Information Technology Reference
In-Depth Information
space savings without losing any information regarding a specific
image. It allows image data to be compressed greatly, and this ap
proach motivates the detailed mechanisms for fax machines. Fax
machines process a page by scanning one line after another, and this
approach to data compression allows an efficient transmission of
the images.
files when surfing the Web. How do these image
files work?
GIF
is an abbreviation for Graphics Interchange Format and
specifies a particular file format copyrighted by CompuServe
Incorporated. Several variations are possible, but a picture stored
in a .gif format basically uses one or more color tables, where each
table is limited to 256 colors. Although .gif files also can take ad
vantage of predefined font tables, the main usage of .gif files arises
in pictures containing relatively few colors. When pictures contain
direct colors without a great deal of shading, the abbreviation of
colors through a color table such as that used for .gif files can be
particularly effective.
Images can also be saved as
PostScript
files or “.ps” files, a
framework based on drawing commands. For example, the
“moveto” command traces a line from a current position to a speci
fied new one, and the “arc” command draws an arc (including a cir
cle), given its center, radius, and so on. To supplement these basic
commands, PostScript also allows general shapes to be defined and
then used in multiple places. Thus, fonts for characters can be de
fined once (like font tables, but with drawing instructions), remem
bered, and then used as often as desired. This format is particularly
useful when a picture can be rendered efficiently as a drawing or
when the image contains many of the same elements (e.g., text us
ing the same fonts).
.jpg
or
JPEG
files, created by the Joint Photographic Experts
Group (and hence the acronym), are based largely on what people
actually see. It turns out, for example, that human vision relates
more to luminance than specific color, and the
JPEG
approach takes
this into account. The specifics are quite complex, but the idea is to
divide a picture into blocks of 8 by 8 pixels. These blocks are then
