[R] persp plot question..

[Ross Ihaka]

On Thu, Oct 26, 2000 at 11:46:43AM +0100, Colin Gillespie wrote:
> Dear All,
> 	I have been trying to to do this for a few days now. I can generate
> persp plot OK and can generate it in colour using the command
> 
> persp(x,y,z,col=terrain.colors(20))
> 
> However, I was wondering if it is possible to shade the 3d surface like a
> contour plot. i.e. black for large z, white for small z, say

Here is a quick and trivial example which shows you how.

	#  Create a simple surface  f(x,y) = x^2 + y^2

	nx <- 21
	ny <- 21
	x <- seq(-1, 1, length = nx)
	y <- seq(-1, 1, length = ny)
	z <- outer(x, y, function(x,y) x^2 + y^2)

	#  Average the values at the corner of each facet
	#  and scale to a value in [0, 1].  We will use this
	#  to select a gray for colouring the facet.

	hgt <- 0.25 * (z[-nx,-ny] + z[-1,-ny] + z[-nx,-1] + z[-1,-1])
	hgt <- zmid / max(zmid)

	#  Plot the surface with the specified facet colours.

	persp(x, y, z, col = gray(1 - hgt))

You can use something similar to select colours from one of the
other standard palettes.  E.g

	persp(x, y, z, col=cm.colors(20)[floor(19*hgt+1)])

Hope this helps.

	Ross


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| Ross Ihaka                         Email:  ihaka at stat.auckland.ac.nz |
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