[R] smoothed contour lines

[William Dunlap]

> Greg Snow Greg.Snow at imail.org
> Wed Jan 7 17:58:07 CET 2009
> 
> Here is one possible approach to get you started (this is not a final
answer):
> 
> x <- seq(-3,3)
> y <- seq(-3,3)
> 
> z <- outer(x,y, function(x,y,...) x^2 + y^2 )
> 
> tmp <- contourLines(x,y,z)
>
> contour(x,y,z, lty=0)
> lapply(tmp, function(l) {
> 	x <- l$x
>	y <- l$y
>	if( length(x) > 2 ){
>		if( isTRUE( all.equal( c(x[1],y[1]),
c(x[length(x)],y[length(y)])))) {
>			xspline(x[-1],y[-1], -1, FALSE)
>		} else {
>			xspline(x, y, -1, TRUE)
>		}
>	} else {
>		lines(x,y) # or whatever else should go here
>	}
> } )
>
>
>You can play with the settings to xspline to control the properties 
>of the curves, also it will look better if you thin some of the points
> from contourLines (the points that are nearly identical cause the
small loops).
> 
> Hope this helps,
>
> -- 
> Gregory (Greg) L. Snow Ph.D.
> Statistical Data Center
> Intermountain Healthcare
> greg.snow at imail.org
> 801.408.8111

It often works better to fit a smooth surface to the data, evaluate
that surface on a finer grid, and pass the result to contour.  This
ensures that contour lines don't cross one another and tends to avoid
the spurious loops that you might get from smoothing the contour lines
themselves.

Thin plate splines (Tps from library("fields")) and loess (among others)
can fit the surface.  E.g., with your data try
   library("fields")
   xy<-as.matrix(expand.grid(x=x,y=y))
 
contour(predict.surface(Tps(as.matrix(expand.grid(x=x,y=y)),as.vector(z)
)))
These surface fitters have arguments to control the smoothness/flatness
of the
surface.   

Bill Dunlap
TIBCO Software Inc - Spotfire Division
wdunlap tibco.com 

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Andrea Storto
> Sent: Tuesday, January 06, 2009 9:52 AM
> To: r-help at r-project.org
> Subject: [R] smoothed contour lines
> 
> Hi all,
> 
> I'm trying to draw a contour plot
> with rounded (smoothed) contour lines instead of sharp angles;
> when the grid consists of only a few points
> in x- and y- axis, the resulting contour
> is in facts rather ugly since very sharp angles may appear.
> 
> I did not find any way to do it,
> by using either "contour" or "contourplot" (from the lattice package),
> I wonder if there exist a way for smoothing the angles,
> apart from artificially increasing the grid resolution,
> 
> Thanks in advance
> 
> Andrea