The function abs(y) with a threshold of 0 gives me a wafer-thin mint* -
however if I add any complexity to the iso (e.g. noise3d), it thickens up. I
can get around this by translating another copy of the iso y*0.01 and
differencing the two, but is there another way to easily keep just a surface
skin of an iso?

The example below demonstrates this and, when rendered, raises another
problem - why is the shadow of this iso-surface incomplete?

* source this quote.

#version unofficial MegaPov 0.5;

#include "colors.inc"

light_source{<0,0,0> color rgb<1,1,0.96>*0.75 translate <-30, 30, -30>}
camera{location  <2.0, 3.0, -2.0> look_at   <0,0,0>}
plane{y, -1 pigment{White}}

#declare isoLayer =
isosurface{
  #declare f1 = function {"ridgedMF" <1,2,5,0.5,10>}
  #declare f2 = function{pigment{wrinkles}}
  function{abs(y) + f1(x*5, y*5, z*5)/5 + f2 - 1}
  max_gradient 3
  eval
  threshold 0
  sign 1
  accuracy .001
  contained_by{sphere{0,1}}
  scale<1,1,1>
}

intersection{
  difference{
    object{isoLayer}
    object{isoLayer translate y*-0.01}
  }
  cylinder{0, 0.9*y, 0.9}
  pigment{Green}
}

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