[x264-devel] Clipping min-keyint

Wed Sep 28 18:38:11 CEST 2016

Hi Bugmaster,
You tested min-keyint=keyint vs. no-scenecut.  You can't achieve fixed GOP
with x264 unless you set min-keyint=keyint AND you disable scenecut.  If
scenecut is not disabled, x264 overrides your min-keyint setting, forcing
min-keyint=(keyint/2)+1.

Without our patch it's not possible to enable scenecut for fixed GOP length
encoding, and you certainly won't see any benefits from setting
min-keyint=keyint while leaving scenecut enabled.

Variable GOP is always going to be more efficient and have better quality at
scene changes than fixed GOP encoding.   The question is whether, for fixed
GOP, it is better to detect scene changes or not.  Today, if someone
requires fixed GOPs with x264, they are forced to disable scenecut.  The
patches we've developed for x265 improve encoding of fixed GOPs by allowing
scene detection to function, inserting I frames as needed at scene changes,
and optimizing slice types before and after the scene change.  These
improvements aren't final, but our users are already seeing significant
benefits.  We'll share the x264 patch and our results as soon as they're
ready.

By the way... 4 or 5 second GOPs would be more typical than 1 second GOPs.

Tom Vaughan
MulticoreWare
(developers of x265)

-----Original Message-----
From: x264-devel [mailto:x264-devel-bounces at videolan.org] On Behalf Of
BugMaster
Sent: Tuesday, September 27, 2016 11:46 PM
To: Mailing list for x264 developers
Subject: Re: [x264-devel] Clipping min-keyint

On Tue, 27 Sep 2016 19:04:37 -0700, Tom Vaughan wrote:
> Hi Bugmaster,
> There are many companies that use x264 and x265 in applications that
> require a fixed GOP length.  This can be for a variety of reasons, but
> typically it is either to comply with a system standard, or just to
> simplify adaptive bitrate streaming (all audio, video and metadata
> segments will be the same length, always).  Some of these users were
> puzzled as to why we forced them to disable scene cut detection in
> order to enable a fixed GOP length.  So we've made the change for
> x265, and we're developing a patch for x264 to do the same thing.

> So, what can scenecut do for you if you have a fixed GOP length?  Two
> things.  First, we know that inter-prediction isn't effective across
> scene boundaries.  So the last frame of a scene will not benefit from
> bi-prediction, and the first frame of a new scene will need to be
> coded with Intra prediction.  With scene detection working properly,
> x264 and x265 can make better frame/slice type decisions before and
> after the scene cut, choosing more optimal frame types and reference
> frames.  Second, I'm told by experts that you can code a frame with
> intra-coded blocks more efficiently if you encode it as an I frame
> (due to coding syntax).  So it's better to make the first frame of a
> new scene a non-IDR I frame instead of a large P frame.

> The way we specify a fixed GOP is simply to specify the same value for
> keyint and min-keyint.  These parameters are already in the API, and
> so no API change is needed.

> Optimizing slice types for all content is really key to encoding
> efficiency.
> We're doing what we can to optimize this in our lookahead, and we'll
> contribute any improvements back to x264.

> Tom

Hi Tom,
My internal testing of "min-keyint=keyint" vs "no-scenecut"
contradicts to such statements and instead of quality gain I was getting
quality hit with "min-keyint=keyint". That is at least with sources I used
and encoding settings with equal bitrates around --crf 18.
So without another testing results your statements are baseless or even
false.

Settings:

Non-fixed GOP (base):
--preset veryslow --psy-rd 0 --ssim --psnr --keyint 30

Fixed GOP "no-scenecut" (v0):
--preset veryslow --psy-rd 0 --ssim --psnr --keyint 30 --no-scenecut

Fixed GOP "min-keyint=keyint" (v1):
--preset veryslow --psy-rd 0 --ssim --psnr --keyint 30 --min-keyint 30

Fixed GOP "no-scenecut b-adapt=1" (v2):
--preset veryslow --psy-rd 0 --ssim --psnr --keyint
30 --no-scenecut --b-adapt 1

Fixed GOP "min-keyint=keyint b-adapt=1" (v3):
--preset veryslow --psy-rd 0 --ssim --psnr --keyint 30 --min-keyint
30 --b-adapt 1

Results:

azumanga (640x480):
     |    SSIM   |    APSNR   |    OPSNR  |
-------------------------------------------
base | 0.9983669 | 49.2820000 | 48.2990000|
v0   | 0.9982809 | 49.0514819 | 48.1124217|
v1   | 0.9982638 | 49.0267895 | 48.0302368|
v2   | 0.9982533 | 48.9990000 | 48.0300000|
v3   | 0.9982181 | 48.8677568 | 47.8401351|
-------------------------------------------

scherzo_inferna (1920x1080):
     |    SSIM   |    APSNR   |    OPSNR  |
-------------------------------------------
base | 0.9720802 | 42.4280000 | 41.9480000|
v0   | 0.9718534 | 42.3870713 | 41.8956372|
v1   | 0.9717174 | 42.3999341 | 41.8924678|
v2   | 0.9719181 | 42.3816765 | 41.8881952|
v3   | 0.9715764 | 42.4302790 | 41.8827348|
-------------------------------------------

SourceCode (768x432):
     |    SSIM   |    APSNR   |    OPSNR  |
-------------------------------------------
base | 0.9837157 | 47.1410000 | 45.3310000|
v0   | 0.9836656 | 47.1082194 | 45.3094387|
v1   | 0.9835531 | 47.0779036 | 45.2659036|
v2   | 0.9833698 | 47.0335714 | 45.2138571|
v3   | 0.9832497 | 46.9939939 | 45.1635671|
-------------------------------------------

Result logs and encoded samples:
http://www.datafilehost.com/d/0d1a2416
http://rusfolder.com/45309357
Choose to download for free (with Ads) and without any managers.

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