Motion blur problems with ground plane shadow

[Yngvai]

I've got an animation of a car peeling out on the ground that I'm compositing into a live action plate. It's a simple setup...car model and ground plane with shadow catcher. If I render it without motion blur, everything is fine.



However, if I render it with motion blur, the car wheel appears detached from the ground and is floating above the shadow.



How do I fix this? I know it's the Octane motion blur causing the problem because parts of the animation where there's no motion blur, then the shadow is fine. It's only the sections where motion blur appears.


I'm using Octane with Blender.

[Yngvai]

OK, I've found a fix for the issue.

The way the car is modeled (I didn't create this model), the wheels just barely touch the ground. Here's a render of the car driving away with no motion blur. You can see the wheels just touch the ground at the peak of the curvature.



So when the wheels spin with motion blur, the gap will appear between the wheels and the ground.



However, a true car tire will have some deformation from the weight of the car so that more of the tire touches the ground, but the car tires on this model obviously don't do that.

My fix was to just slightly sink the model just a tad below the ground plane, as a true car tire would somewhat flatten a bit where it touches the ground anyway. Now, when I apply motion blur, there is no more gap.

[Yngvai]

I'll also note that I found that increasing the "steps" in the object properties from 1 to 3 on the tires also helped. But I think a combo of both solutions is best as sinking the car a bit also adds some "weight" to it rather than having it sit on the tips of the tires.