Cycles doesn't interpolate material properties driven by a frame driver when rendering motion blur #84753

New Issue

Leon Schittek · 2021-01-15T17:41:16+01:00

Leon Schittek commented

2021-01-15 17:41:16 +01:00

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

open new default startup file
animate the position of the default cube (two keyframes are enough, but the effect is more obvious, when the cube is moving a lot between frames)
change the material of the default cube, so that it colors change based on the frame. e.g:
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
Change render engine to Cycles and activate motion blur (optional: increase the motion blur steps in the object properties of the cube to confirm that limited steps isn't the issue)
Render a frame of the cube in motion:
** Result: The cube and its motion blur trail will have one constant color
Optional: recreate expected behavour in Eevee:
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

**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: ![cycles_no-subframe-color-change.png](https://archive.blender.org/developer/F9580606/cycles_no-subframe-color-change.png) Same scene with Eevee: As expected the motion blur accounts for the color change of the object: ![eevee_subframe-color-change.png](https://archive.blender.org/developer/F9580599/eevee_subframe-color-change.png) **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](https://archive.blender.org/developer/F9580609/cycles_no-motion-blur-color-change.blend) **Exact steps for others to reproduce the error from startup file** 1. open new default startup file 2. animate the position of the default cube (two keyframes are enough, but the effect is more obvious, when the cube is moving a lot between frames) 3. change the material of the default cube, so that it colors change based on the frame. e.g: **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* 4. Change render engine to Cycles and activate motion blur (optional: increase the motion blur steps in the object properties of the cube to confirm that limited steps isn't the issue) 5. Render a frame of the cube in motion: ** Result: The cube and its motion blur trail will have one constant color 6. *Optional:* recreate expected behavour in Eevee: **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

Leon Schittek commented

2021-01-15 17:41:16 +01:00

Added subscriber: @lone_noel

Sam Brubaker commented

2021-01-15 21:16:07 +01:00

Added subscriber: @rocketman

Sam Brubaker commented

2021-01-15 21:16:07 +01:00

Changed status from 'Needs Triage' to: 'Archived'

Sam Brubaker closed this issue

2021-01-15 21:16:07 +01:00

Sam Brubaker commented

2021-01-15 21:16:07 +01:00

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

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

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Cycles doesn't interpolate material properties driven by a frame driver when rendering motion blur #84753