[MPlayer-users] What are b_frames for and should I use them for my mpeg2 encodings(for DVD) ?

James Courtier-Dutton james.dutton at gmail.com
Sun May 22 22:20:28 CEST 2005

On 5/20/05, VJ <vijay.s.gill at gmail.com> wrote:
> Hi,
>    As the subjects says, I want to learn what B-frames are, what is
> their role and will they benefit my mpeg2 encodings which eventually
> end up on burnt DVD's?
> Regards from
> VJ

I-Frames are encoded frames that can be decoded as a stand alone entity.
P-Frames (Predicted) use a previous frame and just code motion vectors
and changes from the previous frame.
B-Frames (Predicted) use both a previous frame and a following frame to encode.
I am talking about "previous" and "following" frames in display order.
They do not appear in the stream in display order.

So, if you have a scene where not much changes between frames, then
B-Frames are ideal as they provide the best quality with the minimum
of space.
Any rapid action scene is unlikely to benefit from B-Frames.

Encoding B-Frames is only possible using a 2-pass process.

The introduction of B-Frames into a stream results in the Frames in
stream order not being the same as the display order, so applying
video editing to streams with B-frames is a lot harder to do than to a
stream with only I and P frames. In a stream with only I and P frames,
the stream frame order is the same as the display frame order.


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