January 6, 2003
Circles are very easy to create and very common
to use. In most of drawings you draw one or more circles. There
are 6 standard methods to create circles. You can later use
PROPERTIES command to enhance the area or circumference of the
circle [Figure 1].
Figure 1
Viewing Problems
If you zoom in a drawing the circles might
appear to be polygons. An instance remedy is to regenerate the
drawing. If you are using multiple viewports you can invoke
REGENALL command to regenerate all viewports at the same time.
There are two solutions to prevent this problem:
-
Invoking the VIEWRES command
-
Setting the WHIPARC system variable
Invoke VIEWRES command and assign a proper value
to control the view resolution for circles and arcs. The more
the value is the better the quality of circles will be. It also
increases the regeneration time. The acceptable values lie
between 1 and 20000.
WHIPARC is the latest solution to this problem.
If you enable the variable (i.e. set it to 1), zoom factor will
not affect the quality of circles appearance. However, redraw,
object selection, and OSnaps become slower. By enabling WHIPARC,
the VIEWRES has no effect on circles. The default value for
WHIPARC is 0 (i.e. disabled).
Geometrical Problems
If you rotate a circle about its center-point
nothing happens. Circles retain their original quadrants. Thus
the angular location of quadrants cannot be changed. The
quadrants are not affected by changing the Snap angle either
[Figure 2].
Figure 2
You are not allowed to stretch a circle with
grips. Stretching a circle results in scaling the circle about
its center point. It is obvious that a stretched circle is an
ellipse.
This single clue leads us to a proper answer for
our problems. If you replace circles with ellipses you can
rotate their quadrants, and stretch them with the help of grips.
Ellipses are enhanced types of curves in AutoCAD so despite of
the VIEWRES or WHIPARC values, changing the zoom factor will not
affect their appearance.
I have developed a program that replaces circles
with their equivalent elliptical form. The appearance of the
drawing will not change. But later you are allowed to rotate the
quadrants [Figure 3]. Also, the stretch is possible, and the
quality of view is guaranteed.
Figure 3
I have also implemented a command that aligns
quadrants of an ellipse with horizon. To use the commands, visit
http://www.geocities.com/cad_tips/downloads.htm#autolispvlispprograms
and download C2E, ALEPS, and STC files. The C2E command replaces
selected circles with their equivalent ellipses. You can use
ALEPS to align selected ellipses with horizon. If you invoke STC
you can simultaneously alter circles to ellipses and stretch
them.
Note that Tangent OSnap is not fully compatible
with ellipses.
About the Author
Alireza Parsai is the founder of cadpanel.com. He is a
mechanical engineer with more than 12 years experience with
AutoCAD. Alireza is an Autodesk Authorized Author, the AutoCAD
consultant for Al Khawarizmi Institute, a contributing editor
for Augiworld magazine, a writer for CAD Digest, an instructor
for AUGI Training Program, and an elected speaker for Autodesk
University 2002. He has written 7 books in Farsi, 3 booklets in
English, and more than 30 articles for different magazines and
newsletters. You can reach him at
alireza.parsai@cadpanel.com or visit his website
www.cadpanel.com.
