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. 

You will need to ensure that you have the SOuP plugin for Maya loaded for this tutorial to work.

Houdini

We will use an alembic file from Houdini. It has point velocity in the v channel (point vector attribute 'v' coming from the geometry you are exporting in Houdini to store the velocity, which should be a vector of 3 floats) which calculates motion blur on a mesh where the point numbers are in motion. 

Maya

  • Open the alembic file (.abc) in Maya. You can open it from the File>Open window or from the Pipeline Cache>Open Alembic menu.

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 50 (the attached scene is from 24 to 48).
  • We now need to create a Color Set for the alembic geometry. Select the mesh and go to Color>Color Set Editor. Click on New to create a new color set. Rename it to colorSet1

Ensure that the Color Set is  set to RGB and not RGBA, otherwise you will get an incorrect result.

 

  • Select the geometry and enter colorSet1 in the Motion Vector Source field.

 

  • Go to the soup shelf. Click on the first nodes icon and select arrayToPointColor.

 

  • Open the Node Editor window and make the following connections.
  1. AlembicNode.outPolyMesh -> arrayToPointColor.inGeometry
  2. AlembicNode.prop[0] -> arrayToPointColor.inRgbaPP
  3. arrayToPointColor.outGeometry -> createColorSet.inputGeometry
  4. createColorSet1.outGeometry -> RENDER.inMesh

Alembic node connected to arrayToPointColor and fluid surface geometry

 

  • 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 exports 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 colorSet1 to Color Attr Name attribute of the aiUserDataColor node.

 

 

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