Mike Williams <mik### [at] nospamplease> wrote:
: If the point isn't on the surface, then there's obviously no normal at
: that point.

  I think that the idea is that if the "threshold value" was set so that the
surface for that function would pass through that point, then what would be
the normal vector. It's the normal vector to the equipotential surface of
that function which passes through that point (or something similar). It is
a well-defined concept.
  (And I answered the original question in another article.)

-- 
#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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