# Paged Teleprompter Burn-In Subtitle Design

**Date:** 2026-09-22  
**Scope:** Rewrite the ASS subtitle generation in `clip_exporter.rs` to use a paged teleprompter model instead of the current per-line rolling model.

## Problem

The current rolling subtitle implementation generates 3 independent ASS Dialogue events per spoken line (Active → Context → Disappear), each with `\move` animations. When a new line arrives, both the new and previous lines scroll simultaneously, creating a jarring "2-line block jump" instead of a natural reading flow. The active line always resets to the bottom position, breaking the top-to-bottom reading direction users expect.

## Design

### Core Model: 2-Line Paged Blocks

Group spoken lines into **pages of 2 lines each**. Each page is a **single ASS Dialogue event** containing both lines separated by `\N` (ASS hard line break). Karaoke `\k` tags flow continuously from line 1 through line 2 within the same Dialogue event.

**Reading flow:** The viewer reads the top line (karaoke highlighting progresses left-to-right), then naturally drops to the bottom line (karaoke continues), then a cross-fade transitions to the next page.

### Page Construction

Given a sequence of `SpokenLine` structs extracted from the VTT:

1. Pair lines into consecutive groups of 2: `[line0, line1]`, `[line2, line3]`, …
2. If the gap between two lines within a page exceeds 2 seconds, split them into separate pages instead of combining.
3. If the total line count is odd, the final page contains a single line.

### Karaoke Tag Stitching

For a 2-line page with lines A and B, the Dialogue text is:

```
{\k<dur>}word1_A {\k<dur>}word2_A ... {\k<dur>}lastword_A\N{\k<dur>}word1_B ... {\k<dur>}lastword_B
```

The `\k` duration for each word equals the time from that word's VTT start to the next word's VTT start. For the **last word of line A**, its `\k` duration extends to the start of line B's first word — this naturally covers any gap (silence/pause) between the two lines without any special gap-filler logic.

The `\N` is purely a visual line break and does not interrupt the karaoke timeline.

For lines without `<c>` word timing tags, the entire line text gets a single `\k` equal to the line's full spoken duration.

### Cross-Fade Transitions

Pages transition via a 300ms cross-dissolve:

- **Outgoing page:** ASS end time extended by 300ms past its natural end. Uses `\fad(*, 300)` for a 300ms fade-out.
- **Incoming page:** ASS start time moved 300ms before its natural start. Uses `\fad(300, *)` for a 300ms fade-in.
- The 300ms overlap produces a smooth cross-dissolve between pages.

Specific `\fad` values:
| Page Position | `\fad` value |
|---|---|
| First page | `\fad(0, 300)` |
| Middle pages | `\fad(300, 300)` |
| Last page | `\fad(300, 0)` |

When pages are separated by a long gap (>2s), the outgoing page fades out and the incoming page fades in independently — no visual overlap, just a clean silence gap.

### Positioning

Each Dialogue uses `\an2\pos(cx, y_bottom)` — bottom-center anchor. With `\an2`, the 2-line `\N` block renders with line 2 at the anchor point and line 1 stacked above it. **No `\move` tags are used.** Pages are static in position; transitions are opacity-only via `\fad`.

Layout constants (matching existing style):
- `cx = PlayResX / 2 = 960`
- `y_bottom = PlayResY - margin_v = 1080 - 40 = 1040`

### Edge Cases

| Case | Handling |
|---|---|
| Odd number of lines | Last page has 1 line (single-line Dialogue, no `\N`) |
| Gap >2s between consecutive lines within a pair | Split into separate single-line pages |
| Non-speech cues (`[Music]`, `[Applause]`) | Standalone single-line Dialogue, no karaoke, just `\pos` |
| Lines without `<c>` word timing | Single `\k` tag covering the line's full spoken duration |
| Only 1 spoken line total | Single-line Dialogue with `\fad(0, 0)` |

### ASS Header and Style

Unchanged from the current implementation:
- `PlayResX: 1920`, `PlayResY: 1080`, `ScaledBorderAndShadow: yes`
- Style parameters (font size, colours, border style, outline, margin) derived from user's `CaptionStyle` settings
- `Alignment: 2` (bottom-center) in style definition, reinforced with `\an2` override in each Dialogue

### What Gets Removed

The following components of the current implementation are replaced entirely:
- `RollingLayout` struct (position calculation for `\move` animations)
- Per-line 3-phase Dialogue generation (Active/Context/Disappear)
- All `\move` tags
- All `\clip` tags
- Phase-based timing calculations

### What Gets Added

- `build_page_karaoke_text(lines: &[&SpokenLine]) -> String` — stitches word timings from 1-2 lines into a single karaoke text with `\N` separator
- Page grouping logic in `vtt_to_ass_with_karaoke()` — pairs lines, handles gap-based splitting
- Cross-fade timing logic — computes `\fad` and adjusted start/end times per page

### What Stays the Same

- `SpokenLine` struct and `extract_spoken_lines()` — line extraction from VTT is unchanged
- `build_karaoke_text()` — per-line word timing extraction (reused internally by the new page builder)
- `parse_vtt_word_timings()` — VTT `<c>` tag parser
- ASS header/style generation
- `build_ffmpeg_args_burnin_subs()` — FFmpeg argument construction (unchanged, still uses `ass=` filter)
- All other export logic (lossless, muxed subtitles, progress reporting)

## Files Changed

| File | Change |
|---|---|
| `src-tauri/src/services/clip_exporter.rs` | Rewrite `vtt_to_ass_with_karaoke()` to use paged model. Remove `RollingLayout`. Add `build_page_karaoke_text()`. |

Single file change. No frontend, no new dependencies.