What's GOP???

[lnetzel]

I have been using TMPGewnc for a while now and I find the picture to turn out WAY better if I use the setting Force picture type settings and then Autoscan the clips... with a 150 something...

I started to use that scanning thing after readeing atutorial somewhere but I would never have guess it would make an inprovement...

Now... I guess this has something to do with scen switching and TMPGenc "knowing" what to encode next.. but I'm not sure

Then we have this thing with I, P and B pictures in GOP... what's that?

[kinneera]

I use a drastically different GOP for animation. I adjust the I-frame timing slightly depending on whether I'm encoding film or camcorder footage. If you're curious as to why, try searching the forum for the term GOP, you should find plenty of previous discussion.

[lnetzel]

Okay.. have figured out what the difference is between I, P and B picture frame type is.

I is a full data frame
P is a bit less
B is even less

NOW.. I scanned a bit from Office Space and save the text file and the first few rows look like this..

0,I
21,I
24,I
27,I
32,I
447,I
480,I
601,I
614,I
637,I
668,I
732,I
766,I
800,I
839,I
860,I
962,I
1134,I
1149,I
1200,I
1202,I
1215,I
1227,I

This is only I frames... nothing else.. what does this mean if I use this as the Force Picture Type... does this mean all these frames will be I frames and the rest P and B frames or what??? What frames will be P and what will be B and what or who decides that?

Please help... when I have figured this out I will write a guide to this GOP thang cause I feel you have a lot of quality to gain from knowing what you're doing here.

[kinneera]

This is only I frames... nothing else.. what does this mean if I use this as the Force Picture Type... does this mean all these frames will be I frames and the rest P and B frames or what??? What frames will be P and what will be B and what or who decides that?

Well, it just means that's where your I-frames will be placed. (Clearly, it is detecting scene changes to place I-frames). P and B frames will be placed in a regular pattern between them, specified by you.

Please help... when I have figured this out I will write a guide to this GOP thang cause I feel you have a lot of quality to gain from knowing what you're doing here.

I wish you the best of luck in this endeavor, but you may find it to be a difficult task indeed. There really isn't any magic bullet or single setting that will be appropriate for all purposes. The very need for B frames is even contested at sufficiently high bitrates. Still, I'm sure it would be very handy to at least have a guide posted permanently somewhere that at least explains what I, P and B frames are, since the question comes up a lot.

[technogeek]

if GOP is so important in determinign quality of Video then someone please tell me the correct IPB frame sturture and their number to be used while encoding MPEGs with Tmpeg encoder.

[kinneera]

Don't become too enamored with the GOP. Modifying it really will only have a refining effect at best. Other encoding factors (in a rough order of significance) such as bitrate, resolution, and filters used will have a far greater effect on end result quality.

The roughly ideal GOP for film (23.976/24 fps) is 1-3-2 (IBBPBBPBBPBB) and for broadcast (29.97/30 fps) 1-4-2 (IBBPBBPBBPBBPBB). This is because it produces an I frame twice every second. To tell the truth, I've never seen a recommendation for PAL, but I would trust the encoder's default. For animation, custom GOPs can be useful, mainly by reducing B frames. For example, I've found I get good results in TMPGEnc using 1-7-1 (IBPBPBPBPBPBPBP). Note that again, I maintain the I-frame timing with that sequence (my source is NTSC broadcast).

Some encoders may use scene change detection for I-frame placement (usually an option that can be turned on or off). In CCE, this appears to be default behavior, whereas in TMPGEnc, it is not. In that case, only the pattern you specify for P and B frames will actually be utilized by the encoder.