Did you ever figure this out? Same problem here in both 2013 and 2014. Hi all, I'm pretty new to Maya and I've just begun working with mental ray. I'm trying to create some lit fog/ light rays so I'm using a volume primitive with fog and a spotlight to try and create this effects. I've adjusted all my setting so ray traced shadows and illuminated fog works etc but I still have one problem. When I try to render it using mental ray it doesn't pick up the fog at all when the "illuminated" setting is enabled on the fog. All I get is a really grainy picture with what looks like a water mark symbol in the center of the screen. I've tries messing with samples and every other setting I can find on Render setting and no matter what I do it's not making a difference. Someone please help me, this is driving me up the wall!