In three dimensions, there are eight surrounding grid points.
To combine their respective influences
will require a trilinear interpolation (linear interpolation
in each of three dimensions).
In practice this means that once we've computed
the 3t2-2t3 cross-fade function
in each of x,y and z, respectively,
then we'll need to do seven linear interpolations
to get the final result.
Each linear interpolation a+t(b-a)
requires one multiply.
In the diagram to the left,
x,y,z are in the right,up and forward directions, respectively.
The six white dots are the results of the
first six interpolations:
four in x,
followed by two in y.
Finally, a seventh interpolation in z gives the final result.
