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Mantaflow no longer renders motion blur
Confirmed, NormalPublicBUG

Description

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 longer renders with motion blur in Cycles despite the "Use Speed Vectors" option is checked.

Exact steps for others to reproduce the error

  • Either open the attached blend file:


...or create a super simple scene yourself by adding a cube as a fluid domain, another cube inside it as inflow. Set both to be of fluid type. Under the "Mesh" options check "Use Speed Vectors" and also activate motion blur in Cycles.

  • 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.Tue, Jun 30, 4:07 PM

I cannot reproduce this with the current development versions of Blender: https://builder.blender.org/download/

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.Tue, Jun 30, 4:34 PM
Germano Cavalcante (mano-wii) changed the task status from Needs Triage to Confirmed.Tue, Jun 30, 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 ;)