Setting a curve to Cyclic causes the Geometry Nodes to evaluate the Normal direction of the curve incorrectly. #91953

New Issue

Jason schleifer · 2021-10-05T03:54:34+02:00

Jason schleifer commented

2021-10-05 03:54:34 +02:00

System Information
Operating system: macOS-11.6-x86_64-i386-64bit 64 Bits
Graphics card: AMD Radeon Pro 560X OpenGL Engine ATI Technologies Inc. 4.1 ATI-4.6.20

Blender Version
Broken: version: 3.0.0 Alpha, branch: master, commit date: 2021-10-04 07:43, hash: 8c55333a8e
Worked: (newest version of Blender that worked as expected)

Short description of error
If you turn a curve Cyclic, Geometry Nodes calculates the normal direction of the curve incorrectly. This is easiest to see if you use the Curve to Mesh node to create a mesh, display the normals of the curve in Edit mode, manipulate the curve to give it a unique shape, and then tweak the cyclic value.

Exact steps for others to reproduce the error

Create a Bezier Circle
Create a new Geometry Node for the circle.
Add a Curve to Mesh Node.
Add a Curve Line node.
Set the Curve Line END values to 0.2, 0, 0.
Connect the Curve line to the Profile Curve of the Curve to Mesh node.
Connect the Group Input -> Geometry output to the Curve in the Curve to Mesh node.
Connect the Curve to Mesh -> Mesh output to the Group Output Geometry input.

Now, select the curve and switch to edit mode.
In the Viewport Overlays, increase the display of the Normals until you can see them for the curve.
Adjust the curve until it's twisted.
Now in the Properties Panel for the curve, under Active Spline toggle on and off Cyclic. Notice that the mesh doesn't line up with the normals when Cyclic is on.

or

Open .blend file
Turn on Cyclic

cyclicOffset.blend

**System Information** Operating system: macOS-11.6-x86_64-i386-64bit 64 Bits Graphics card: AMD Radeon Pro 560X OpenGL Engine ATI Technologies Inc. 4.1 ATI-4.6.20 **Blender Version** Broken: version: 3.0.0 Alpha, branch: master, commit date: 2021-10-04 07:43, hash: `8c55333a8e` Worked: (newest version of Blender that worked as expected) **Short description of error** If you turn a curve Cyclic, Geometry Nodes calculates the normal direction of the curve incorrectly. This is easiest to see if you use the Curve to Mesh node to create a mesh, display the normals of the curve in Edit mode, manipulate the curve to give it a unique shape, and then tweak the cyclic value. ![cyclic_offset.gif](https://archive.blender.org/developer/F10771266/cyclic_offset.gif) **Exact steps for others to reproduce the error** - Create a Bezier Circle - Create a new Geometry Node for the circle. - Add a Curve to Mesh Node. - Add a Curve Line node. - Set the Curve Line END values to 0.2, 0, 0. - Connect the Curve line to the Profile Curve of the Curve to Mesh node. - Connect the Group Input -> Geometry output to the Curve in the Curve to Mesh node. - Connect the Curve to Mesh -> Mesh output to the Group Output Geometry input. ![image.png](https://archive.blender.org/developer/F10771255/image.png) Now, select the curve and switch to *edit* mode. In the Viewport Overlays, increase the display of the Normals until you can see them for the curve. Adjust the curve until it's twisted. Now in the Properties Panel for the curve, under Active Spline toggle on and off Cyclic. Notice that the mesh doesn't line up with the normals when Cyclic is on. --- or - Open .blend file - Turn on Cyclic [cyclicOffset.blend](https://archive.blender.org/developer/F10771290/cyclicOffset.blend)

Jason schleifer commented

2021-10-05 03:54:34 +02:00

Added subscriber: @JasonSchleifer

Pratik Borhade commented

2021-10-05 08:23:04 +02:00

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

Pratik Borhade commented

2021-10-05 08:23:10 +02:00

Added subscriber: @PratikPB2123

Orestis Konstantinidis commented

2021-10-05 10:08:51 +02:00

Added subscriber: @OrestisKonstantinidis

Jacques Lucke commented

2021-10-29 12:54:50 +02:00

Added subscriber: @JacquesLucke

Jacques Lucke commented

2021-10-29 12:54:50 +02:00

Changed status from 'Confirmed' to: 'Archived'

Jacques Lucke closed this issue

2021-10-29 12:54:50 +02:00

Jacques Lucke commented

2021-10-29 12:54:50 +02:00

Unfortunately, this is a limitation currently. The old code to compute normals did not work entirely correct on cyclic splines. We tried to keep the geometry nodes implementation close to the original implementation while still fixing the bug (didn't even know how to write the algorithm so that it has the same bug..). This is also mentioned in the original patch: D11621. In the long term the fixed implementation should be used all over Blender of course.

Here is an example where the old algorithm failed and geometry nodes does the correct thing on the same curve:
old_normal_calculation_buggy.blend

Unfortunately, this is a limitation currently. The old code to compute normals did not work entirely correct on cyclic splines. We tried to keep the geometry nodes implementation close to the original implementation while still fixing the bug (didn't even know how to write the algorithm so that it has the same bug..). This is also mentioned in the original patch: [D11621](https://archive.blender.org/developer/D11621). In the long term the fixed implementation should be used all over Blender of course. Here is an example where the old algorithm failed and geometry nodes does the correct thing on the same curve: [old_normal_calculation_buggy.blend](https://archive.blender.org/developer/F11595590/old_normal_calculation_buggy.blend) ![image.png](https://archive.blender.org/developer/F11595595/image.png)

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Setting a curve to Cyclic causes the Geometry Nodes to evaluate the Normal direction of the curve incorrectly. #91953