One of the example isosurfaces in my tutorial renders very differently
in beta 3 than it did in betas 1 and 2. In the previous versions, and in
MegaPOV it produces the expected cylinder (it's a sphere scaled
infinitely in the x direction by substituting "0"  for "x"). In 3.5b3
the surface does not appear.

I've tried adding "evaluate 1,10,0.99" and it still comes out blank, and
suggests that the max_gradient should be 0.000.

POV 3.5b3, AMD K6-2 500, W98se, 128Mb

=================================
camera { location  <0, 0, -4> look_at <0, 0, 0>}

light_source {<-100,200,-100> colour rgb 1}

#declare  S = function {x^2 + y^2 + z^2 - 1}
                     
isosurface {
  function { S(0,y,z) }
        max_gradient 4
        contained_by{sphere{0,2}}
        pigment {rgb 1}
}
=================================

If I change it to "function { S(x*0,y,z) }", which is obviously an
identical function, it renders correctly (and very much quicker). Weird.
S(x,0,z) and S(x,y,0) behave correctly, it's just S(0,y,z) that goes
wrong.

It looks like the details of the function aren't important, it's the
"function Foo(0,..,..)" that provokes the strange behaviour.

-- 
Mike Williams
Gentleman of Leisure

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