jansb000 <jan### [at] planetnl> wrote:
> Note: I created the object manually, but I understand the nature of bicubic
> patches and I am 99.9999% certain that the individual patches line up
> smoothly.

  It's not enough for the patches to line up, their slopes should also be
the same at the common edge.
  Achieving this is rather simple (even though perhaps not immediately
obvious). Suppose we name the points in the two patches like this:

 1  5  9 13 17 21 25
 2  6 10 14 18 22 26
 3  7 11 15 19 23 27
 4  8 12 16 20 24 28
===========
Patch 1  ===========
         Patch 2

  The points 13-16 are the ones which form the common edge of the two
patches (ie. they have to be the same points in both).
  In order to get the same slope at both edges, all the triplets 9-13-17,
10-14-18, 11-15-19 and 12-16-20 should lay in the same lines (that is,
for example the points 9, 13 and 17 should lay on the same line, ie. they
must be co-linear).
  The points 1-8 and 21-28 don't affect this edge and can be anything (of
course if we want to attach more patches to these two, they have to follow
the same rule with them).

-- 
#macro M(A,N,D,L)plane{-z,-9pigment{mandel L*9translate N color_map{[0rgb x]
[1rgb 9]}scale<D,D*3D>*1e3}rotate y*A*8}#end M(-3<1.206434.28623>70,7)M(
-1<.7438.1795>1,20)M(1<.77595.13699>30,20)M(3<.75923.07145>80,99)// - Warp -

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