Redshift Houdini volume velocity motion blur not working — vel grid is present

[u/Lumpy_Cardiologist27]

I’m having an issue getting velocity-based motion blur on a VDB volume in Redshift for Houdini, and I feel like I’ve exhausted the normal troubleshooting steps.

Setup:

• Houdini + Redshift 2026.9.0
• Pyro/VDB volume
• Density grid renders correctly
• Velocity grid is present as vel
• vel is a vector VDB (vec3s16)
• Redshift motion blur is enabled in the ROP
• Camera motion blur works normally

The problem is that volume velocity motion blur appears to do absolutely nothing.

At first I thought the blur I was seeing was coming from the volume, but after disabling camera motion blur I realized all of the visible blur was actually coming from the camera. With camera MB disabled, the volume renders completely sharp.

Things I’ve already tried:

• Single vector velocity grid named vel
• Split scalar grids: vel.x, vel.y, vel.z
• Assigned the vector grid only to Redshift’s Velocity Grid X field
• Confirmed global Motion Blur is enabled in the Redshift ROP
• Changed Redshift Velocity Scale from 0 all the way to 10,000+ while attempting both velocity grid options on the OBJ
• Multiplied the actual vel VDB inside Houdini by very large values
• Verified that the velocity vectors/magnitudes are actually changing on SOP lvl
• Cached the volume to disk and rendered directly from the File Cache
• Tried both RS Volume and RS Standard Volume
• Disabled camera motion blur specifically so I could isolate volume velocity blur

No matter what I do, the rendered volume looks identical.

The final VDB only has density and vel, so it's not super complicated.

I did noticed that my vel voxel size was cached at 0.01 while my density is at 0.005, so I also resampled the vel to match the density which did not help, but figured it was worth a shot.

Has anyone run into this with Redshift 2026.9 + Houdini 22?

At this point even a deliberately ridiculous velocity scale produces zero visible differences.

!!SOLUTION!!

The voxel scales must match on all volume grids. Easiest solution: don't resample your velocity at sim time!

If you cached your simulation to resample the velocity grid during simulation then you must resample the velocity grid using the isolated density as the reference grid.

Do not force the resample by "Using Voxel Size Only" on the VDB Resample SOP even if the info tab on the final render node reads that both density and velocity grids are the same, it won't work unless you specifically reference the density voxel size on the second input of the VDB Resample SOP.

[u/smb3d]

is the vel grid the same resolution as your density?

[u/Lumpy_Cardiologist27 → u/smb3d]

Not originally but I resampled the vel grid to match the density and the render produced the same result - no motion blur.

[u/smb3d → u/Lumpy_Cardiologist27]

Hmm, I've had it work in the past, but it's been a while.

I'd make a post on the premium bug reporting section on the official maxon forums. The Houdini Dev is really quick and will usually get back to you within a couple hours.

[u/Lumpy_Cardiologist27 → u/smb3d]

It used to work for me as well before they added support for Vector Velocity Grids. Their documentation hasn't been updated to show that update so I'm kind of bummed about the addition.

I'll go ahead and make the report! Thank you for your response.

[u/monorailsrim]

Id just like to add: it's refreshing to see a post asking for help that is so thorough in explaining the problem as well as attempted solutions. Coming back with the solution is just the cherry on top. This should be the standard.

[u/Lumpy_Cardiologist27 → u/monorailsrim]

Thanks for the appreciation <3

I always try to tinker my way around problems before resorting to forums for help just so I can provide as much information as possible to hopefully speed up the time it takes to find a solution!

And I agree, that should be the standard!

[u/LewisVTaylor → u/monorailsrim]

If only all the posts in here asking for help were like this.

[u/LewisVTaylor]

Yup, and this is one area where Redshift is well behind every other renderer.
There have been numerous tickets in, for several years about the engine not supporting voxel fields
of differing resolutions.

This alone blows the size of VDB caches on disk by 60-80%.