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Motion Blur missing on Alembics with changing topology / point counts
Confirmed, NormalPublic


System Information
Operating system: Linux-5.4.0-45-generic-x86_64-with-debian-bullseye-sid 64 Bits
Graphics card: GeForce GTX 1080 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 450.51.06

Blender Version
Broken: version: 2.91.0 Alpha, branch: master, commit date: 2020-09-01 12:39, hash: rB526fbdec3b6a

Short description of error
Motion Blur is missing when the imported Alembic contains vertex normals.

Exact steps for others to reproduce the error

  • Unpack the ZIP and open the attached blend file:

  • Press F12:

  • The object on the left has (a bit but still visible) motion blur, the one on the right doesn't.

The only difference between the two Alembics is that the first one doesn't have the "N" attribute on its vertices.

Event Timeline

Kévin Dietrich (kevindietrich) changed the task status from Needs Triage to Confirmed.Tue, Sep 1, 4:05 PM

The issue seems to be that somehow the Cycles mesh has a different number of vertices than the Blender mesh. In the viewport statistics the Mesh is said to have 243 vertices, while the Cycles one has 520, and we read only 243 velocity vectors.