Motion Blur and animated objects with changing vertex counts / vertex positions

[greymiura]

The parametric object in the scene below is generated from a C4D rectangle spline with an animated width parameter which means that the vertex count of the extrude object changes and individual vertex indices jump around from here to there over time.



While you'll get a clean topology for every single frame, V-Ray seems to trace the positions of the individual vertices for motion blur interpolation and the shading gets messed up.

This may be some kind of expected behaviour rather than an issue but is there a way to tell V-Ray to consider the overall geometry of an object rather than the position of its single vertices for motion blur? Or is this just the way motion blur works?

You may download the scene file here:

https://we.tl/t-SS0mQUKRFp

[deyan.hadzhiev]

Hi greymiura, what you observe is expected. Particularly:

This may be some kind of expected behaviour rather than an issue but is there a way to tell V-Ray to consider the overall geometry of an object rather than the position of its single vertices for motion blur? Or is this just the way motion blur works?

V-Ray traces moving triangles and quads during the motion blur sampling. For the rendering, all mesh geometry is only a collection of triangles. There is a small exception here - you can have different amount of vertices between frames, as long as the triangle/quad count remains the same (e.g. you can have motion blur with geometry that is teared between frames, as long is it retains its triangle count).

Given the limitations above, in your example scene you can achieve stable results with motion blur, by ensuring the primitive count is stable between frames. With some experimentation in your example scene, setting the Extrude Tessellation to Delaunay and having Uniform Intermediate Points in the Rectangle spline achieves stable triangle count between frames: