Most of the time ffdshow's H.264 decoder does not seem to decode the 2 B frames that preceed the first I frame (in presentation order) in the stream. It DOES seem to decode the 2 B frames most of the time at the start of a FILE, but if you start streaming data from an I-frame located elsewhere in a file then the 2 preceding B frames are NOT decoded (as if it was an open GOP). But Mainconcept and Elecard always seems to decode the 2 preceeding B frames no matter which I-frame in the file you start streaming from (as if all GOPs were closed).
This was originally raised on the doom9 forum, where Haruhiko Yamagata replied "They are simply dropped without any flags. It's a bug then. I'll re-read the spec and think about the fix."
The original post can be found here: