Philippe Debar <phd### [at] wanadoobe> wrote:
> "hollow" should work too.

  IMHO it's not the best solution.
  You should understand how plane works and it should be completely crystal
clear which side of the plane you are defining is inside and which is
outside. "Fixing" the warning by putting 'hollow' in the plane definition
is more like hiding the problem, not fixing it. It can teach you wrong
principles. You should have a good, real reason to use 'hollow', and IMHO
getting rid of the warning is not one.
  Defining the plane so that the scene is located *outside* it is the best
solution. It helps you visualizing better what's happening.

  A quick recapitulation: The first parameter of 'plane' defines its normal
vector. The normal vector points outside the plane.
  That is, if you for example define a 'plane { z,0 }', then everything
which is located anywhere with a positive z coordinate is outside the plane,
while things located at a negative z coordinate are inside the plane.
'plane { -z,0 }' inverts this.
  The second parameter tells an offset. The plane is translated by this
amount along the normal vector (ie. in the direction of the first parameter).
Usually you want to move the plane backwards, ie. in the opposite direction
of the normal vector (very common when defining floors and walls), which
naturally means that this parameter should be negative.
  Thus, eg. a floor is usually defined like 'plane { y, -5 }', which creates
a plane at y=-5 so that everything above it is outside.

-- 
#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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