The Velocity element generates a rendering that contains information about the motion of objects in the frame.
You can use the Velocity rendering to generate motion blur when you use a composition application such as Combustion or Flame. There are plug-ins for compositors that generate motion blur; for example, those created by RE:Vision Effects, Inc.
The advantages of rendering a Velocity element are that the composition application might give you finer control over the motion blur than 3ds Max does. You can render a “beauty” frame that does not contain motion blur, and the Velocity element might be quicker to render than using one of the 3ds Max motion-blur effects.
Another use of the Velocity element is to re-time clips rendered in 3D. There are specialized applications that allow you to re-time an image sequence, using velocity data to generate more accurate inbetween frames.
In the Velocity rendering, the motion information is saved as RGB color information: Red saves movement on the X axis, green saves Y-axis movement, and blue saves Z-axis movement, relative to the plane of the rendered frame.
Controls on the element's rollout let you improve the precision of the motion data saved in the rendering.