Motion Vector AOV

Motion blur using the motion_vector AOV (rollover image)

 

This tutorial aims to demonstrate how to use the motion_vector AOV in a pool ball scene. We will talk about the advantages and disadvantages of using this technique compared to true 3D motion blur. The motion_vector AOV outputs a color channel that shows object movement within the scene. This AOV can be used by post-processing software to calculate a 2D motion blur effect. The advantage is that it is usually quicker to render compared to true 3D motion blur.

 

Creating The Pool Ball Render

Before we render the Motion Vector AOV, we must first separate the pool ball from the background. We will render the pool ball (with the motion_vector AOV) separately and composite them together in a post-processing program like Nuke.

  • Before rendering the pool ball separately, we must enable Instantaneous Shutter in the Motion Blur settings. The reason being is that we don't want motion blur in the render, but we still want the motion velocity information in the motion vector AOV.  
  • Select the geometry in the scene (except the pool ball that we want to motion blur) and disable their primary_visibility. You should just see the pool ball rendered on its own.

Pool ball render (primary_visibility disabled for the rest of the scene)

Creating the Background Layer

  • Now disable primary_visibility for the pool ball that we want to motion blur. When rendered, you should see the background objects and only the shadow of the motion-blurred pool ball.

Background render (primary_visibility disabled for the pool ball)

Composite

  • Open the two EXR files in a post-processing package (in this case Nuke).
  • Connect a  'Vector Blur' node to the pool ball render. We will use the Vector Blur node to blur the pool ball using the 'Motion Vector AOV'.

 

  • Select the VectorBlur node. Change the uv channels to motionvector, and the mv presets to Arnold. Enable uv alpha and choose rgba.alpha to prevent the blur from being clipped by the alpha of the motionvector AOV.

Vector Blur node settings in Nuke

 

  • Use a 'merge' node (operation set to 'over') with its A input as the Vector Blur and it's B input as the background render.

 

Finally, we should see the pool ball render composited onto the background render as follows:

Limitations

There are are some limitations when rendering a 2D motion vector compared to rendering Arnold's native 3D motion blur. Some of these limitations include motion blur in a reflective surface, deformation blur, no true 3D motion blur effects such as a wheel spinning that is not facing the camera, and time-lapse effects using motion-blurred light sources. 

Reflected motion blur is not possible when using Motion Vector blur (rollover image)

The motion vector AOV has a problem handling deformation motion blur. If you need to render a proper motion vector AOV for deformation motion blur, you must create a custom AOV with a motion_vector shader attached to it.

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