Motion Vector AOV

Motion blur using 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.

The scene file can be downloaded here.

 

Creating The Pool Ball Layer

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

  • Before rendering the separated pool ball layer, 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.  
  • Create a render layer for the pool ball. Select the geometry in the scene (except the pool ball that we want to motion blur) and disable their 'Primary Visibility' in the attribute editor. When we render the layer, we should just get the pool ball on its own as in the image below.

Pool ball render (primary visibility disabled for the rest of the scene)

Creating the Background Layer

  • Create a new render layer for the background. Assign the geometry to the render layer and 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)

Adding the Motion Vector AOV

  • With the pool ball render layer activated, select the AOV tab in the Render Settings window. Under 'AOV Browser', create a Motion Vector AOV. Make sure that the AOV filter is set to closest.
  • Under AOVs, to the right of the newly created Motion Vector AOV, there is a downward pointing arrow. Click on that and choose 'Select Driver' from the menu.

Select Driver by clicking on the downward arrow to the right of the Motion Vector

 

  • The Arnold driver should appear in the attribute editor. Select 'Merge AOVs'. This will combine the Motion Vector AOV with the rendered EXR as one file.

Enable 'Merge AOVs' stores the AOVs in one EXR file

  • Render both render layers (background and pool ball) as EXR files.

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 Vector Blur 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 Motion Vector 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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