Normals generated by Curve to Points node aren't always perpendicular to the curve #94522

New Issue

slowburn · 2021-12-31T00:29:48+01:00

slowburn commented

2021-12-31 00:29:48 +01:00

System Information
Operating system: Windows-10-10.0.19043-SP0 64 Bits
Graphics card: NVIDIA GeForce RTX 3060 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 497.09

Blender Version
Broken: version: 3.1.0 Alpha, branch: master, commit date: 2021-12-26 20:08, hash: 20b438d523

Short description of error
For curves with sharp corners, like poly splines or bezier splines with vector handles, the normals are not perpendicular to the straight curve segments, but bend around corners. This makes it impossible to properly align instances to the curve.

Exact steps for others to reproduce the error

Create curve with sharp corners
Add Geometry nodes modifier and use Curve to Points node to generate points
Use either Normal or Rotation output to drive the rotation of instances, like in the example

**System Information** Operating system: Windows-10-10.0.19043-SP0 64 Bits Graphics card: NVIDIA GeForce RTX 3060 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 497.09 **Blender Version** Broken: version: 3.1.0 Alpha, branch: master, commit date: 2021-12-26 20:08, hash: `20b438d523` **Short description of error** For curves with sharp corners, like poly splines or bezier splines with vector handles, the normals are not perpendicular to the straight curve segments, but bend around corners. This makes it impossible to properly align instances to the curve. ![instance_rotate.jpg](https://archive.blender.org/developer/F12782541/instance_rotate.jpg) **Exact steps for others to reproduce the error** - Create curve with sharp corners - Add Geometry nodes modifier and use Curve to Points node to generate points - Use either Normal or Rotation output to drive the rotation of instances, like in the example

slowburn commented

2021-12-31 00:29:48 +01:00

Added subscriber: @slowburn

Pratik Borhade commented

2021-12-31 11:15:51 +01:00

Added subscriber: @PratikPB2123

Jacques Lucke commented

2022-01-10 13:25:24 +01:00

Added subscriber: @JacquesLucke

Jacques Lucke commented

2022-01-10 13:25:24 +01:00

Changed status from 'Needs Triage' to: 'Needs User Info'

Jacques Lucke commented

2022-01-10 13:25:24 +01:00

Please provide the .blend file to make it easier for us to reproduce the problem.
What's probably happening here is that the normal is interpolated between the control points. To work around this you could resample the curve to a poly spline that does not have intermediate points.

Please provide the .blend file to make it easier for us to reproduce the problem. What's probably happening here is that the normal is interpolated between the control points. To work around this you could resample the curve to a poly spline that does not have intermediate points.

slowburn commented

2022-01-10 19:16:51 +01:00

Here's the file. As you can see it happens with Poly splines as well.
And you're right, there's some interpolation going on, I'm just not sure if it's intentional. According to the manual, Curve to Points node should return a proper normal at each generated point instead of some interpolated value from the control points. Or at least there should be a way to control when this interpolation happens.
curve_normals.blend

One workaround I found is to scale the control handles to 0, but it doesn't eliminate the problem completely. And it would be nice to have a proper fix anyway.

2022-01-10_20-13-37.mp4

Here's the file. As you can see it happens with Poly splines as well. And you're right, there's some interpolation going on, I'm just not sure if it's intentional. According to the manual, Curve to Points node should return a proper normal at each generated point instead of some interpolated value from the control points. Or at least there should be a way to control when this interpolation happens. [curve_normals.blend](https://archive.blender.org/developer/F12799341/curve_normals.blend) One workaround I found is to scale the control handles to 0, but it doesn't eliminate the problem completely. And it would be nice to have a proper fix anyway. [2022-01-10_20-13-37.mp4](https://archive.blender.org/developer/F12799516/2022-01-10_20-13-37.mp4)

Hans Goudey commented

2022-01-11 20:42:12 +01:00

Added subscriber: @HooglyBoogly

Hans Goudey commented

2022-01-11 20:42:12 +01:00

I guess a fix is to make a different version of of the interpolation of attributes to evaluated points, but I think there are multiple use cases here.

Few control points, large angles, and vector handles: interpolation is unwanted, since it makes a sharp curve smooth.
Smooth curve with many control points and no purposeful hard angles: interpolation might be desired, since the goal is to look smooth anyway.

Or maybe it would make sense to require having a higher resolution curve for the second case, and always avoid interpolation.

To change this, I think the sample curve node and the curve to points node would have to be refactored. We might also have to redefine the tangents as the direction vector from one evaluated point to the next (which might make sense anyway, since we decided that a point attribute value conceptually applies to the following edge).

I guess a fix is to make a different version of of the interpolation of attributes to evaluated points, but I think there are multiple use cases here. 1. Few control points, large angles, and vector handles: interpolation is unwanted, since it makes a sharp curve smooth. 2. Smooth curve with many control points and no purposeful hard angles: interpolation might be desired, since the goal is to look smooth anyway. Or maybe it would make sense to require having a higher resolution curve for the second case, and always avoid interpolation. To change this, I think the sample curve node and the curve to points node would have to be refactored. We might also have to redefine the tangents as the direction vector from one evaluated point to the next (which might make sense anyway, since we decided that a point attribute value conceptually applies to the following edge).

Hans Goudey commented

2022-01-17 06:37:03 +01:00

Changed status from 'Needs User Info' to: 'Confirmed'

Hans Goudey commented

2022-01-31 23:55:28 +01:00

Since the solution isn't obvious and would have to be discussed, I think this is more of a design task.

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Normals generated by Curve to Points node aren't always perpendicular to the curve #94522