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It is possible to use velocity vertex maps to control the motion blur of changing meshes (vertex maps are scalar float maps and not vector maps). You can enable / disable vector motion blur, define vertex maps, unit and scale. C4DtoA calculates a time delta from the initial render frame in each motion step and calculates vertex positions using the formula:

  pos = initPos + delta * scale * mapValue

where mapValue is the vector read from the user defined vertex maps. If the unit is set to 'per frame' delta it is frames (seconds * fps).

Enable

Enable/disable motion blur by the object.

Use Motion Vector

When enabled the motion blur will be calculated using a velocity vector. By default, C4DtoA will not motion blur an alembic cache of a mesh that has a changing topology. However, C4DtoA 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.

Find maps

You can select how to define vertex maps, by link or by name.

  • By link: you have to drag & drop the vertex maps of the motion vector channels to the link fields. This mode can be used if the maps are visible in the object tree. 
  • By name: this mode is useful when the object is a generator and the maps are generated to the polygon object, so they are not visible in the object tree.

X,Y,Z map 

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 vertex map or drag & drop the vertex maps on 'by link' mode. C4DtoA will use the vertex velocity attribute to calculate the motion blur. The point vector attribute coming from Houdini, for example, may be called 'v_1', 'v_2', 'v_3'.

  • X map: vertex map of the x velocity component.
  • Y map: vertex map of the y velocity component.
  • Z map: vertex map of the z velocity component.

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.


Workflow

In this example we will use a scene that has been exported from Real Flow with velocity point vertex data. In C4DtoA we can use motion vectors to apply motion blur settings to the fluid.


  • Select the Vertex Map Tag in the Object Manager to view the X, Y, Z velocity maps in the viewport.


  • Add an Arnold Tag to the mesh and go to the Motion Blur tab.


  • Drag  and drop the Vertex Map Tags from the Object Manager onto the respective X, Y and Z Motion vector map fields below and enable 'Use motion vector'.

Make sure that the vertex maps are exported. You can go to the Vertex Map tab to check the settings. All vertex maps are exported by default.


  • Increase the Motion Vector Scale to exaggerate the motion blur effect when rendering.


  • Below you can see the results when rendering each motion vector map independently.

You can download this motion vector example scene file here.

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