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[Cycles] Motion Blur of fast rotating objects "randomly" glitches
Open, Confirmed, MediumPublic

Description

System Information
Operating system: Windows-10-10.0.17134 64 Bits
Graphics card: GeForce GTX 1070 with Max-Q Design/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 419.35
CPU: Intel i7-8750H @ 2.20 GHz
RAM: 16 GB

Blender Version
Broken: version: 2.80 (sub 74), branch: master, commit date: 2019-07-05 16:36, hash: rB699193713fc0
Worked: version: 2.79 (sub 0), branch: master, commit date: 2018-03-22 14:10, hash: f4dc9f9d68b

Short description of error
Motion Blur of fast rotating objects "randomly" glitches.
Basically I was rendering some car animation in 2.80 and I noticed that in some frames the wheel motion blur was glitching as if there was a sudden change in the wheel rotation speed.
More below.

Exact steps for others to reproduce the error

Open the .blend file in Blender 2.80 and render the animation.
If opened and rendered in Blender 2.79 the animation gives the expected result.

A comparison between the renders in 2.79 and 2.80. The wheel is rotating at a constant speed on the x-axis, no easing. No other variables are animated. Of course I'm expecting the usual temporal aliasing/wagon wheel artifact, but those jumps in 2.8 just look wrong.

Comparison between same "glitched" frame [Frame006] rendered in 2.79 and 2.80.

2.792.80

If relevant, in the given example the rotation on the x-axis for the wheel is:

Frame numberX RotationGlitching?
005785.413dN
006939.169dY
0071092.93dN

A test render from my project, here the (invisible) car is moving and the camera is parented to the car rig. The wheels are animated via a rig using bones.

I've been doing some testing, but no matter what I try I always get the same result. Of course, depending on the speed of the wheel, the frames in which it happens vary.
I tried playing with Position Center/Start/End on Frame, shutter, rendering with CPU and GPU, using quaternions and eulers. I really can't understand, I think it's a Cycles related problem.
It almost looks like there's a limit when calculating angles differences between frames for motion blur, or some kind of clamping/wrapping/overflow/(?)

I also tried older 2.8 builds (oldest build I've been testing with: 2.80 (sub 41), branch: blender2.7, commit date: 2019-01-21 00:28, hash: dd3f5186260e) but the bug is already there.
Same thing on other Windows 10 machines.

I think that it's a bug and not a desired behaviour.

Details

Type
Bug

Related Objects

Event Timeline

Giacomo Balma (iamlab) updated the task description. (Show Details)
Sebastian Parborg (zeddb) lowered the priority of this task from Needs Triage by Developer to Confirmed, Medium.

Waiting for a fix, I found a temporary solution that helps hiding this artifact (it worked for me at least). Try increasing the Motion Blur Steps for each spinning object,using 6-7 did the job for me.

I can confirm, And while it's not that noticeable on a plain spinning object, it's very bad on an animation like this:

Pablo suggested poking. So i'm poking :) Increasing motion blur steps doesn't help.

confirmed. This is a major issue!

Test scene still does not work properly with 2.81 build 14 Oct 2019