blender/blender

Sample Volume node's Position input is off by half the voxel width #113265

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opened 2023-10-04 23:20:36 +02:00 by Cornus-Ammonis · 4 comments

Cornus-Ammonis commented

2023-10-04 23:20:36 +02:00

System Information
Operating system: Windows-10-10.0.19045-SP0 64 Bits
Graphics card: NVIDIA GeForce RTX 3090/PCIe/SSE2 NVIDIA Corporation 4.6.0 NVIDIA 536.23

Blender Version
Broken: version: 4.1.0 Alpha, branch: main, commit date: 2023-09-29 20:55, hash: 90ec11b823e8

Short description of error
The Sample Volume node's Position input is off by half the voxel width.

Exact steps for others to reproduce the error
This issue is easiest to notice when used with Simulation Nodes because the error accumulates over time. In the attached blend file, a checkbox controls whether to apply a fix to the error or not. If the fix is not applied, the mesh will drift in the -X, -Y, -Z direction over time. When working correctly, the mesh does not drift. In order to fix it, half the voxel width is subtracted from the Sample Volume node's position input.

**System Information** Operating system: Windows-10-10.0.19045-SP0 64 Bits Graphics card: NVIDIA GeForce RTX 3090/PCIe/SSE2 NVIDIA Corporation 4.6.0 NVIDIA 536.23 **Blender Version** Broken: version: 4.1.0 Alpha, branch: main, commit date: 2023-09-29 20:55, hash: `90ec11b823e8` **Short description of error** The Sample Volume node's Position input is off by half the voxel width. **Exact steps for others to reproduce the error** This issue is easiest to notice when used with Simulation Nodes because the error accumulates over time. In the attached blend file, a checkbox controls whether to apply a fix to the error or not. If the fix is not applied, the mesh will drift in the -X, -Y, -Z direction over time. When working correctly, the mesh does not drift. In order to fix it, half the voxel width is subtracted from the Sample Volume node's position input.

newfeaturesbug.blend

1.3 MiB

113265.blend

956 KiB

Cornus-Ammonis added the

labels 2023-10-04 23:20:37 +02:00

Jesse Yurkovich commented

2023-10-05 01:05:44 +02:00

Member

Confirmed and I've attached a slightly simplified/more visual example .blend showing the offset. This is similar to #85312 in that OpenVDB has a different idea of what the center of a voxel is compared to blender. This seems like a similar situation almost.

Confirmed and I've attached a slightly simplified/more visual example .blend showing the offset. This is similar to #85312 in that OpenVDB has a different idea of what the center of a voxel is compared to blender. This seems like a similar situation almost.

Jesse Yurkovich added

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and removed

labels 2023-10-05 01:05:56 +02:00

Iliya Katushenock commented

2023-10-05 17:21:19 +02:00

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@LukasTonne /

Iliya Katushenock added the

label 2023-10-05 17:21:28 +02:00

Lukas Tönne commented

2023-10-06 15:12:28 +02:00

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I don't think the point sampling is actually wrong. It uses the position inputs as they are, so a position (0, 0, 0) would sample right at voxel (0, 0, 0), assuming there is no translation of the grid. OpenVDB grids don't care which point inside a voxel the coordinate represents, but generally we'd consider this to be the voxel center.

Some of the volume nodes offset the grids by half a voxel to compensate for this:

Volume Cube changes the grid transform
Volume to Mesh modifies the mesh vertex positions afterwards

Crucially, the Mesh to Volume, Mesh to SDF Volume, and SDF Volume Sphere node do not add any translation. I believe this is where the "drift" in the original file comes from: the mesh is converted back and forth to a volume but the Volume to Mesh node adds an offset that is not reverted by the Mesh to Volume node.

I don't think the point sampling is actually wrong. It uses the position inputs as they are, so a position (0, 0, 0) would sample right at voxel (0, 0, 0), assuming there is no translation of the grid. OpenVDB grids don't care which point inside a voxel the coordinate represents, but generally we'd consider this to be the voxel center. Some of the volume nodes offset the grids by half a voxel to compensate for this: - [Volume Cube](https://projects.blender.org/blender/blender/src/commit/9a654c04dea839ef8da6ef006abc54a90fff86d3/source/blender/nodes/geometry/nodes/node_geo_volume_cube.cc#L168) changes the grid transform - [Volume to Mesh](https://projects.blender.org/blender/blender/src/commit/9a654c04dea839ef8da6ef006abc54a90fff86d3/source/blender/blenkernel/intern/volume_to_mesh.cc#L106) modifies the mesh vertex positions afterwards Crucially, the _Mesh to Volume_, _Mesh to SDF Volume_, and _SDF Volume Sphere_ node do **not** add any translation. I believe this is where the "drift" in the original file comes from: the mesh is converted back and forth to a volume but the _Volume to Mesh_ node adds an offset that is not reverted by the _Mesh to Volume_ node.

Cornus-Ammonis commented

2023-10-06 21:04:22 +02:00

Author

Wouldn't you expect a loop of only Mesh to Volume -> Volume to Mesh to exhibit the same drift if that's the case? That does not occur in my testing.

Wouldn't you expect a loop of only Mesh to Volume -> Volume to Mesh to exhibit the same drift if that's the case? That does not occur in my testing.

m2v-v2m.blend

915 KiB

Brecht Van Lommel referenced this issue

2024-03-05 18:42:32 +01:00

OpenVDB import volume voxel offset #119082

Bart van der Braak added

and removed

labels 2024-08-14 13:07:55 +02:00

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