Hidden emitters casting shadows

SnowSultan

This is a little obscure, but hopefully I can explain it well enough. I added a Linear Point light and changed the shape to a Rectangle, 100x100 size. Emitter visibility is off, so it casts a light in the shape of a square but the square itself is not visble. However, if a Spotlight is added to the scene, the invisible square emitter still casts a shadow if hit by the spotlight. Is there any way to prevent this aside from moving the emitter out of the way?

I actually don't want to do that because what I'm trying to do is cast directional shadows from the same direction as a linear point light, and I've put a spotlight right behind the linear light.

Thanks in advance for any help.

Tobor

This is pretty much how it works. When "hidden," the emitter is simply not viewed directly. It can still be viewed in reflections, and other lights will cast a shadow from it.

Since you're using an enlarged emitter, its output is now fairly non-specular, so here's a good time to use an emissive geometry. You can cast a light from it, and make it hidden by dialing out the Opacity Cutout value to 0.0001. That will also prevent it from another light behind casting a shadow.

 

SnowSultan

Oh, so now I should just use a primitive set as emissive and the opacity at .0001 instead of a light? I'll give that a try, good idea. Thanks very much!

Tobor

If you want a rectangle, use the plane, set it to 1 meter square (same as 100x100cm emitter for the light you used), and set its divisions to 1, the lowest number. This will help improve efficiencies a bit.

Tip: It's always best to apply the Emissive shader, rather than the Uber Base, followed by adjusting the Emission Color. When you use the Emissive shader, counter-productive settings are not used, and makes for a faster render. Remember the emissives start at 2700K color temperature. You may find it easier to set that to 0, and then control the color of the light (if not white) using the Emission Color.

mjc1016

Tobor said:

 Remember the emissives start at 2700K color temperature. You may find it easier to set that to 0, and then control the color of the light (if not white) using the Emission Color.

I find that using color temperature easier for things like ceiling light panels/troughs...the ubiquitous florescent lights that you can pick a known temperature for.  For other kinds of lights/emitters, just use the emissive color, because trying to dial in 'bioluminescent' glow by temperature is not easy.