On 09-02-2019 04:45 PM, Carl Eugen Hoyos wrote:
2019-02-09 7:49 GMT+01:00, Gyan <ffm...@gyani.pro>:
On 09-02-2019 02:26 AM, Carl Eugen Hoyos wrote:
2019-02-08 6:08 GMT+01:00, Gyan <ffm...@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.
Do you mean just the defaults in the vpx wrapper? Ok. Changing the lib
default will affect encoders which make use the bitrate value, like mjpeg.
Is it expected that constant crf with vp8 leads to broken output?
Not broken, simply unexpected for those used to how x264/5 handle it in
ffmpeg.
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
Yes, this returns a few non-lossless frames.
Is the encoder meant to be used for actual encoding?
Or does -crf 0 have several meanings depending on specific options
and vp8 is just bad?
We're talking about VP9. VP8 has no lossless mode. Yes, this is a
production encoder - Youtube uses it to generate VP9 streams for
uploaded videos.
Like I mentioned earlier, CRF + bitrate triggers constrained quality.
If the rate ceiling is high enough, the result is equivalent to
unclamped CRF, which is what the wrapper does when avctx bitrate is 0,
set its own bitrate target to 1 Gbps.
In any case, -lossless shouldn't be deprecated unless the workaround
produces identical results.
Gyan
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