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Normal Disappear Problem

Hi, All:

I have the following parametric equation for an unit sphere:

x = cos(u)sin(v)
y = sin(u)sin(v)
z = cos(v)

0<=u<2*Pi ; 0<=v<=Pi

Then I use

normal = (Dx/Du, Dy/Du, Dz/Du) CROSS (Dx/Dv, Dy/Dv, Dz/Dv) to get the
normal vector.

I get the follwoing after calculation (with normalization):

normal =  [sin(v) ^2 cos(u), sin(v)^2  sin(u), cos(u)^2  cos(v) sin(v)
+  sin(u)^2  cos(v) sin(v)]

Now when u=0, v=0 , Normal = (0,0,0)! How can it be? We know the fact
that a sphere should have normal everywhere.


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