[FFmpeg-devel] [PATCH] ffmpeg: add -map_channel option.

[Clément Bœsch]

On Mon, Oct 24, 2011 at 06:29:10PM +0200, Michael Niedermayer wrote:
> On Mon, Oct 24, 2011 at 04:19:10PM +0200, Michael Niedermayer wrote:
> > On Mon, Oct 24, 2011 at 02:05:55PM +0200, Clément Bœsch wrote:
> > > On Wed, Oct 19, 2011 at 11:42:15PM +0200, Michael Niedermayer wrote:
[...]
> > > > also shouldnt rematrix be independant of the existence of a channel_map
> > > > 
> > > 
> > > Well, AFAIU, the rematrix is meant to automatically set the audio gain
> > > levels for each channel; the channel mapping is "incompatible" with this,
> > > it's all about reordering, or maybe with another phrasing: ppl won't
> > > expect the levels to change (the number of output channels being fixed by
> > > the number of -map_channel and not a specific -ac option)
> > 
> > if your inputs represents 5.1 and your output represenst 7.0 you need
> > the rematrix code
> 
> a better and simpler example is stereo input and mono output
> 

In case of stereo → mono with map_channel, you're likely to have something
like:
    ffmpeg -i stereo.wav -map_channel 0.0.0 left.wav -map_channel 0.0.1 right.wav

Since you mean the other usage, you would do something like:
    ffmpeg -i mono.wav -map_channel 0.0.0 -map_channel 0.0.0 stereo.wav

By doing so you're expecting to have a copy of the mono channel in each
output channel; why would you want to force the resample here since you
can do the exact same operation with rematrix using:
    ffmpeg -i mono.wav -ac 2 stereo.wav

map_channel is all about placing and reordering, I don't think users will
expect the level of the channels to change, but I may be wrong.

-- 
Clément B.
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