The issue is caused by extreme rotation values. Motion blur typically solves rotations along the shortest arc between frames. Which implies the maximum rotation you can have between frames which solves correctly is 179.9 degrees or else you'll get the 'rewind' effect. Mental ray does have some built-in logic to solve on a window a little larger than 179.9, but probably not beyond 360 degrees. Mental ray is largely dependent on information coming from the host application to resolve these kinds of things
In your case, the difference between frames is 421,039.2 degrees which is waaaaay beyond that solvable range. The motion blur arc is so stretched out it's wrapping around itself hundreds of times like thread wrapped around a spool. Your shutter open/close times are so close together it reveals only a very tiny portion of that path which results in the strange arc you see. The solution is to reduce the number of rotations by a multiple which retains the persistence-of-vision of the wheels appear to rotate correctly, but where the delta between frames is less than 360 degrees. Matt From: [email protected] [mailto:[email protected]] On Behalf Of Arvid Björn Sent: Wednesday, July 18, 2012 12:58 AM To: [email protected] Subject: Re: Car wheel motion blur Here, I've recreated the problem in the simplest possible setup, no weird rotation order and it's centered on the null, just draw a region to see the problem. File: http://www.stopp.se/arvid/motionblurbug.scn Different rotation speeds of the ball yield different arcs though, so sometimes you'd get lucky to hit a number that doesn't show the problem, which is why the problem appear at random for us. The problem was solved by skinning the geo to the null instead of parenting it, which invokes deformation motion blur, but the scene gets more complex and the rig gets heavier. Please see if I've missed something in my settings, I'd love to solve the problem without skinning! Cheers On Tue, Jul 17, 2012 at 6:09 PM, Manny Papamanos <[email protected]<mailto:[email protected]>> wrote: I did a quick test and the blur seemed to look good in MR on a simple cube spinning on a moving cube hierarchy. Be sure you don't use 'offset' in the MB settings or the wheels or they may look like they are coming out of the wheel wells. Check the transforms on your 'centers' on the wheels and all the parents for weirdness. If something weird is based on the parents, one way out of this is to plot and then use that motion on the wheels. Also, perhaps playing with the 'rotation order' may help, I would make sure that the spinning axis is first on the list. -manny|SI support From: [email protected]<mailto:[email protected]> [mailto:[email protected]<mailto:[email protected]>] On Behalf Of Arvid Björn Sent: Tuesday, July 17, 2012 10:20 AM To: [email protected]<mailto:[email protected]> Subject: Re: Car wheel motion blur Thanks, I tried your suggestion, but there's nothing wrong with the parenting. It is related to parenting, but that's not where the problem lies. It works fine with deformation motion blur with the exact same center of rotation, so that proves that the problem is in how transformation motion blur is being evaluated. When evaluated per point with a global vector, everything's fine. I wish I could force MR to do that even though I'm not actually deforming anything. On Tue, Jul 17, 2012 at 1:56 PM, Alok Gandhi <[email protected]<mailto:[email protected]><mailto:[email protected]<mailto:[email protected]>>> wrote: Most probably the motion samples are creating an arc due to parenting. What you need to do is to check this is take the fcurves and scale them quite a bit. This will allows you to see in "slo-mo" the trajectory of the wheel as it moves along. If you see the undesired arcing then I would suggest changing the parenting so the center of motion is at the center of the wheels.
