Chapter 7 Digital Prepress 147
black, and two spot colors) and a master image.
The desktop color separations 1.0 (DCS 1.0) format
creates fi ve separate fi les, one for each process color
(CMYK), and a data or master fi le. The DCS format
supports grayscale, RGB, spot color, and CMYK
color models. DCS fi les print faster than standard
EPS fi les and can contain both bitmap and vector
graphics information.
Graphics Interchange Format (GIF)
The graphics interchange format (GIF) supports
raster images and only handles up to 256 colors.
GIF fi les offer lossless data compression, which
makes them particularly effective for drawn images,
animations, and images used on the Internet.
Lossless data compression will be discussed later
in this chapter.
Portable Document Format (PDF)
The Adobe® portable document format (PDF)
has become a standard for electronic document
distribution throughout the world. PDF is a universal
fi le format that preserves all aspects of a native fi le,
regardless of the application or platform used to
create the PDF fi le. Anyone using Adobe® Reader®
can view, navigate, and print the fi le exactly as the
author intended.
Unlike the complex, continuous stream of data
in PostScript fi les, PDF fi les are simple, compact,
vector fi les. They process quickly and can be sent
across the Internet or a network for remote proofi ng
or printing. PDF fi les are also page-independent,
so single pages can be replaced or altered without
reprocessing the other pages. Page independence
also allows printing pages in any order from a single
fi le. The PDF fi le is also self-contained, meaning that
the fi le has all the fonts and other resources needed
to image it.
File Names
File naming conventions are often overlooked, or
even ignored. However, carefully naming fi les helps
keep work organized. Whether creating a standard
in-house convention or following recommendations
from a printer, the fi le format must be consistently
applied.
Computer platforms and programs are subject
to their own conventions. Even though some of the
latest operating systems allow fi le names up to 255
characters, fi le names should be limited to fewer
than 20 characters with a three-character extension.
Other general rules for fi le naming include:
• Use only alphanumeric characters; symbols
should be avoided.
• File names should not begin with a space.
• Each fi le name should be unique.
• Use the appropriate fi le extension to identify fi le
type, such as .tiff, .eps, or .pdf.
To avoid confusion, revised fi les should not be
submitted with the same name as the original fi le. If
you are using OPI software, however, it is important
that the fi le names remain the same as the original.
The fi le name serves as the link to the high-resolution
image, and changing the fi le name requires that the
link be reestablished, which causes delays.
File Compression
Before sending digital data to a printer, most
publishers compress, or reduce, the size of the
fi les. Some programs automatically compress the
fi le when it is converted and decompress when it
is viewed. Compressed fi les require less storage
space, allow more effi cient data management, and
can be transmitted faster because redundancies and
other unnecessary elements are eliminated from the
original fi le.
Lossless Compression
A lossless compression algorithm refers to a
data compression process in which no data is lost.
The PKZIP compression technology is an example
of lossless compression. The fi les are often referred
to as ZIP fi les and typically have .zip as the fi le
extension. PKZIP fi les with an .exe extension are
self-extracting fi les, which can be unzipped simply
by opening the fi le. Decompressing either of these
types of fi les is called unzipping.
For most types of data, lossless compression
techniques can reduce the fi le size by about 50%.
Lossless algorithms used for image compression
assume that the likely value of a pixel can be inferred
open press interface (OPI) system: A computer
confi guration and software that allows the designer to use
low-resolution images when creating document layouts in
page composition programs, and high-resolution images
automatically replace the low-resolution images when the
fi le is sent to an output device.
lossless compression algorithms: A mathematical
formula for image compression that assumes that the
likely value of a pixel can be inferred from the values of
surrounding pixels.