What does persp() return?

I want to plot some 3D points on top of the grid produced by persp().

On 2/22/01, Paul Murrell <paul at stat.auckland.ac.nz> wrote in R-help:

In S-Plus, persp() returns a value that can be used to transform 3D 
locations to 2D, but this sort of thing is not (yet) available in R.

But persp() does return something (in R-1.5.1): a 4x4 matrix which in the C
code is called the "viewing transformation matrix", VT.  Can VT be used to plot
additional points?  If not, is such a thing in the works?

Paul also suggested Uwe Ligges's "scatterplot3d" package, which is very nice
but seems better at producing points than grids.

Thanks!

-- David Brahm (brahm at alum.mit.edu)
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