How to make an object transparent to the freestyle pass? In Blender 2.75a, I'm making a render using Cycles and the Freestyle pass. My scene contains a mesh with a smoke modifier that I'm rendering through a volumetric shader. 

The boundaries of this mesh (the box, in the example below) are meaningless to the extents of the render, therefore the mesh should neither have its edges drawn by freestyle nor cover the lines of the things behind or inside it.

I don't know if it's possible to tell freestyle to completely ignore the mesh, and how to do it.

This is what happens if I render a sample scene with cycles+freestyle+volumetric rendering and without further options (left) and with the box assigned to an excluded object group (right)



Neither of them is the expected result: I'd like to have the freestyle lines on the sphere but not on the box. Is it possible?



Add after TLousky answer

As you can see, even if I choose to render freestyle all the objects except the box using an "Inclusive" group, the "phantom" of the box still covers the edges of the sphere and partially of the cone and torus (exactly as in the example above where I used an Exclusive group containing only the cube instead)
The source of the problem is that Freestyle view map has (can see) the smoke domain geometry. This is a typical problem. Transparent mesh like glasses can be solved with the same method.


Put torus, cone and sphere on a new render layer (mesh on layer 1). This render layer will be rendered with freestyle.
Isolate the smoke domain cube to another render layer (ie: mesh on layer 2). Turn off freestyle on this render layer or don't render this layer at all.


That should solve the smoke domain cube from being rendered.