Module mp4 

Pure-Rust fragmented-MP4 (fMP4) muxer for the built-in recorder.

Hand-rolled rather than pulled in as a dependency for two reasons: ffmpeg-sys-next’s avformat feature would add libavformat as a hard runtime dependency of every build, and the pure-Rust mp4 crates only write moov-trailing progressive files, which lose everything on a crash. Fragmented MP4 needs no trailer and no seeking — each moof+mdat pair is self-contained — so a file truncated by a crash or SIGKILL stays playable up to the last fragment, and the writer works on any Write sink.

Timestamps are caller-supplied wall-clock microseconds (damage-driven capture emits sparse, irregular frames), carried at a 90 kHz track timescale with one sample per fragment: tfdt anchors every sample at its true capture time, so variable framerate needs no constant-rate lie. The sample duration is only known once the NEXT frame arrives, so one sample is always buffered and flushed a frame behind (a clean stop closes it with the median observed duration); on SIGKILL at most that one buffered frame is lost — every fragment already written remains playable.

Codec support is deliberately split: [annexb_to_sample] and the H.264-specific parameter-set capture live in H264SampleBuilder, while the fragment/box writer below is codec-agnostic (bytes + sync flag + timestamps + a ready-made stsd sample entry). HEVC or AV1 recording later means a new sample builder emitting an hvc1/av01 entry, not a new muxer.

Structs

BuiltSample

One converted access unit ready for the fragment writer.

FragmentWriter

Codec-agnostic fMP4 fragment writer: ftyp+moov once, then one moof+mdat pair per sample, each anchored at its wall-clock decode time via tfdt.

H264SampleBuilder

H.264-specific front end: captures SPS/PPS from the stream, gates output on the first IDR, converts Annex-B access units to AVCC samples, and builds the avc1 sample entry.

Mp4Stats

Aggregate counters reported when the writer finishes.

TrackConfig

A codec’s contribution to the init segment: its stsd sample entry (with decoder configuration record) plus the display dimensions for tkhd.

Functions

parse_sps_dimensions

Coded frame dimensions parsed out of an H.264 SPS NAL (with its NAL header byte).

split_annexb

Split an Annex-B elementary stream into NAL payloads (start codes removed, emulation prevention bytes kept — the RBSP layer is only unescaped where a parser needs it).