I was wondering if any of you kind fellows could help me with a

problem...

I am currently writing some software for modelling involute gears in

3D: the 3D processing being handled by Microsoft's DirectX. My

problems start when I come to cast light onto the model. To

effectively light an object I need to know the normal vectors at each

of the vertices. The parametric equation I am using to generate the

tooth profile is:

Xi = r

***(cos(t)+t***sin(t))

Yi = r

***(sin(t)-t***cos(t))

Where t is the parametric variable and r is the radius of the gear's

base circle.

Could anyone tell me how I could go about calculating the normal

vectors? I think it has something to do with taking a unit vector to

the tangent line (as calculated by differentiation?), but I'm clueless

how to actually go about doing this. Any help would be much

appreciated.

Many thanks,

Dax