Le 2022-06-23 à 13:46, Chris R a écrit :
> The attached image shows two renderings of the same scene.  The right image is
> the same as the left, except that I added a completely transparent box, whose
> purpose was to support media with some ground fog:
> 
> box {
>     -size/2, size/2
>     texture { pigment { color rgbt 1 } }
>     translate <0, size.y/2, 0>
> }
> 
> As you can see, the box shows up as a mirror instead of being transparent.
> 
> The scene is fairly complex, but here are a few details that might help point
> out to the experts what the problem might be:
> 
> 1.  The whole scene is inside of a mesh-based sky sphere, but I verified that
> the problem shows up even if that is turned off.
> 2.  I have enabled radiosity, but I verified that the problem shows up without
> it as well.
> 3.  The light source is a sun emulator using a parallel area_light at a far
> distance from the scene
> 
> I also tried using rgbf 1.0 as the pigment for the box, but I get the same
> problem.
> 
> Any ideas?  It's driving me crazy
> 
> -- Chris R.

That looks like it could be a coincident surface issue.
Try placing the box slightly lower so that it's bottom face is just 
under the ground or slightly higher so that it don't touch the ground.
Some options :
box {
    -size/2-0.1*y, size/2 // lower
    texture { pigment { color rgbt 1 } }
    translate <0, size.y/2, 0>
}

box {
    -size/2+0.1*y, size/2 //higher
    texture { pigment { color rgbt 1 } }
    translate <0, size.y/2, 0>
}

box {
    -size/2, size/2
    texture { pigment { color rgbt 1 } }
    translate <0, size.y/2-0.1, 0> // lower
}

box {
    -size/2, size/2
    texture { pigment { color rgbt 1 } }
    translate <0, size.y/2+0.1, 0> //higher
}

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