N-Sphere radius output of Voronoi node is broken for 1D settings #89494

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Florian · 2021-06-28T10:05:09+02:00

Florian commented

2021-06-28 10:05:09 +02:00

System Information
Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: NVIDIA GeForce RTX 3080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 465.89

Blender Version
Broken: version: 2.93.0, branch: master, commit date: 2021-06-02 11:21, hash: 84da05a8b8
Worked: (newest version of Blender that worked as expected)

Short description of error
The "N-Sphere Radius" output of the Voronoi node is supposed to return half the width of the Voronoi cell, but the output values seem to be wrong

Exact steps for others to reproduce the error
This picture shows, from top to bottom:

Distance To Edge output of a 1-D Voronoi node
The N-Sphere Radius output of the same node (note how the edges line up, it is the same Voronoi pattern)
The Distance To Edge divided by N-Sphere radius: this should produce a pattern with peaks of equal height (as the distance to edge is maximum at the cell center and equal to the sphere radius there)

Here is the node setup for point 3):

And the .blend file: voronoi_bug_1_v01.blend

Edit: When looking at what was changed in the source code for another bug in the Voronoi node, it seems to me that it may be actually the same error: https://developer.blender.org/rC7f5d86c43150628588d9173076f3c88b4ab5d720

**System Information** Operating system: Windows-10-10.0.19041-SP0 64 Bits Graphics card: NVIDIA GeForce RTX 3080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 465.89 **Blender Version** Broken: version: 2.93.0, branch: master, commit date: 2021-06-02 11:21, hash: `84da05a8b8` Worked: (newest version of Blender that worked as expected) **Short description of error** The "N-Sphere Radius" output of the Voronoi node is supposed to return half the width of the Voronoi cell, but the output values seem to be wrong **Exact steps for others to reproduce the error** This picture shows, from top to bottom: 1) Distance To Edge output of a 1-D Voronoi node 2) The N-Sphere Radius output of the same node (note how the edges line up, it is the same Voronoi pattern) 3) The Distance To Edge divided by N-Sphere radius: this should produce a pattern with peaks of equal height (as the distance to edge is maximum at the cell center and equal to the sphere radius there) ![image.png](https://archive.blender.org/developer/F10204407/image.png) Here is the node setup for point 3): ![image.png](https://archive.blender.org/developer/F10204409/image.png) And the .blend file: [voronoi_bug_1_v01.blend](https://archive.blender.org/developer/F10204411/voronoi_bug_1_v01.blend) Edit: When looking at what was changed in the source code for another bug in the Voronoi node, it seems to me that it may be actually the same error: https://developer.blender.org/rC7f5d86c43150628588d9173076f3c88b4ab5d720

Florian commented

2021-06-28 10:05:09 +02:00

Added subscriber: @LordoftheFleas

Wannes Malfait commented

2021-06-28 14:35:02 +02:00

Added subscriber: @wannes.malfait

Wannes Malfait commented

2021-06-28 14:35:02 +02:00

I think the current implementation is correct. I think your reasoning is wrong at this point: the feature point is where the maximum distance is to the edges. In general the center point between two edges is not a feature point. I admit that it is confusing to think about, and I might have made a mistake myself. Hopefully the image below makes it a bit clearer.

With the node setup below you can see what the n-sphere radius is: half the distance from the closest feature point to the the second closest feature point. Notice how the circles always touch one of the edges.

I think the current implementation is correct. I think your reasoning is wrong at this point: the feature point is where the maximum distance is to the edges. In general the center point between two edges is not a feature point. I admit that it is confusing to think about, and I might have made a mistake myself. Hopefully the image below makes it a bit clearer. With the node setup below you can see what the n-sphere radius is: half the distance from the closest feature point to the the second closest feature point. Notice how the circles always touch one of the edges. ![image.png](https://archive.blender.org/developer/F10204730/image.png)

Florian commented

2021-06-28 15:31:18 +02:00

You are correct, especially when considering the higher dimensional case: it is generally impossible to find an N_sphere that touches all of the edges / hyperplanes of a cell, so centering the sphere on the feature point at least ensures consistency for all dimensions. (I should have read the documentation more closely here).

By the way is there a simple method to access the width of the cell in the 1D case from the existing outputs?

You are correct, especially when considering the higher dimensional case: it is generally impossible to find an N_sphere that touches *all* of the edges / hyperplanes of a cell, so centering the sphere on the feature point at least ensures consistency for all dimensions. (I should have read the documentation more closely here). By the way is there a simple method to access the width of the cell in the 1D case from the existing outputs?

Wannes Malfait commented

2021-06-28 17:17:52 +02:00

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

Wannes Malfait closed this issue

2021-06-28 17:17:52 +02:00

Wannes Malfait self-assigned this 2021-06-28 17:17:52 +02:00

Wannes Malfait commented

2021-06-28 17:17:52 +02:00

By the way is there a simple method to access the width of the cell in the 1D case from the existing outputs?

I don't think there is a way to do it with the outputs provided. If we had F3 as an output you could do (F2+F3)/2, but I don't immediately see a way to calculate F3 from the current outputs. If you really need it, you're probably better off creating a 1D voronoi from scratch, where you calculate this value as well, but that doesn't qualify as "simple" :)

> By the way is there a simple method to access the width of the cell in the 1D case from the existing outputs? I don't think there is a way to do it with the outputs provided. If we had F3 as an output you could do `(F2+F3)/2`, but I don't immediately see a way to calculate F3 from the current outputs. If you really need it, you're probably better off creating a 1D voronoi from scratch, where you calculate this value as well, but that doesn't qualify as "simple" :)

Florian commented

2021-06-28 21:20:29 +02:00

Just to close this off (and for future reference), I think I managed to extract at least the distance between feature points (which is enough for my own purposes). The cell width could be done similarly:

voronoi_feature_distance_1_v01.blend

Just to close this off (and for future reference), I think I managed to extract at least the distance between feature points (which is enough for my own purposes). The cell width could be done similarly: ![image.png](https://archive.blender.org/developer/F10204991/image.png) [voronoi_feature_distance_1_v01.blend](https://archive.blender.org/developer/F10204993/voronoi_feature_distance_1_v01.blend)

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N-Sphere radius output of Voronoi node is broken for 1D settings #89494