Bezier Patch Assignment

I wrote a class to represent a Bezier Surface.  I read the input control points from a text file into this class (one instance per patch).  When rendering the class will create n traces in s and t.    In each trace 10*n line segments are used to give a smooth appearance.  At the intersections of traces in s and t normal points are calculated as offset points from the sample points in the normal direction.  The normals are calculated by taking the partial derivate in s and in t and then forming the cross product.  The normals can be turned on and off by pressing the 'n' key.  The partial derivative in s is calculated by replacing the vector [s^3 s^2 s 1] with [3s^2 2s 1 0] and likewise for t.

Here are the screenshots:

Large number of lines per patch:

Few lines per patch:

Normals in pink at intersection points of patch lines.