Chapter 7 Computer-Aided Drafting and Design
145
assembly, or product in 2D or 3D form. The
process involves many changes and refi nements
before a design is fi nalized. With a CAD system,
design changes can be made and evaluated
quickly.
Computer graphics programs fall into one
of two classifi cations. These classifi cations are
based on how the actual images are created.
Images in a computer graphics program may
be created with vector objects or raster objects.
Drawings created in a CAD program are made
up of vector objects. Vector objects are made up
of lines (vectors) and arcs and are defi ned with
point coordinates in space. See Figure 7-3A.
Raster objects are defi ned using tiny shapes
of data called picture elements, or pixels. See
Figure  7-3B. Pixels are arranged in a fi xed,
precise manner. Each pixel is the same size
and shape. The number of pixels making up an
image defi nes the resolution, or visual quality,
of the image. Raster objects are also known
as bitmap graphics. Image editing programs
are commonly used to alter bitmap graphics.
Televisions and computer monitors are examples
of devices that use raster displays.
A vector-based drawing can be converted
to bitmap form in several different ways. The
most basic way is to export a drawing fi le from
a CAD program as a bitmap fi le. The fi le can
then be imported into a different program for
editing. For example, a 3D model created with
a CAD program may be converted to a bitmap
fi le and edited to add special effects, such as
lighting and shadows. If a vector drawing is
output as hard copy, it can be converted to
bitmap form with a scanner. A scanner is an
automatic digitizing device. It analyzes the lines,
circles, and other graphic elements of the drawing
and converts the objects into computer data.
Referring to Figure 7-3, vector images are
edited by modifying the individual lines and
arcs making up the drawing. Raster images
are modifi ed by editing the individual pixels
making up the image. In creating or editing a
vector object, such as a line, it can be helpful to
visualize the object as an entity defi ned with
coordinates. Most CAD programs provide a
drawing grid that can be used to defi ne objects
with coordinates. This method of drawing is
similar to the graph method used in manual
drafting. The graph method, discussed in
Chapter 3, is used to enlarge or reduce drawings
with coordinate grids. When using a CAD
program, objects can be drawn to the desired
size by defi ning coordinates or other parameters
or by scaling an original object to a different
Vector Image
A
Raster Image
B
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Y
Pixel
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Figure 7-3. A comparison of vector and raster displays.
Copyright Goodheart-Willcox Co., Inc.