On Saturday 07 October 2006 03:52, Nick Rout <nick@rout.co.nz> wrote 
about 'Re: [gentoo-user] {OT} dvdrip permissions error, alternative?':
> On Saturday 07 October 2006 06:50, Grant wrote:
> > > >  I think I'll stick with:
> > > >
> > > > dd if=/dev/dvd of=image.dvd
> > >
> > > That won't work on CSS scrambled discs. You'll copy the scrambled
> > > data but not the key. Instead, use vobcopy followed by growisofs.
> >
> > What about this (it's what I've been doing):
> >
> > lsdvd && dd if=/dev/dvd of=image.dvd
> >
> > Is there any advantage to creating an ISO filesystem out of the image
> > if you aren't going to burn it?
>
> yes. ease of transfer, keping everyting togther. still playable with
> xine dvd://path/to.iso
>
> why are you naming it image.dvd instead of image.iso?
>
> > Also, is there any way to compress
> > the image without doing any kind of transcoding or that type of
> > reprocessing?
>
> the mpeg streams on the dvd ARE compressed. thats what the mpeg codec
> does. You can try the usual suspects - zip, bzip etc, but you won't get
> far.
>
> programs that reduce the size of DVD9 so that they will fit on a DVD5
> usually requantise the stream, I am not sure what that means, but it is
> much quicker than transcoding to another codec like xvid. xvid will, of
> course, give you a much smaller avi file.

Increasing the quantization increases the number of pixels represented by a 
single data element in the stream.  From what I understand, MPEG2 does 
very JPEG like compression, meaning that square groups of pixels with 
almost the same color will be stored as a single data element with the 
average color (and maybe some hinting).  Increasing the quantization 
further merges some of these groups together, resulting in both quality 
and size reductions.

> the analogy with flac is not really appropriate. flac is a lossless
> compression, you start with an uncompressed wav file and end up with a
> losslessly compressed audio file. A DVD is already lossy compressed to
> mpeg2, so it is not logical to make an analogy with flac.

Agreed.  There are a number of lossless video codecs out there ( 
http://en.wikipedia.org/wiki/List_of_codecs#Lossless_data_compression_2 ), 
though certainly none are in wide use.  I'd wager that since your are 
starting w/ an already compressed stream, converting to a lossless format 
would actually make the video larger.  With DVD it isn't practical to 
transfer uncompressed video, perhaps we might see some uncompressed video 
available with HD-DVD or Blu-ray.

-- 
"If there's one thing we've established over the years,
it's that the vast majority of our users don't have the slightest
clue what's best for them in terms of package stability."
-- Gentoo Developer Ciaran McCreesh