Re: [Leica] Archiving Leica photos on CDrom (long)

[James J Dempsey <jjd@jjd.com>]

Alfred Breull writes:
> You may also keep the thumbnails uncompressed on your CD for a 
> quick picture identification, but compress the original picture 
> files via arj or pkzip. Compression programs usually reduce the 
> original files onto 30 or 40 percent.

You have to be a little careful here when talking about compression rates.
There are two basic kinds of compression:  lossless and lossy.  Losless
compression means that no information is lost when compressed.  When you
uncompress, you get the exact same bits back -- bit for bit.

Lossy compression can compress much more, but you don't get the exact same
bits back when you uncompress -- some information is lost.  This usually
isn't a problem for snapshots or web production, but it might be for
producing art-quality 8x10s.

Lossless compression is used in PNG, TIFF, and GIF.  (Unfortunately GIF has
the added disadvantage that it supports only 256 color (8-bit) images, not
true color.)  Lossless compression generally reduces the size of your image
file by a factor of 2x, but that number can vary wildly depending on the
subject matter.  For examples, pictures of snowmen in the snow (i.e. mostly
white) compresses very well while average images will compress less.

JPEG uses lossy compression, but can offer almost arbitrarily large
compression rates depending on how much quality you are willing to give up.
Generally, if you compress by a factor of 4 or 8 most people can't tell the
difference on computer-viewed images without looking very carefully.
However, digital artifacts will likely be apparent in shadow detail when
printed on a quality printer.

Kodak PhotoCD also offers lossless compression, and that is how I have been
digitally storing my images.  It has the other nice property of storing the
images in multiple resolutions so you don't always have to deal with a 50MB
file when you want to print a snapshot.  Unfortunately, PhotoCD is a
proprietary format and the only software that could write PhotoCD format
files (Build-It/Arrange-It from Kodak) has been discontinued.  This means
that if you want to write PhotoCD, you have to use a service provider who
has a Kodak workstation.  (I complained to Kodak to no avail.)

Finally, if you are into digital imaging, web production or are just
interested in photography, I recommend you check out Phil Greenspun's
Photo.net web site at: http://photo.net/photo/ .  He has lots of good tips
about digital production for the web and about photography in general.

                       --Jim Dempsey--
                         jjd@jjd.com
                       http://jjd.com/