[MPlayer-users] what are b frames?

[Frank Boehme]

christopher j bottaro wrote:
> 
> [Automatic answer: RTFM (read DOCS, FAQ), also read DOCS/bugreports.html]
> couldn't find much about em in the documentation or the man pages...where can
> i go for info about em?

A key frame (K-frame) is a fully encoded still picture. A D-frame is a
frame which can only be decoded if a number of previous frames have
already been decoded and stored in a buffer. Such frames contain only
certain diffs to previous frames.

Now, a B-frame is similar to a D-frame but it can only be decoded if
also certain *future* frames have already been decoded and buffered. It
contains diffs to previous and future frames. Often, codecs which make
use of B-frames, reorder the sequence of frames in the stream so that
upon decoding no read-ahead accesses are required.

>  what do they have to do with going from dvd to divx?

These issues are important when it comes to cutting video.

> pros, cons, etc...?  i think i read something about they make encoding more
> efficient (does that mean faster?) and that they can improve the quaility of
> high motion/action scenes...

Well, the less K-frames you have, the shorter the resulting video stream
will get. But, as you van imagine, scenes which fast moving parts cannot
efficiently be encoded with non-K-frames.

Also, some codecs write only K-frames (I think mjpeg works like that).

F

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