Motion Vector Source

 

By default, MtoA will not motion blur an alembic cache of a mesh that has a changing topology. However, MtoA can use the velocity attribute as a source to derive motion blur data from a mesh that has a changing topology using a motion vector. 

Motion Vector Source

Use this velocity attribute as the source to derive the motion blur data. You must use the name that the velocity was exported as a per point vector. MtoA will use the vertex velocity attribute to calculate the motion blur. The point vector attribute coming from Houdini, for example, may be called 'v'. The default name is 'velocityPV'.

Motion Vector Unit

Choose between 'Per Frame' or 'Per Second'.

Motion Vector Scale

A float scale field (with a default value of 1) to do time scaling effects. 

Motion Blur and Meshes with Changing Topology

This short tutorial will go through the process of exporting an Alembic file with motion data stored in a vertex color channel from Houdini and rendering it in Maya with MtoA. We will use the velocityPV attribute to convert per vertex attributes with which MtoA can use to apply motion blur to the Alembic geometry. This geometry has topology that changes from frame to frame. 

Houdini

We will use an Alembic file from Houdini. It has point velocity in the v channel converted to a point color attribute velocityPV which calculates motion blur on a mesh where the point numbers are in motion, details on generating a compatible alembic file are available here

Maya

  • Open the Alembic file (.abc)  in Maya. 
  • You can't use the menu to do this as there is no option to import the color data that is in the cache, instead use the following MEL script, changing the path for where you have saved your fluid.abc file.

    AbcImport Command

Alembic file opened in Maya

 

  • Scrub through the timeline in order to view the animation. In this case, the animation range is from frame 1 to 100 (the attached scene is from 40 to 60).
  • We can view the color per vertex data that is in the alembic. Select the mesh and go to Color>Color Set Editor and select the velocityPV(RGB) attribute.
  • Select the geometry and make sure the Motion Vector Source field is set to velocityPV
  • make sure the Motion Vector Unit is set to Per Second
  • Finally, make sure Export Vertex Colors is enabled, otherwise, our velocityPV attribute will not be exposed to Arnold

 

  • Open the Arnold Render Settings window. Enable Motion Blur and render the scene. You should now see that the mesh is rendering with motion blur. 

When velocity blur is enabled, the translator will only export two keys.

Color

UserDataColor

Color information can also be exported to the Alembic file from Houdini and then extracted and rendered with MtoA in Maya.

  • Add Cd to Color Attr Name attribute of the aiUserDataColor node.

 

 

 

 

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