[FFmpeg-devel] lavc/libvpxenc: Deprecate lossless option

[James Zern]

On Sat, Feb 9, 2019 at 3:15 AM Carl Eugen Hoyos <ceffmpeg at gmail.com> wrote:
>
> 2019-02-09 7:49 GMT+01:00, Gyan <ffmpeg at gyani.pro>:
> >
> >
> > On 09-02-2019 02:26 AM, Carl Eugen Hoyos wrote:
> >> 2019-02-08 6:08 GMT+01:00, Gyan <ffmpeg at gyani.pro>:
> >>>
> >>> On 08-02-2019 03:31 AM, Carl Eugen Hoyos wrote:
> >>>> .
> >>>> No strong opinion here, I hadn't realized that -crf 0 only works with
> >>>> newer versions.
> >>>>
> >>>> Can you explain why crf alone has no effect and needs -b:v 0?
> >>>> Isn't this a bug both with libvpx and libaom?
> >>>>
> >>> With crf present but VBV params absent, VPX operates using a constrained
> >>> Q RC mode , where the target bitrate acts as a ceiling. Since acvodec
> >>> has a non-zero default -b of 200 kbps, this produces undesirable
> >>> effects. If set to 0, VPX switches to constant quality.
> >> Yes.
> >> This looks like a bug to me.
> >
> > If there's a bug, it's in libav, in that, we can't signal when a value
> > is assigned by the user or via user-initiated logic, versus being
> > assigned as a default fallback. So it's a design conflict, and the
> > workaround has been long documented.
> >
> > Maybe just before the next major bump, a new field should be
> > introduced in AVOptions which registers if the field was
> > populated at the behest of the user.
>
> We could set the default for "b" to "0" without a major bump.
>
> Is it expected that constant crf with vp8 leads to broken output?
>
> >>> I do see this block though,
> >>>
> >>>       if (avctx->codec_id == AV_CODEC_ID_VP9 && ctx->lossless == 1) {
> >>>           enccfg.rc_min_quantizer =
> >>>           enccfg.rc_max_quantizer = 0;
> >>>       } else {
> >>>           if (avctx->qmin >= 0)
> >>>               enccfg.rc_min_quantizer = avctx->qmin;
> >>>           if (avctx->qmax >= 0)
> >>>               enccfg.rc_max_quantizer = avctx->qmax;
> >>>       }
> >>>
> >>>
> >>> Looks like the quantizer range is disabled only by using the deprecated
> >>> option, or has this changed?
> >> Is your question "Isn't 'lossless 1' necessary for lossless encoding"?
> >
> > Yes. A quick glance at vpx HEAD indicates it is, although
> > -qmin & -qmax 0 should also work.
>
> Given that it works for most frames, I wonder what the technical
> background is...
>

I believe the option was added as a convenience in vp9. The parameter
here matches what vpxenc uses, but min/max-q=0 is equivalent.