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

[Fu, Linjie]

> From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of
> Anton Khirnov
> Sent: Saturday, April 11, 2020 16:43
> 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 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".

Indeed, I noticed this while attempting to select rc_mode by avctx->bit_rate.
It makes sense to distinguish them, and I've changed it.

> 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).

I'm afraid it doesn't have one if we leave the bitrate unset:
[OpenH264] this = 0x0x55a61f8dddd0, Error: Invalid bitrate settings in total configure, bitrate= 0

so I'll assign a default target bitrate to 2M.

- Linjie