Maniphest T76009

Follow Path constraint works only on curves with Cyclic U option activated
Closed, Archived
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Assigned To
Richard Antalik (ISS)
Authored By
Maurizio Memoli (mmemoli)
Apr 22 2020, 10:33 PM
Tags
  • BF Blender
  • User Interface
  • Documentation
Subscribers
Germano Cavalcante (mano-wii)
Maurizio Memoli (mmemoli)
Richard Antalik (ISS)
Ryan Inch (Imaginer)

Description

System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce GTX 1080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 441.20

Blender Version
Broken: version: 2.82 (sub 7), branch: master, commit date: 2020-02-12 16:20, hash: rB77d23b0bd76f
Worked: (optional)

Short description of error
If the constraint is not set on fixed position the constrained object will stay at the start position even if the offset is on a different value.
The constraint will work fine if the curve is set to Cyclic U.
This may not be considered a bug, but the "Fixed position" option isn't really working as the name describes.
It is merely remapping the frames value to a range that goes from 0.0 to 1.0. This will cause a slide of the constrained object on the curve when the curve moves and it will not keep its parametric coordinates, it will just keep the position on the percentage of the length of the curve.

Exact steps for others to reproduce the error
For Cyclic U:
Create a Nurbs Curve
Create an Empty
Add a follow path constraint
Set the offset value to 50
in the curve option tab, under Active spline activate Cyclic U

Fixed position:
A simple way to recreate the issue is to create a curve with 6 control points, place the constrained object between the first and second control point and move the sixth control point. You will notice the constrained object moving even if the curve is not actually deforming in that area.

Event Timeline

Maurizio Memoli (mmemoli) created this task.Apr 22 2020, 10:33 PM
Germano Cavalcante (mano-wii) added a subscriber: Germano Cavalcante (mano-wii).Apr 23 2020, 4:37 PM

Could you provide a simple file showing the problem?

Richard Antalik (ISS) closed this task as Archived.Apr 23 2020, 6:47 PM
Richard Antalik (ISS) claimed this task.
Richard Antalik (ISS) added projects: User Interface, Documentation.
Richard Antalik (ISS) added a subscriber: Richard Antalik (ISS).

Please reffer to documentation before reporting: https://docs.blender.org/manual/en/dev/animation/constraints/relationship/follow_path.html

In first issue I would agree though that wording is quite bad, but it is correct still. Tooltip is also quite misleading.

Offset

The number of frames to offset from the “animation” defined by the path (by default, from frame 1).

At frame 1 you are at beginning of the curve. If you offset animation by +50 frames empty will correctly start moving from frame 51.

Regarding second issue, descriprion is quite clear. Tooltip seems to be fine as well.

Fixed Position

Object will stay locked to a single point somewhere along the length of the curve regardless of time.

I will close this report, because I can't see bug in actual behavior, and tag projects, so they can update description of property if they see this as something worth fixing

Ryan Inch (Imaginer) added a subscriber: Ryan Inch (Imaginer).Apr 23 2020, 9:53 PM

Offset:
I can confirm that when the curve's Cyclic U property is active the animation starts immediately from frame 1 as opposed to what's set in offset.
@Richard Antalik (ISS) This looks like a bug to me.

Fixed Position:
As far as I can tell this is working properly. The "Fixed Position" is defined by percentage, so it makes sense that the object changes position when the curve is edited (for example, made longer).

Richard Antalik (ISS) added a comment.Apr 23 2020, 10:38 PM
In T76009#916414, @Ryan Inch (Imaginer) wrote:

Offset:
I can confirm that when the curve's Cyclic U property is active the animation starts immediately from frame 1 as opposed to what's set in offset.
@Richard Antalik (ISS) This looks like a bug to me.

Sorry I must have misread report. In any case that is still not a bug IMO, because when curve is cyclic, motion paths will go from -infinity to +infinity.
Therefore offset is now really just "phase shift".

Maurizio Memoli (mmemoli) added a comment.EditedApr 23 2020, 11:08 PM

@Richard Antalik (ISS)
I should have gone through the documentation. Sorry for the rush bug report.
But still isn't the change of behavior still when the curve turns Cyclic considered a bug?
It would make more sense having the evaluation time parameter in the constraint itself instead of the curve.

I know this is not the place to make suggestions to improve the software, but having objects sliding on the curve and not having the option to keep them locked on the parameter it is kind of crippling for the rigging workflow.