[comp.graphics] Find Surface Normal Which Goes to a Point

finn@sunshine.cad.mcc.com (Chris Finn) (12/12/89)

Here is a problem to which I would like to know a fast and
robust solution.
	Given a point (x1,y1,z1), and a surface represented
by z = f(x,y), find the point (or points) on the surface where
the normal points towards (x1,y1,z1). The spatial derivatives
of f(x,y) are easily computed.
	The points and surfaces to which I would like to apply
this are relatively non-pathological (i.e. f(x,y) != a hemisphere
with x1,y1,z1 at the center).

Thanks,
Chris Finn
MCC CAD Program, P.O. Box 200195, Austin, TX 78720 [512] 343-0978
ARPA: finn@mcc.com
UUCP: {uunet,harvard,gatech,pyramid}!cs.utexas.edu!milano!cadillac!finn