It's kind of esoteric subject.

I want to rendering smoke without rendering geometry.

Because i want to rendering seperately and compositing.

I have some reason for doing like this. It's important for me.
(I know there's pass for doing like this however i don't really want to rendering sametime because it's too much to taking render time.)

but if rendering smoke without geometry. there's no way to blending two of them.

becasue some geometry is front of volume, something`s don`t.

Is there any way to creating matte for perfectly blending geometry and volumetric?

I hope opengl rendering can making this matte.

Not sure if this helps, but I have used density to great solid and non solid groups. For example I have a simulation with steel posts holding a copper wire. A current melts the wire. The density of the steel never changes, but as the copper transitions from solid to a volume it's density changes. I used this density change to identify what is to be rendered as a volume. I could use a blast node on the solid group to remove anything not a volume.

I have also done this with temperature.

Normally you would just include all the objects that you want to use as matte in Forced Matte on Mantra

Not in the viewport though