How do we interpolate a surface normal? Keep in mind that a normal is a unit vector. We can’t just interpolate the x,y,z components of the normal, because we
wind up with a non-unit normal. Here’s a
simple example:
N1 = (0, .436, -.9). N2
= (0, -.436,.9)
If we take the average of these, we get (0,0,.9), which
is not a unit
normal. We have to normalize this to
get (0,0,1).