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From: olm@informatik.uni-kiel.dbp.de (Olaf Mehlberg)
Newsgroups: comp.lang.c
Subject: Re: Help w/Plotting a circle...
Message-ID: <4218@snoopy.informatik.uni-kiel.dbp.de>
Date: 14 Jun 91 10:21:15 GMT
References: <1991Jun11.154335.130@sugaree.uu.net> <8433@awdprime.UUCP> <1667@seq.uncwil.edu>
Organization: Universitaet Kiel, Germany
Lines: 41

In <1667@seq.uncwil.edu> session@seq.uncwil.edu (Zack C. Sessions) writes:

>sjb@piobe.austin.ibm.com (Scott J Brickner) writes:

>>In general, though, a quick optimization for any of the circle drawing
>>algorithms is to only COMPUTE 1/8th of the points, and use reflection
>>about the various axes x=0, y=0, y=x, and y= -x for the remainder.

>Umm, wouldn't that be compute 1/4 of the points and then use the
>reflection method to get the other points? Also, when using the
>reflection method to obtain the remaining points, you need to consider
>the coordinates of the center of the circle, if it isn't (0,0).
>The computation is also elementary trigonometry which I won't go
>into since any elememtary book on trig in your library should
>be all you need.

I agree, you have to look for the coordinates of the center, but
1/8th of the points is enough. 
How to do assuming (0/0) as center: (* is the center c is a
computed point r a reflected point)
			         !     c 
                                 !      c 
                                 *-------c
                                 !      r
				 !     r
compute the 1/8 c, reflect them at the --- line and you have 1/4
of the circle. reflect this at the ! line and you have 1/2 of the
circle. flip x/y coordinates and you get the missing 1/2 of the
circle. Then move all coordinates into the right center.

I hope this is correct, sounds bit too simple ;-0

Olaf Mehlberg
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