Here’s a quick tip I use almost all the time now, mostly because I really like the look and it’s easy and quick to setup, but the main advantage of an AO (ambient occlusion) pass is spotting problem areas in the geometry – especially holes and flipped normals you might have missed
Setting up the AO pass with VRayLightMtl & VRayDirt is real easy and fast. You don’t need to turn on GI or use a VRay Physical Camera for this (if you do, make sure to tick off the exposure option). After I import the model into 3d Studio Max, or just finished modeling inside 3dsmax, I run this quick pass to check the model for geometry bugs such holes and flipped normals i might have missed.
In the image below you’ll see how those problem areas will show in such a pass. This will only work if the Emit light on back side option in the VRayLightMtl is off allowing for all those problem areas to appear pure black since they are not emitting any light.
The material settings
Using VRayDirt map as color, making sure Emit on back side is off.
Setting pure white and pure black for colors and radius to 200cm gives me good result, but you can play with it.
Check out Peter Guthrie’s VRay Dirt Tutorial at his blog for more cool uses for this great VRay map.