Path: utzoo!utgpu!attcan!uunet!tut.cis.ohio-state.edu!ucbvax!decwrl!adobe!schiller
From: schiller@adobe.com (Steve Schiller)
Newsgroups: comp.graphics
Subject: Re: Tangents to Three Circles
Message-ID: <1069@adobe.UUCP>
Date: 12 Aug 89 00:16:45 GMT
References: <859@mrsvr.UUCP>
Sender: news@adobe.COM
Reply-To: schiller@adobe.COM (Steve Schiller)
Organization: Adobe Systems Incorporated, Mountain View
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Actually, in the general case there are eight circles
tangent to three given circles.  (As people have
pointed out there are degenerate cases with less
solutions.)  The reference I used to solve this
problem was a book published around 1900 on conics.
Conics seemed to be a hot topic around then.  Anyway,
I suggest you go to the largest library near you
and try to find your own tome on conics.  Then you
have to translate the mathematical concepts into
numerically stable code.  It was several years ago
when I wrote this code and unfortunately I don't
really remember how I did it - or I would tell you,
but it was a fairly complicated piece of code.

 Steve Schiller