Regenerating keyframes?

[Velocity 7]

I'm just wondering, is it possible to regenerate keyframes in a file that's already been encoded? I don't want to have to recompress the file just to get the same keyframes, but rather have a program that parses an AVI or OGM XviD file, searches out which frames are good keyframes, then makes them as such.

If at all possible, is there a modified VirtualDub or something I can use that can do this as well? Re-creating keyframes when saving the file rather than having to re-compress to do the keyframe generation...(this of course means discarding the old keyframes as well, if they even exist)

EDIT: Suddenly this thread looks like it fits more in the VDub forum than it it does here, even if it's an XviD file I'm working on. Please move the thread.

[toraneko]

that could indeed be a useful tool in order to split video files, as VirtualDub (with "direct stream copy") has to start clips on key-frames.

the question is : can a P-frame or a B-frame be "compressed" as a key-frame without any loss (as any difference will produce a "ghost" effect until next key-frame) ?

[crusty]

I think recreating keyframes might be possible but would produce quite some quality loss for the resulting clip. It would also mean a change to the codec used, because Vdub in itself can't reencode anything without the proper codec. And a rekeying like this, even if it is possible, HAS to be supported by the codec, which it isn't right now.

[toraneko]

DivX, XviD and others are all (more or less) MPEG-4 compliant codecs. which means XviD decoders can decode DivX material and vis versa.
in the worst case, quality loss will only occure from the "new inserted key-frame" to the next (regular) key-frame.
we are not talking about "re-encoding" anything. in fact, it would be simplier to use a new little software which will have to
1°) copy the video stream from the beginning to the frame before "the pre-key-frame"
2°) insert a "P-frame/B-frame transmuted into key-frame"
3°) copy the video stream from the next frame to the end

PS : sorry for my bad BAD english...

[sysKin]

Originally posted by toraneko
1°) copy the video stream from the beginning to the frame before "the pre-key-frame"
2°) insert a "P-frame/B-frame transmuted into key-frame"
3°) copy the video stream from the next frame to the end
In theory, it would work and would result in what crusty said - visible quality loss until next I-frame.
In practice, problems start with b-frames: not only picture information (which can be re-created using i-frame to some extend) but also motion information (which is absent in i-frames) is used to decode them.
All b-frames before the transcoded p-frame would be lost (or broken).

Only a P-frame could be changed this way: b-frame could not, because that would break entire decoding order (next p-frame would suddenly get a new, different reference).

In my humble opinion it's better to encode correctly to begin with ;) but dev-api-3 does not support forced keyframes, I know :(

Radek

[toraneko]

that means GK doesn't do anything when I edit the "1st pass stats" file (with the "graph" and stuff) ?

[Velocity 7]

Okay, so perhaps it isn't as simple then; how about a modification to VirtualDub (aka VirtualDubMOD) that analyzes the motion information in the entire video to have keyframes reconstructed, then strip all keyframe information from the video and re-insert? Basically it's using a new "Save to AVI" command, except it would be "Save to AVI with new keyframes" and would only be enabled if "Direct Stream Copy" is checked.

Of course, in a case like this, this means that the same codec that was used for encoding needs to be present and available in order to accomplish something like this; VirtualDubMOD needs to be able to analyze all the information correctly and use the proper codec (e.g., XviD) to re-create the same type of keyframes, though in different positions.

I have absolutely no knowledge in P, B, and I-frame stuff, so hear me out. >_>