You might consider converting to "higher quality" formats (e.g. dv) *before* editing. sure it will be compressed again when rendered, but at least you can minimize the loss by avoiding the I-and-B-frame problem.
good luck georg On Thursday, 5. July 2007 16:57:38 [EMAIL PROTECTED] wrote: > On Thu, 5 Jul 2007, Pier Luigi Conte wrote: > > Does it exist a method to don't compress again (during the render phase) > > an originally already compressed video? > > Allowing Cinelerra to pass-through frames where possible is a wishlist > feature. At the moment, it doesn't exist. You can get that effect with a > lot more effort using some other tools instead of Cinelerra, but even in > the case where you're just cutting (without doing fancier effects), some > recompression is unavoidable because of the way MPEG compression works. > > The thing is, the MPEG stream contains a few frames called I-frames that > are complete in themselves, but most of the frames are B-frames, which > are stored by storing the differences between the current frame and nearby > I-frames. If your cut eliminates an I-frame, then you MUST recompress the > B-frames that depend on it. (Things called P-frames also exist but > considering them wouldn't really clarify the discussion.) > > For example, suppose you have two video streams that look like this: > > Stream 1: I0 B1 B2 B3 I1 B4 B5 B6 I2... > Stream 2: I3 B7 B8 B9 I4 B10 B11 B12 I5... > > You want to cut from Stream 1 after frame B5, to Stream 2 before frame B8, > so as to get this sequence of frames: > > I0 B1 B2 B3 I1 b4 b5 b8 b9 I4 B10 B11 B12 I5... > > The problem is that frames B4 and B5 depend on frame I2, which isn't > included in your output, and frames B8 and B9 depend on frame I3, which > isn't included in your output. (I showed them with lowercase letters to > make it clearer). So you have to recompress those frames at least. > You've also got an unusually long interval between frames I1 and I4 (I4 is > the fifth frame after I1 whereas the original streams had an interval of > four), and although that doesn't break any rules in this particular case, > there *are* constraints on how long the interval is allowed to be, and > it's easy to imagine cases where cutting could violate those constraints > and you've have to recompress some I-frames too. > > There are also long-term bit rate contraints in things like DVDs, which > you're allowed to violate for a short time but not for a long time, and in > the worst case, cutting together two pieces that are near the limit could > push you over and force you to recompress a substantial chunk of video. > So avoiding recompression where possible is not just a matter of "skip > over the recompression code" - making such an effort actually work, is a > hard problem. -- dr.k.g.hooss schoepfung & wandel wissenschaftliche medienberatung breite strasse 6-8, d-23617 luebeck www.schoepfung-und-wandel.de _______________________________________________ Cinelerra mailing list [email protected] https://init.linpro.no/mailman/skolelinux.no/listinfo/cinelerra
