Hello,

I want to create a half spherical shell using an isosurface like this:

difference{
    isosurface{function{0.1-exp(-abs(pow(x*x+y*y+z*z,0.5)-0.75)/0.1)}
        threshold 0 accuracy 0.0001 max_gradient 10
        texture{pigment{rgb yellow}}
    cylinder{<0,0,0>,<0,0,-1>,1 texture{pigment{rgb blue}}}
}

which generally works. The yellow outer face as well as the blue cross section
are visible and indicate that, in principle, the function gives a spherical
shell. However, the inner face of the shell is invisible - even more than
invisible, the background of the scene is visible in this direction through the
shell. Adding "interior_texture" does not help, the rays just find no object if
they hit the inner face. Increasing accuracy and gradient does also not fix the
problem.

By the way: I cannot use just the difference of two sphere, because I would like
to give the shell a more detailed 3d-structure by adding some other functions to
the isosurface.

Thank you in advance!

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