Remux without re-encoding (stream copy)¶

Most container changes don't need a transcode. Moving mp4 → mkv, or joining segments that already share a codec, is a stream copy: read the demuxed packets, optionally fix their bitstream framing, and mux them straight back out — no decode, no encode. It's fast, lossless, and needs no codec at all, so it works for any codec in either licence variant, including streams the engine can't itself decode (spec 0013).

The mechanism: name the input streams to copy in Map with unbracketed specifiers ("0:v", "0:a:0", "0:0" — an input index, a media type, an optional per-type index) and set the codec to the CodecCopy sentinel. Bracketed labels ("[vout]") still mean graph pads that get encoded, so copy and transcode mix freely in one job.

Change the container, keep the bytes¶

cmd := afmpeg.Command{
    Inputs: []afmpeg.Input{{Path: "in.mp4"}},
    Outputs: []afmpeg.Output{{
        Path:       "out.mkv",
        Map:        []string{"0:v", "0:a"},
        VideoCodec: afmpeg.CodecCopy,
        AudioCodec: afmpeg.CodecCopy,
    }},
}
_, err := rt.RunJob(ctx, fs, cmd)

The video and audio are copied through byte-for-byte; only the wrapper changes. Any bitstream filter the target container requires (for example H.264's NAL framing differs between mp4 and MPEG-TS) is auto-inserted by the muxer — you don't have to reason about it for the common case.

Copy one stream, re-encode another¶

Because an unbracketed Map entry means copy and a bracketed one means a graph pad, a single output can do both — here the video is remuxed untouched while the audio is normalised and re-encoded:

cmd := afmpeg.Command{
    Inputs:        []afmpeg.Input{{Path: "in.mp4"}},
    FilterComplex: "[0:a]loudnorm[aout]",
    Outputs: []afmpeg.Output{{
        Path:       "out.mp4",
        Map:        []string{"0:v", "[aout]"},
        VideoCodec: afmpeg.CodecCopy, // copied
        AudioCodec: "aac",            // encoded from the [aout] pad
    }},
}

Join like-codec segments (concat)¶

WithConcatInput presents a playlist of like-codec files as one continuous input via the concat demuxer — a stream-copy join, distinct from the concat filter (which decodes and re-encodes):

cmd := afmpeg.NewCommand(
    afmpeg.WithConcatInput("part0.ts", "part1.ts", "part2.ts"),
    afmpeg.WithOutput("joined.ts",
        afmpeg.Map("0:v"), afmpeg.Map("0:a"),
        afmpeg.VideoCodec(afmpeg.CodecCopy), afmpeg.AudioCodec(afmpeg.CodecCopy)),
)

The segments must share codec and parameters — that's the demuxer's requirement, and a mismatch surfaces as a typed error. MPEG-TS is the natural concat-copy source (its packets carry continuous timestamps); joining mp4 segments with audio can hit mp4's priming-sample timestamp discontinuity at a boundary, so concat-copy is happiest on stream-friendly containers.

Override or disable the bitstream filter¶

The muxer's auto-insertion is right for the common case. When you need to force a specific filter (or force none), set it per copied stream, keyed by the Map entry:

afmpeg.WithOutput("out.ts",
    afmpeg.Map("0:v"), afmpeg.VideoCodec(afmpeg.CodecCopy),
    afmpeg.BitstreamFilter("0:v", "h264_mp4toannexb"), // explicit; "none" disables
)

Notes¶

• Copy can only cut on keyframes. Trimming a copied stream to an exact frame needs a decode the copy path skips; for a frame-accurate cut re-encode with SeekAccurateTo (extract a clip, spec 0014).
• Container reach. Copy lands on mp4/mkv/webm in the lean profile; MPEG-TS, HLS/DASH segmenting, and fragmented MP4 are available in the intermediate profile (spec 0015) — see package for streaming.
• Licensing. Copy touches no codec library, so it stays in the LGPL default and even reaches codecs the engine doesn't ship a decoder or encoder for.