blender/blender

Edge Angle Node -> Signed Angle returns wrong sign #101979

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opened 2022-10-21 14:08:35 +02:00 by Emil Forslund · 5 comments

Emil Forslund commented

2022-10-21 14:08:35 +02:00

System Information
Operating system: Windows-10-10.0.22000-SP0 64 Bits
Graphics card: GeForce RTX 2070 with Max-Q Design/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 462.59

Blender Version
Broken: version: 3.3.1, branch: master, commit date: 2022-10-04 18:35, hash: b292cfe5a9
Worked: Never (to my knowledge)

Short description of error
The Edge Angle Node computes the angle between the two faces connected by an edge. If one of the faces stretch behind the edge, then the node may report the edge as being convex even though the edge is concave.

Exact steps for others to reproduce the error
The .blend-file: debug_edge_angle_sign.blend

Illustration of the problem:

In the example, I have two meshes that share a geometry-node setup that highlights convex edges in blue and concave edges in red. A single vertex is different in the two input meshes, but that vertex is not part of any of the highlighted edges. It still affects the signed angle of the concave edge though, making it appear convex.

**System Information** Operating system: Windows-10-10.0.22000-SP0 64 Bits Graphics card: GeForce RTX 2070 with Max-Q Design/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 462.59 **Blender Version** Broken: version: 3.3.1, branch: master, commit date: 2022-10-04 18:35, hash: `b292cfe5a9` Worked: Never (to my knowledge) **Short description of error** The Edge Angle Node computes the angle between the two faces connected by an edge. If one of the faces stretch *behind* the edge, then the node may report the edge as being convex even though the edge is concave. **Exact steps for others to reproduce the error** The .blend-file: [debug_edge_angle_sign.blend](https://archive.blender.org/developer/F13737715/debug_edge_angle_sign.blend) Illustration of the problem: ![debug_edge_angle_sign.png](https://archive.blender.org/developer/F13737729/debug_edge_angle_sign.png) In the example, I have two meshes that share a geometry-node setup that highlights convex edges in blue and concave edges in red. A single vertex is different in the two input meshes, but that vertex is not part of any of the highlighted edges. It still affects the signed angle of the concave edge though, making it appear convex.

Emil Forslund commented

2022-10-21 14:08:35 +02:00

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Added subscriber: @pyknic

Added subscriber: @pyknic

Germano Cavalcante commented

2022-10-21 23:39:49 +02:00

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

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

Germano Cavalcante commented

2022-10-21 23:39:49 +02:00

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Added subscriber: @mano-wii

Added subscriber: @mano-wii

Emil Forslund was assigned by Pratik Borhade

2022-10-22 07:56:09 +02:00

Philipp Oeser removed the

Nodes & Physics

label 2023-02-10 08:43:32 +01:00

Lukas Tönne commented

2023-03-20 15:04:28 +01:00

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The edge angle node needs a reference point on both adjacent faces to tell if the face is convex (positive angle) or concave (negative angle). It calculates a tangent vector from the edge center to the reference point and the dot product with the other face's normal.

The reference point currently used is the center of the entire polygon, which means that a concave polygon can generate a point behind the other adjacent face, leading to a flipped angle, even when the local tangent direction should yield the opposite sign.

For a more robust detection of concavity the generated face triangles must be used, because those can define a valid local tangent direction through the 3rd vertex.

To illustrate the point, here is an example where the normals of the faces are exactly the same, and only the reference point makes the difference between a convex and a concave edge angle:

The edge angle node needs a reference point on both adjacent faces to tell if the face is convex (positive angle) or concave (negative angle). It calculates a tangent vector from the edge center to the reference point and the dot product with the other face's normal. The reference point currently used is the center of the entire polygon, which means that a concave polygon can generate a point _behind_ the other adjacent face, leading to a flipped angle, even when the _local_ tangent direction should yield the opposite sign. For a more robust detection of concavity the generated face triangles must be used, because those can define a valid local tangent direction through the 3rd vertex. To illustrate the point, here is an example where the normals of the faces are exactly the same, and only the reference point makes the difference between a convex and a concave edge angle: ![image](/attachments/61c45160-7b48-4b05-86a9-51e0fe0c90ed) ![image](/attachments/d38bac59-8ebd-4ebd-b107-0681098a575d)

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Iliya Katushenock commented

2023-03-20 15:10:40 +01:00

Member

Yes, it seems to be good to accept the fact that this can only work correctly for a flat polygon, after which a branch in the looptris direction can be found for any edge

Yes, it seems to be good to accept the fact that this can only work correctly for a flat polygon, after which a branch in the looptris direction can be found for any edge

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Reference: blender/blender#101979

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