On 1/7/2006 12:47 PM, Martin Erwig wrote:

> Considering the R function/plot shown below, I wonder whether

> it is possible to do the following changes:

>

> (1) Change the color of each point to be picked from

> list of colors according to its z-value. (The range

> should be from blue (z=0) to red (z=1).) The grid

> should then be omitted. [I have seen "terrain.colors", but

> don't know how to use it for this purpose.]

>

> (2) Add two lines to the surface for, say z=0.8 and

> z=0.3. [Can contour or contourLines be used?]

>

>

> ---

>

> x <- seq(0, 1, length = 50)

> y <- x

> f <- function(x,y) { sin ((1-x)*y) }

> z <- outer(x,y,f)

>

> persp(x, y, z,

> theta = 30, phi = 30,

> shade = 0.3, col = "red"

> )

It's not that hard, but there are a couple of tricks to it. The

colorRamp() and colorRampPalette() functions give you the way to go from

z values to colors. persp() wants output in the form colorRampPalette

gives, so use that.

But the col option to persp() wants one color per facet, not one per z

value, so you need to be a little careful calculating them. However,

this should give what you want:

colfn <- colorRampPalette(c("blue","white","red"))

# You don't need white in the middle, but I think it looks better

cols <- colfn(256)[1 + 255*z[1:49, 1:49]]

trans <- persp(x, y, z, theta=30, phi=30, col = cols)

To add the lines, use contourLines() to calculate them, then trans3d()

(from the ?persp examples) to convert them to coordinates that can be

plotted. E.g.

clines <- contourLines(x,y,z, levels=c(0.8, 0.3))

trans3d <- function(x,y,z, pmat) {

tr <- cbind(x,y,z,1) %*% pmat

list(x = tr[,1]/tr[,4], y= tr[,2]/tr[,4])

}

for (i in seq(along=clines)) lines(trans3d(clines[[i]]$x, clines[[i]]$y,

clines[[i]]$level, trans))

> ---

>

>

> Finally, I would also produce a flattened 2D map

> of the same function, i.e. a map in which each point

> (x,y) is mapped to a color in a range according to

> f(x,y). Also two lines for f(x,y)=c1 and f(x,y)=c2

> should be added.

Use image() for that, e.g.

image(x,y,z)

and then contour():

contour(x,y,z,levels=c(0.8,0.3),add=T)

Duncan Murdoch

______________________________________________

[hidden email] mailing list

https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide!

http://www.R-project.org/posting-guide.html