moq-transcode

Just-in-time live transcoding for hang broadcasts.

Consume a source broadcast, publish a derivative broadcast next to it: the derivative catalog advertises lower renditions (rungs) of the source video plus relative references back to the source renditions, so a player picks from the combined ladder and the transcoder never proxies what it doesn't re-encode.

Nothing is encoded until someone asks, Cloudflare-style just-in-time per rung:

• Subscribe to a rung and the transcoder decodes the source live, resizing and re-encoding group for group until the last subscriber leaves. Every active rung of a source shares one subscription and one decoder, so decode cost scales with sources, not ladder depth.
• Fetch a specific group and the transcoder fetches that same group from the source and transcodes just that group. Output groups mirror source group sequence numbers 1:1, so group N of every rung is the same content as source group N and rendition switches land cleanly.

The codec work is moq-video: hardware where available (NVDEC + NVENC on Linux, VideoToolbox on macOS, Media Foundation on Windows), openh264 as the H.264 software fallback. H.264, H.265, and 8-bit 4:2:0 AV1 source renditions are eligible when a matching decoder is available. On an NVIDIA GPU the pipeline is fully GPU-resident: NVDEC decodes and scales in hardware and NVENC encodes the CUDA frame in place, with no CPU copies. Other decoders fall back to CPU scaling.

Library

One entry point: run(source, output, config). The caller owns session setup and where the derivative is announced; the transcoder only fills the output broadcast.

let mut config = moq_transcode::Config::default();
// The derivative is announced at `<source>/transcode.hang`, so the source
// renditions are referenced one level up.
config.source = Some(moq_net::PathRelativeOwned::from("..".to_string()));

let output = origin.create_broadcast(
    format!("{path}/transcode.hang"),
    moq_net::broadcast::Route::new().with_announce(true),
)?;

moq_transcode::run(source, output, config).await?;

Only renditions strictly below the source survive the ladder: a 480p source is never transcoded up to 720p, and a same-height rung is only offered when it undercuts a known source bitrate.

Example

Publish something first (e.g. moq publish camera from moq-cli), then:

cargo run -p moq-transcode --example transcode -- \
    --url http://localhost:4443/anon --source my-broadcast

The derivative appears at my-broadcast/transcode.hang.