Re: [Mjpeg-developer] closed GOP's with B frames

[Scott S. <sco...@gm...>]

On 1/23/06, Andrew Stevens <and...@mn...> wrote:
> Hi Scott,
> > I've seen several commercial DVD's with closed GOP's that have actual
> > B frames preceding the I frame.
>
> > I must assume (though I have no way of testing) that these B frames
> > comprise only of forward predictors.
>
> If the B-frames *precede* the I frame they must belong (if it is legal MP=
EG)
> to the previous GOP. Sounds like they're simply being 'mis-placed' in the
> VOBU of the closed GOP by the multiplexer. What are their temporal refere=
nces
> vs that of the I frame?

they are temporally before the I frame.  I=3D2, B=3D0, B=3D1, P=3D5, etc

Heck on Bullitt the first GOP has preceding B frames! And the GOP is
marked closed.

> This basically mildly bonkers.  A GOP with n initial B frames starts eith=
er:
>
> I(0)P(n+1)B(1)...B(n)   (Closed GOP) or
>
> I(n)B(0)...B(n-1)P(n+1) (Open GOP)

choice 3:

I(2)B(0)B(1)P(5) (Closed GOP)

:-)

me likey dvd

The only logical conclusion I can come up with is that the B frames
only contain forward predictors, not backward or bidrectional
predictors.  It seems they really like B frames.

I've decided that B frames are not to enhance compression, but instead
to assist in FF/REW and seeking.  You can FF by only showing P frames,
and it doesn't require any extra decoding power over what is already
necessary to decode a movie.  Same for REW, except you need to be able
to buffer all the P frames for a GOP (and thus be decoding 1 GOP
behind what you're displaying).  Seeking is enhanced because you only
have to decode the P frames to get to the middle of a GOP.  Oh and of
course there is less DCT error when you use B frames.

It matches that Steven has seen -- that B frames don't really save you
any space.  Sure they're smaller but then the P frames are larger --
it's all a tradeoff.  The bigger thing is that by using B frames you
can have larger GOPs (because of the seeking and DCT error cases that
I noted above), and larger GOP's generally mean better compression.
--
        Scott