Project Points onto Three-Dimensional Perspective Plots
DESCRIPTION:
Project points onto a three-dimensional perspective plot.
USAGE: 
perspp(x, y, z, from.persp)
OPTIONAL ARGUMENTS:
x:
vector giving the x coordinates of points to be projected onto the current perspective plot.
y:
vector of the y coordinates.
z:
vector of the z coordinates.
from.persp:
object returned from prior call to persp function.
VALUE:
plot structure containing components names x and y representing the projection of the input coordinates into 2-dimensional coordinates to match a current perspective plot.
DETAILS:
perspp uses information set by calling function persp; hence perspp can only be called in the same S-PLUS session following a call to persp.
SEE ALSO:
persp , points , text .
EXAMPLES: 
zout <- persp(switzerland)  # perspective plot from default viewpoint
points(perspp(0,0,0, zout))  # plot and label the origin
text(perspp(0,0,0, zout)," Origin", adj = 0)