Cycles modifier changes the shape of the F-Curve #83023

New Issue

Lucas Boutrot · 2020-11-25T22:09:29+01:00

Lucas Boutrot commented

2020-11-25 22:09:29 +01:00

System Information
Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: GeForce GTX 1060/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 457.30

Blender Version
Broken: 2.80, 2.92.0 Alpha (hash: 8330e19cb2)
Worked: 2.73, 2.79

Short description of error
When putting a modifier "cycles" on a F-Curve created with default settings, its shape changes with rotation on the keyframes tangents.

Exact steps for others to reproduce the error
bug_anim_curves.blend

Open the file
Add a "cycles" modifier on the selected F-Curve
The shape changes

**System Information** Operating system: Windows-10-10.0.19041-SP0 64 Bits Graphics card: GeForce GTX 1060/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 457.30 **Blender Version** Broken: 2.80, 2.92.0 Alpha (hash: `8330e19cb2`) Worked: 2.73, 2.79 **Short description of error** When putting a modifier "cycles" on a F-Curve created with default settings, its shape changes with rotation on the keyframes tangents. **Exact steps for others to reproduce the error** [bug_anim_curves.blend](https://archive.blender.org/developer/F9375072/bug_anim_curves.blend) - Open the file - Add a "cycles" modifier on the selected F-Curve - The shape changes

Lucas Boutrot commented

2020-11-25 22:09:29 +01:00

Added subscriber: @thornydre

Germano Cavalcante commented

2020-11-26 13:56:26 +01:00

Added subscriber: @mano-wii

Germano Cavalcante commented

2020-11-26 13:56:26 +01:00

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

Germano Cavalcante commented

2020-11-26 13:56:26 +01:00

I can confirm the problem.
Although there are many ways to get around it, the curve handle should not be affected by the modifier in that way.
I tested it on blender 2.79 and it works there.
It seems that the problem was introduced in Blender 2.80.

I can confirm the problem. Although there are many ways to get around it, the curve handle should not be affected by the modifier in that way. I tested it on blender 2.79 and it works there. It seems that the problem was introduced in Blender 2.80.

Alexander Gavrilov commented

2020-11-27 11:09:39 +01:00

Added subscriber: @angavrilov

Alexander Gavrilov commented

2020-11-27 11:09:39 +01:00

This is an intentional documented feature , intended to allow one to create an actual smooth cyclic curve using auto handles. Otherwise all keyframes on the boundary*must// be manually tweaked to achieve a smooth transition.

You could argue that 'modifiers' shouldn't do that, but curve cyclicity is a fundamental property that should not be a modifier in the first place, and originally wasn't. In a very old version of blender before modifiers it was an extrapolation type, next to Constant and Linear, and it is in effect still in the extrapolation menu.

If you really intend a discontinuous curve, enable the Use Influence option on the modifier - even if the actual influence is 1, the special handling is turned off. Or switch the handle type at the boundary to aligned.

P.S. The real bug here is that the curve doesn't turn into a straght line - it's the natural interpolation for just two points. Mathematically the handles of the first and last keyframes of a cyclic curve are the same, so probably there are more unknowns than equations and the solver becomes confused.

This is an intentional [documented feature ](https:*docs.blender.org/manual/en/dev/editors/graph_editor/fcurves/modifiers.html#trivially-cyclic-curves), intended to allow one to create an actual smooth cyclic curve using auto handles. Otherwise all keyframes on the boundary*must// be manually tweaked to achieve a smooth transition. You could argue that 'modifiers' shouldn't do that, but curve cyclicity is a fundamental property that should not be a modifier in the first place, and originally wasn't. In a very old version of blender before modifiers it was an extrapolation type, next to Constant and Linear, and it is in effect still in the extrapolation menu. If you really intend a discontinuous curve, enable the Use Influence option on the modifier - even if the actual influence is 1, the special handling is turned off. Or switch the handle type at the boundary to aligned. P.S. The real bug here is that the curve doesn't turn into a straght line - it's the natural interpolation for just two points. Mathematically the handles of the first and last keyframes of a cyclic curve are the same, so probably there are more unknowns than equations and the solver becomes confused.

Sybren A. Stüvel commented

2020-11-27 12:05:03 +01:00

Added subscriber: @dr.sybren

Sybren A. Stüvel commented

2020-11-27 12:05:03 +01:00

This appears to be the effect of 8bdc391c54. The automatic handles attempt to keep the curve smooth, but this fails because of the discontinuity in the keyframe value at each end of the cycle. This is controlled by the handle type (set to Auto Clamped in the example) as well as the curve's Handle Smoothing setting.

Top top bottom, these are:

Before adding the Cycles modifier
After adding the Cycles modifier, with the handles still at Auto Clamped.
After setting the handles to Aligned, and then adding the Cycles modifier.

You could argue that this smoothing shouldn't happen when there is a discontinuity in the FCurve value, but that's not how things are implemented at the moment. Possible improvements can be discussed in the design task #54724 (Workflow improvements for creating Animation Loops).

The slight wobble you see in the curve shouldn't happen, though, which is something @angavrilov will look into (discussed this on Blender Chat).

This appears to be the effect of 8bdc391c54. The automatic handles attempt to keep the curve smooth, but this fails because of the discontinuity in the keyframe value at each end of the cycle. This is controlled by the handle type (set to Auto Clamped in the example) as well as the curve's Handle Smoothing setting. ![image.png](https://archive.blender.org/developer/F9388995/image.png) Top top bottom, these are: - Before adding the Cycles modifier - After adding the Cycles modifier, with the handles still at Auto Clamped. - After setting the handles to Aligned, and then adding the Cycles modifier. You could argue that this smoothing shouldn't happen when there is a discontinuity in the FCurve value, but that's not how things are implemented at the moment. Possible improvements can be discussed in the design task #54724 (Workflow improvements for creating Animation Loops). The slight wobble you see in the curve shouldn't happen, though, which is something @angavrilov will look into (discussed this on Blender Chat).

blender-admin commented

2020-11-28 12:44:10 +01:00

This issue was referenced by b3f20eed6e

This issue was referenced by b3f20eed6e85249d15724543743cb049e7991622

Alexander Gavrilov commented

2020-11-28 13:57:41 +01:00

Changed status from 'Confirmed' to: 'Resolved'

Alexander Gavrilov closed this issue

2020-11-28 13:57:41 +01:00

Alexander Gavrilov self-assigned this 2020-11-28 13:57:41 +01:00

Lucas Boutrot commented

2020-11-28 14:10:24 +01:00

Thanks a lot for the answers, sorry for this report, but at least it was probably not completely useless :) Cheers !

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Cycles modifier changes the shape of the F-Curve #83023