[x264-devel] Fixed GOP, keyint and keyint-min

[Tom Vaughan]

We looked into this, and we concluded that scene detection is useful even
for fixed GOP length encoding.  When the encoder detects scene changes in
the lookahead, it can better understand where inter-prediction will work,
and where it won't work (so it can avoid wasting time searching for
inter-prediction modes that won't work).  It can also insert a non-IDR I
frame at the start of the new scene, if that turns out to be more efficient
than using a P frame (because the first frame at the start of a new scene is
unlikely to find lower rdcost inter-prediction references to a previous
frame, and a P frame that contains mostly I blocks compresses less
efficiently than an I frame).

We changed this behavior in x265.  So for x265, if you want fixed GOP
lengths, you can just set max-keyint = keyint.  Scenecut is independent, and
it's on by default.

-----Original Message-----
From: x264-devel [mailto:x264-devel-bounces at videolan.org] On Behalf Of
BugMaster
Sent: Wednesday, August 23, 2017 12:18 PM
To: Mailing list for x264 developers
Subject: Re: [x264-devel] Fixed GOP, keyint and keyint-min

On Wed, 23 Aug 2017 21:08:14 +0200, Lars Kiesow wrote:
> Hi everyone,
> today we had some discussion about forcing a fixed GOP size using x264
> with scenecut enabled to get additional I-frames. E.g. having an IDR
> frame exactly every 50 frames with some additional non-IDR I-frames in
> between if scenecut detects that having one would make sense.

> ...

Hi. There is no point in having fixed GOP with scenecut enabled i.e.
if you want fixed GOP you SHOULD disable scenecut. If you don't need really
fixed GOP (usually less than 100 frames) but only some sync points between
different encodeds of same video (say every 1000
frames) than you better use forced frame types (i.e. qpfile way of
x264 cli).

_______________________________________________
x264-devel mailing list
x264-devel at videolan.org
https://mailman.videolan.org/listinfo/x264-devel