How to get illumination pass from a Smoke Sim with Cycles? Cycles can now render smoke and fire simulation but for compositing need, I want the illumination pass from the fire. The aim is to composite this pass with a tracked footage. 

I already try with the diffuse indirect pass but there is the ground plane with it so it's hard without alpha to get a proper image.
One solution is to add a material index to your fire's material, and then, on the compositor,  you can take the Emission pass, and multiply it with the Object Index one.



Go to the Properties panel, then switch to the Materials tab, and under the Settings subsection, change the material index.



Then, still on the Properties panel, switch to the Render Layers tab, go to the Passes subsection and activate Material Index and Emission.



Then, in the compositor, multiply the output of a IDMask set to 1 and the Emission pass.