I'd like to think of volume lighting as more of an atmospheric effect, something entirely different than focal distance. The depth pass can be used to map simple fog in post, if you decide to take that route. I'm thinking about it now and I don't think the Maya volume primitives work in MR :s
And about the steps texture varying, you can use a triple switch node, I did a short vid recently since some had a similar situation.
Last edited by GecT; 04-09-2012 at 04:49 AM.
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