[mythtv] Lossless MPEG2 editing!

Nigel Pearson nigel at ind.tansu.com.au
Mon Jan 24 19:14:47 EST 2005

	Cory, thanks for your clarifying thoughts here.
I didn't realise about PTSs.

> - GOP vs. non-GOP accurate edits.  MPEG2 streams have I, P, and B 
> frames. Only the I frames can "stand alone."  The other two rely on 
> previous/future combinations of other frames to generate a frame.

	Yes, but...

a) If I want to chop say 5 seconds off the end of a sequence,
the edit needn't and on an I-frame. If playing forward, it will
be frame accurate. If playing backwards from the end, there is
garbage until the next I-frame, but from my point of view that
is an imperfect scenario anyway (e.g. if the audio is even able
to be played back, it sounds strange, so a video glitch is OK)

b) If I want to take a few seconds out of the middle of a sequence,
then there are 2 simple "hacky" scenarios:

1) Frame accurate start of cut, non-frame-accurate end of cut
(join sequence at next I frame like the crude original utility)

2) Frame accurate start and end of cut, by generating a new
I-frame at the end of the cut

	Now, these are far from perfect. They both change the bitrate
of the original stream, and neither would seek backwards correctly,
but I think they are a workable alternative to transcoding.
Until someone codes up a self-contained correct/perfect solution!

Nigel Pearson, nigel at ind.tansu.com.au | "In this city I confess
Telstra BI&D, Sydney, Australia       |  god is mammon, more is less
Office: 8255 4222    Fax:  8255 3153  |  off like lemmings at the gun
Mobile: 0408 664435  Home: 9792 6998  |  I know better, still I run"

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