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View Full Version : GOP closed/open ???? What does it mean ?


apfraats
4th January 2006, 22:19
I know some basics but not all.

I know a GOP is one I frame followed by P and B frames untill then NEXT I frame comes in. Then a new GOP starts.

Navigation should always start with a GOP, so an I-frame or you get a short blocky introduction, becuase the I-frame is missing.

But what about CLOSED and OPEN GOP's ?

I know a GOP can be CLOSED or OPEN, but do not know exactly what the diffrence is, to be honest, and how to see the difference at my level of thinking (not too high :) )

JDOBBS stated that the use of ALL GOP CLOSED can hurt quality, and I needed that for DVD-LAB-PRO, because it's stating you should set youre encoder to 'CLOSE ALL GOPS'...


If anyone can explain without too much useless details, I will be happy to read it.

Thanks in advance.

jptheripper
4th January 2006, 22:44
try the encoder forum

bigotti5
4th January 2006, 22:53
If all b-frames from one GOP references only to I- and P-frames of the same GOP then it is commonly called a closed GOP (all frames can be decoded without the need of another GOP).

Guest
5th January 2006, 06:13
I know a GOP is one I frame followed by P and B frames untill then NEXT I frame comes in. Then a new GOP starts. That is not correct. A GOP can contain multiple I frames. A new GOP starts with the next GOP header.

manono
5th January 2006, 14:37
B-Frames are Bidirectional frames. They can get information from the previous or following I or P-Frames. With a closed GOP, the B-Frame(s) at the end can only use data from the previous I or P-Frame, and as a result, they will be larger than they might be if the GOP is open. With an open GOP, it can "look" beyond the GOP boundary for the data. As a result, they'll be smaller usually. The upshot is that the quality will be better by a little bit for the same file size with open GOPs. And jdobbs, as always, was right.

Closed GOPs are useful only if you are planning to do some editing of the MPEG. I think ILVU or angles might also require closed GOPs. I could be wrong about that though. I could be wrong about anything I've said. But that's my story and I'm sticking to it.

If I'm not mistaken, although DVDLab Pro might warn you, it does allow you to go ahead and author, doesn't it?

There's more info at the bottom of this page, under Interframe Compression (although not about open and closed GOPs):

http://www.iis.ee.ic.ac.uk/~frank/surp99/report/kht97/work.html

In the Weighting section is enough information about matrices to make your head spin.

mpiper
6th January 2006, 07:51
The best way I remember the Closed/Open GOP issue is this:
Closed GOPs make playing a DVD in Fast Rewind easier.

Open GOPs mean that it the last B-frame has to look at both previous and upcoming frames to create the full image for display on screen. (small frame size, since it borrows from other frames)

Closed GOPs only need to look at previous frames, making each GOP (Group of Pictures) self-contained (and with no borrowing from external frames, the frame size goes up). No need to look anywhere but at one GOP to render any frame in the group.

The reason Closed GOPs are needed for Editing and Multi-angle videos is as Manono stated.
For editing, the closed GOPS make it easier to conduct frame-accurate edits, since there is no worry of lost data if the previous or following GOP fets cut off.
For Multi-angle, you have a main storyline, with selected "chapters" where the alternate video starts playing. If the GOPs were open, and you hit a "chapter-branch," the player would have to load the GOP at the end of the chapter, the following normal GOP and the alternate following GOP to get a fully-rendered B-frame (regardless of which "storyline you were watching). Having a Closed GOP guarantees that no matter where you are in the storyline, each pack of frames is self-contained and can play without extra overhead being loaded.

So, again, in a nutshell; For the average video Open GOPs are the way to go, unless you have a DVD player that doesn't like playing in fast-rewind. Other than that, there is NO reason to use Closed GOPs if you can avoid it.

Laters!
Mike