Cindy Shaw wrote:

They're talking about slide scanners, PhotoCDs, etc. on another
listserv, and I was wondering if the following quote is true:

"The point is, once compressed with JPEG, a lossy image compression
model, your image is forever degraded to some degree and while you can convert
image formats, you can never replace the image data which is gone."

Cheers,
Cindy
--------------------------------------------------

Cindy,

I'm am absolute neophyte, but from my leapfrog reading of the highly
recommended book Real World Scanning and Halftones (written by David
Blatner and Steve Roth and published by Peachpit Press, Berkeley, 1993), I
believe the statement is accurate.  My understanding is that once an image
is compressed via a lossy image compression model (which JPEG is), any
information that is eliminated by that compression model is lost
irretrievable.  I do believe that when saving something as a JPEG out of
Photoshop, you are given a choice of levels of compression to selectively
control the loss.

The above-cited book has this additional info on the subject (pp. 187-188):

        "The most common lossy compression scheme is currently JPEG (for
Joint Photographic Experts Group), though different programs implement the
method differently, with varying results.  Another method is fractal
compression, which is potentially a better system but can take
substantially longer to compress (and isn't widely supported).  Lossy
compression is based on mathematical algorithms that are too complicated to
include . . . However, one thing you should know is that these are
asymmetrical compression methods--it usually takes much longer to compress
an image than it does to decompress it.
        JPEG++ is a slightly different format that only products from Storm
Technology can write, and that all programs that support JPEG can read.  It
lets you selectively avoid compressing parts of an image, while doing
normal JPEG compression to the rest of the image.  The [sic] could be
helpful, for instance, if you had to send an image of a medical X-ray over
a telephone line.  You could compress everything but the most important
parts of the image at a high level, and compress the important parts at a
low level.
        Here are a few things to remember when working with JPEG.  First,
note that images with hard, high contrast, and angular areas are most
susceptible to develop artifacts from JPEG compression.  For example, a
yellow square on a green background in a lower-resolution image would look
pretty miserable after compression.  Second, compressing and decompressing
images repeatedly can make images worse than just doing it once (this is
reportedly a problem specifically with QuickTime's JPEG compression, but
not with most others)."

With all due credit to David Blatner, Steve Roth, and Peachpit Press . . .

Christina

Christina L. Jordan, Principal Illustrator
University of California, Berkeley
College of Natural Resources
Dean's Office
101 Giannini Hall #3100
Berkeley, CA 94720-3100
Phone: 510/642-4167, FAX: 510/642-4612
E-mail: [log in to unmask]