ftp.cdrom.com/pub/cdrom//cdrom.tar/cdrom/textinfo/pcd_scan.txt

Text File | 1993-11-08 | 7KB | 121 lines

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Subject: PhotoCD vs Highend scans
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Hi all,

Here's a tidbit of information on Photo CD from Kodak.  I wrote the following
article for a local prepress trade publication in late July of this year.  I
gives some insight to the generation of Photo CD images.  Hope you find the
information fun and useful :)

Neka


While Compact Disc - Read Only Memory (CD-ROM) storage technology has been in
widespread use for a number of years, the emergence of Kodak's Photo CD
technology may prove to be the biggest ambassador of compact disc technology
to-date.  Photo CD will help other Compact Disc (CD) technologies, such as
Compact Disc - Interactive (CD-I) and Full Motion Video (FMV) on CD to gain a
foothold, helping them to become as commonplace in U.S. homes as the
omnipresent videocassette recorder.
When Kodak first introduced the Photo CD player in late 1990, Photo CD was
envisioned as the beginning of paperless photography.  Connected to
television set or color monitor, the Photo CD player allows a user to display
digitally encoded photos, and can even provide limited special effects, such
as controllable zoom and pan of each image.
Almost immediately, color publishers saw it as an inexpensive means of
acquiring high resolution scans.  At less than two bucks a piece, Photo CD
images are truly a bargain when considering the quality of the image.
Each Photo CD image is stored on the disk as an "Image Pac" at five different
pixel resolutions--192 by 128, 384 by 256, 768 by 512, 1536 by 1024, and 3072
by 2048 using a color encoding system known as YCC.  Presently, Photo CD
images can only be generated from 35mm film negatives or slides.
Kodak just began shipping a larger format scanner to photofinishers which
will generate Photo CD images from 2 1/4 and 4 by 5 transparencies.  Known as
Photo CD Pro, the new format is being developed specifically to address the
needs of professional photo labs and color prepress firms.
Kodak's original vision was to have images scanned to look like the original
scene.  Referred to as scene space, the objective was to render the picture
to meet the viewer's expectations, or, as the viewer remembered the scene in
the minds eye.  Many professional photographers objected to the scene space
approach.  Professional photographers make conscious decisions about a film's
color tendencies and how that will affect the final image.  Using the scene
space approach negated their conscious selection of film type.  In response,
Kodak released two "universal" film term specifications, one for Kodachrome
and one for Ektachrome.  The film terms are designed to preserve the
tendencies of the original color transparencies.

The primary difference between highend scans and Photo CD is the scanning
technology use for generating a Photo CD image. The Photo CD scanner uses
Charge Coupled Device (CCD) mechanisms.  Thus, Photo CD suffers from the same
deficiencies in capturing shadow and highlight detail as does the typical
desktop CCD scanner.  Photo Multiplier Tube (PMT) drum scanners have a
greater density range (0.0 to 3.9) which allows them to capture more detail
in the shadows and highlights. In effect, more detail equals more data, and
more data yields better picture quality.  Secondly, and a bit
more complicated, Photo CD images are designed to be viewed on a television
using Kodak's YCC format.  To bring a Photo CD image into your favorite image
editing application on the computer, a conversion must take place.  Kodak
first released a conversion utility called Photo CD Access which converts
native YCC to RBG format. Kodak also offers PhotoEdge, which adds some
additional image manipulation capabilities.  However, the YCC to RGB
conversion performed by either Photo CD Access or PhotoEdge does not provide
an accurate conversion of the original image. The poor conversion is due to
the algorithms used to convert the image, which assumes the target conversion
is RGB video, or simply put, broadcast video
.  Unfortunately, images prepared for broadcast video don't reproduce well
for print.  Users will experience a gain in the mid-tone values, and a
clipping of highlight values.  Additionally, image
s converted using Photo CD Access or PhotoEdge are optimized for a white
point of 6500o K as opposed to the standard prepress viewing environment of
5000o K.  The white point differential between the two standards will show up
as an off-color image when printed.  To address the short-comings of Photo CD
Access and PhotoEdge, Kodak also provides a new plug-in for the Macintosh
called Photo CD Acquire.  For $59.95, Photo CD Acquire allows the user to set
white point and gamma targets. CD Acquire also provides for image cropping,
color correction, and monitors targets which optimize conversion algorithms.

Adobe Photoshop 2.5 allows for the acquisition of Photo CD images through
it's "Open As..." menu selection. However, be aware that while it functions
similarly to Photo CD Access, it doesn't provide the same options as Photo CD
Acquire.  The last important consideration that must be accounted for
concerns the conversion process as well.  One must remember that every time
an image is converted into a different color space,
YCC to RGB to CMYK, data is lost.

The Long and Short of it...
A word of caution to art directors, designers, and prepress production
personnel. The old saying "You get what you pay for" definitely holds true
here.  High-end drum scans are still the way to go for critical, high-quality
color work, and Kodak agrees.  Photo CD images are great for pleasing
color-budget-minded publications such as catalogs and spec sheets.  In fact,
the economy of Photo CD images allows designers and other electronic image
developers to produce four-color publications when they could not have done
so before.