> In other words, they just mustn't overlap?

Right. The only points that will actually be precisly where the
control-point is placed are the four corners, everything else is used as
control points a la the typical bicubic interpolations. So, like I said, to
line patches up, the edges of bordering patches need the same vertices. The
inner four control points are crucial to create a seamless surface. If a
patch on the left side has control points "below" the surface, the patch
will move upwards at the edge. Hence, the control points on the next patch
need to be "above" the surface in such a way that the angle at the edges are
the same.

There's an image on this page which shows that the control points just build
a bounding cage:
http://www.ece.eps.hw.ac.uk/~dml/cgonline/hyper00/curvesurf/bicub.html

As you can see, the relation/curvature along the edge is determined by the
cage, and joined bicubic-patches should have the same angle/steepness at
edges to avoid seams.

Regards,
Tim

-- 
aka "Tim Nikias v2.0"
Homepage: <http://www.nolights.de>

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