Gradients, math and graphs

[David Bovill]

Forgotten my high school math. I'm creating bevels and gradients using the
imagedata and HSV values - so I change the velocity (and/or saturation) of
the HSV values to simulate  lighting on a beveled surface - to get the
appearance of a 3D rounded button... hope you get the idea - now what I need
is the maths for various types of curve and i need to be able to skew these
curves to simulate light coming from an angle...

Does anyone know a useful web source for the math needed here - I guess its
log and or sign based curves?