[FFmpeg-devel] [RFC] Channel layouts

[Michael Niedermayer]

On Fri, Aug 29, 2008 at 04:28:00PM +1000, Peter Ross wrote:
> Hi.
> 
> This patch adds the notion of channel layouts to libavcodec.
> 
> Summary of new concepts:
> 
> * Channel IDs: We give each speaker a notional bit index.
>   e.g. CHANNEL_FRONT_LEFT=0, CHANNEL_FRONT_RIGHT=1, CHANNEL_BACK_CENTER=9
> 
> * Channel Layout: An ORing together of Channel IDs.
>   e.g. ((1<<CHANNEL_FRONT_LEFT)|(1<<CHANNEL_FRONT_RIGHT))
>   The resulting layout is identical to the dwChannelMask value found in
>   WAVEFORMATEXTENSIBLE. A channel layout of zero implies 'no statement'.
> 
> * Chanels are stored with the FFmpeg 'samples' array according to ID order
>   e.g. left comes before right.
> 
> * Encoders will indicate their supported channel layouts in AVCodec, in the
>   same way we do for pixel and sample formats.
> 

This suggestion has a few problems.
1. it is limited to 32 (or 64) fixed channel "positions", 18 are already used
   in this initial patch, that is more than a 1/4 of the available space
   is alraedy used up. This reminds me a little of a famous quote of how
   much memory will always be enough for everyone ...
2. It breaks down once x,y,z positions of ideal speakers are available
   instead of "front left" vs "back center"
3. When audio is returned by decoders as samples of all channels interleaved,
   like it is done currently, then a fixed order means the user app must
   reorder unless the destination (SDL, OSS, ALSA, ...) and finally and most
   importabtly the hardware can handle our order. This of course is irrelevant
   for planar audio which may be what many decoders will output in the future.

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I have often repented speaking, but never of holding my tongue.
-- Xenocrates
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