I've been trying to expand my understanding of mental ray with Boaz Livny's book, as i'm doing a job involving GI and FG. The workflow suggested is to preview with a tone mapping lens shader and then render out 32bit channels - without any tone mapping, as render channels ignore the tonemapping shader - and apply tonempping to the channel in 2d during the composite.
All straight forward, except the characters in my scene are using a skin shader - and in the irradiance channel, anything with the skin shader (any factory supplied skin shader or skin shader compound) turns up black. It's fine in the main pass and is recieving indirect illum, just not showing up in the irr channel.
This would seem to be punching a bit of a hole in the workflow. And whilst i could forgo the separate channel route and just render a tonemapped beauty pass, i would like to know whether this is something i'm doing wrong, or whether it's a current limitation in MR.
on a side note, i recommend the Boaz Livny mental ray book for artists who want to expand their knowledge of MR.