Digital BW, The Print

It is not just an issue of speed. There is an information theory theorem upon which the 
Lempel-Ziv compression algorithm is based. It says that for arbitrary data, there is an 
optimal compression. Once files get pretty big - and just about
anything that we would  care to compress counts as pretty big - on average you can't do
better than Lempel-Ziv  coding.  That's what is used in a standard zip and unix compress
files.

It's one of those "obey the speed limit of light, it's the law!" kind
of things.

The best you can do lossless is usually about 2-fold compression.  That is why we accept 
lossy coding for images. With lossy coding we all know you can do much better, and the 
price paid is relatively small. But it is not lossless and the picture will continue to degrade 
as you make changes and resave.

There may be better compression for certain types of images, if one can make  
assumptions, but that's not usually a useful strategy across many images. Examples where 
it can help is in high resolution "line art" where the vast majority pixels are white. It may 
turn out that one can make more assumptions about data in images than one can, for 
example, do with an arbitrary program file.  But, in general, it just does not seem like this 
is the case. That is why you don't see lots of better coding schemes being used. On  
average they are just not better.

Costa Colbert

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, The Wogster
<wogsterca@y...> 
wrote:
> Anthony G. Atkielski wrote:
> > The Wogster writes:
> > 
> > 
> >>For archiving though, where you are not looking at the image on a
> >>regular basis, you can use other compression mechanisms.  For
example if
> >>you can compress a 600MB file down to say 50MB for archiving, and
then
> >>decompress it again later, who cares?  Even if it takes an hour
to 
> >>compress/decompress.
> > 
> > 
> > Agreed.  But what file formats would be in this category?
> > 
> > Theoretically, you could compress most image files enormously
with no
> > loss, given time and space to do an absolutely optimal
compression.
> > I've never heard of a file format that is designed for this
purpose,
> > though.
> > 
> 
> The best I have seen is around 50% compression, having tried a few 
> general purpose compression formats, with a ~6MB TIFF file, I would 
> expect that a compression format designed specifically, would have 
> better luck, even in a lossless form.
> 
> The real issue comes down to, if your scanning a 4x5 negative, and
then 
> post processessing, do you need to store the files forever on the 
> computer? As long as you still have the negative, then you can
always 
> rescan and repeat the processing can be done again, to give a new
result.
> 
> When I did printing is the darkroom, I rarely kept printing notes,
other 
> then that on the back of the contact print, so I knew where to
start 
> from.  This was often because, the print I wanted 6 months or 3-4
years 
> later might be very different, considering the mood I was in.
> 
> We sometimes forget in print photograpy that, like music and
theatre, 
> the art is double staged, the negative (or initial digital image)
is the 
> symphony (or script), the print is the performance.
> 
> W