Re-using GI & FG Maps Part 2.vtm
NOTE: There appeared to be a discrepancy in the class demo where although both GI & FG rendered fine without maps, after generating and using the maps, the scene became blotchy again. To overcome this, try upping the FG values to Very High. Initial render times will sky rocket but if you are only making a FG map then it can be discounted.
There is also a forum link here that discusses this subject in great depth if you can get past the arguments...