Vector motion blur getting ahead of itself

Hi,

I’m having a bit of a problem with the vector motion blur on a relatively simple scene where I have a fast moving object appearing from one side of the frame and then stopping quite abruptly in the middle of the screen. I’ve rendered the scene out in Blender using the EXR format with an embedded vector pass. Using the vector motion blur in Fusion works just fine until the object reaches the point where it stops moving. The frame before the stop occurs seems as though it’s anticipating the object to keep on going along its trajectory since the motion blur in that frame reaches areas where the object will never even go. It’s as though it somehow fails to acknowledge that the movement is about to slow down rather quickly in the next frames and instead "predicts" a future for the object that it will never have. Is this a known issue and is there anything that can be done about it?

I think you hit the nail on the head: there is no anticipation of what's going to happen. A velocity factor is just that: an indication of the velocity at the current frame. Otherwise it would've been called an acceleration vector

I could be wrong though, there are more knowledgeable people than I am on this forum.

That's quite right. This is one of those cases where a post motion blur shows its limitations. You could animate the Scale, but since there is no center bias control in the VMB, you'll shorten the blur behind the object as well as in front of it. Probably the best solution is to simply paint out the leading edge of the blur. If the object stops very suddenly, you should only need to do it on one or two frames.

Thanks for the clarification. I sort of had hopes it'd have something akin to a "lookahead" function found in many audio limiters... If only it could play Marty McFly for a couple of frames

I ended up rendering the motion blur in the 3D app. I did think about masking out those portions of the blur in Fusion that extend to areas the moving thing will never reach and it's indeed a viable option since you only have to do it for a couple of frames. Maybe next time