---------------------------Syskin@Doom9---------------------------------
first b-frame is packed together (in an avi) with its future reference (a p-frame most likely). Future reference is transmitted first because it has to be decoded first, but the frame you actually want to *see* is the b-frame. So, packing delivers the b-frame immiedietly, so it can also be decoded and shown on time.
Without packing, decoder gets future reference, but has to wait for the b-frame before it can display anything. This creates a one-frame lag, which makes such avi difficult to seek in (for example) virtualdub.
---------------------------Syskin@Doom9---------------------------------
B-Frame is a hack of AVI，Designed by DivX，to handle B Frame in AVI.
新版的xvid.ax(DShow)已经解决了这个问题，不会出现这个画面
但vfw(xvidvfw.dll)还有这个问题，因为VFW不允许解码后面的frame，只能按顺序解码当前Frame。
但是不管有没有这个画面，都会造成1 frame的lag。
