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Mantaflow: motion blur does not work on liquids cached with OpenVDB format
Confirmed, NormalPublicBUG


System Information
Operating system: Linux-5.4.45-050445-generic-x86_64-with-debian-buster-sid 64 Bits
Graphics card: GeForce GTX 1080 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 450.36.06

Blender Version
Broken: version: 2.90.0 Alpha, branch: master, commit date: 2020-06-29 16:20, hash: rB5d31ef082057
Worked: 2.83.1

Short description of error
A FLIP fluid mesh no renders with motion blur in Cycles (despite the "Use Speed Vectors" option is checked) when OpenVDB cache is used.

Exact steps for others to reproduce the error

  • Open the attached blend file or create a super simple scene yourself by:
- add a cube as a fluid domain,
- add another cube inside it as inflow.
- Set both to be of fluid type.
- Under the "Mesh" options check "Use Speed Vectors"
- Activate motion blur in Cycles.
- Set cache type to OpenVDB
  • Go to frame 20 (where the falling liquid should show motion blur) and press F12

No motion blur
This exact workflow rendered with motion blur in 2.83.1 official
Eevee also doesn't show motion blur
If I open the cached VDB sequence in Houdini I can see a velocity field and vdb particles containing motion blur vectors.

Event Timeline

Germano Cavalcante (mano-wii) changed the task status from Needs Triage to Needs Information from User.Jun 30 2020, 4:07 PM

I cannot reproduce this with the current development versions of Blender:

See the blur at the bottom of the fluid:

In the file I had to choose a cache format, otherwise the simulation was not baked.

Go to File → Defaults → Load Factory Settings and then load your file to see if you still can reproduce this issue.

Thanks for investigating.

I can reproduce the problem with both, my own compile of master and the very latest buildbot build (both on Linux).
Loading factory defaults doesn't help, caching to VDBs doesn't help as well BUT caching to "Uni Cache" works, even when setting the mode to "Replay" to use the "live" version.
So it looks like a problem (Linux only?) that likely started to appear when Sebbas changed the VDB caching to cache multiple fields / grids?

Germano Cavalcante (mano-wii) changed the task status from Needs Information from User to Needs Triage.Jun 30 2020, 4:34 PM
Germano Cavalcante (mano-wii) changed the task status from Needs Triage to Confirmed.Jun 30 2020, 4:45 PM
Germano Cavalcante (mano-wii) changed the subtype of this task from "Report" to "Bug".

I can confirm with OpenVDB.

Nice! Good to hear that I'm not the only one ;)

Small update: Since 6b6e2e742ff4 motion blur works again when using "Uni" as the format for volumes. Motion blur support with OpenVDB remains to do.

EDIT: Motion blur with "Uni" caching seemed to work before too - in that case this was more of a clean up, e.g. the mesh file were written twice per frame under different names before.

Germano Cavalcante (mano-wii) renamed this task from Mantaflow no longer renders motion blur to Mantaflow: motion blur does not work on liquids cached with OpenVDB format.Oct 13 2020, 11:15 PM