Hiya!

I'm struggling to get per object motion blur to work in Karma. I've made a sample hip attached.

I've got two static spheres in a scene, the camera orbiting quickly around them, one of them I want blurred, the other I do not.

I've got two Render Geometry Settings nodes with the following settings:
- Enable Motion Blur enabled, Samples from Stage disabled, Tranform samples = 2 (enough to get blur)
- Enable Motion Blur disabled, Samples from Stage disabled, Tranform samples = 1 (i.e., do not blur)

In Karma I have Per Object motion on by default, and motion samples from stage enabled (thinking it will pick up the settings from my nodes above.

Alas both objects are blurred instead of just one!

Help greatly appreciated!

Cheers,
Jayson

I don't think there's an easy way to accomplish what you're describing, short of rendering two passes: one with motion blur, one without motion blur.

The Render Geometry Settings controls how the renderer will handle possible motion data specified on the *geometry*, but in your case the geometry (i.e., the spheres) are static, so there's no effect of enabling/disabling things here.

Effectively what you're asking for is to have the camera moving (between shutter open&close) when generating rays that intersect with one object, but not moving when generating rays that intersect with another object. But there's a bit of a chicken-and-egg situation here which needs you to know for which pixels you should be generating which rays, and to know that you'd need to render both ways, which brings me back to the starting solution of "render this in two passes".

Gotcha! Thanks Rob, that makes a lot of sense.

It's been suggested on the academy discord that I extract the camera velocity and subtract it from my object's velocity to counteract the motion. I've seen Steven Knipping do something similar with parenting in obj context so I'll give those ideas a go and report back to help anyone else finding this thread

Back soon, cheers!

Yes, if you're able to counter-animate, that should work too. I will, however, give my personal warning that this can get very fiddly/fragile very quickly. Not saying "don't do this", just "don't be surprised if this proves tricky".