Zero-dependency pure-Rust MP4 muxer for recording applications.

Muxide turns correctly-timestamped, already-encoded audio/video frames into a standards-compliant MP4 — pure Rust, zero runtime deps, no external tooling required for muxing.

Why Muxide Exists

If you are building a recording pipeline in Rust, you already know the tradeoffs:

• FFmpeg is powerful, but usually implies an external binary (distribution, process orchestration, and “what was this build configured with?” questions).
• Low-level MP4 writing APIs exist, but they require container expertise (sample tables, interleaving, seeking/indexing, and box layout).
• Many “minimal” approaches fall down on fast-start layout, strict validation, or production-grade ergonomics.

Muxide exists to solve one job cleanly and completely:

Take already-encoded audio/video frames with correct timestamps and produce a standards-compliant, immediately-playable MP4 using pure Rust.

Nothing more. Nothing less.

Core Invariant

Muxide enforces a strict contract:

• Input must be already encoded (H.264/H.265/AV1 video; AAC/Opus audio)
• Timestamps must be correct (monotonic PTS; provide DTS explicitly for B-frames)
• Muxide does not guess or “fix” broken streams

In return, Muxide guarantees:

• A valid ISO-BMFF (MP4) file
• Correct sample tables and offsets
• Optional fast-start layout (moov before mdat)
• No post-processing or external tools required

If the input violates the contract, Muxide fails fast with explicit errors.

Features

Video Codecs

• ✅ H.264/AVC (Annex B format)
• ✅ H.265/HEVC (Annex B format with VPS/SPS/PPS)
• ✅ AV1 (OBU stream format)

Audio Codecs

• ✅ AAC (ADTS format)
• ✅ Opus (raw packets, 48kHz)

Container Features

• ✅ Fast-start (moov before mdat) for web-friendly playback
• ✅ B-frame support via explicit PTS/DTS
• ✅ Fragmented MP4 for DASH/HLS streaming
• ✅ Metadata (title, creation time)

Philosophy

• ✅ Zero runtime dependencies (only std)
• ✅ Pure Rust (no unsafe, no FFI)
• ✅ Thread-safe (Send + Sync when writer is)
• ✅ Extensive test suite (unit, integration, and property tests)
• ✅ MIT OR Apache-2.0 (no GPL)

Note: no_std is not supported. Muxide requires std::io::Write for output.

Quick Start

use muxide::api::{MuxerBuilder, VideoCodec, AudioCodec, Metadata};
use std::fs::File;

fn main() -> Result<(), Box<dyn std::error::Error>> {
let file = File::create("recording.mp4")?;

let mut muxer = MuxerBuilder::new(file)
.video(VideoCodec::H264, 1920, 1080, 30.0)
.audio(AudioCodec::Aac, 48000, 2)
.with_metadata(Metadata::new().with_title("My Recording"))
.with_fast_start(true)
.build()?;

// Write encoded frames (from your encoder)
// muxer.write_video(pts_seconds, h264_annex_b_bytes, is_keyframe)?;
// muxer.write_audio(pts_seconds, aac_adts_bytes)?;

let stats = muxer.finish_with_stats()?;
println!("Wrote {} frames, {} bytes", stats.video_frames, stats.bytes_written);
Ok(())
}

Other Codecs

// HEVC/H.265 - requires VPS, SPS, PPS in first keyframe
let mut muxer = MuxerBuilder::new(file)
.video(VideoCodec::H265, 3840, 2160, 30.0)  // 4K HEVC
.build()?;
muxer.write_video(0.0, &hevc_annexb_with_vps_sps_pps, true)?;

// AV1 - requires Sequence Header OBU in first keyframe
let mut muxer = MuxerBuilder::new(file)
.video(VideoCodec::Av1, 1920, 1080, 60.0)
.build()?;
muxer.write_video(0.0, &av1_obu_with_sequence_header, true)?;

// Opus audio - sample rate is always 48kHz per Opus spec
// (input sample_rate parameter is accepted but internally normalized)
let mut muxer = MuxerBuilder::new(file)
.video(VideoCodec::H264, 1920, 1080, 30.0)
.audio(AudioCodec::Opus, 48000, 2)  // Opus requires 48kHz
.build()?;
muxer.write_audio(0.0, &opus_packet)?;

What Muxide Is Not

Muxide is intentionally opinionated. It does not:

• Encode or decode audio/video
• Transcode between codecs
• Read/demux existing MP4 files
• Perform heuristic timestamp correction
• Support non-MP4 containers (MKV, WebM, etc.)
• Handle DRM or encryption

Muxide is the last mile: from encoder output to a playable file.

Example: Fast-Start Proof

The faststart_proof example demonstrates a structural MP4 invariant:

• Two MP4 files are generated from the same encoded inputs
• One with fast-start enabled, one without
• No external tools are used at any stage

$ cargo run --example faststart_proof --release

output: recording_faststart.mp4
layout invariant: moov before mdat = YES

output: recording_normal.mp4
layout invariant: moov before mdat = NO

When served over HTTP, the fast-start file can begin playback without waiting for the full download (player behavior varies, but the layout property is deterministic).

This example is intentionally minimal:

• Timestamps are generated in-code
• No B-frames/DTS paths are exercised
• The goal is container layout correctness, not encoding quality

Who This Is For

Muxide is a good fit if you are:

• Building a screen recorder, camera app, or capture pipeline
• Writing a video editor/exporter in Rust
• Shipping MP4 output without bundling external binaries
• Generating fMP4 segments for streaming
• Operating in environments where external tooling is undesirable

Muxide is probably not a fit if you need encoding/transcoding, container introspection/editing, or legacy codec support.

Performance

Muxide is designed for minimal overhead. Benchmarks run on a standard development machine:

Scenario	Time	Throughput
1000 H.264 frames	264 µs	3.7M frames/sec
1000 H.264 frames (fast-start)	362 µs	2.8M frames/sec
1000 video + 1500 audio	457 µs	2.2M frames/sec
100 4K frames (~6.5 MB)	14 ms	464 MB/sec

Run benchmarks yourself: cargo bench

In practice, encoding is always the bottleneck—muxing overhead is negligible.

Documentation

• API Reference (docs.rs)
• Design Charter
• API Contract

License

Dual licensed under MIT OR Apache-2.0.

Muxide is designed to be boring in the best way: predictable, strict, fast, and invisible once integrated.