memo
Jan. 21, 2024 17:32:35
I'm trying to get velocities blur (and motion vectors AOV) to work in Karma, Houdini 20.0.59, and just cannot.
Attached is a test case hipfile and screenshot.
I tried:
1. Particle system with @v attrib copied to @velocities. Velocity Blur enabled. Cache node after my render settings. No motion blur at all.
2. Animated mountain and transform SOP on sphere. Random huge @velocities attribute (also copied to @v to visualize, see screenshot below). Velocity Blur enabled. I'm getting motion blur, but it's the correct (i.e. deformation) blur, and ignoring my velocities attributes.
Even though I can see the velocities attribute in my scene graph! (and they are the random huge ones).
Why are the velocities being ignored?
P.S. I also want motion vectors AOV, but it keeps appearing black. See attached hip.
blakshep
Jan. 21, 2024 18:39:30
In usd you need time samples to make effects like motion blur happen as far as i know.
In 20 i think they improved somehow how to create them, try to search on that
cdordelly
Jan. 21, 2024 19:17:18
Hi!
You need to enable the Velocity Blur property on your points using the Render Geometry Settings LOP:

Cheers!
tamte
Jan. 21, 2024 19:25:05
1. no need to rename v@v to v@velocities, v@v is usually correctly imported as velocities
2. the reason you are not getting velocity MBlur is that you are using Scene Import, which will author /geo1.primvars:karma:object:vblur based on the Object's settings, in your case 0, so either set
- /obj/geo1 Render/Sampling/Geometry VElocity Blur to Velocity Blur
or
- override /geo1.primvars:karma:object:vblur to 1 or block it using Edit Properties or Geometry Render Settings
since Velocity Blur Settings on Karma Render Settings is used only for objects that don't have this property explicitly set
memo
Jan. 21, 2024 21:55:01
Thank you!! Yes both methods worked!
P.S. All the documentation I found said that there needed to be a velocities attributes. So seeing that I had no motionblur, out of desperation I tried that.
matt_barker
April 2, 2026 02:19:49
tamte
the reason you are not getting velocity MBlur is that you are using Scene Import, which will author /geo1.primvars:karma:object:vblur based on the Object's settings, in your case 0, so either set
- /obj/geo1 Render/Sampling/Geometry VElocity Blur to Velocity Blur
Thanks tamte,
I lost too much time trying to find a solution for this! I'll add this to my gotchyas list for Solaris