using existing motion vectors

[MvB]

Hi,

is it possible to use existing motion vectors in a mpeg2 file to convert a movie into xivd or divx without doing motion search?

I've heard dvd2one does the high-low bitrate conversion of mpeg2 video material this way in unbelievable speed.

Maybe it's possible to use this technique too for mpeg2 -> mpeg4 conversion?

if this question goes totally out of line: Sorry, but i'm not exprerienced enough in this topic to decide wether this question is bull$$$t or not...

[sh0dan]

Reusing motion vectors have been discussed, but the same problem arises as with hinted-ME. The main problem being that there are different optimal motion vectors for different quantizers.

That means that a motion vector at high quality is not always the best, when encoding for lower quality.

Another issue is related to modifying the video source. If you do any kind of cropping or scaling, the motion vectors becomes mostly unuseable, since it would be very hard to maintain them through such a process. Furthermore you'd have unuseable MV's along all edges.

For MPEG2 -> MPEG2 direct transcode it might be useable, but you would still get better quality when letting the encoder do the ME.

[UGAthecat]

what if you were to just use the vector information from the original mpeg file as a guide for the ME of your encoder to start working from.
That way you aren't totally screwing the encoder by forcing it to use values that may not be optimal for it, your just skipping 1 or 2 checks per macroblock for it.

As to this working when resizing the video, I think it would be possible, but you would have to 'resize' the vectors also which could prove to be quite a lot of math, and maybe difficult math to boot. then again, since you could just be using the vector information as a guideline you could use some MV equivilant to point sampling to make the 'resizing' fast. all this resizing code would most likely make the gains gotten by skipping a couple checks per block useless tho.

thats all the pocket change I have for today I think :)