Prevent Black Appearance Not Working Properly on Rectangular Lights

[apjasko]

I'm not sure if I'm the only one who has this issue, but I can't seem to preserve the direct appearance of rectangular lights. The prevent black apparent just doesn't seem to work in these instances. Is this normal for rect lights?

[TomG]

All seems working normally here. The one thing I can think of that you might be experiencing is when Directionality is zero, the non-illuminated side of the light will still show as black (and Prevent black appearance is greyed out). You can either add the slightest amount of directionality, or use Emit on both sides, in that instance.

Can you share screengrabs of the scene set up, light settings etc?

[apjasko]

I attached a screenshot with my light selected.

The light is shining through on the pop-jets as I'd like them to, and when I set the directionality to 0, or a low enough number that cone of light hits the physical camera, it's visible. The check box doesn't seem to work on the rectangular light. I've had this issue in the past with rect lights and I was able to get by with disc lights instead.

[TomG]

So this is in a case where the CoronaLight is not directly visible, but only seen through Refraction?

There might be a bug here, would be up to the developers to say for sure. Attaching a simple test scene. If the Light is viewed through the box with Refraction (even with IOR of 1.0), then it does not show up even with Prevent Black Appearance. Check the "Thin (no refraction)" and the Light does show up. A white self illuminated plane at the same location always shows up.

So it looks like "Prevent black appearance" may not work as expected through Refraction.

For your scene set up, where the light itself isn't really casting light into the scene but only intended to show up in the water jets via refraction (if I am understanding the image correctly!), would replacing it with a plane with a Self Illumination material work? Alternatively, if there is some purpose for it being a light other than to add white into the refraction of the water (volumetrics maybe?), you could use both a Light and a Plane together to achieve both effects.

[maru]

Yes, the "prevent" option will not work in refractions, and probably reflections either. This is currently a limitation, and it should be fixed in the upcoming releases, as it is often requested.