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Two Kinds of Computer Graphics
There are two kinds of computer graphics -
raster (composed of pixels) and vector
(composed of paths). Raster images are more commonly called bitmap
images.
A bitmap image uses a grid of individual pixels where each pixel
can be a different color or shade. Bitmaps
are composed of pixels.
Vector graphics use mathematical relationships between points and
the paths connecting them to describe an image. Vector
graphics are composed of paths.
The image to the left below is representative of a bitmap and the
image to the right is representative of a vector graphic. They are
shown at four times actual size to exaggerate the fact that the
edges of a bitmap become jagged as it is scaled up:
Bitmap Image:
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Vector Graphic:
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With Adobe® Systems' introduction of the PostScript®
page-description language computers could display fonts and images
using point-to-point math rather than by pixels alone. The advantage
to using a page-description language such as PostScript becomes
clear when you scale an image up. The larger you display a bitmap,
the more jagged it appears, while a vector image remains smooth
at any size. That is why PostScript and TrueType® fonts always
appear smooth - they are vector-based.
The jagged appearance of bitmap images can be partially overcome
with the use of "anti-aliasing". Anti-aliasing is the
application of subtle transitions in the pixels along the edges
of images to minimize the jagged effect (below left). A scalable
vector image will always appear smooth (below right):
Anti-Aliased Bitmap Image:
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Smooth Vector Image:
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Bitmap images require higher resolutions and anti-aliasing
for a smooth appearance. Vector-based graphics on the other hand
are mathematically described and appear smooth at any size or resolution.
Bitmaps are best used for photographs and images
with subtle shading. Graphics best suited for the vector format
are page layout, type, line art or illustrations.
Wherever possible use the vector format for all your type, line
art and illustrations and only use bitmaps for photos or images
with complex or non-uniform shading. If the graphics application
recognizes native vector files such as those created by Deneba Canvas™
(a filename with an extension of .CNV- for versions 6 and
7), Adobe Illustrator® (a filename with an extension of .AI),
CorelDRAW® (a filename with an extension of .CDR), or Macromedia®
FreeHand® (a filename with an extension of .FH8 - for
version 8), then use them first.
The EPS File Format
If the graphics application you are using cannot
read native vector files the next best thing would be to save them
as EPS (Encapsulated PostScript) files. These are self-contained
PostScript files which contain the same mathematical descriptions
as the vector files they are made from. Even bitmaps can be saved
in the EPS file format. EPS files are supported by most all graphics
applications. It is the most portable format for this reason. It
is best to use EPS files for all line art and illustrations because
they can be reproduced at any size or resolution and still display
exactly as they were drawn. Use them wherever native vector files
cannot be used.
A Little of Both In Today's Applications
Today's graphic artist has to master both skills
- image editing and illustration. In fact, Adobe Photoshop®
- an image editor - incorporates vector based paths which can be
exported as native vector files. Four of the major illustration
programs - Deneba Canvas, Adobe Illustrator, CorelDRAW and Macromedia
Freehand - allow bitmaps to be embedded in the vector files they
create.
I believe that the ideal page layout program of
the future would be primarily a vector-based application which would
create, import, display and print native vector objects side by
side with bitmaps. As computing power becomes more affordable I
believe this will happen.
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Images and text Copyright © 1998 by Mike Doughty, All
Rights Reserved. This tutorial reproduced here with permission.
For more tutorials and help, visit Mike's Sketchpad at http://www.sketchpad.net
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