DVD>DVD4 tecnical questions

[Mighemag]

Hi!
Somebody told me that it is possible to reduce a 9GB DVD into a 4GB DVD without modifying or recompressing the video file. Video quality keeps being the same as the 9GB DVD.
This person also told me that this is possible by compressing the VOB file (just as DropStuff or DropZip do with an image or a text file). Is it true?

On the web site of ffmpeg I read that “ ffmpegX, with DVD>4DVD tool, achieving compression of a VOB file larger than 4GB into 4GB without reencoding”.
They say without reencoding!, thanks to the uses of the M2VRequantiser engine (with kind authorization from Metakine.com).
So, the quality of a 9GB DVD remains exactly the same in a 4Gb DVD, with the DVD>4DVD ffmpegX tool ?
Thus, can I really compress my movie (2 hours length) with a bit rate of 8 Mbps, create my DVD larger than 4 GB (two hours at 8 Mbps produce an mpeg2 file which is certainly larger than 4 GB) and after that reduce the VOB file of the DVD into a 4GB?

Probably there's something I haven't understood quite well.
Could you help me?
Thank's in advance,
Mighemag

[major]

This kind of compression is lossy, so the 4GB version will not look as good as the 8GB one, though the quality is often quite good. Also, if you encode the video yourself, then encoding directly to 4GB will always look better than encoding to 8GB and then recompressing it back to 4GB.

[noki]

Somebody told me that it is possible to reduce a 9GB DVD into a 4GB DVD without modifying or recompressing the video file. Video quality keeps being the same as the 9GB DVD.

ha-ha funny .
if true why have D9s at all? or for that matter why have D10, D14, and D18s just reduce the video file without modifying it AT ALL and put in on a D5. hey, sounds good to me let's do it
BTW never mind the dual-layers burners, i have a triple-layer one
now start a new post staring with...

Somebody told me

[Mighemag]

Dear Noki,
I’ m happy I made you laugh. Your days would never be the same without my topics.
Please just wait until “somebody tells me” some other things and be sure that I'll write you!

Anyway, I have to thank Major for the helpful answer. But thus, what is the meaning of without reencoding?
Is there any manual or text about technical specifications on how software like ffmpeg can reduce the GB of a movie file and the requantiser function?
Thanks
Bye

[major]

Without reencoding means without going through a complete encoding process. This is possible because the source is already mpeg-2 and the motion vectors can be kept unchanged.