necessary to direct the electron beam so it strikes the
correct phosphor dots as the electron gun scans the
screen (fig. 3-9).
Color monitors are available from standard EGA
levels to pixel levels of 2,048 by 2,048. The monitor
that is selected for your imaging system must match the
display card in the computer, since it is the display card
that limits the resolution of the monitor.
DIGITAL-IMAGE FILE STORAGE
Color digital images take up an extremely large
amount of memory when they are stored. Methods,
such as optical media, have been developed to
overcome this storage problem. Optical media is very
suitable for storing digital photographs. Some
examples of optical media include the following: Write
Once, Read Many (WORM) disks, erasable optical
disks, and optical memory cards. An example of
nonerasable memory is the Kodak Photo CD; this CD
allows high-quality color images to be stored for
archival and retrieval purposes.
One Kodak Photo CD can store up to 650
megabytes. This equates to 100 high-resolution, color
digital images when stored in compressed form. These
images are stored at five different resolution levels,
ranging from 128 by 192 pixels for a proof, or thumbnail
sketch, to a high resolution 2,084 by 3,072 pixels
(compressed) full-color image. These images can be
imported using photo software packages, then they can
Figure 3-9.Cross section of a CRT.
be manipulated, printed, or placed in various layout
Image compression makes it possible to take a large
color-image file and reduce its size. This reduces the
amount of memory required to store it or decreases the
time required to transmit it. Compression can reduce
the amount of memory needed by a factor from five to
one hundred. Various compression-decompression
chips, add-on boards, and software are available in the
Image compression is made possible because in a
typical digitized image, the same information appears
several times. For example, areas of the same color in
different parts of the image or straight lines contain the
same information. This duplication of information
values, or REDUNDANCY, can be identified in three
types as follows:
Spatial redundancy. This results from
dependence among neighboring pixel values.
Spectral redundancy. This results from an
association of color (RGB) planes.
Temporal redundancy. This results from a
correlation between different frames in a
sequence of images.
The most common compression program has been
formed by the Joint Photographic Experts Group
(JPEG). The technique used in JPEG compression
allows the user to select the compression ratio.
High-compression ratios generally result in low
image quality. This low image quality is a result of
avoiding the risk of losing data as the image
compression ratio is increased. The amount of image
compression depends on the amount of redundancy that
exists in an image. When a compressed image is
reconstructed (uncompressed) and the pixel values are
identical to the original image, the compression is
known as lossless. When discrepancies occur between
the original and the reconstructed image, the
compression is called lossy. Lossless compressions can
be achieved with compression ratios of up to 5 to 1.
Files that are compressed may be identified by the file
A number of methods for making digital
photographs are used. Some of these technologies
include the thermal-dye transfer, inkjet, thermal-wax