Hi all,

I use white vertical ground fog to simulate depth cue for molecular
visualization. A defined a solid transparent sphere that should not contain
fog, because the hollow statement is outcommented (see below).

But objects within this transparent sphere show strange kind of fog. Instead
of white fog (that should not penetrate the transparent sphere) these
objects have "black" fog (see middle green sphere in expample below).

Is there any explanation for this observation or even better is there any
way to get rid of this black fog inside the trasnsparent sphere?

Thanks and best, Simon


///////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////
camera {direction<0.0,0.0,  -2.835>
 location <0.0 , 0.0 , 0.0>
 right 1.3333333653*x up y
 }

light_source {
<1000.0001,1000.0001,1450.31260>
color rgb<1.0000,1.0000,1.0000>
}

plane{z , -4000
pigment{color rgb<1.0000,1.0000,1.0000>}
finish{phong 0 specular 0 diffuse 0 ambient 1.0}}

// set global atmospheric fog effect in the scene.
// at the fog distance, there will be 63% visibility
fog {
 fog_type 2               // 1=constant, 2=ground_fog
 distance 10
 color rgb<1.0000,1.0000,1.0000>  // can also have 'filter' and 'transmit'
 fog_offset -880.00
 fog_alt 0.001
 up <0.0,0.0,1.0>
}


// three green spheres
#declare tex1 = texture { pigment { rgb <0.001,0.999,0.001> } finish {
diffuse 0.75 ambient 0.28 phong 0.75 phong_size 20.0 specular 0 } }
object {
    sphere {<0,0,-920>,50}
    texture {tex1}
}
object {
    sphere {<130,0,-920>,50}
    texture {tex1}
}
object {
    sphere {<-130,0,-920>,50}
    texture {tex1}
}

// transparent large sphere, solid, should not contain fog
// fully encloses middle green sphere, and partially left
// and right green spheres
#declare tex2 = texture { pigment { rgbt <1,1,1,1> } finish { diffuse 0.0
ambient 0.0 phong 0.0 phong_size 0.0 specular 0 } }
object {
    sphere {<0,0,-800>,140}
    texture {tex2}
    //hollow
}
///////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////

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> Is there any explanation for this observation or even better is there any
> way to get rid of this black fog inside the trasnsparent sphere?

	It looks like it's a bug in at least the 3.5/3.6 series.  You might give
a try to POV-Ray 3.7 beta which gives a different, more expected result -- yet
I'm not sure whether it is completely correct.

	- NC

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By the way, note that your script can be simplified; for instance:

> plane{z , -4000
> pigment{color rgb<1.0000,1.0000,1.0000>}
> finish{phong 0 specular 0 diffuse 0 ambient 1.0}}

background{color rgb 1}  // unless you need to cast shadows on the background...

> object {
>     sphere {<0,0,-920>,50}
>     texture {tex1}
> }

sphere{<0,0,-920>,50 texture {tex1}}

	- NC

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Hi Nicolas, thanks a lot for the 3.7 beta version hint, there I get what I
would expect, the strange black fog is indeed gone!
Best, Simon

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