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Imagine the following scene, top down view with a simple object emitting light (cycle) on a large floor (plane).

Now what i'd like to achieve is to only render the object and the emission of light but not the plane (tranparent bg). of course if I hide the plane there is no emission visible since there is no surface to refect it.

I think maybe using green screen effect (making the plane green) could do the trick but I don't think it will be very clean for a effect like that. I wonder if there is any other way it could be done ? I tested transparent material but again no emmision visible then.

edit : picture to illustrate what I doenter image description here what i see enter image description here what i'd like to see in a way less ugly version (done with my awesome skill in gimp) (but since its a white glow on a white background you can't see it here damnit) enter image description here

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  • $\begingroup$ if you remove the world surface shader ? $\endgroup$ – lemon Jul 5 '16 at 7:44
  • $\begingroup$ I don't get it ? why the world surface shader since the world is transparent ? $\endgroup$ – eephyne Jul 5 '16 at 7:47
  • $\begingroup$ not sure to understand. You want to see only the parts of the plane that receive light, am I right ? $\endgroup$ – lemon Jul 5 '16 at 7:55
  • $\begingroup$ only want to see the light. don't want the plane at all, but the light on it yes. I guess a image would be more efficient $\endgroup$ – eephyne Jul 5 '16 at 8:07
  • $\begingroup$ yes please, add an image $\endgroup$ – lemon Jul 5 '16 at 8:09
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Note : this answer is obsolete due to the comments. I don't delete it as it still produces a result.

Use the compositor.

The setup can be like this :

enter image description here

On the left, the rendered input (here, all is black around, because the world has no material or light set to 0).

The color ramp is here to convert eventual colored parts of your scene to black and white.

Alpha over puts a transparency (the first image slot) over the rendered image, depending on the B&W chanel.

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  • $\begingroup$ thats a nice idea but it only work in this specific case, I was more hoping for something that would work in "any case". if for example, I have 3 object emitting three light of different color $\endgroup$ – eephyne Jul 5 '16 at 19:20
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If I understand you correctly, you want to show the light being emitted in a scene, but without necessarily having an object reflect that light to show it.

The only way that can happen is if the atmosphere itself is dense/dirty enough to reflect that light, and that means volumetrics.

enter image description here

Compositing can 'filter' your way out of trouble, but volumetrics generally achieves better photorealistic and light-interactive results. In this scene I have three floating cubes with different color emission shaders.

The secret sauce is the surrounding globe. The node editor shows a typical Volumetric material set-up applied to it. Volume Scatter scatters emitted light; Volume Absorption adjusts light's penetration through the material. By adjusting both you can vary the density of the atmospheric environment as needed.

(I stuck a plane in there for reference, but it won't be rendered. To hide an object from ever rendering, select the object, go to Object > Ray Visibility panel and deselect 'Camera'.)

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  • In the render panel (1) : set the film to transparent
  • In the compositor (2) : indicate use alpha
  • You can also (3) switch the output to render layer (so the compositor is not used)

enter image description here

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    $\begingroup$ I did try to redo this one and i don't have a transparent plane at all. you don't indicate how to set the plane transparent here,and thoses settings are kind of the default one (except for transparent bg) $\endgroup$ – eephyne Jul 7 '16 at 5:49
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    $\begingroup$ @lemon, Can you update this answer please? Like eephyne I don't know how you got the ground plane to be transparent. $\endgroup$ – Mario Feb 2 '18 at 18:01

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