Please explain Philip's use of "negative" area light
 

In Philip's tutorial on Front Projection, he creates two area lights, with one being a clone of the other...

He sets the second area light to a NEGATIVE one hundred percent intensity value (-100%), and the light is placed in the exact same location as the "positive" (+100%) area light...

I know that a negative light will extract light from a scene, but in this case it seems like one light would just cancel the other out (?). But the render shows plenty of light, with no apparent "cancellation" occuring.

Can someone explain the reason for placing two lights in the same spot, and then giving one light a negative value of the other?

Best Regards,
Don Z.

There was a distant light in the scene as well so that's where the illumination was coming from. The area lights were cloned and one inverted so as to provide only shadow casting. So illumination was provided by the distant light and shadows provided by the cloned area lights. Cast shadows was turned off for all lights except the 'positive' area light.

Follow-up to "negative light" question:
 

>
> illumination was provided by the distant light and shadows provided by the cloned area lights.
> Cast shadows was turned off for all lights except the 'positive' area light.


Hmm... That still doesn't explain the purpose of the negative light. It cannot cast a shadow. Why not just use the positive area light? Why try to "take away" the positive light with a negative light?

With two identical lights set at 'inverse' intensities the result is no effect on the scene whatsoever, unless the positive light has shadows enabled, whereby the lights cancel each other out everywhere except the shadow area. Thus we get the softer shadows of an area light without them affecting diffuse or specular shading. Philip chose to have the Distant Light source for illumination only and no shadow casting and used the 2 area lights for their softer shadows. Now he can control the shadows by altering the intensity offsets of the area lights, ie, 50% and -50% would render 'half-transparent' shadows.

Thanks. The technique makes sense now.