[FFmpeg-user] Confusion over stream specifier on -map, -codec, and -filter_complex

[Mark Filipak]

On 25/07/2024 00.56, gqqnb2005 wrote:
> Basically, I want to figure out how to specify streams for the option -map,
> -codec,  and -filter_complex.
> 
> ffmpeg -i INPUT -map 0:1 out.wav
> 
> Is 0:1 a stream specifier (
> https://ffmpeg.org//ffmpeg.html#Stream-specifiers-1)? Which form is it?

Yes. Input 0, stream 1 (i.e., the 2nd stream, of any type). 'map' wires things to the output. No 
'map', no output.
> ffmpeg -i INPUT -map 0 -c copy -c:v:1 libx264 -c:a:137 libvorbis OUTPUT
> 
> v:1 and a:137 seem to follow the syntax of stream specifier because a and v
> are stream_type.

Yes, the 2nd video stream. There can't be 138 audio streams, so that has to be wrong.

> ffmpeg -i in.flac -filter_complex 'acrossover=split=1500[LOW][HIGH]' -map
> '[LOW]' low.wav -map '[HIGH]' high.wav
> 
> Is '[LOW]' in the syntax of stream specifier?

In a sense, yes. It's a 'plug'. You connect plugs within a filter complex and to the output 'map' 
like they are physical equipment.

--Mark.