About a year ago I started work on a problem that appeared to
be very simple and turned out to be quite difficult.  I am wondering if
anyone on the net has seen this problem and (hopefully) some published 
solutions to it.

	The problem is to draw an outline of a surface defined by two
roughly parallel cubic splines.  For inputs the problem essentially
starts with two sets of points where each set of points is on the 
edge of an object which we treat as two dimensional, i.e. only extant
between the edges, but which exists in three dimensional space.  To draw 
the object we 

1) fit a cubic spline through the points.  Each spline is effectively
	computed as a sequence of line segments approximating the
        curve.  Each spline has an equal number of segments.  We assume
	that the nth segment along each spline is roughly, but not
	exactly, the same distance along each spline by any reasonable
	measure.
2) Take each segment (n) along each spline and match it to the nth segment
	of the opposing spline.  Use the pair of segments to form two
	triangles which will be filled in to color the surface.
3) Depth sort the triangles
4) Take each triangle in sorted order, project onto a 2D pixmap, draw
	and color the triangle.  Take the edge of the triangle that is
	along the edge of the surface and draw a line along that edge
	colored with a special "edge color"

	It is the edge coloring in step 4 that is at the heart of the
problem.  The idea is to effectively outline the edge of the surface.
The net result however generally has lots of breaks and gaps in
the edge of the surface.  The reasons for this are fairly complicated.
They involve both rasterization problems and problems resulting
from the projecting the splines.  If anything about this problem
sounds familiar we would appreciate knowing about other work in this
area.