[FFmpeg-devel] [RFC] Channel layouts

[Aurelien Jacobs]

Peter Ross wrote:

> On Fri, Aug 29, 2008 at 04:28:00PM +1000, Peter Ross wrote:
> > Hi.
> > 
> > This patch adds the notion of channel layouts to libavcodec.
> 
> Patch updated. Thanks for the feedback.
> 
> Test wav/wma files for 7.1 and 5.1 can be found here:
> http://www.microsoft.com/windows/windowsmedia/howto/articles/Multichannel.aspx
> 
> Audio coordinates space: my feeling is that this would be best implemented as
> an alternate/supplemental method of identifying the channel layout.

I agree. A function to map a list of coordinates to a list of channel
position should work fine in all common cases.
That can be added when needed.

> +void avcodec_get_channel_layout_string(char *buf, int buf_size, int nb_channels, int64_t channel_layout)
> +{
> +    if (channel_layout==0)
> +        channel_layout = avcodec_guess_channel_layout(nb_channels);
> +    if (nb_channels==1 && channel_layout==CHANNEL_LAYOUT_MONO) {
> +        snprintf(buf, buf_size, "mono");
> +    }else if (nb_channels==2 && channel_layout==CHANNEL_LAYOUT_STEREO) {
> +        snprintf(buf, buf_size, "stereo");
> +    }else if (nb_channels==4 && channel_layout==CHANNEL_LAYOUT_QUAD) {
> +        snprintf(buf, buf_size, "quad");
> +    }else if (nb_channels==6 && channel_layout==CHANNEL_LAYOUT_5POINT1) {
> +        snprintf(buf, buf_size, "5.1");
> +    }else if (nb_channels==8 && channel_layout==(CHANNEL_LAYOUT_5POINT1|CHANNEL_LAYOUT_STEREO_DOWNMIX)) {
> +        snprintf(buf, buf_size, "5.1+downmix");
> +    }else if (nb_channels==8 && channel_layout==CHANNEL_LAYOUT_7POINT1) {
> +        snprintf(buf, buf_size, "7.1");
> +    }else if (nb_channels==8 && channel_layout==CHANNEL_LAYOUT_7POINT1_WIDE) {
> +        snprintf(buf, buf_size, "7.1(wide)");
> +    }else if (nb_channels==10 && channel_layout==(CHANNEL_LAYOUT_7POINT1|CHANNEL_LAYOUT_STEREO_DOWNMIX)) {
> +        snprintf(buf, buf_size, "7.1+downmix");
> +    }else{

Maybe this slightly ugly code could be replace with a
lookup table.
Something like:

static const struct {
    int      nb_channels;
    int64_t  layout;
    char    *string;
} layout_map[] = {
    {  1, CHANNEL_LAYOUT_MONO                                 , "mono"        },
    {  2, CHANNEL_LAYOUT_STEREO                               , "stereo"      },
    {  4, CHANNEL_LAYOUT_QUAD                                 , "quad"        },
    {  6, CHANNEL_LAYOUT_5POINT1                              , "5.1"         },
    {  8, CHANNEL_LAYOUT_5POINT1|CHANNEL_LAYOUT_STEREO_DOWNMIX, "5.1+downmix" },
    {  8, CHANNEL_LAYOUT_7POINT1                              , "7.1"         },
    {  8, CHANNEL_LAYOUT_7POINT1_WIDE                         , "7.1(wide)"   },
    { 10, CHANNEL_LAYOUT_7POINT1|CHANNEL_LAYOUT_STEREO_DOWNMIX, "7.1+downmix" },
    {  0 }
};

for (i=0; channel_layout_mapping[i].nb_channels; i++)
    if (nb_channels    == layout_map[i].nb_channels &&
        channel_layout == layout_map[i].layout) {
        snprintf(buf, buf_size, layout_map[i].string);
        break;
    }

Aurel