Processing: Containers and Streams

Common Processing Operations

5.1 Remuxing

Copy the compressed streams from an MP4 container to an MKV container:

ffmpeg -i input.mp4 -map 0 -c copy output.mkv

-c copy does not decode or re-encode streams, so performance is normally limited by storage I/O. The target container must support the original video, audio, and subtitle formats.

5.2 Software Transcoding

Convert to H.264 video and AAC audio:

ffmpeg \
  -i input.mkv \
  -c:v libx264 \
  -preset medium \
  -crf 23 \
  -c:a aac \
  -b:a 192k \
  output.mp4

• preset controls the tradeoff between encoding speed and compression efficiency; it does not directly represent image quality.
• crf controls constant quality. With x264, lower values produce higher quality and larger files.
• libx264 and libx265 require FFmpeg to be built with the corresponding external libraries.

5.3 Stream Selection

Select the first video stream and the second audio stream:

ffmpeg \
  -i input.mkv \
  -map 0:v:0 \
  -map 0:a:1 \
  -c copy \
  output.mkv

Continue processing even if the requested audio stream is missing:

ffmpeg -i input.mp4 -map 0:v:0 -map 0:a:0? -c copy output.mp4

The ? marks the mapping as optional.