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Cycles Motion Blur 'explodes'
Closed, ResolvedPublic

Description

System Information
Operating system: Kubuntu 18.10
Graphics card: GTX970

Blender Version
Broken: 2.80, 3600bb79d05d, 2019-03-15
Worked: (?)

Short description of error

In a scene with an animated camera which is tracking to an animated target, motion blur 'explodes' on certain frames.

Exact steps for others to reproduce the error

I have a .blend file attached; it contains a simple scene with a cube, an animated camera tracking to an animated empty. When rendering frame 39 with Cycles and motion blur enabled (default settings), the motion blur 'explodes'. Other frames seem fine. If I disconnect the Environment Texture node from the Background in the world shader, the motion blur seems fine.

I am not 100% sure this isn't some sub-frame gimbal lock issue I should avoid in animation, but since it manifested often with Blender 2.8 I suspect this is a bug.

Event Timeline

William Reynish (billreynish) lowered the priority of this task from 90 to 50.

Can reproduce this. Seems strange.

@Brecht Van Lommel (brecht) feel free to re-assign if the issue is not with Cycles itself.

Update: motion blur does not 'explode' when I set Motion Blur 'Position' to either 'Start on Frame' or 'End on Frame'.

With Start on Frame the problem appears when 0.18 <= shutter <= 0.35.
With Center on Frame it happens when 0.36 <= shutter.