R help - Oct 2008 - how to evaluate a cubic Bezier curve (B-spline?) given the four control points

I'm trying to use R to determine the quality of a cubic Bezier curve
approximation of an elliptical arc.  I know the four control points
and I want to compute (x,y) coordinates of many points on the curve.
I can't find anything in either the base distribution or CRAN that
does this; all the spline-related packages seem to be about *fitting*
piecewise Bezier curves to a data set.  Presumably, internally they
have the capability I need, but it doesn't seem to be exposed in a
straightforward way.  Help?

You could look at the xspline function.  It approximates b-splines or Bezier
curves given control points and shape parameters.  It can either plot the spline
or return a bunch of point on the curve for comparison to other values.

Hope this helps,

--
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org
801.408.8111

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Zack Weinberg
> Sent: Friday, October 10, 2008 4:11 PM
> To: r-help at r-project.org
> Subject: [R] how to evaluate a cubic Bezier curve (B-spline?) given the
> four control points
>
> I'm trying to use R to determine the quality of a cubic Bezier curve
> approximation of an elliptical arc.  I know the four control points
> and I want to compute (x,y) coordinates of many points on the curve.
> I can't find anything in either the base distribution or CRAN that
> does this; all the spline-related packages seem to be about *fitting*
> piecewise Bezier curves to a data set.  Presumably, internally they
> have the capability I need, but it doesn't seem to be exposed in a
> straightforward way.  Help?
>
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