In this video we will cover how to create volumetric scatter fog with Arnold for Maya, and how to comp it in Nuke 8. As of the creation of this video Arnold MtoA is in Release Candidate 3, and there is no dedicated AOV pass for breaking out volumetric scatter fog nor will there be by the release of 1.0. There are however two ways to isolate your scatter for further manipulation in Nuke.
The first being the old faithful render layers, and layer overrides with Mattes. All you have to do is build your scene with scatter to your liking, then create a separate render layer and activate the "Matte" function in all of your Ai_Standard shaders, and render your render layers accordingly. The second method is great if you did not remember to break out your scatter with a render layer. All you have to do is combine your standard AOV passes which as it turns out by default do not account for the scatter, then use a merge function with your full beauty pass as the A pipe, and your combined AOV passes as you B pipe with a "Minus" function. In effect this method subtracts your combined AOV passes from your beauty pass, leaving behind only the volume scatter. That's it!
Once you have your volume scatter broken out you can use useful nodes like grades and Hue Shifts to recolor or achiver effects like turning a light on and off with a dim function.
I hope you find this technique useful, and please remember to like us on Facebook, and follow us on twitter @themaxdepth