source here begins at 4.043 s). `amix` cannot emit output before both inputs
have frames, so the voiceover would be delayed by that offset. When ffprobe
reports an offset the filter becomes:

```
asetpts=PTS-STARTPTS,adelay=<offset_ms>:all=1,aresample=async=1
```

`asetpts` rebases the stream to zero and `adelay` reinserts the offset as real
silence, so the background audio stays where it belongs while `amix` receives
frames from both inputs starting at zero.

`duration=first` with `orig_mix` listed first means the mix runs as long as the
film, not as long as the voiceover.

### Pass 2 — stream-copy mux

Video, original audio, the mix, and every subtitle track are copied into the
final MKV with `-c copy`. Nothing is re-encoded, and it runs at several hundred
times real time.

Two flags carry the weight here:

**`-max_interleave_delta 0`**, placed **after the maps and before the output
path**. It is an output-muxer option; before `-i` it silently configures the
demuxer and does nothing. Without it, sparse subtitle tracks make the muxer
abandon interleaving after its default 10-second window, and the lektor audio
ends up written hundreds of megabytes away from the video it belongs with —
measurably fine to ffmpeg, but silent in any player that reads sequentially.

**MKV output**, not MP4. MKV copies subtitles of any codec without conversion,
carries DTS and TrueHD in track 0, and avoids the `mov_text` conversion that
made large multi-subtitle files abort mid-mux.

### Why two passes

Doing the filtering and the subtitle muxing in one command fails on sources with
many subtitle tracks. The sparse subtitle streams stall the muxer while the
audio filter graph keeps producing, and ffmpeg aborts partway through with:

```
Error while filtering: Invalid argument
Failed to inject frame into filter network: Invalid argument
```

The message points at audio, but audio is not the cause. Splitting the work
keeps the filter graph free of subtitle streams and makes the final mux a pure
copy. It is also about ten times faster.

## Progress reporting

`_run_progress()` adds `-progress pipe:1 -nostats` before the output path, reads
`out_time_us=` lines from stdout, and rewrites a single line with `\r`. A daemon
thread drains stderr concurrently, so ffmpeg cannot deadlock on a full stderr
pipe while the main thread reads stdout.

## Design rules

Worth preserving, since each was learned by breaking it:

1. **Per-clip processing before assembly**, never on the assembled track.
2. **Never trim clips** to the subtitle window.
3. **Keep every clip file** — `--regen` depends on them.
4. **FLAC throughout**, encoding only the final voiceover track.
5. **Two audio tracks**, with the original stream-copied and untouched.
6. **Verify muxed output by byte position**, not only by volume. A track can
   measure perfectly and still be unplayable.