What are B-frames

[ToiletDuck]

You know after all the movies I have encoded I never found out what b-frames where. What are they? Why was divx 5.0.2 better because of b-frames?

[Teegedeck]

My understanding of this is not sooo deep, but perhaps to give you a first impression:

Without B-frames, we only have frames that relate to no other frame (this would be an I-frame) or their immediately preceding frame (that's what we call a P-frame). I-frames store a complete video-frame, P-frames only store the information about what in the video-frame has changed, compared to the preceding frame (I-frame or P-frame), mainly by describing motion in vectors.

Now, B-frames are a bit different; they are 'calculated' upon knowing what frame came before AND what frame comes after them. Effectively less information needs to be stored in the B-frame because only the difference between the frame before and the frame after it will be described. But more important here is that the frame after a B-frame is pretty much independent from the B-frame, it doesn't relate to the B-frame but to the frame before the B-frame. Complex. :rolleyes:

DivX5 (and XviD) use this for saving space: A heavily compressed B-frame doesn't hurt the following frame, whereas a heavily compressed I- or P-frame would also hurt all following frames till the next I-frame because these following frames need information from that frame.

Arrrgh! What am I trying to explain this , other people did better:
http://forum.doom9.org/showthread.php?s=&threadid=16935
and
http://forum.doom9.org/showthread.php?s=&threadid=19436

P.S. Better take a look at the FAQs in the forum before asking basic stuff. And if you don't find anything, do a search.

[Justinus]

@ToiletDuck
Read the book below:

Peter Symes "Video Compression Demystified" McGraw-Hill, USA, 2001 (ISBN 0-07-136324-6)

Good luck ;)

[ToiletDuck]

thanks alot. While the first explination wasn't the best for me. I have a strange feeling that I won't find an easier answer anywhere:( oh well. Thanks a lot anyway. you guys are swell