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

["Martin Storsjö" <martin@martin.st>]

On Mon, 22 Jan 2024, Tomas Härdin 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?
>
> 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

FWIW, it's possible for an encoder to individually override the defaults 
for fields like these. See e.g. x264_defaults in libx264.c, where it 
overrides the default bitrate to zero.

// Martin
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