craft-libs/crates/media/README.md

craft-media

Media containers and bit-level I/O for the Craft apps: an MSB-first bit reader and writer, clean-room ISO BMFF (MP4), QuickTime (MOV) and Matroska (MKV, WebM) demuxers and muxers, written from the public specifications. No unsafe, no panics on malformed input, builds for wasm32-unknown-unknown. Dependencies: thiserror (for isobmff), serde (optional).

Codecs go in separate crates that depend on this one: craft-audio for audio (in a follow-up), and a video codec crate later. What both need lives here: the containers, the bit I/O, one ByteSource trait, and the shared identifiers that codec crates use to describe a file and configure a decoder.

Features

Feature	Module	Enables
bitstream	craft_media::bitstream	none
isobmff	craft_media::isobmff	bitstream, thiserror
matroska	craft_media::matroska	none
(none)	Container, CodecId, CodecParams, PcmFormat, xiph_lace, ByteSource, ReadSeekSource	none
serde	Serialize/Deserialize on Container, CodecId, CodecParams and PcmFormat	serde

default = ["isobmff"], so bitstream and isobmff build. matroska is opt-in: it is about as much code as isobmff, and most users of MP4/MOV (LightCraft, FigCraft, SoundCraft, EffectCraft) do not read Matroska. The features are independent.

craft-media = { git = "https://github.com/storytold/craft-libs", rev = "<commit>", features = ["matroska"] }

A crate that needs only bit I/O (a codec) uses:

craft-media = { git = "https://github.com/storytold/craft-libs", rev = "<commit>", default-features = false, features = ["bitstream"] }

Boundaries

• In scope: bit-level reading and writing, and container demuxing and muxing (boxes, sample tables, sample entries and codec configuration records) for audio and video tracks.
• Out of scope: codecs. Audio codecs are in craft-audio; video codecs (H.264, HEVC, ProRes and others) stay in the apps until a video crate exists. Containers only FilmCraft reads (MXF, MPEG-TS/PS, AVI) are not here yet.
• Anything both the audio and the video codec crates need belongs here, so the two never depend on each other.

Origin and API

The code comes from FilmCraft's filmcraft-bitstream, filmcraft-isobmff and filmcraft-matroska crates at storytold/filmcraft@6c3948a. Public items keep their names and signatures, so FilmCraft can turn all three crates into re-exports (pub use craft_media::isobmff::*;), except for the renames below. Our own names use US spelling; names taken from a specification keep its spelling.

Renamed:

FilmCraft	craft-media
matroska::Colour	matroska::Color
matroska::VideoInfo::colour	matroska::VideoInfo::color
matroska::VideoInfo::colour_space	matroska::VideoInfo::color_space

The element IDs element_ids::COLOUR and COLOUR_SPACE and the field isobmff::VpcConfig::colour_primaries keep the spellings of the Matroska and VP9 specifications.

Output that differs on valid input:

• Matroska lacing applies only to tracks with a DefaultDuration, and a lace ends at a timestamp gap. Readers space the frames of a lace by DefaultDuration, so FilmCraft's laces moved frames that followed a gap.
• The sample budget covers the whole file, so a multi-track file with more than 2²⁶ samples in total fails with TooLarge (ISO BMFF: all tracks and fragments; Matroska: all tracks, and at most 2¹⁶ TrackEntries). FilmCraft capped ISO BMFF per track and Matroska not at all.
• MOV and MP4 PCM chunks end at 1 MiB, as ffmpeg writes them; FilmCraft wrote each contiguous run of one track as one chunk. The reader splits a PCM chunk above 16 MiB into samples of at most 16 MiB; FilmCraft made it one sample, cut at 4 GiB.
• MOV PCM at 65,536 Hz and above, and unsigned 16-bit PCM, is written as lpcm (sound description v2), and signed 8-bit PCM as twos. FilmCraft wrote 0 Hz for the former and labeled the signedness wrongly for the latter two. raw under a sound handler reads as PCM.
• Matroska BlockDuration is rounded to the nearest tick; FilmCraft truncated it.
• A Matroska TimestampScale above 2³² is ignored, like a zero scale.
• matroska::MkvWriter writes MuxingApp = craft-media, and MuxOptions::default() has writing_app = craft-media (FilmCraft: filmcraft-matroska and filmcraft). Apps set writing_app to their own name.

Other differences:

• bitstream::BitReader::read_ue_plus (from DeckCraft's copy) is new.
• isobmff::boxes is new: FilmCraft's private box iterator and byte reader, made public (see Box reader).
• BitReader::peek, BitReader::read_bits and BitWriter::write_bits accept widths above 32 without a debug assertion: peek and read_bits return the low 32 bits of the field (use read_bits_u64 for wide fields), and write_bits writes leading zero bits, at most 64 bits in all.
• bitstream::length_prefixed_nals returns the new BitError::InvalidNalLengthSize for a length_size outside 1 to 4 (as FilmCraft does since filmcraft#2953) and cannot overflow on a hostile length prefix.
• isobmff::PcmConfig::bytes_per_frame saturates at u32::MAX for a hostile channel count.
• matroska: a cluster timestamp above i64::MAX saturates at i64::MAX (FilmCraft wraps it to a negative value), and adding a block's relative timestamp saturates instead of overflowing (a debug-build panic in FilmCraft).
• matroska::Error::TooLarge is new: indexing a file past the limits above returns it.
• matroska::MuxOptions::max_lace_frames is clamped to 1 to 256; FilmCraft wrote a corrupt frame count above 256.
• isobmff::Mp4Writer::set_sample_entry returns an error when the new entry changes the codec, the media type or the PCM frame size, and Error::NoSuchTrack for a bad index. FilmCraft accepted any entry.
• isobmff::Mp4Writer::write_sample returns an error when a track would pass 2³² − 1 samples (PCM: frames), the most stsz can count; FilmCraft clamped the count.
• isobmff::ByteSource and matroska::ByteSource are one trait, craft_media::ByteSource (with ReadSeekSource, which was Matroska's). A type FilmCraft implements both traits for keeps one of its two impls.
• isobmff::Track and matroska::Track have codec_params(), both returning a CodecParams (CodecId::Unknown for a codec without an identifier). The MP4 one describes the first sample's sample description; codec_params_for(description_index) gives any other, and has_mixed_descriptions() says whether the samples use more than one. isobmff::CodecConfig has codec_id() and extradata(), and OpusConfig::opus_head().

bitstream

• BitReader: MSB-first reader with Exp-Golomb (ue(v), se(v), te(v)), as used by H.264/HEVC/AV1 headers.
• BitWriter: the matching writer, for encoders and header rewriting.
• unescape_rbsp / escape_rbsp: remove or insert H.264/HEVC emulation-prevention bytes.
• annexb_nals / length_prefixed_nals: split Annex-B byte streams and AVCC/HVCC samples into NAL units.
• crc32: CRC-32 (IEEE), as used by Matroska and PNG.

isobmff

Demuxing

use craft_media::isobmff::{open, TrackKind};

let file = std::fs::File::open("clip.mov")?;
let mp4 = open(&file)?;                        // any ByteSource: &[u8], Vec<u8>, Arc<[u8]>, File, ...
let v = mp4.track_of_kind(TrackKind::Video).unwrap();
let track = &mp4.tracks[v];
let i = track.sample_at_presentation_time(1000).unwrap();   // track timescale, edit-adjusted
let key = track.sync_sample_before(i);          // decode from here
let bytes = mp4.read_sample(&file, v, key)?;    // raw sample (AVCC/HVCC length-prefixed NALs, ...)

• ByteSource (len, read_at(offset, buf)) is the input abstraction. Implementations exist for [u8], Vec<u8>, &T, Arc<T>, Box<T>, and std::fs::File on non-wasm targets.
• open reads only top-level box headers, ftyp, moov and every moof. It never reads mdat. The one exception is the first sample of a tmcd track, which it reads (4 bytes) to get the timecode start frame.
• Mp4File: brands, is_quicktime, movie timescale/duration, tracks, metadata, fragmented, fragment_count.
• Track: id, kind (Video/Audio/Timecode/Subtitle/Other), handler, timescale, duration, language, tkhd fields (size, matrix, volume, enabled), entries: Vec<SampleEntry>, samples, edits, edit_offset, references (tref), sdtp, sample_groups (sbgp+sgpd, raw), pcm_chunked. Helpers: codec(), video(), audio(), presentation_pts(i), sample_at_pts(pts), sample_at_presentation_time(t), sync_sample_before(i), sync_samples().
• Sample: offset, size, dts, pts (dts + ctts), duration, is_sync (all samples are sync when stss is absent), description_index. All times are raw media time in the track timescale.
• Edit lists: edits holds the elst entries unchanged. edit_offset is leading empty edits (rescaled to media timescale) − first non-empty edit's media_time, so presentation time = pts + edit_offset. This matches ffprobe's timestamps for the common cases (B-frame delay, AAC priming, empty-edit offsets). Multi-segment edits, dwells and rate changes are exposed raw for the caller to interpret.
• Uncompressed PCM (lpcm, sowt, twos, in24, in32, fl32, fl64, raw , ipcm, fpcm) is merged to one Sample per chunk (pcm_chunked = true), with duration in frames. This avoids one table entry per audio frame. A chunk above 16 MiB becomes several samples of at most 16 MiB.

Parsed boxes

ftyp, moov/mvhd, trak/tkhd, edts/elst (v0/v1), tref, mdia/mdhd (v0/v1, ISO and Mac language codes), hdlr (ISO and Pascal names), minf/stbl: stsd, stts, ctts (v0/v1, read as signed), stss, stsc, stsz/stz2 (4/8/16-bit), stco/co64, sdtp, sbgp/sgpd. Also mvex/trex, moof/traf/tfhd/tfdt/trun (every tfhd base-offset mode and multiple moofs), and udta metadata (QuickTime ©xxx atoms, meta/ilst, and meta/keys (mdta)). uuid, free, skip, wide, sidx and mfra are skipped. Boxes with 64-bit sizes and with size 0 (runs to the end of the file) are supported.

Sample entries → CodecConfig

Format	Config
avc1/avc3 + avcC	Avc(AvcConfig): profile, level, length size, SPS/PPS, extension bytes
hvc1/hev1 + hvcC	Hevc(HevcConfig): all header fields plus NAL arrays (vps()/sps()/pps())
av01 + av1C	Av1(Av1Config)
vp09 + vpcC	Vp9(VpcConfig)
apch apcn apcs apco ap4h ap4x	ProRes { fourcc }
jpeg mjpa mjpb	Jpeg { fourcc }
AVdn AVdh	Dnx { fourcc }
mp4a + esds (in wave too)	Aac(AacConfig): ASC bytes + decoded object type/rate/channels; Mp3 for OTI 0x69/0x6B
.mp3/.mp2	Mp3: MPEG-1/2 audio of any layer (each frame header names it; Track::codec_params() reports Mp1, Mp2 or Mp3 from the first frame)
PCM fourccs	Pcm(PcmConfig): bits, float, endianness (enda, lpcm flags, pcmC), channels, rate
alac	Alac { cookie }
Opus + dOps	Opus(OpusConfig)
fLaC + dfLa	Flac(FlacConfig) (STREAMINFO decoded)
ac-3/ec-3	Ac3 { dac3 } / Eac3 { dec3 }
tmcd	Timecode(TimecodeConfig): flags (drop frame), timescale, frame duration, fps, start frame, reel name; format_frame()
anything else	Unknown { fourcc, raw }

Video entries also parse colr (nclx/nclc/ICC), pasp, clap, fiel, gama and btrt into VideoParams. Audio entries parse QuickTime sound description v0/v1/v2 into AudioParams.

Robustness

The parser does not panic on malformed input; every error comes back as an Error. Table counts are checked against the bytes in their box before anything is allocated. A constant-size stsz count is clamped to what fits in the file. trun counts are checked, and the whole file (all tracks and fragments) is capped at 2²⁶ samples. moov/moof are capped at 1 GiB. Compressed movie headers (cmov) are rejected with Error::Unsupported.

Muxing

use craft_media::isobmff::*;
let mut w = Mp4Writer::new(std::io::Cursor::new(Vec::new()), WriterOptions::new(Brand::Mp4))?;
let v = w.add_track(TrackConfig::new(SampleEntry::avc(avcc, 1920, 1080), 90_000))?;
let mut acfg = TrackConfig::new(SampleEntry::aac(asc, 2, 48_000), 48_000);
acfg.media_start = Some(1024);                 // AAC encoder delay → edit list
let a = w.add_track(acfg)?;
w.write_sample(v, WriteSample { data: &au, duration: 3000, composition_offset: 6000, is_sync: true })?;
// ... interleave samples in decode order ...
let bytes = w.finish_faststart()?.into_inner(); // or finish() for moov-at-end

• Mp4Writer<W: Write + Seek> streams samples into a single mdat. finish() appends moov. finish_faststart() (needs W: Read + Write + Seek) moves the media data forward in place and writes moov before it. If mdat goes past 4 GiB the header becomes a large-size header, and chunk offsets switch to co64 when needed.
• Brands: Brand::Mp4 writes isom with isom iso2 avc1 mp41 mp42. Brand::Mov writes qt , uses QuickTime hdlr/Pascal names, and sets wide as the placeholder box.
• Tables written: stts, ctts (v1 when an offset is negative), stss (omitted when every sample is sync), stsc, stsz (constant or table), stco/co64. Consecutive samples of one track share a chunk; a PCM chunk ends at 1 MiB.
• Sample entries come from SampleEntry (avc, hevc, prores, jpeg, aac, pcm, or any demuxed entry). Remuxing copies codec config, colr, pasp, clap, fiel, gama and btrt. PCM is written as sowt/twos (16-bit) or raw (8-bit) or sound description v2 lpcm in MOV, and as ipcm/fpcm + pcmC in MP4. A PCM write_sample call can hold any whole number of frames, and the track timescale must equal the sample rate.
• Edit lists: set TrackConfig::edits explicitly, or use media_start to get a single edit.
• Timecode: add_timecode_track(video, TimecodeConfig, start_frame) adds a tmcd track (with gmhd) and a tref/tmcd link from the video track.
• WriterOptions::metadata writes ©xxx text atoms to udta.
• FragmentedWriter<W: Write> writes fMP4. The init segment is ftyp + moov with mvex/trex. Each flush_fragment() then writes moof (mfhd, traf with default-base-is-moof tfhd, tfdt v1, trun with per-sample duration, size, flags and composition offset) followed by mdat.

Box reader

isobmff::boxes gives direct access to the box format, for files and boxes the demuxer does not model: CR3 uuid boxes, HEIF meta items, JPEG 2000 (JP2, JPX, JPM, MJ2) boxes.

use craft_media::isobmff::boxes::{BoxIter, Reader};

for b in BoxIter::new(&bytes) {
    let b = b?;
    if b.kind == b"uuid" && b.user_type == Some(CANON_UUID) {
        for c in BoxIter::within(&bytes, b.payload_range()) { /* CMT1..CMT4, THMB */ }
    }
}

• BoxIter::new(data) yields the sibling boxes in data; BoxIter::within(data, range) yields those in data[range] with offsets relative to data, so a walker keeps file offsets as it descends. Each BoxRef has kind, user_type (the 16-byte extended type of a uuid box), offset, header_len, payload, payload_range(), range() and end().
• 64-bit largesize and size 0 (runs to the end of the range) are handled. A malformed header or a box larger than the range yields one Err and ends the iteration; allow_truncated() instead yields that last box cut at the end of the range, with truncated set. A 32-bit zero terminator (QuickTime) ends the iteration.
• BoxHeader::parse(buf) reads one header (up to BoxHeader::MAX_LEN = 32 bytes), for scanning a file through ByteSource without loading it.
• Reader is the bounds-checked big-endian cursor the demuxer uses: u8 to u64, i16 to i64, f64, u24, fourcc, full_header (version and flags), bytes, skip, rest and count (an entry count checked against the bytes left, before allocating).
• The iterator does not recurse. The caller bounds nesting depth and box count.

Limitations

• presentation time = pts + edit_offset only applies the leading empty edits and the first media edit. Multi-segment edit lists, dwell edits and non-unity rates are exposed raw. ffmpeg handles an edit that starts mid-frame differently: it snaps to the frame.
• Other data references (external alis/url files), encrypted entries (encv/enca, sinf, senc), cmov, and QuickTime reference movies are not supported.
• sidx/mfra are ignored, because the demuxer scans every top-level box. stsd entries with a QuickTime color table are not decoded.
• Subtitle and text tracks (tx3g, c608, wvtt) and mp4v show up as Unknown.
• Muxer: one mdat, no chunk-size or duration limits outside PCM (the caller controls interleaving), no sdtp, sgpd/sbgp or cslg output, ISO-style meta/ilst metadata is not written, and the fragmented writer has no PCM, sidx or mfra.

matroska

A clean-room Matroska (MKV) and WebM demuxer, plus a minimal muxer, written from RFC 8794 (EBML), RFC 9559 (Matroska), the Matroska codec mappings and the WebM container guidelines. No GPL or LGPL code (FFmpeg, libmatroska, mkvtoolnix) was consulted; FFmpeg is a test oracle only.

Demuxing

use craft_media::matroska::{Demuxer, TrackKind, open};

// Sample tables in the same shape as isobmff (any ByteSource: &[u8], Vec<u8>, Arc<[u8]>, File, ...)
let file = std::fs::File::open("clip.mkv")?;
let mkv = open(&file)?;
let v = mkv.track_of_kind(TrackKind::Video).unwrap();
let k = mkv.keyframe_before(v, 2_000_000_000).unwrap();   // sample index
let frame = mkv.read_sample(&file, v, k)?;

// Streaming packets and seeking (from a ByteSource, any Read + Seek, or a byte slice)
let mut d = Demuxer::from_reader(std::io::BufReader::new(std::fs::File::open("clip.webm")?))?;
let sp = d.seek(v, 2_000_000_000)?;          // lands on the keyframe with pts <= 2 s
while let Some(p) = d.next_packet()? {        // file order, all tracks
    // p.track, p.pts (ticks), p.pts_ns, p.duration, p.keyframe, p.data
}

• Open (open, open_with(src, &OpenOptions { index, verify_crc })): EBML header (DocType matroska or webm), the first Segment, and its level-1 elements: SeekHead (followed for elements after the clusters), Info, Tracks, Cues, Chapters, Tags, Attachments (listed with offset and size; payloads are not read). With index: true (the default) every Cluster is scanned, element and block headers only, to build per-track Sample tables (offset, size, pts, duration, keyframe, cluster, block offset, lace index, BlockAdditional location) and keyframe lists. With index: false, open stops at the first Cluster.
• Timestamps: Matroska ticks are TimestampScale ns (usually 1 ms); Track::timebase is that as a reduced fraction of a second. pts is in ticks, pts_ns in exact nanoseconds. CodecDelay is subtracted (RFC 9559), as FFmpeg does, so Opus and Vorbis priming gives negative pts. Laced frames get block ts + i × (BlockDuration or DefaultDuration) / n; when neither is known, later laces repeat the block timestamp and have duration == 0 (Packet::lace tells which).
• Keyframes: the SimpleBlock keyframe flag; a BlockGroup is a keyframe when it has no ReferenceBlock.
• Lacing: Xiph, fixed-size and EBML. Unknown sizes: a Segment (live or streamed output) and Clusters end at the next top-level element. CRC-32: optional verification of level-1 elements and clusters; mismatches go to MkvFile::crc_errors and are not fatal.
• Damage: an undecodable or oversized element starts a byte scan for the next plausible level-1 element (a Cluster whose first child is Timestamp, or a sized metadata element); a truncated final block is dropped. Problems are listed in MkvFile::warnings.
• Limits: indexing stops with Error::TooLarge past 2²⁶ frames in the file, past one frame per file byte (each frame counts its stored size, and at least one byte), or past 2¹⁶ TrackEntries. Level-1 elements read at most the file's length in all, so SeekHead entries that point into one another are skipped with a warning.
• Seeking (Demuxer::seek(track, time_ns)): uses the sample index when present, otherwise Cues (refined by scanning the cued cluster for the exact keyframe), otherwise builds the index by scanning all clusters. Demuxer::keyframe_index(track) returns the keyframe list.
• Block additions: MkvFile::read_sample_addition returns the BlockAdditional with BlockAddID 1 of an unlaced block (in WebM, the VP8/VP9 alpha layer).
• Tracks: number, UID, type, flags, DefaultDuration, name, language (and BCP 47), CodecID, CodecPrivate, CodecName, CodecDelay, SeekPreRoll; VideoInfo (pixel and display size, crop, interlacing, stereo, alpha mode, pixel_aspect(), display_rotation()), Color (matrix, range, transfer, primaries, subsampling and siting, MaxCLL/MaxFALL, SMPTE 2086 mastering metadata); AudioInfo (rate, output rate, channels, bit depth); ContentEncodings.
• Content encodings: header stripping is undone in every packet and in read_sample. zlib, bzlib and lzo compression and encryption are reported (Track::frames_readable() == false); packets are then returned as stored and read_sample fails with Unsupported.
• matroska::ebml (public) has the EBML primitives: variable-length integers, element headers, typed values, CRC-32 and lacing. matroska::element_ids lists the element IDs.

Codec mapping (Codec)

CodecID	Codec	Payload
V_MPEG4/ISO/AVC	Avc	avcC (CodecPrivate)
V_MPEGH/ISO/HEVC	Hevc	hvcC
V_VP8, V_VP9	Vp8, Vp9	VP9 CodecPrivate features
V_AV1	Av1	av1C
V_APV	Apv	apvC
V_PRORES	ProRes	FourCC; frames get the 8-byte size + icpf header re-added
V_MJPEG, V_MS/VFW/FOURCC (MJPG)	Mjpeg
V_MS/VFW/FOURCC (other)	VfwFourcc	BITMAPINFOHEADER
A_AAC, legacy A_AAC/MPEG{2,4}/...	Aac	AudioSpecificConfig (synthesized for legacy IDs, SBR included)
A_OPUS	Opus	OpusHead
A_VORBIS	Vorbis	three headers split from the Xiph-laced CodecPrivate
A_FLAC	Flac	fLaC + metadata blocks
A_PCM/INT/LIT, A_PCM/INT/BIG, A_PCM/FLOAT/IEEE	Pcm	bit depth from Audio
A_AC3, A_EAC3, A_MPEG/L3, A_MPEG/L2, A_MPEG/L1	Ac3, Eac3, Mp3, Mp2, Mp1
S_TEXT/UTF8	SubRip
S_TEXT/WEBVTT, D_WEBVTT/* (WebM)	WebVtt	WebM blocks keep id\nsettings\npayload
S_TEXT/ASS, S_TEXT/SSA	Ass	script header
anything else	Other(id)	Track::codec_private

Codec::name() matches FFmpeg's codec_name for these.

Muxing

MkvWriter::new(w: Write + Seek, Vec<TrackSpec>, MuxOptions), then write_frame(track, pts_ns, keyframe, data, duration_ns) per frame, then finish(). It writes the EBML header, Segment, SeekHead, Info (with Duration), Tracks, Clusters of SimpleBlocks (a new cluster at a video keyframe after cluster_duration_ns), BlockGroups for frames whose duration differs from the track's default_duration_ns, optional audio lacing (Xiph, EBML, fixed-size; for tracks with a default_duration_ns, at most 256 frames, ending at a timestamp gap), and Cues (video keyframes, or the first frame of each track per cluster). MuxOptions::doc_type = "webm" writes WebM. CodecPrivate is fixed when the writer is created, so a codec whose configuration arrives with the first frame (AV1 from rav1e) creates the writer after that frame.

Limitations

• Only the first Segment is read (no linked or chained segments, no ordered-chapter playback).
• zlib, bzlib and lzo compressed and encrypted tracks are detected, but their frames are not decoded. EncryptedBlock is ignored.
• TrackTimestampScale (deprecated) is ignored.
• Lace timestamps without any duration information repeat the block timestamp (FFmpeg derives them from codec parsers).
• Muxer: no CRC-32, Chapters, Tags, Attachments, content encodings or block additions.

Containers, codecs and codec parameters

Always built, with no dependencies, so codec crates share them:

• Container: the file format (Wav, Aiff, Flac, Mp4, QuickTime, Matroska, WebM, Ogg, Caf, Adts, MpegAudio, Ac3), with name().
• CodecId: audio and video codecs (Aac, HeAac, Opus, Eac3, Speex, …, H264, Hevc, Av1, ProRes, Dnxhd, Apv, Theora, …, Unknown), with name(), is_audio(), is_video().
• PcmFormat: the layout of uncompressed samples, built with PcmFormat::int(bits, signed, big_endian) or PcmFormat::float(bits, big_endian).
• CodecParams: what a decoder needs about a track, whatever its container: codec, extradata, tag (the container's four-character code, such as a ProRes profile), the audio fields sample_rate, channels and pcm (PcmFormat), and the video fields width and height. Built with CodecParams::new(codec) and the with_* methods, or by the demuxers' codec_params(), which every container returns in the same form. extradata follows Matroska's CodecPrivate, as ffmpeg's extradata does: the AudioSpecificConfig for AAC, the OpusHead for Opus (MP4's dOps is converted), Xiph-laced headers for Vorbis (xiph_lace, None for more than 256 packets), fLaC + metadata blocks for FLAC, the avcC/hvcC/av1C/apvC record for video.

All are #[non_exhaustive]: new formats and fields are minor changes.

ByteSource

craft_media::ByteSource (len, read_at(offset, buf)) is what every demuxer reads from. It is always built, so a crate that uses only bitstream can name it too. isobmff::ByteSource and matroska::ByteSource are the same trait. It is implemented for [u8], Vec<u8>, &T, Arc<T>, Box<T> and std::fs::File (not on wasm). ReadSeekSource adapts any Read + Seek; it is Send + Sync when the reader is Send. Implement the trait once for a custom source (a web Blob, a network range reader) to use every container with it.

Tests

• Unit tests cover the bit reader and writer, the box reader (64-bit, size-0, uuid, truncated and hostile sizes), hand-built stbl edge cases, codec configuration round trips and allocation guards. For Matroska: EBML integers, typed values, CRC-32, every lacing mode, codec mapping, a hand-built file (unknown-size Segment and Clusters, header stripping, BlockGroups, block additions, color and mastering metadata, Chapters, Tags, Attachments, CRC-32, Cue-less seek, hostile cluster timestamps) and mux-demux round trips.
• tests/roundtrip.rs: property tests that write random multi-track files (progressive and fragmented) and read them back.
• tests/oracle_demux.rs and tests/oracle_mux.rs compare the demuxer and muxer with ffprobe and ffmpeg on generated fixtures in target/fixtures/isobmff/ ($CRAFT_MEDIA_FIXTURES_DIR overrides it). Fixtures are never committed.
• tests/oracle_matroska.rs compares every packet (stream, size, keyframe flag, pts, duration), codec names, time bases, dimensions and audio parameters with ffprobe on fixtures in target/fixtures/matroska/: H.264+AAC, VP9+Opus WebM, FLAC, Vorbis, SRT+ASS, WebVTT WebM, Cues at the front, a streamed file without Cues, HEVC with HDR color, ProRes+PCM, MJPEG+AC-3, AV1. Seeks must land on the latest keyframe at or before the target. The muxer's laced output is read back by ffprobe. A damaged file must resync.
• tests/fuzz.rs and tests/fuzz_matroska.rs: deterministic mutation fuzzing; nothing may panic. fuzz.rs also walks every box of a mutated file as a container and checks that each payload range stays inside its parent.

ffmpeg and ffprobe are test oracles only, never linked. The helpers are in craft-media-testkit (an unpublished crate, shared with craft-audio and the video crate to come). They are looked up in $CRAFT_MEDIA_FFMPEG / $CRAFT_MEDIA_FFPROBE, then PATH, then common install directories. When a tool is missing, the oracle tests print SKIPPED and pass; CRAFT_MEDIA_REQUIRE_ORACLES=1 turns a skip into a failure. CI (.github/workflows/media.yml) sets it and installs Ubuntu's ffmpeg. craft-audio's oracle tests use the same variables. Generate every fixture ahead of time with cargo test -p craft-media --all-features -- --ignored generate_fixtures.