GOP explaination please.. :-)

[davidian]

Does anyone feel like explaining all about GOP and I, B, P frames..? If it's complicated and takes time, perhaps there is some nice homepage where I can read all about it?

Obviously, changing the GOP stucture in Tmpeg, the results change. How can I fiddle with these settings to obtain maximum quality?

Please help..
and, if it's too complicated - leave it-

[w00kiee]

http://bmrc.berkeley.edu/frame/research/mpeg/faq/mpeg1.html

i have to say although it is a bit technical its the simplest faq ive seen... but if thats too technical... here is a quick explanation

I - intra frame.. basically an exact (well fairly close) still frame but compressed heavily

p - predicted.. frame is predicted from the last I or reconstructed P frame. it has the differences from the last I or P in 16x16 segments (macroblocks) and if the differences are enough (say fast motion scene) would end up equivelant to an I frame although it gets less bitrate than the I frames

b - bidirectional frames with little info. just enough to guess what should be there from the past I or P frame and next I or P frame. these frames are given the least share of the bitrate pie.

GOP = group of pictures - basically its the set of frames from I frame to I frame - here is a gop setup

IBBPBBPBBPBBPBB

at the beginning of each gop set is a header. you arent stuck with the structure above and can add more p's and b's and go longer between I's but thats probably the most dvd friendly way to do it

quick faq.. there is more to it but its all you really need to know unless you plan to make a player/encoder

[davidian]

Thanks man for taking the time! I'll look into it later

[xzarkad]

I have answered a similar question once, and I have got an extremely complete answer. Please check:

https://www.videohelp.com/phpBB/viewtopic.php?topic=55722&forum=3

[xzarkad]

Sorry: TYPO. I mean: I have asked a similar question....

[w00kiee]

koalabears tutorial (the link xzarkad posted above) uses plainer english to describe the process (koala is very knowledgable) you should read both links as they both describe the frames well

all in all its nice fyi but messing with the gop generally doesnt help things much. the defaults the encoders give you are well thought out...


<font size=-1>[ This Message was edited by: w00kiee on 2001-12-30 17:05:33 ]</font>

[vitualis]

The default GOP settings (for VCD) given by most encoder are probably the most appropriate.

You could mess around with the GOP settings if you want, of course, but it is unlikely that you will improve the quality of the video (indeed, it will probably go the other way).

Ultimately, it is a balance between the number of I-frames and the number of P and B frames. I frames give good quality (no temporal compression), but require a lot of bits (because of no temporary compression). P and B frames require much less bits, but may not be appropriate depending on video content (e.g., if there is a large change between frames). Furthermore, I frames are needed for random access.

Too many I-frames starve the average bitrate per frame. Too many P and B frames mean you don't get the benefit of I frames.

In the environment of limited bitrate (VCD and SVCD), a balance of I frames and the other frames are needed to achieve the best possible quality (and you probably can't do much better than the default settings for VCD).

Regards.

_________________
Michael Tam

<font size=-1>[ This Message was edited by: vitualis on 2001-12-31 01:54:07 ]</font>