[FFmpeg-devel] [PATCH v4 2/9] lavc/libopenh264enc: add default gop size and bit rate

[Fu, Linjie]

> From: Martin Storsjö <martin at martin.st>
> Sent: Tuesday, April 28, 2020 14:28
> To: Fu, Linjie <linjie.fu at intel.com>
> Cc: FFmpeg development discussions and patches <ffmpeg-
> devel at ffmpeg.org>
> Subject: RE: [FFmpeg-devel] [PATCH v4 2/9] lavc/libopenh264enc: add
> default gop size and bit rate
> 
> On Tue, 28 Apr 2020, Fu, Linjie wrote:
> 
> >> From: Martin Storsjö <martin at martin.st>
> >> Sent: Tuesday, April 28, 2020 03:28
> >>> static const AVCodecDefault svc_enc_defaults[] = {
> >>> +    { "b",         "0"     },
> >>> +    { "g",         "120"   },
> >>>     { "qmin",      "-1"    },
> >>
> >> Why do you hardcode a value for g here, but put the default bitrate value
> >> in the code above? Wouldn't it be clearer to have both defaults here at
> >> the same place?
> >>
> > A default value in svc_enc_defaults[] would help to distinguish between
> > "the user specified the bitrate to be <x>" vs. "the user did not specify
> anything
> > about the target bitrate", as Anton has suggested in [1].
> >
> > Considering about your suggestions in patch 1/9, IMO it seems to be more
> reasonable
> > to keep the uiIntraPeriod untouched. The libopenh264 library would fill the
> default
> > value of uiIntraPeriod to 0, and as a consequence the gop size would be
> rather large.
> >
> > Updated the default "g" to "-1", same as libx264 did.
> > (Note that it's not acceptable for bitrate, since bitrate = 0 as default in
> library is not
> > valid)
> 
> If we have the option of not setting the bitrate on the encoder, then yes,
> it's better to avoid setting one in svc_enc_defaults. But in this case, if
> no bitrate was chosen by the user, we end up enforcing a default value
> anyway - just that the default is not written in the global defaults table
> (libavcodec/options_table.h) and not in svc_enc_defaults, but instead in
> code.
> 
> If we actually need to set a bitrate, it's IMO better to put this default

Indeed we have to set the bitrate.

> in the table, especially if we have other defaults like gop size there.
> 

In previous discussion in [1], we seems to come to an alignment that this would
help to determine whether it's specified by user, and hence allow libopenh264enc
to select the RC_MODE through settings like avctx->bit_rate, max/min/avg bitrates
if rc_mode is not set explicitly.

VAAPI encoder has the similar logic in [2].

I'm okay with either solution, if above logic is not going to be implemented.

[1] https://lists.ffmpeg.org/pipermail/ffmpeg-devel/2020-April/260215.html
[2] https://github.com/FFmpeg/FFmpeg/blob/master/libavcodec/vaapi_encode.c#L1542

- Linjie