"Black Hole"... umm, had that occur in Unity. no fun.

Yep, even at 0 bias it still leaks. Got to go to high positive value to suppress it. But then it creates a "black hole" sucking in your shadows :alien:

True... a small extrude clears it up, but the real problem is "shadow bias" and occasionally "bounce intensity."

Plain made objects do not play along with directional light no matter how well or where you weld them. Either add enough thickness to the structure to suppress leaking or use other types of lighting.

Not relying on 3dxchange's (Smooth & Weld) has anyone actually tried exporting the "Leaking Object" into say Blender and "Weld" it shut.
If any light leaks through a (properly) welded object... time to use new lighting.

I have always had this issue with iClone. I was hoping I was missing something and someone might have a solution.
But that's how I've always dealt with it...using a Spotlight and adjusting it's angle and falloff.



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I did a test with a cone within the box and found that even with lights off there was still gray. So I changed the background to black to have a clear starting point.

Regardless, there is still some light leaking at certain angles.

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Yes, if I adjust the shadow bias enough to get rid of the issue  I lose the shadows I wanted.  The workaround for me is to use a spotlight in place of the directional light.  Still I wonder what causes this issue.

Have you checked your shadow bias?

I had this problem. Welding would not help if the box walls do not have a substantial thickness.

Bring a default iClone box to the scene, scale it and go inside. Play with direct light and notice it is leaking all over the place.
Now try to make a large box with iClone Wall prop, go inside and rotate the light - leaking is gone, even though walls are not welded together.
Though it is a good practice to always have your props mesh solid - welded.