[FFmpeg-devel] [PATCH 03/10] lavc/libopenh264enc: add default gop size and bit rate

[Anton Khirnov]

Quoting Fu, Linjie (2020-04-10 15:33:04)
> > From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of
> > Anton Khirnov
> > Sent: Friday, April 10, 2020 18:14
> > To: FFmpeg development discussions and patches <ffmpeg-
> > devel at ffmpeg.org>
> > Subject: Re: [FFmpeg-devel] [PATCH 03/10] lavc/libopenh264enc: add
> > default gop size and bit rate
> > 
> > Quoting Linjie Fu (2020-04-06 13:14:46)
> > > Signed-off-by: Linjie Fu <linjie.fu at intel.com>
> > > ---
> > >  libavcodec/libopenh264enc.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > >
> > > diff --git a/libavcodec/libopenh264enc.c b/libavcodec/libopenh264enc.c
> > > index c7ae5b1..3ff5be7 100644
> > > --- a/libavcodec/libopenh264enc.c
> > > +++ b/libavcodec/libopenh264enc.c
> > > @@ -335,6 +335,8 @@ static int svc_encode_frame(AVCodecContext
> > *avctx, AVPacket *avpkt,
> > >  }
> > >
> > >  static const AVCodecDefault svc_enc_defaults[] = {
> > > +    { "b",         "2M"    },
> > > +    { "g",         "120"   },
> > 
> > Why these values specifically? 
> 
> According to the default settings in nvenc (b = 2M) and vaapi encoder for h264 (g = 120).
> 
> >What happens if we leave them unset?
> 
> It would be 200kbps bitrate with gop size = 12.
> 
> There would be too much IDR frames with in small bitrate, which leads to
> a poor quality. 
> 
>  (I should have added such statements in the commit message)

I mean we could override the defaults to "-1" like is done for e.g.
x264, then we can distinguish between "the user specified the bitrate to
be <x>" vs. "the user did not specify anything about the target
bitrate".
And if the user did not specify the bitrate we could leave libopenh264
to choose bitrate on its own or use some other rate limiting mechanism
like constant QP or something CRF-like (don't know if it has that).

-- 
Anton Khirnov