"Chris B" <c_b### [at] btconnectcomnospam> wrote:
> "gadoid" <hansen at tkgate.org> wrote in message
> news:web.46e05e106d67f0f14e9a00c00@news.povray.org...
> > The code below generates five spheres which should be invisible, yet three
> > of them are visible from their interaction with the environment.  The
> > spheres become visible when you apply a rotation to a texture that uses a
> > texture_map  containing clear (rgbt <1,1,1,1>) components as in the
> > T_Electron texture below.  If the texture is not rotated, then the sphere
> > remains invisible.  Rotation applied to a clear texture not part of a
> > texture_map also remains invisible.
> >
> > It also seems to be the case that the surface behind the spheres must have
> > a
> > "normal" for the effect to occur.  The effect goes away if you remove the
> > "normal" block in the texture of the  "floor".
> >
> > ... snip ...
> >
> > #declare T_Clear = texture {pigment { Clear }}
> >
> > #declare T_Electron =
> >  texture {
> >    crackle
> >    texture_map {
> >        [0.00   T_Clear]
> >        [1.00   T_Clear]
> >    }
> >  }
> >
> > ... snip ...
> > sphere { <0,1,0>,    0.2 hollow texture { T_Electron rotate 90*y }}
> >
>
> I'm taking a bit of a guess here, but maybe it relates to using a texture
> that only contains a pigment for a crackle texture map. It looks a bit to me
> like the sphere is acquiring a crackle normal. If you define T_Clear and
> T_Electron as pigments and use a pigment_map the problem seems to go away.
> If this is the cause though, I don't know why your texture would go clear at
> a zero and 180 degree rotation around y.
>
> Regards,
> Chris B.

Hmmm.  It seems to me to be more of a refraction-like effect, and changing
"crackle" to anything else does not change the way it renders.  I've placed
an image at http://www.tkgate.org/povray/electrons.png (this is a test shot
for a larger scene) that has the problem in it.   When you look through the
clear portion of the "electrons", you can see that the  waves seem shifted
as though the sphere had an 'ior' in it.  The "electrons" in this image are
made with an internal shiny metal sphere, and a co-located larger sphere
with the T_Electron texture to get the lightning cage effect.  If I don't
rotate the "electrons", the clear portion behaves and does not cause a
problem.

This is the actual texture I am using:

#declare P_ElectricBlue = pigment { color  rgb <0.178, 0.122, 1> };

#declare T_Lightning =
  texture {
    pigment { P_ElectricBlue }
    finish {
      ambient 5
      brilliance 100.0
    }
  }


#declare T_Clear =
texture { pigment { Clear }}

#declare T_Electron =
  texture {
    //    gradient y
    crackle
    turbulence .3
    texture_map {
      [0.000 T_Lightning ]
      [0.0400 T_Lightning ]
      [0.0800 T_Clear ]
      [0.00 T_Clear ]
      [1.00 T_Clear ]

    }
    scale 0.1
  }

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