A basic remuxing example:
ffmpeg -i video -i audio -c copy -aspect 16:9 output.webm
-c copy
will enable stream copy mode and therefore will avoid re-encoding.Because
-aspect
was used with-c copy
, the aspect ratio will be stored at the container level; not in the video stream. I'm not sure if WebM supports this, or if players will pay attention to it, but I will assume it does.