Chapter 7 Computer-Aided Drafting and Design 153
process involves many changes and refine- fi
ments 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 moni-
tors 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 file fi
from a CAD program as a bitmap fi le. The file fi
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 for 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 modified 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 defined with
coordinates. Most CAD programs provide a
drawing grid that can be used to define objects
with coordinates. This method of drawing is
similar to the graph method used in manual
Vector Image
A
Raster Image
B
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X
Y
Pixel
Figure 7-3 A comparison of vector and raster displays.