Re: [Mjpeg-developer] mpeg2enc, I-frames, and GOP's?

[Bob M. <mce...@dr...>]

Andrew Stevens [and...@ne...] wrote:
> On Friday 16 Aug 2002 6:08 pm, Bob McElrath wrote:
> > So I just downloaded the neato tool GOPchop, which uses the mpeg2dec
> > library to do its mpeg decoding.  When operating on an lavtools
> > (mpeg2enc) generated file (an mpeg-2 file), I get this message about 20
> > times (on a 2 hour video):
> >
> > buildGOPs @ 669720(0.07%): Whoa: I-Frame number 2 seen in the same GOP!?
> >  (You might want to re-multiplex your MPEG2-PS file first.)
> >
> > Could the lavtools mpeg2enc be generating bogus GOP's?
>=20
> It *could* but I think it is more likely that GOPchop is making some=20
> assumptions about GOP structure that aren't necessarily always true.
> E.g.  Two B frames between I/P frames or a fixed GOP length.
> The error message text suggests the author isn't 100% up on his MPEG=20
> syntax (re-multiplexing won't change GOP structure).

I think you are correct.  GOPchop also generates mpeg's that cannot be
played by mplayer.  :(

> When you activate variable GOP-length in mpeg2enc this less common
> structures can show up.   Easy way to check... set -g and -G to 12 and see
> if the messages disappear.=20

Cheers,
-- Bob

Bob McElrath (rsm...@st...)=20
Univ. of Wisconsin at Madison, Department of Physics

"No nation could preserve its freedom in the midst of continual warfare."
    --James Madison, April 20, 1795