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

[Gyan]

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.
>> 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.

>> Also, with libvpx v1.7.0-1758, I get different results for -crf 0 -b:v
>> 0  vs only -lossless 1, with the latter producing a slightly larger
>> file, and its result showing a slightly larger SSIM score. Although
>> neither produces a SSIM of 1, like libx264. Not truly lossless?
> Please provide your input sample.
I can reproduce the difference in result with 
fate-suite/h264-conformance/src19td.IBP.264

although the `-lossless 1` encoding does return as lossless in SSIM.

Gyan