Motion Blur shrinking my geometry

[Yannick]

Hi,
I am trying to have some motion blur on a spinning wheel. However my rendered geometry is smaller that an image of the same object without motion blur.
I am using motion blur through a physical camera.
If I do the same motion blur without a physical camera, the final image is the same size as the original geometry.

It looks like the speed of the spinning geometry changes the size of object on the final image.

Anyone knows why this is happening?

Thanks,

Yannick

[vlado]

You will probably need to use more geometry samples for the motion blur of the object - you can go this with the V-Ray object properties.

Best regards,
Vlado

[Yannick]

Thanks Vlado.
I did that and it solve the problem.
Could you explain why it is doing this?

[vlado]

By default V-Ray uses only 1 motion blur segment, which gives you linear motion blur. For fast rotating objects, the start and end points of the line that approximates the motion lie on the circle for the movement, but since they are connected with a straight line, the object appears shrunk. With more segments, the circular movement is approximated better. Something like the picture below.



Best regards,
Vlado

[Yannick]

Thanks for the explanation.