Cycles doesn't interpolate material properties driven by a frame driver when rendering motion blur #84753
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Reference: blender/blender#84753
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System Information
Operating system: Darwin-18.7.0-x86_64-i386-64bit 64 Bits
Graphics card: Intel Iris OpenGL Engine Intel Inc. 4.1 INTEL-12.10.16
Blender Version
Broken: version: 2.91.0, branch: master, commit date: 2020-11-25 08:34, hash:
0f45cab862
(same behaviour with 2.93 LTS and 2.92 alpha build)
Worked: -
Short description of error
When setting up a material with its color (or other properties) driven by a frame driver cycles doesn't seem to interpolate the frame driver when calculating motion blur. This is inconsistent with how the motion blur in Eevee behaves.
Example: Rendering a material with color change driven by frame:
With Cycles: The cube and the motion blur trail have a consistent color:
Same scene with Eevee: As expected the motion blur accounts for the color change of the object:
Exact steps for others to reproduce the error
You can open the provided blend file and render the current frame with cycles. To confirm the desired effect with Eevee change the render engine to Eevee and render the frame again.
cycles_no-motion-blur-color-change.blend
Exact steps for others to reproduce the error from startup file
Add a Value Node Add a driver to the Value Node by typing '#frame' into the value field.
Add a Math Node set to Fraction and plug the value node into it. Add a Color Ramp and create a colorful gradient to make the effect more obvious
Feed the Math Node into the Color Ramp Plug the Color Ramp into an Emission Shader and plug that into the Material Output
** Result: The cube and its motion blur trail will have one constant color
change render engine to Eevee activate motion blur in the render settings
** increase the number of motion blur steps to 20 or higher to make the effect more noticeable
Added subscriber: @lone_noel
Added subscriber: @rocketman
Changed status from 'Needs Triage' to: 'Archived'
This is a known issue. Whereas Eevee will actually render subframes for each step, Cycles uses a different method which unfortunately does not account for material changes between frames.
It's disappointing, but not a bug per se. Sorry to shoot you down