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From Odwiki

MotionBlur : Velocity Motion Blur

There are three types of motion blur supported by Houdini, TransformationBlur, VelocityBlur and DeformationBlur.

Velocity blur uses the VelocityAttribute (v) to estimate the motion on deforming geometry. The velocity attribute can be created using a variety of [SOPs] including the TrailSOP, the AttribCreateSOP, and the PointSOP. Velocity is expressed as distance per second.

Instead of having to cook the geometry at the end of the shutter time (as with DeformationBlur), the deformation on the geometry is estimated by using the velocity of each point. This gets around several of the issues with DeformationBlur, but has its own set of problems.

Since the deformation of the geometry is approximated by using the velocity, the actual deformation performed in the next frame may not be accurately reflected in the velocity blur. For example, consider a particle which is moving down and is just about to hit a ground plane. The velocity blur will estimate that the particle will continue moving down through the plane without considering the collision.

Note: VelocityBlur also takes TransformationBlur into account