How to use object velocity to make motion blur

[burr]

Could someone write out a quick tutorial (step by step) on how to achieve this? When I check vray's motion blur to be "on" it takes too long to render and I get strange artifacts here and there.
thanks

[Morbid Angel]

there is nothing really to write...its very simple, all you do is under gbuffer rollout check objects velocity and then render your object without motion blur into RPF file, where rpf will contain data of the motion blur which can be used in post, however the motion blur in post is 2d so it doesnt carry true 3d tracing of the object, but render speed is way better.
Its a good solution for quick shots but I wouldnt recomend it.