how to fade out volumetric material?

What am I doing wrong? The idea is to fade out the fog

  • at the outer radius = distance + width
  • AND at the inner radius = distance - width

Fading out at the outer radius works fine: I can see how the sphere mask works. Instead of filling the bounding box, the volumetric fog has a round shape at the outer radius.

But the inner radius and the Sphere Mask with OneMinus doesn’t work. I can’t create a whole in the fog anywhere.

This is how it looks. The fog is everywhere, but I want to create a hole, i.e. a donut shape.

Maybe someone can confirm that, for unspecified reasons, volumetric materials never has holes.

At least, using sphere masks I wasn’t able to create holes inside my fog. Maybe there is some logic behind it.