Re: [FFmpeg-devel] [PATCH] liavcodec: add bit-rate support to RoQ video encoder

[Victor Luchitz <vluchits@gmail.com>]

On Mon, Jan 22, 2024 at 4:57 PM Tomas Härdin <git@haerdin.se> wrote:

> > >
> > > >  -    if (frame->quality)
> > > > -        enc->lambda = frame->quality - 1;
> > > > -    else
> > > > -        enc->lambda = 2*ROQ_LAMBDA_SCALE;
> > > > +    if (avctx->bit_rate <= ROQ_DEFAULT_MIN_BIT_RATE) {
> > > > +        /* no specific bit rate desired, use frame quality */
> > > > +        if (frame->quality)
> > > > +            enc->lambda = frame->quality - 1;
> > > > +        else
> > > > +            enc->lambda = 2*ROQ_LAMBDA_SCALE;
> > > > +    }
> > >
> > > This looks like a bit of a janky way to switch between qscale and
> > > bitrate. Isn't there a way to detect whether an option has been set
> > > explicitly? At the very least this behavior should be documented in
> > > doc/encoders.texi
> > >
> >
> > Originally, the code just checked for bit_rate !=
> > AV_CODEC_DEFAULT_BITRATE,
> > which required including options_table.h, which in turn produced a
> > bunch
> > of compilation warnings about certain fields being deprecated. None
> > of the
> > other codecs include that file + many simply check the bit_rate field
> > against
> > magic constants.
>
> grepping for 200000 didn't reveal anything like that. Do you have a
> specific example of an encoder that does this?
>

~/ffmpeg/libavcodec# grep -RI bit_rate . | grep 000 | grep -v 1000 | wc -l
38

Most of the cases here are comparisons against magic constants without
any comments about how they were chosen or what they do, none whatsoever.


> Perhaps we could move AV_CODEC_DEFAULT_BITRATE somewhere else, to avoid
> pulling in a bunch of unrelated stuff. Maybe that doesn't need to hold
> up this patch though. Tbh the way bitrate is defaulted to a value,
> which makes it impossible to differentiate between a user-supplied -b
> 200k an no -b at all, is even more janky. The default is also
> ridiculously low..
>
> I know some encoders like libvpx allow specifying both quality (-crf)
> and bitrate at the same time
>
> /Tomas
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-- 
Best regards,
 Victor Luchitz
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