"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.

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