[R] constructing a matrix for rgl plots

[ravi]

Hi,
rgl plots seem to require the z object in the form of a matrix. I would like some help in constructing this matrix when I cannot use the outer function. Let me explain.
library(rgl)
library(plot3D)
x <- 1:10
y <- 1:20
fun1 <- function (x,y) {x^2+y^2}
z <- outer(x,y, fun1)
open3d()
surface3d(x,y,z,col="red",theta=50, phi=20)

This works fine. However, when I have a data frame of x, y and z with z as a vector, how do I proceed?
z <- x^2+y^2
> surface3d(x,y,z,col="red",theta=50, phi=20)

Error in surface3d(x, y, z, col = "red", theta = 50, phi = 20) : 
  At least one coordinate must be a matrix 

How do I get z in the form of a matrix?
Thanks.