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

[wm4]

On Thu, 9 Jul 2015 14:10:40 +0200
Hendrik Leppkes <h.leppkes at gmail.com> wrote:

> 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).

I'd prefer that too.

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

Then I guess it doesn't matter, as it won't cause regressions. Thanks
for the explanation.