Artifact with cycles (interaction with moving spotlight and motion blur) #103602
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Reference: blender/blender#103602
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System Information
Operating system: Windows-10-10.0.19045-SP0 64 Bits
Graphics card: NVIDIA GeForce GTX 1050 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 516.94
Blender Version
Broken: version: 3.4.1, branch: blender-v3.4-release, commit date: 2022-12-19 17:00, hash:
55485cb379Worked: (newest version of Blender that worked as expected)
Short description of error
I wouldnt describe it as a error, more as a artifact in cycles, basically when you parent a light to an object with motion blur, it doesnt behave as expected, it clips into objects and deforms due to the motion blur, there is no easy way to explain this, its better to show it, keep in mind that the spot light in these examples is completely Static! its just parented to the empty that also is parented to the camera, what is moving is the empty, light in real life doesn't behave this way!
i first noticed this in a render i did for my animation
render: https://drive.google.com/file/d/1l4jIvidlHOeqxlpCxUgLo2zBUd56sGIz/view
project file: https://drive.google.com/file/d/1RiMK3vCwIT9YLO_l9om3Hj0mA3TpMs7v/view?usp=sharing
so i did a test render and the problem pressists
render: https://drive.google.com/file/d/1As0zdwAkwGMstVtOOxSRfaSu8qGX2FZ_/view?usp=sharing
project file: https://drive.google.com/file/d/18rMRXSX5O5DWmUX6L8GiiGkbDCUSt6OA/view?usp=sharing
All the best! If this report is invalid please inform me why, because i cant find a work around this issue and there is no information online regarding this problem. Thank you.
Added subscriber: @kiko-3
Added subscriber: @mod_moder
Changed status from 'Needs Triage' to: 'Needs Developer To Reproduce'
The strength of the motion blur depends on the speed
I've changed the speed change of the empty so that the drop is more pronounced:
Here is my render of your example:
uhkbuytfbxtffddd0001-0110.mp4
And it seems that the problem of understanding here is:
That the spotlight we see in the camera is not the light of the source! You can't see the laser light from the side.
This is the light of the mist illuminated by your lamp. And the fog is still. This creates a difference in speed.
But this creates another question. Why directional light in a dense environment can be taken into account for motion blur...
Im not quite sure i understand, could you please explain it in other words?
what i understood is that blender is taking the "fog light" into consideration and making a blur around it, so it kinda leaves a trail of light where it passes....but the thing is that this happends even whitout any fog at all!, looks like the light gets pushed back,when motion blur is applied.
These are rendered with the fog completely deleted, and the lights are exacly on the same place but somehow the spaceship is being lit from behind, this issue only occurs when motion blur is enabled.
this is what is supposed to look like,the thing is...to look like this i have to remove the motion blur and motion blur is an essential piece to the realism of any animation.
and thank you for the quick awnser!
Just encountered this in a similar scene (airplane takeoff) in 4.0 and 4.1.
I've been able to replicate it in a fresh scene. All objects are parented to an animated empty, except the checkered background. The empty is moving right with camera also being locked on it.
Without motion blur everything looks correct, as if there wasn't any movement.
However with motion blur the light seems to move backwards.
This becomes even more apparent at higher speeds, the light is completely behind the cylinder.
I've also tested not having the objects parented and using both drivers and keyframes for movement.
The same problem happens in Eevee.
I haven't found any workaround for this, so I will have to accept it in my project. The lights will be moving depending on camera shake too.
One possible workaround for now is using mesh emitters instead of light objects. This has obvious limitations: cumbersome, won't work in Eevee, probably won't give you long light trails. But it should solve at least the most visible cases where the light would get submerged into a mesh.