This command:

ffmpeg -i left.mp4 -i right.mp4 \
       -filter_complex "[0:v][1:v]hstack=inputs=2[v]; \
                        [0:a][1:a]amerge[a]" \
       -map "[v]" -map "[a]" -ac 2 output.mp4

Returns this error:

Stream specifier ':a' in filtergraph description
[0:v][1:v]hstack=inputs=2[v]; [0:a][1:a]amerge[a] matches no streams.

Make sure the original video file contains an audio track, otherwise use this:


Which means "skip the audio track if doesn't exist".

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