It is possible to use velocity vertex maps (a color map or three float maps) to control the motion blur of changing meshes. C4DtoA calculates a time delta from the initial render frame in each motion step and offsets vertex positions using the formula:

  pos = init pos + delta * scale * motion vector

where motion vector is the vector read from the user defined vertex maps. If the unit is set to 'per frame', delta is in frames instead of seconds.

Enable

Enable/disable motion blur by the object.

Use Motion Vector

When enabled the motion blur will be calculated using a velocity vector defined by vertex maps.

Vertex map type

Specifies the type of the vertex map used, which can be a color map or three float maps (X, Y, Z channels).

You can also specify if the vertex maps are assigned 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 / Vector map 

The map(s) which define the velocity per vertex.  For instance the point vector attribute coming from Houdini, may be called 'v_1', 'v_2', 'v_3'.

Unit

Specifies if the velocity is interpreted as a 'per frame' or 'per second' value.

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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