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cottongin 8ad2f1c800 chore: stage all pending work — caption styling, media server, processing modal, docs, summaries

Includes:
- Extended caption styling (font, shadow, dimmed color, bg toggle)
- Media server, subtitle downloader, VTT parser, processing modal
- Waveform tiers, thumbnail/timeline improvements, transport controls
- Hybrid download model, dependency management, clip export enhancements
- 21 chat summaries, 2 implementation plans, 2 design specs

Co-authored-by: Cursor <cursoragent@cursor.com>

2026-09-22 10:48:16 -04:00

3.1 KiB

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Karaoke Burn-In for Exported Subtitles

Date: 2026-09-21 23:11
Task: Convert word-timed YouTube VTT captions to ASS format with karaoke \kf tags so burned-in subtitles have the same word-by-word highlight effect as the preview.

Changes Made

1. VTT-to-ASS Converter with Karaoke Tags

src-tauri/src/services/clip_exporter.rs — Added several new functions:

vtt_to_ass_with_karaoke() — Main converter. Reads raw VTT, generates a complete ASS file with:
- [Script Info] header (1920x1080 play resolution)
- [V4+ Styles] section with the user's CaptionStyle baked in (PrimaryColour, SecondaryColour at 60% alpha for the "dim/upcoming" look, BackColour, Outline, Shadow, Alignment, BorderStyle=4)
- [Events] section where each cue is a Dialogue line with \kf<centiseconds> tags for word-level karaoke fill animation
- Graceful fallback: cues without word-level timestamps render as plain text (no \kf)
parse_vtt_word_timings() — Rust equivalent of the TypeScript parseWordTimings(). Extracts (word, start_time) pairs from YouTube's <timestamp><c> word</c> format.
format_ass_timestamp() — Formats seconds as ASS timestamp H:MM:SS.cc (centiseconds).
hex_to_ass_color_with_alpha() — Like hex_to_ass_color() but with a specific alpha byte.
find_timestamp_tag_pos() / find_next_timestamp() — Helpers for parsing VTT timestamp tags without regex.

2. Updated Burn-In Export Path

In export_single_clip_with_subs(), the burn-in path now uses:

VTT → vtt_to_ass_with_karaoke() → .ass file → subtitles= filter

Instead of the previous:

VTT → sanitize_vtt_for_ffmpeg() → plain VTT → subtitles= filter

Since the style is now embedded in the ASS file header, caption_style is passed as None to build_ffmpeg_args_burnin_subs() (no force_style needed — avoids potential conflicts between the ASS header and force_style overrides).

3. Updated UI Note

src/lib/components/ExportDialog.svelte — Changed the burn-in note from "word-by-word highlighting is preview-only" to "Captions will be burned into the video with your style settings".

Files Modified

File	Change
`src-tauri/src/services/clip_exporter.rs`	Added VTT-to-ASS converter, word timing parser, ASS timestamp formatter, updated burn-in path
`src/lib/components/ExportDialog.svelte`	Updated burn-in note text

Build Verification

cargo check — passes (only pre-existing warnings)
cargo test — all 43 tests pass

Technical Notes

ASS karaoke \kf<N> means "fill this word over N centiseconds", transitioning from SecondaryColour to PrimaryColour. This matches the preview behavior where upcoming words are dimmed and spoken words are bright.
The ASS SecondaryColour is set to the same text color with 0x99 alpha (~60% transparent), matching the preview's opacity: 0.4 for upcoming words.
No regex crate was needed — the VTT word timing pattern is simple enough to parse with manual string operations.
The subtitles ffmpeg filter handles ASS files natively (same libass backend), so no filter change was needed.

3.1 KiB Raw Blame History