Re: [FFmpeg-devel] [PATCH] avcodec/libvpxenc: enable dynamic max quantizer parameter reconfiguration

[James Zern <jzern-at-google.com@ffmpeg.org>]

On Mon, Mar 28, 2022 at 7:05 AM Danil Chapovalov <danilchap@google.com> wrote:
>
> On Thu, Mar 24, 2022 at 7:27 PM James Zern <jzern@google.com> wrote:
> >
> > On Thu, Mar 24, 2022 at 6:12 AM Danil Chapovalov
> > <danilchap-at-google.com@ffmpeg.org> wrote:
> > >
> > > ---
> > >  libavcodec/libvpxenc.c | 6 ++++++
> > >  1 file changed, 6 insertions(+)
> > >
> > > diff --git a/libavcodec/libvpxenc.c b/libavcodec/libvpxenc.c
> > > index dff1d06b0e..463a658bb0 100644
> > > --- a/libavcodec/libvpxenc.c
> > > +++ b/libavcodec/libvpxenc.c
> > > @@ -1625,6 +1625,12 @@ static int vpx_encode(AVCodecContext *avctx, AVPacket *pkt,
> > >      vpx_svc_layer_id_t layer_id;
> > >      int layer_id_valid = 0;
> > >
> > > +    if (avctx->qmax >= 0 && enccfg->rc_max_quantizer != avctx->qmax) {
> > > +        struct vpx_codec_enc_cfg cfg = *enccfg;
> > > +        cfg.rc_max_quantizer = avctx->qmax;
> > > +        vpx_codec_enc_config_set(&ctx->encoder, &cfg);
> > > +    }
> > > +
> >
> > Jan, I think this was what you were suggesting, no?
> > The docs could be updated to note qmax can be changed per-frame
> > [1][2]. Saying that, it does seem a bit unbalanced to only do qmax
> > here.
> >
> > [1] https://ffmpeg.org/ffmpeg-codecs.html#libvpx
> > [2] https://git.ffmpeg.org/gitweb/ffmpeg.git/blob/HEAD:/doc/encoders.texi#l2000
>
> As I understand, docs describe command line options.
> I do not plan to expose changing qmax per frame as a command line
> option, I do not see how that can be reasonably done. My intent is to
> change max qp when ffmpeg is used as a library.
>

That is true, I was thinking about it from the library perspective.
libx264 doesn't mention this behavior but the codec options section
does talk about setting these from the API, so I think a note is
worthwhile as this is the only point of documentation for the codec
wrapper.

> I agree it looks unbalanced to change just the qmax, but that is the
> only parameter I currently need for my usecase. Personally I prefer to
> only add features that are planned to be used.
> Are there any particular configuration settings you want me to make
> configurable,
> or do you think it is better to support all settings that can be configurable?

Since libvpx supports a reconfigure it should be safe to check the
options again. There is some complexity in that, though, so we can
leave this as a targeted approach similar to libx264 for now unless
others have a strong opinion.
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