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AutoCAD Tutorial

Circle Enhancement

By Alireza Parsai, 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:

  1. Invoking the VIEWRES command

  2. 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

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