[Help] Gop Structure

[toryang]

How to create this kind of GOP Structure... IDK how to create this 8.5 avg/13 max

[sanlyn]

Create it from what?

There is no "8.5" GOP structure; that number is an average figure for the VOB file that GSpot is analyzing. A GOP is a "Group of Pictures", so you can't have "8.5" pictures. GSpot is saying that the GOP organization in the VOB file varies from time to time, with 13 being the max GOP size for your sample video. The GOP size is usually set during encoding from source to MPEG2.

[toryang]

It is created from Original Anime Video source from DVD

[sanlyn]

Same answer. The GOP setting is usually specified when encoding.

[FulciLives]

You set the limit based on what you are encoding i.e., the frame rate.

With progressive NTSC (which is 23.976fps) the norm is to use 12 max
That's MOSTLY what I encode these days so I don't know the numbers off the top of my head for PAL (25fps) and NTSC that is 29.970fps OTHER THAN they can use a higher max GOP than 12 (that much I do know). Actually come to think of it I believe that a max of 15 is common with the other two.

[sanlyn]

AFAIK the min and max for PAL and NTSC are the same. At least, it's that way in my MPEG encoders. I don't even remember the exact figures, but I use between 12 and 15, usually based on how much motion or detail I'm dealing with. I've seen 10 used for encoding fast motion with somewhat cleaner encoded and playback results.

[toryang]

I know a little bit of Gop structure's, But that 8.5 avg/13 max is questioning my mind how they put that kind of gop...

[sanlyn]

8.5 is an average. You know how to get an average, right? GSpot sees that the GOP size is changing during the course of the video, which is not unusual. Some segments were encoded into large GOP's (13 being the max used, according to GSpot), and some segments used smaller groups. If you add several figures together and divide by the number of figures, the result is an average value. You can't set a GOP of "8.5".

[toryang]

Oh i see... So i cannot do an averegare?

Sorry im only newbie for GOP's

[El Heggunte]

For DVD-Video:

25fps: max GOP length = 15

29.97fps: max GOP length = 18

23.976fps: max GOP length = 14
(even though most encodes have set 12 frames as the upper limit)

[sanlyn]

Thanks, El Heggunte, for those numbers. I think 15 is as high as I've gone.

Oh i see... So i cannot do an averegare?

No, you can't set an "average" GOP. You could play with some GP numbers, encode some segments at one GOP and other segments at other GOPs, then calculate until you get the average you want. It would take forever. Why bother?

[FulciLives]

Maybe the OP doesn't realize that in the MPEG-2 encoder you set the MAX limit for the GOP but then the encoder uses different GOP lengths through out the encoding BUT it will never be MORE THAN whatever the MAX is set to yet often times it will be less. This depends on the content itself. Anyway it's not like you set the max to 12 and each and every GOP structure is 12. Some will be 12 and some will be less and that is why the AVERAGE is always going to be less than the MAX setting.

[manono]

How to create this kind of GOP Structure... IDK how to create this 8.5 avg/13 max

Those are very screwy numbers. I checked a few of mine and they're uniformly 12 max; N=12, M=3 (79%) in that box. I suspect it's because you chose the first VOB to check and it's only 10 seconds long and it's logos or some other crap with fast edits so the GOPs wind up being very short and in no way representative of a whole episode. Open an episode VOB and notice the difference.

Anyway, there's no reason at all you'd want to recreate those figures for your own encodes. And just using the 'normal' settings might result in what you're showing us for that particular VOB anyway. Did you try?

[toryang]

@manono
All vobs of that video have the same structure...

So i think i will be using those default gops...


Sorry for this post... Coz my mind blown out after i saw those kind of gops...

[toryang]

@sanlyn

I don't know how to get an average... Could you tutor me about average gops?

I am using, Rovi Totalcode, FFMPEG, Mainconcept Reference, Etc. I tried it all but no one has given me that kind of GOP.

[DB83]

This is crazy.

It is like saying Rembrandt painted great pictures and I want to paint like that.

The simple answer is that you can not.

You have already been told in the simplest terms possible how how mighr be able to create that. But do the maths. If the maximum is 13 and you have 2 gops then the second one would have to be 4. Keeping it simple, if you have 4 gops then one permutation is 3*7 + 13. But a video has 00's of gops so you are on a loser to try to replicate it.

Just stick to standard methods of encoding and do not lose any more sleep over this.

[toryang]

Thanks for the info DB83 now i know i cannot replicate those gops...

[sanlyn]

Maybe the OP doesn't realize that in the MPEG-2 encoder you set the MAX limit for the GOP but then the encoder uses different GOP lengths through out the encoding BUT it will never be MORE THAN whatever the MAX is set to yet often times it will be less. This depends on the content itself. Anyway it's not like you set the max to 12 and each and every GOP structure is 12. Some will be 12 and some will be less and that is why the AVERAGE is always going to be less than the MAX setting.

Makes sense to me. What I see happening on many videos is this: the better encoders I use have an option for detecting scene changes and creating a new I-frame at scene changes. Some are more efficient at detection than others, but in that case let's say you have a GOP with a scene change in it. You might have specified GOP=12. But a scene changes at frame 8, where the encoder creates a new full-iimage I-frame. This means that there are now 2 GOP's, one with 7 frames and a new one with 12. I see this happening in video all the time, even on DVD's I record off cable with my DVD recorder. I even see GOP's with only a key frame (i.e, a GOP of 1). So if you factor all those scene changes into the equation, the average GOP isn't going to be anything near 12.