Newsgroups: comp.graphics
Path: utzoo!utgpu!watserv1!watcgl!rhbartls
From: rhbartls@watcgl.waterloo.edu (Richard Bartels)
Subject: Re: curve represenation
Message-ID: <1990Nov16.152901.27626@watcgl.waterloo.edu>
Keywords: bezier or b-spline
Organization: University of Waterloo
References: <1990Nov14.160528.10333@progress.com> <1990Nov14.193732.570@watcgl.waterloo.edu> <4012@csccat.cs.com>
Date: Fri, 16 Nov 90 15:29:01 GMT
Lines: 30

In article <4012@csccat.cs.com> larry@csccat.UUCP (Larry Spence) writes:
>
>The table below doesn't mention rational vs. non-rational curves.  An important
>point is that non-rational cubics cannot exactly represent conics.  Of course,
>Mr. Bartels knows this, but Mr. Shukla may not.
>

True. I knew it.  I'm the fellow who submitted the inclusion in the
"most frequently asked questions" posting about doing circles with
splines.  I just thought I'd get out easily, since the original questions
offered only integral (non-rational) choices.

>>Thus it is that one sees Bezier representation as a frequent industry
>>choice for representation and interchange.
>
>I'm not a CAD guru, but I had the impression that many CAD systems are using 
>NURBS as an exchange format (i.e., IGES format).
>

I'm not an IGES guru, either.  If it's NURBS in IGES, then I've learned
something.  Industry types I consult with stick to Bezier for a
variety of reasons.  One of them had to do with passing the surface
model off to other users, which they do in IGES format.  But, on reflection,
the reasons they give me for sticking with Bezier in this regard could also
be interpreted as "we don't have machine-tool drivers that understand
rational splines."  Their own modeler, in fact, understands rational splines,
but they avoid using any other weights than 1.0000000.  :-)

-Richard