filmcraft/crates/captions/README.md

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 <v Speaker> 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 carrying caption data, omitting empty frames, 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 <i> / <u>, extension blocks for long subtitles, start-of-programme (TCP) subtracted.
TTML .ttml (IMSC1)	yes	yes	Writes the IMSC1 Text profile (TTML2 namespaces): <p> per cue, <br/>, styled <span>s for <i> / <b> / <u>, 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 Ticks. 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 tick timing

When ttp:tickRate is omitted and ttp:frameRate is present, tick offsets use the effective frame rate (frameRate times frameRateMultiplier) times subFrameRate. The rate stays rational, so a 30-tick offset at 30 times 1000/1001 fps is exactly 1.001 seconds. An explicit tickRate takes precedence; without frameRate, the default is one tick per second. This follows W3C TTML1, third edition (2018), §6.2.10.

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. The April 2006 TTaf1 draft, §§7.2.1 and 12.1.5–12.2.1 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.

MCC writer

The writer merges the scheduled CEA-608 and CEA-708 frames in timestamp order. Frames without caption data are omitted, including during a displayed caption; its explicit erase/hide command still lands at its out point. CDP sequence counters advance per emitted packet, wrapping at 16 bits. Export work and output size therefore follow the scheduled caption data, not the time between cues or their duration (#2811). Empty-frame insertion is an optional MCC export behavior; see EZTitles' MCC export documentation, "Write empty frames". FilmCraft does not offer continuous padding for consumers that require a packet on every video frame.

Regression tests cover sparse output, both caption streams individually and together, drop-frame and non-drop timecodes, exact cue round trips, and timestamps near the tick storage limit. An external FFmpeg 9.0.2 decode of the two-cue regression fixture preserved text and all four timestamps (2.002–4.004 s and 120.120–122.122 s); the combined-stream export has 29 data lines, down from 3,614. No media fixtures are committed.

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/it/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.