library(rgl)

## Define the quadratic form
A <- 2; B <- 1; C <- 2
Q <- function(x1,x2){
  A*x1^2+2*B*x1*x2+C*x2^2
}

## Define the density
D <- A*C-B^2;
f <- function(x1,x2) {sqrt(D)/(2*pi)*exp(-Q(x1,x2)/2)}
## Prepare 3D plot data
x1 <- seq(-2,2,.1);
x2 <- seq(-2,2,.1);
z <- outer(x1,x2,f)

## Plot the density
open3d()
ncolors <- 20
col <- terrain.colors(ncolors)
persp3d(x=x1,y=x2,z=z,theta=20,phi=50,col=col)