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Rather atrocious artefacts in Cycles render: only when motion blur enabled
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Description

System Information
Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: GeForce GTX 1060/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 461.72

Blender Version
Broken: version: 2.92.0, branch: master, commit date: 2021-02-24 16:25, hash: rB02948a2cab44
Worked: (newest version of Blender that worked as expected)

Short description of error
[Rather atrocious artefacts in Cycles render: only when motion blur enabled]

Exact steps for others to reproduce the error
[Load the attached scene, point it at the attached Alembic file (with the same 'MeshSequenceCache' parameters: Frame Offset=-150), and try rendering with and without motion blur in Cycles, you should get the same horrendous artefact on or around frame 161. As you can see in this clip, it renders fine in Cycles without motion blur, and also fine in Eevee with motion blur. And obviously you can't 'see' the issue in the viewport in any mode.

So this is an Alembic, created in Blender (2.79) from a rigid body sim, and apparently completely 'clean' unless rendered in Cycles with motion blur enabled.
It looks deeply/suspiciously similar to a motion blur artefact I reported about 3 years ago (T57669) apparently deemed 'low priority', but as I am now getting this issue (or something extremely similar) with an entirely different type of source object (and it is also completely wrecking an animation I am trying to do for a real job), I'm wondering if this will ever get fixed, or will this just remain a 'skeleton in the closet' which may well occur again.
I would be extremely happy to be told there is a simple 'fix' for this issue, and that it is in fact a completely different issue, but it seems only to be an issue if Cycles is used with motion blur, so I'm at a complete loss as to what to do, or change, to fix it.
As I found this in an animation I'd spend a couple of days rendering (Open VDBs involved), and there were several un-useable frames scattered here and there throughout the sequence; I would be just as happy not have to completely re-do the rigid body sim, re-export the alembic, and then spend another day or so rendering, just to discover another few random frames are un-useable, and I have to go back to the drawing board again is it were.]
[Based on the default startup or an attached .blend file (as simple as possible)]


Event Timeline

Mark Spink (marks) updated the task description. (Show Details)

Tried it in V3 Alpha, same results. This version-

Thansk for the report,
This is the same problem as described in T82286: Alembic motion blur still fires random velocity vectors and crashes Blender.
It is quite possible that the problem originates from the ShatteredMoon05a.abc file, so if the problem is no longer reproducible with newer-generated Alembic files, I don't think there is much we can do.
An important piece of information is: Has this ever worked well in any version?

I will merge the reports since they are the same.

No idea! I'm stuffed as the 2.79 I'm using to create the file is the Fracture Modifier branch, to enable the rigid body sim, so I'm stuck with 2.79's alembic export (when if ever will FM get into the 2.9x branch, or something better please?!?!?)...
So, guestimating that I should render F300-320 (as there's no 'frame offset' parameter that works like in 2.9x), and the issue does still show in 2.79.