Render fire simulation without fire being visible in the render for lighting the surrounding geometry

Question

Is the above possible?

I've rendered the fire as a separate pass. I want to now render surrounding geometry acting as light catcher objects illuminated by the fire, but the fire not being visible. As the fire is dynamic its lighting of surrounding geometry will vary.

I know a workaround will be just to use and animate separate lights but this is not what I want to do if there is another option as the work has already been done on the fire simulation.

Edit: example picture added, see following low res render, you can see the fire is illuminating the arms and fingers from the fire illumination alone (you can see pixels on the arm are yellow or orange), when I disable the fire domain from render the associated lighting disappears off the arms. I have just now parented a point light to the green sphere to fake the lighting as a workaround, but if there's a way to do it with the fire I'd like to know.

Answer 1

You can use the light path node along with the mix shader and a transparent shader to achieve this effect.

In this demonstration scene, the sphere is casting light on this cube.

This is where you are now

And this is the solution

In this node setup the light path ray checks whether the ray is the first ray from the camera which is hitting the suface. If yes, then it outputs 1 and if no, then zero. Using this as a factor for the mix shader you can change between a transparent shader and the shader you want to.