Hide object but leave reflections

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Hi all. I'm just getting started in Houdini and this is my first scene and I've been working on it for two months now. Now I've got to the lighting and shadows and I can't solve the problem with the light in my scene. It seems that the solution should be obvious, but I do not understand in which direction I should move.
How to cast shadows and light (from the explosion) into the background? Do I understand correctly that you need to create a surface and create a shader for it? Or is it done in another way?
If this is done with a shader, then how to create a shader in such a way that the object becomes invisible, but casts shadows and reflections?

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If you are using Mantra: Go to the render tab on the fire geometry. On Render Visibility, either type -primary or use the drop down "Invisible to Primary Rays (Phantom)"
If you are using Redshift: Go to Redshift OBJ>Visibility tab on the fire geometry and uncheck "Primary Ray Visible".
If you are using something else: I don't know.
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robsdesign
If you are using Mantra: Go to the render tab on the fire geometry. On Render Visibility, either type -primary or use the drop down "Invisible to Primary Rays (Phantom)"
If you are using Redshift: Go to Redshift OBJ>Visibility tab on the fire geometry and uncheck "Primary Ray Visible".
If you are using something else: I don't know.
Forgot to mention, I use Mantra.
Perhaps I did not put it correctly, I need the light created by the fire to be moved to the background, but the grid should not be visible.
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It's been a while since I've used Mantra so I haven't looked at the updated workflows for this. ie. Last time I checked, it was done using Extra Image Planes on the Mantra ROP but after a quick search, I can see the following;

https://www.sidefx.com/docs/houdini/nodes/lop/backgroundplate.html [www.sidefx.com]
https://www.sidefx.com/docs/houdini/gallery/shop/vopmaterial/shadowMatte.html [www.sidefx.com]

Not sure if there is a non-Karma version of the first link but that looks like the more recent workflow.
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