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Old 11th November 2018, 10:13   #1  |  Link
Forteen88
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x265: Are new versions = encoding speed ?

Hi. I wonder, in the new versions of x265 that are coming out, are they mostly (like 95%) for speed, or are there many video-quality improvements also?
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Old 11th November 2018, 15:29   #2  |  Link
excellentswordfight
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Quote:
Originally Posted by Forteen88 View Post
Hi. I wonder, in the new versions of x265 that are coming out, are they mostly (like 95%) for speed, or are there many video-quality improvements also?
Even though this question probably is best answered by someone closer to the development of x265. But tbh I have seen very little improvements in both speed and/or quality for some time now, I have basically seen the same speed/quality for my use cases since the new lambda tables in 2.4. I have glanced on most of the patchnotes after that and from what i've seen most of them includes bug fixes and added features (chunk encoding this year for example), exluding the avx512 though, but the gains there was somewhat disappointing. I hope I'm wrong, but I view x265 to be very mature in these regards at this point.

Last edited by excellentswordfight; 11th November 2018 at 15:34.
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Old 12th November 2018, 00:09   #3  |  Link
Blue_MiSfit
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There were recently some nice fixes for some really nasty VBV bugs that made a huge difference in my workflows.
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Old 12th November 2018, 18:49   #4  |  Link
benwaggoner
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There have been some features that directly improve quality for some scenarios since 2.4. Also, performance improvements ARE quality improvements, because you can use a slower preset and get improved quality at the same encoding time. All the analysis reuse stuff can really speed up encoding a 12-stream adaption set. And chunked encoding is huge.


Specific to quality

2.5: --tune grain
2.7 --radl and --gop-lookahead
2.8 --refine-intra 4 and VBV Lookahead fix
2.9: fix rowStat computation and disable noise reduction with vbv


Only the VBV Lookahead and rowStat fixes would be on by default in a basic encode; the others are context specific. Tune grain only for really grainy stuff, radl only for fixed GOP, and I'm not really sure what gop-lookahead is for (can't I just set --keyint and --min-keyint?).

But all things together, on a good AVX512 capable system, for a lot of scenarios one can now use --preset-slower in the time that --preset slow took in 2.3. Heavily leveraging the analysis reuse for multibitrate encoding, I'm sure it can get up to 3-4x for some scenarios.
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Old 13th November 2018, 20:51   #5  |  Link
MGarret
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--gop-lookahead is for overriding default --keyint if it finds new scene change within that additional lookahead that you specify. It's for adaptive gop, not for fixed.
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Old 16th November 2018, 17:40   #6  |  Link
benwaggoner
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--gop-lookahead is for overriding default --keyint if it finds new scene change within that additional lookahead that you specify. It's for adaptive gop, not for fixed.
Right. I'm just fuzzy on the difference between using --keyint 60 and --keyint 50 --gop-lookahead 10.

Those would both have a maximum GOP duration of 60 frames. Would the --gop-lookahead approach yield a bias towards having GOPs closer to 50 or something?
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Old Yesterday, 16:38   #7  |  Link
MGarret
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In theory --gop-lookahead option should work like that. It should bias to 50 and then sometimes extend it to 60 for a scenecut but I found that x265 likes to save bits by coding many scenechanges as P and not IDR/I so this option is basically useless.
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Old Today, 06:02   #8  |  Link
Blue_MiSfit
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What's the benefit of the chunked encoding method in x265?

I've always done this by simply using ffmpeg to encode chunks of a source and then stitching the result, e.g.

Code:
ffmpeg -ss 30 -i input.mov -c:v libx265 -t 10 output.mp4
From the x265 docs, this sounds weird:
Quote:
--chunk-start <integer>
First frame of the chunk. Frames preceeding this in display order will be encoded, however, they will be discarded in the bitstream. This feature can be enabled only in closed GOP structures. Default 0 (disabled).
When would this be useful?
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