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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.enter image description here

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  • $\begingroup$ Please provide images. And if you have an object that is reflective, then is it okay if the fire is seen in that mirror? $\endgroup$ Jul 19, 2023 at 11:42
  • $\begingroup$ Image added with further description, I always render my passes out as separate sequences rather than relying on blenders render layers. $\endgroup$
    – Helpme
    Jul 19, 2023 at 12:02

1 Answer 1

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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 enter image description here

And this is the solution

enter image description here

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.

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  • $\begingroup$ Many thanks, this worked exactly as required. $\endgroup$
    – Helpme
    Jul 20, 2023 at 16:50

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