Jumping object when rendering with Motion Blur

Hi there

I have an issue when rendering with Motion blur in Cycles. Animated wheels of a car which stands almost still, start jumping around the rotation axis at certain reasonable speed when rendered in animation using Motion Blur. No issues when rendered without motion blur. They jump in a triangle way and the higher the speed, the larger the amplitude is.

I suppose when Blender analyses the position of pixel between two frames, it may happen that at some speed pixels go one way, but at higher speed it may seem that they go backwards, because of rotation. But what to do then? I don’t think its an excuse for such a unusable behaviour… I was animating the burnout at the speed which is much lower than in reality…

I don’t have a rendered example of this particular issue, I had to slow it down so that it was more or less not noticible, but even smoke around is still jumping like crazy… The wheels were jumping the same way when rotated faster.


Ok, I found that the source of all the problems was the smoke domain… A car and rotating wheel are inside the domain, and looks like although it seams that domain is transparent having only some smoke inside, it completely obstructs all the auxiliary passes, like Vector pass. So Blender was trying to perform Motion Blur blindly which resulted in crazy motion. Once the scene was rendered with smoke domain placed on another layer (and switched off), everything became perfectly normal. Finally…

Now I have to find out how to render volumetrics separately, preserving all the shadows and reflection on interacting objects, to combine everything in post.