[FFmpeg-devel] [PATCH 2/2] avcodev/libdcadec: implement request_channel_layout

[Hendrik Leppkes]

On Thu, Jul 9, 2015 at 1:32 PM, wm4 <nfxjfg at googlemail.com> wrote:
> On Tue,  7 Jul 2015 03:06:24 -0300
> James Almer <jamrial at gmail.com> wrote:
>
>> Signed-off-by: James Almer <jamrial at gmail.com>
>> ---
>>  libavcodec/libdcadec.c | 34 ++++++++++++++++++++++++++++++++++
>>  1 file changed, 34 insertions(+)
>>
>> diff --git a/libavcodec/libdcadec.c b/libavcodec/libdcadec.c
>> index 4bc3765..796826e 100644
>> --- a/libavcodec/libdcadec.c
>> +++ b/libavcodec/libdcadec.c
>> @@ -36,6 +36,8 @@ typedef struct DCADecContext {
>>      int buffer_size;
>>  } DCADecContext;
>>
>> +static int downmix_warned = 0;
>> +
>>  static int dcadec_decode_frame(AVCodecContext *avctx, void *data,
>>                                 int *got_frame_ptr, AVPacket *avpkt)
>>  {
>> @@ -131,6 +133,17 @@ static int dcadec_decode_frame(AVCodecContext *avctx, void *data,
>>      if (exss = dcadec_context_get_exss_info(s->ctx)) {
>>          enum AVMatrixEncoding matrix_encoding = AV_MATRIX_ENCODING_NONE;
>>
>> +        if (!downmix_warned) {
>> +            uint64_t layout = avctx->request_channel_layout;
>> +
>> +            if (((layout == AV_CH_LAYOUT_STEREO_DOWNMIX || layout == AV_CH_LAYOUT_STEREO) && !exss->embedded_stereo) ||
>> +                ( layout == AV_CH_LAYOUT_5POINT1 && !exss->embedded_6ch))
>> +                av_log(avctx, AV_LOG_WARNING, "%s downmix was requested but no custom coefficients are available, "
>> +                                              "this may result in clipping\n",
>> +                                              layout == AV_CH_LAYOUT_5POINT1 ? "5.1" : "stereo");
>> +            downmix_warned = 1;
>> +        }
>> +
>
> What does this mean? Does this happen with FFmpeg's DCA decoder too?
>

It just means that the DCA stream has no embedded downmix
coefficients. It'll downmix using the default coeffs, which can mean
two things, 1) this was intended, and all is fine, 2) downmixing was
not considered, and it may clip.
Personally, I might like a mode where it would downmix if the coeffs
are available, and not downmix if not (so I can downmix clipping-safe
using av/swresample later).

The native decoder would have the same problem, but I don't know if it
even supports embedded downmixes properly.

- Hendrik