Bug: persp and colors (PR#1476)

Hi

I am working with the persp command. I want to change to color of the

facets

according
to their z-value. The colors seems to be transposed, i.e. the color of a

facet A

seems
to be used for a facet B which is a (defined?) way off....

An example:

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)
hgt <- z / max(z)
persp(x, y, z, col = gray(1 - hgt), theta = 70, phi = 40, expand = 0.4)

Now the z-value should define the color, but the color pattern seems to be
moved.

I don't think this is a bug -- I think the problem is that the surface
produced for the z of dimension 21x21 has only 20x20 facets.  For example,
the following is not a solution, but it shows that a 20x20 colour matrix
produces the expected result ...

    persp(x, y, z, col = gray(hgt[1:20,1:20]), theta = 0, phi = 40, expand =
0.4)

... now, what you really want I guess is to calculate hgt to be some
interpolation of the four z values at the four corners of each face.  I am
due somewhere so this calculation is left as an exercise ...

Paul


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