- Set up an override material. The material is a Vray Light Material. In the color slot put a Vray dirt material. The defaults will do fine for a pass to just accentuate edges and add contrast. If you wan't some really dirty looking stuff you can up the radius to something big (i.e. 4') and put a smoke map (or whatever you want) in the radius mask slot.
- Turn off all the lights in your scene. The easiest way to do that is to turn off all your global switches in the render settings.
- Make sure the Vray Environment Light is turned off.
- Set your environment map to pure white.
- Experiment with different settings. Fidgeting with the Vray dirt settings will give a wide variety of results.
If you need more info I also posted this Vray Dirt info more recently.
Here is another video I did covering all different kinds of Vray materials. It might be helpful as well.
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