craft-libs/crates/captions/README.md

craft-captions

Caption and subtitle files behind one neutral model: bytes in, a Document of Cues out, and back. One module per format. Depends on craft-timeline (exact time), craft-xml (TTML) and thiserror.

use craft_captions::{Format, WriteOptions, detect, parse, write};

let bytes = b"1\n00:00:01,000 --> 00:00:02,500\nHello\n";
let format = detect(bytes, Some("srt")).ok_or(craft_captions::Error::NotFormat("caption"))?;
let mut doc = parse(bytes, format)?;
doc.snap_to_frames(craft_captions::FrameRate::FPS_25);
let scc = write(&doc, Format::Scc, WriteOptions::default());
assert!(scc.starts_with(b"Scenarist_SCC V1.0"));
# Ok::<(), craft_captions::Error>(())

• detect(bytes, extension) guesses the format; parse(bytes, format) reads; write(doc, format, options) writes; charset_warnings says what an SCC or MCC export cannot carry.
• Cue: start and end (Tick), text with inline <i> / <b> / <u> tags kept, and the WebVTT speaker, cue identifier and cue settings. Document: the cues, WebVTT STYLE / REGION / NOTE blocks, the language and the reader's warnings.
• cea608 has the CEA-608 character sets, control codes, parity and preamble address codes; scc::decode_pairs turns timed 608 byte pairs (from a file, or from a video stream's caption data) into cues, and scc::schedule does the reverse. mcc has the CEA-708 service coding and the SMPTE ST 334-2 caption distribution packet.
• plain_text strips inline tags and decodes the common entities, for display or for formats without markup.

Time

Tick is an integer count of 1/254 016 000 000 s: a whole number of ticks per millisecond and per frame at every broadcast rate, so no format's times are rounded on the way in. FrameRate is an exact rational. Both are craft-timeline's (craft_timeline::time), re-exported here as craft_captions::time, so a caller that already uses that crate passes its values straight in. No format computes with floating-point seconds.

Limits

parse refuses input over MAX_INPUT (64 MiB) and keeps at most MAX_CUES (1,000,000) cues, with a warning. TTML is read by craft-xml: no DTD processing or entity expansion, at most 128 levels of nesting, and its node and attribute limits. A TTML speaker label is at most 4096 bytes from at most 64 agent references; a language tag at most 35 characters. charset_warnings lists at most 20 characters per stream.

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 (or on every frame, on request), 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) with the document's language as xml:lang (read back on import): <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 frame grid and removes overlaps. SCC and MCC time codes count from Document::start_timecode, a time code label read as a time (label_time), so the label carries across rates: 01:00:00:00 at 25 fps is written as 01:00:00;00 drop-frame or 01:00:00:00 non-drop (label_frame). Readers leave it at zero and keep the file's own time codes, noting the rate and drop-frame mode of its labels in Document::labels; Document::count_from then counts the cues back from a start time code, when every cue is at or after it.

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 individual agent identities are not kept. Files that refer to an agent without defining it read the reference itself as the label, with underscores as spaces. 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.

TTML cue identifiers

TTML and DFXP writers keep valid explicit cue IDs verbatim unless a style, region or earlier cue already uses them. Missing, invalid or repeated IDs receive deterministic replacements that also avoid valid IDs on later cues. Emitted cue, style, region and speaker IDs are unique across the document. Unicode spelling is preserved without normalization.

Identifier validation follows the NCName production in Namespaces in XML 1.0, third edition (2009), §3, using the character ranges in XML 1.0, fifth edition (2008), §2.3.

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; text past 15 rows is cut. Characters come from the 608 basic, special and extended sets (extended characters are preceded by a basic fallback character, as the standard requires); a few have a close stand-in (’ as ', – as -) and others are dropped. charset_warnings names each dropped or replaced character and each caption cut to 15 rows, with how many characters it lost. 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. CEA-708 text uses the G0 and G1 (Latin-1) sets and part of G2 (… ‘ ’ “ ” • ™ and a few more); other characters are sent as ?, and charset_warnings names them. The CEA-708 window holds 12 rows and the writer loads only those (mcc::window_rows), so a longer caption loses its last rows there, and charset_warnings names it too. Export work and output size therefore follow the scheduled caption data, not the time between cues or their duration.

For consumers that need a packet on every video frame (SDI, frame-wrapped MXF with a SMPTE 436M track), WriteOptions::every_frame_until also writes one on every frame without caption data, from the start to that time: CEA-608 null pairs and CEA-708 padding. The packets are 29.97 fps, so they line up with the video only at that rate; the caller decides whether to offer it. It is off by default, covers at most 24 hours (mcc::MAX_PADDED_FRAMES), and can be streamed (write_to).

EBU STL writer

Text is encoded in ISO/IEC 6937 (character code table 00). Characters it has no code for, such as the ellipsis, the em dash, the euro sign and most scripts other than Latin, are written as ?; charset_warnings names each one with how often it occurs and the first caption that has it, at the STL file's own time code.

TTML inherited text styles

On import, italic, bold and underline styles from <body>, <p> and timed <span> roots appear in the cue text as balanced <i>, <b> and <u> tags. Descendant overrides and explicit normal / noUnderline resets are kept, and inherit restores the parent value. Styling attributes are read from the TTML styling namespaces (and those of the 2006 drafts), with unqualified attributes as a compatibility fallback; other namespaces are ignored. Named-style chaining and xml:space="preserve" are not supported. The inline text collector does not recurse. Content style inheritance follows W3C TTML1, second edition (2013).

Literal text and style markup

Cue text combines style tags with character entities, as WebVTT does. The TTML and EBU STL readers keep a literal < as < (and a literal & that would read as an entity as &), so it never turns into a tag. The TTML and STL writers decode the eight entities entity_at knows: &, <, >,  , ", ', ‎ and ‏; other named entities and numeric references stay literal. The SRT writer keeps stored entities next to real style tags, so a reimport does not turn a literal bracket into markup.