In a recent case, a customer was using an ICE tree in the Modeling stack to create a fixed number of particles. Then, he used an animated scalar value in the ICE tree to drive changes in the particle positions. Basically, it was a bunch of “spokes” rotating in a circle.
The problem was that there was no motion blur when he rendered with mental ray (but there was motion blur with 3Delight).
To get motion blur in this type of scene, you have to set the PointVelocity attribute to a value that indicates the direction and distance/second of travel.
Unlike 3Delight, mental ray only takes the motion from the velocity attribute. There’s no delta comparison done.