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How to turn a (mesh) light into a shadow caster only? I want to use a (mesh) light to cast a shadow only, not actually light anything. It's easily done in the blender internal and other software. Question is how to do it in cycles?

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    $\begingroup$ Could you be more precise about what side effects you would like to avoid? Maybe a little example would be helpful. Are you open to the idea of using the compositor? $\endgroup$
    – Carlo
    Jul 23, 2016 at 7:15
  • $\begingroup$ If this light only casts shadow... The lighted part will receive only the environment light and by definition the shadowed part also... so nothing will be visible or I don't understand... or do you want to darken the shadow ? $\endgroup$
    – lemon
    Jul 23, 2016 at 8:30
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    $\begingroup$ I thing this can be done in compositor only. Try make shadow catcher in different render layer by this tutorial and combine all together in compositor. youtube.com/watch?v=99NTQUUGWUY $\endgroup$
    – Shubol3D
    Jul 23, 2016 at 13:07
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    $\begingroup$ Indeed, I want to use a (mesh) light to cast a shadow only, not actually light anything. It's useful sometimes, it's possible easily (a simple switch) in other software, I know. Question is how to do it in Blender? I think the closest answer is by Shubol3D. But maybe somebody knows about a simpler way to do it, if possible at all? $\endgroup$
    – Aardo
    Jul 23, 2016 at 15:38
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    $\begingroup$ please edit your question and clarify what you want to do $\endgroup$
    – user1853
    Jul 23, 2016 at 15:48

1 Answer 1

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1: Negative lighting

Add a lamp and an idetical one but with negative strength and shadow casting disabled:

Positive Lamp

Positive Lamp

Negative Lamp

Negative Lamp

Without the Lamps

Without the Lamps

With the Lamps

With the Lamps

Additional Notes

Because mesh light shadow casting can not be disabled, this works only for other light sources. Requires more samples than without the shadows. These are subtractive shadows, meaning the value of the shadow is subtracted from the color. Multiplicative shadows would look better, but those require spearate render layers and compositing.

2: Compositing

On the render layers tab there is an option for quickly replacing all materials with a single one, which can help with creating a shadow map render for the light by using a simple white material. You could exposure and gamma adjust the result of the render layer and multiply it ower the original render.

Additional Notes

This is a little longer setup, but with material overriding and putting the main lights and the shadow light to separate layers it can be easier. It might be good to disable bounces for the shadow render. This method however, instead of casting only shadows also darkens faces facing away from the light.

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