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View Full Version : GOPeditor "fencepost" question



lsmod
03-09-2003, 04:42 PM
First off, thank you jdiner for the excellent tools. I just completed a test DVD with three programs from three different channels from my DTivo, and everything went great. (Mine is a crossplatform workflow: TyTool6r3/GOPEditor to get an edited mpeg TS, demuxed with jrdmux, patched with DVDPatcher on the XP...copy to Mac, Author in DVDSP, patch .m2v in situ with DVDPatcher over the SMB share off the Mac, and build/format with DVDSP.)

As I'm now ready to start burning my archive of .ty's to DVD, I have a detail question about what GOPeditor really does. We, the users, specify I-frames as cut points. But those I-frames are just fenceposts: What isn't clear to me, and will affect my editing decisions, is what exactly is done with those cut points.

For instance, let's lay out a string of frames. I-frames are numbers, P- and B- frames are letters. (I'm lumping them together for this example.)

1aaaa2bbbb3cccc4dddd5eeee6ffff7gggg8hhhh9

If I cut out a commercial, say, specifying a cut start of 3, and a cut end of 6, what do I get?

It seems like I'd get:
1aaaa2bbbb36ffff7gggg8hhhh9

But I could just as easily see that Josh would have decided to give me:
1aaa27gggg8hhhh9

or

1aaaa26ffff7gggg8hhhh9

I don't have the tools at the moment to do frame-by-frame MPEG analysis, so I thought I'd just ask.

jdiner
03-09-2003, 06:51 PM
Using the terms you put in your message let me make this a bit clearer.

By default in an MPEG-2 stream you get things out of display order, as they come in decode order.

So you get for the very first GOP:

IPBBPBBPBBP ... IBBPBBPBBP ...

So in the first segment only you go I to P...

Then for every GOP segment after that you get IBBPBBP where the first 2 B frames are really part of the previous GOP.

When I do the cutting I cut it such that things still line up.

I2 P2 B2 B2 ... I6 B2 B2 P6 B6 B6 P6

So if you were cutting from 3-6 from the file. You would lose all of 3, all of 4, all of 5, and then with re-alignment have all of 6.

--jdiner

lsmod
03-10-2003, 03:25 AM
OK, so to put in simple terms, the behavior of the editor is that the I-frame at the beginning of the cut is the first one removed, and the I-frame at the end of the cut is the first one shown after the cut.

Trying to get back into the MPEG headspace again...It's been too long.

So to use my original syntax (Which was presentation order, sorry about that), it's:

1aaaa2bbbb6ffff7gggg8hhhh9

Got it, thanks. ;)