[FFmpeg-devel] [PATCH] Clearer wording of audio channel maping limitations

Clément Bœsch ubitux at gmail.com
Wed Dec 21 16:40:28 CET 2011

On Wed, Dec 21, 2011 at 03:23:26PM +0000, Tim Nicholson wrote:
> Since I and others have been caught out by the channel mapping limitations and the documentation
> is not clear about those limitations with regard to  stream mapping from a single file. I offer the attached patch
> in the hope it clarifies things.

> From 2ce04ac3c8fccda673cc08eed94cb57990161f69 Mon Sep 17 00:00:00 2001
> From: Tim Nicholson <tim.nicholson at bbc.co.uk>
> Date: Wed, 21 Dec 2011 15:11:04 +0000
> Subject: [PATCH] Clearer explanation of audio channel mapping restrictions
> ---
>  doc/ffmpeg.texi |    7 ++++---
>  1 files changed, 4 insertions(+), 3 deletions(-)
> diff --git a/doc/ffmpeg.texi b/doc/ffmpeg.texi
> index 553d759..ace4b29 100644
> --- a/doc/ffmpeg.texi
> +++ b/doc/ffmpeg.texi
> @@ -825,9 +825,10 @@ The following example split the channels of a stereo input into streams:
>  ffmpeg -i stereo.wav -map 0:0 -map 0:0 -map_channel 0.0.0:0.0 -map_channel 0.0.1:0.1 -y out.ogg
>  @end example
> -Note that "-map_channel" is currently limited to the scope of one input for
> -each output; you can't for example use it to pick multiple input audio files
> -and mix them into one single output.
> +Note that "-map_channel" is currently limited to the scope of one input stream for
> +each output stream;

Adding "stream" is indeed a good idea. I'd actually even reword it
emphasis that "one output stream is associated with one single input
stream"; I think it's easier to visualize that the output stream can't be
composed of multiple inputs. It enlightens the fact that -map_channel is
(currently) limited to filtering and reordering.

>                      you can't, for example, use it to pick multiple input audio channels
> +contained in different streams (from the same or different files) and mix them into one single
> +output stream in order to turn two separate mono streams into a single stereo stream.

You start that part of the sentence with an "abstraction" ("pick multiple
input audio channels contained in different streams") and end up with a
very specific example; I think it would be better split.

Also, could you keep the 80 columns wrapping?


Clément B.
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