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View Full Version : x264: how to set open GOP?

lazyn00b
25th May 2006, 22:17
Unless I am mistaken, it looks like x264 always closes GOP with P-frame before next I-frame. Is it possible to have open GOP with B-frame before I-frame of next GOP?
Sagittaire
25th May 2006, 22:27
x264 close GOP because x264 detect change scene and in this case you must use closed GOP. Short GOP like for MPEG2 DVD compliant stream open GOP could be usefull but why use short GOP with x264 ... ???
lazyn00b
26th May 2006, 00:18
Need it for BLu-Ray, maybe HD DVD, too. It looks like Blu-Ray only allows for half second GOP.

I'm having trouble with strobing/throbbing/pulsing effect with 12-15 frame GOP, and I heard that open GOP will help a little with this, by allowing the last couple B-frames reference the next I-frame.
sysKin
26th May 2006, 04:14
Need it for BLu-Ray, maybe HD DVD, too. It looks like Blu-Ray only allows for half second GOP.
Are they crazy???
Any reason for this? The only one I can imagine is robustness against disc scratches.
lazyn00b
26th May 2006, 06:59
Are they crazy???
Any reason for this? The only one I can imagine is robustness against disc scratches.

I hear it's because they want FF/RW to work good, and maybe also to reduce memory requirements for 1080p.
akupenguin
26th May 2006, 10:04
I hear it's because they want FF/RW to work good, and maybe also to reduce memory requirements for 1080p.
A long GOP does not cost any more memory.
lazyn00b
27th May 2006, 02:17
A long GOP does not cost any more memory.

I was asking Ben Waggoner over at AVS Forum if 120 frame GOP was OK for HD DVD, and he said:

"120 is waaay too high.

DVD was 0.5 sec, if I might offer a strong hint . Since the real issue is memory utilization, GOP length for hardware systems is typcially definced by time instead of frame count."
Golgot13
27th May 2006, 02:33
GOP must be less than 0.606 second for HD DVD use
(information come from Sonic Scenarist 4 documentation).


Golgot13
GodofaGap
27th May 2006, 06:36
The only reasons I can think of for shorts GOPs are:

-MPEG TS has horrible seeking,
-they want the players to have some form of fast reverse play.
lazyn00b
27th May 2006, 07:04
GOP must be less than 0.606 second for HD DVD use
(information come from Sonic Scenarist 4 documentation).


Golgot13

Wow, thanks! Is the latest Scenarist documentation available online anywhere? I couldn't find it at Sonic's website.
lazyn00b
27th May 2006, 08:40
Hmm, according to this, http://www.dvdforum.org/gen-reqspec.htm , we're also going to need a "pulldown" app for x264/megui in order to make 23.976 fps progressive streams HD DVD compliant.
sysKin
27th May 2006, 11:46
I was asking Ben Waggoner over at AVS Forum if 120 frame GOP was OK for HD DVD, and he said:

"120 is waaay too high.

DVD was 0.5 sec, if I might offer a strong hint . Since the real issue is memory utilization, GOP length for hardware systems is typcially definced by time instead of frame count."

DVD had a GOP limit because of DCT inaccuracies. AVC doesn't have such problems.

More importantly, saying that GOP size affects memory is simply completely false.
bobololo
28th May 2006, 01:57
DVD had a GOP limit because of DCT inaccuracies. AVC doesn't have such problems.

More importantly, saying that GOP size affects memory is simply completely false.

Short gops are intended for fast forward/reverse.