I want parts of a scene to be invisible in final render but still be visible in reflections and light bounces

Question

I'm working on a scene in blender where I have recreated a corridor shot for real. In the scene, I have an object traveling down the corridor. The goal is to just get the object and not the corridor in the final render. I still want to get the reflections and light bounces of the corridor, maybe even shadows if possible!

I'm super new so I've been having a lot of problems. I've tried to use the Holdout node on the materials but that removes the light bounces. Any help is appreciated!

Answer 1

Use the Shadow Catcher feature available in versions 2.78.5 or higher. You don't need to do much.

Set the surrounding elements be a shadow catcher in Cycles Settings> Ray Visibility.

The rendered image will have the shadow and reflections and a transparent background.

Then just overlay the rendered image onto your background using Alpha Over:

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For other ways to do this.

• Using a Layer Mask.

In this example the monkey is in layer 2 and the environment in layer 1. In the layer settings Layer 1 is used as a mask layer.

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• Make the surrounding elements invisible to the camera using Ray Visibility.

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• Make the surrounding elements use a material that is transparent to Camera Ray using Light Paths.

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• Give the material for your object a Pass Index number, enable Material Index in the render layers, and use the Material Index as an ID mask to Set an alpha channel for the rendered image in the compositor.

(click on the image to enlarge)

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• Give the Object a Pass Index number, enable Object Index in the render layers, and use the IndexOB as an ID mask to set an alpha channel for the rendered image in the compositor.

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