# filmcraft-captions Caption files and caption burn-in for FilmCraft (layer L2: depends on `filmcraft-time`, `filmcraft-color`, `filmcraft-project`, `filmcraft-text` and `roxmltree` for TTML). The caption *model* (caption tracks in a sequence, captions, track style) lives in `filmcraft-project` (`caption.rs`) because it is part of the `.fcproj` document. This crate reads and writes caption files, converts them to and from caption tracks, and draws captions. ## Formats | Format | Read | Write | Notes | |---|---|---|---| | SubRip `.srt` | yes | yes | BOM, UTF-16, Windows-1252 fallback, CRLF/CR, missing or wrong indexes, `,` or `.` milliseconds, 1–9 fraction digits, missing hours, SSA coordinates, cues not separated by blank lines. Inline tags are kept as text. | | WebVTT `.vtt` | yes | yes | Cue identifiers, cue settings (verbatim), a leading `` voice span (as the caption's speaker) and STYLE / REGION / NOTE blocks are kept. A missing `WEBVTT` signature is tolerated. | | Scenarist SCC `.scc` | yes | yes | CEA-608 at 29.97 fps, drop-frame or not. Reads pop-on, roll-up and paint-on (channel 1); writes pop-on. | | MacCaption MCC `.mcc` | yes | yes | One SMPTE ST 334-2 caption distribution packet (CDP) per frame in MacCaption's hex text with letter abbreviations. Writes 29.97 fps (`30DF` / `30`) with CEA-608 field 1 (the SCC schedule) **and** CEA-708 service 1 (two-window pop-on emulation). Reads CEA-608 field 1 when present, else CEA-708 service 1; time code rates 30DF, 30, 60DF, 60, 25, 24, 50. | | EBU STL `.stl` | yes | yes | EBU Tech 3264-E: GSI block + 128-byte TTI blocks, `STL25.01` (25 fps) or `STL30.01` (read/written as 29.97 fps, non-drop labels), ISO/IEC 6937 Latin text (accents as diacritic + letter), italics / underline codes as `` / ``, extension blocks for long subtitles, start-of-programme (TCP) subtracted. | | TTML `.ttml` (IMSC1) | yes | yes | Writes the IMSC1 Text profile (TTML2 namespaces): `

` per cue, `
`, styled ``s for `` / `` / ``, `ttm:agent` speakers, exact frame times (`123f`) at the sequence rate. Reads any TTML (TTML1, TTML2, the 2006 `ttaf1` drafts): clock / frame / offset / tick times, `dur`, nested time containers (`body` / `div` begins offset and ends clip their contents; `seq` containers are read as `par` with a warning), timed spans, referenced styles. | | DFXP `.dfxp` | yes | yes | TTML1 with the DFXP presentation profile; clock times in milliseconds (frame-exact after `snap_to_frames`). | All cue times are exact `Tick`s. Milliseconds convert exactly (1 ms = 254 016 000 ticks); SCC frames map through `FrameRate::FPS_29_97`. `Document::snap_to_frames` snaps cues to a sequence's frame grid (the engine does this on import) and removes overlaps. ### TTML speaker metadata TTML and DFXP exports define each distinct speaker label once in the document head and refer to it with a stable, unique `ttm:agent` ID. Names are XML-escaped full-name text with preserved whitespace; punctuation, Unicode and literal underscores survive a round trip. Agent type is `other` because a caption's speaker label does not say whether it denotes a person or a group. On import, agent definitions directly under `head` or `head/metadata` resolve paragraph and timed span speakers. Speaker metadata uses the standard TTML namespace or the April and October 2006 TTaf1 draft namespaces. Agent IDs use `xml:id`, except in the April draft, which uses unqualified `id`. Agent definitions from unrelated application metadata are ignored. Recognized `ttm:agent` attributes take precedence over the legacy local-name `agent` fallback. A full name is preferred, otherwise the first nonempty name (or an explicitly empty name) is used. Multiple agent references become one display label, joined with `, ` in reference order; the project does not retain the individual agent identities. Older FilmCraft files without agent definitions remain readable through the previous underscore-to-space fallback. Other agent metadata, including actor relationships, is not retained. This mapping follows [W3C TTML1, third edition (2018), §§12.1.5–12.2.1](https://www.w3.org/TR/2018/REC-ttml1-20181108/#metadata-vocabulary-agent). The [April 2006 TTaf1 draft, §§7.2.1 and 12.1.5–12.2.1](https://www.w3.org/TR/2006/WD-ttaf1-dfxp-20060427/) defines its earlier metadata namespace and agent ID form. ### SCC writer Each caption is sent as a pop-on load (RCL, ENM, then per row a preamble address code and tab offset for centring, then text; control codes doubled) into non-displayed memory, followed by End Of Caption exactly on the caption's first frame. The load is placed in the latest run of free frames before the caption, so back-to-back captions load while the previous one is on screen. An Erase Displayed Memory clears the caption at its out point unless the next caption replaces it. Text is wrapped at 32 columns, bottom-aligned (last row 15) and centred. Characters come from the 608 basic, special and extended sets (extended characters are preceded by a basic fallback character, as the standard requires); others are dropped. If there is no room to load a caption before its in point (for example a caption at 00:00:00:00), it appears as soon as it is loaded. ## Burn-in `burn::render_caption` lays out a caption with the track style (font size relative to a 1080-line frame, colour, background box, outline, alignment, top/middle/bottom anchor and margin; WebVTT `line:N%` and `align:` settings override them), wraps lines to 90% of the frame width and rasterises it to a small premultiplied linear-light RGBA overlay. `filmcraft-render` composites the overlays of visible caption tracks over the finished frame (Program monitor and export burn-in). Text is set in **Inter SemiBold** (`assets/fonts/Inter-SemiBold.ttf`, SIL OFL 1.1, attributed in `ATTRIBUTION.md`) by the `filmcraft-text` engine: shaped with kerning and ligatures, bidi-ordered, and drawn from its sub-pixel positioned glyph cache. Specifications used: SMPTE ST 334-2 (CDP structure), CEA-608 / CEA-708 (public descriptions of the caption commands), the MacCaption MCC format description (line syntax and abbreviations), EBU Tech 3264-E (1991), ISO/IEC 6937, W3C TTML1 (2nd ed.), TTML2 and IMSC 1.1. No other implementation's code was consulted. ## Tests - Unit tests per module (timestamps, decoding, sloppy SRT, WebVTT blocks and voices, CEA-608 tables, PAC round trips, known SCC pop-on and roll-up streams, layout). - `tests/roundtrip.rs`: property tests — SRT (also with BOM + CRLF), WebVTT (ids, speakers, settings, STYLE blocks), SCC and MCC (drop-frame and non-drop, gaps and back-to-back captions; MCC also through CEA-708 alone), EBU STL (25 and 29.97 fps, accents, extension blocks), TTML (23.976 / 25 / 29.97 / 59.94, ids, speakers) round-trip frame-exactly; DFXP round-trips after snapping to frames; readers never panic on random bytes (also a valid STL header followed by random blocks, and random MCC lines); SRT → VTT → SCC keeps text and frames.