Subdivision modifier does not interpolate zero weights. #82771

New Issue

Will · 2020-11-17T05:35:33+01:00

Will commented

2020-11-17 05:35:33 +01:00

System Information
Operating system: Linux
Graphics card: Intel UHD

Blender Version
Broken: 2.90.1
Worked: ???

Short description of error

The subdivision modifier usually interpolates both value and membership across subdivided vertices.

This allows it to be used in non-destructive, modifier-based workflows that operate on or using vertex groups.

It does not do this, however, when vertex weights are equal to zero. The original vertices retain their (zero-weighted) vertex group membership, while new vertices in between them have no vertex group membership.

An example of why this is problematic is screenshotted below, with the file attached:

Each Subdivision'd sphere has a bumpy texture applied using a Displace modifier after a textured VertexWeightMix modifier.

The rightmost sphere has its vertices weighted to 0.01, and interpolates and behaves correctly.

The leftmost sphere has its vertices weighted to 0.00. It intuitively (and, numerically/extrapolatively) should be indistinguishable to the 0.01 sphere. Because its weights aren't correctly interpolated, it instead ends up with a bunch of spikes at each of its original vertices.

The middle sphere has vertices weighted in a gradient from 0.00 to 0.50. It serves as an example of a feasible real-world application which would be broken by this kind of behaviour. Perhaps the original vertex weights act as a sort of base heightmap for a terrain model, and the changes made by the Subdivision and textured VertexWeightMix modifiers add an extra layer of detail. In this case, the clear and useful behaviour would be for behaviour to be continuous in the transition from 0.01 to 0.00. Instead, there is a sharp cutoff at 0.00, and inconsistent and discontinuous behaviour after that.
ZeroWeightSubdiv.blend
Exact steps for others to reproduce the error

Apply a subdivision modifier to a mesh with zero vertex weights, then check the vertex group membership of its vertices afterwards.

Or download and open the attached .blend file.

**System Information** Operating system: Linux Graphics card: Intel UHD **Blender Version** Broken: 2.90.1 Worked: ??? **Short description of error** The subdivision modifier usually interpolates both value and membership across subdivided vertices. This allows it to be used in non-destructive, modifier-based workflows that operate on or using vertex groups. It does not do this, however, when vertex weights are equal to zero. The original vertices retain their (zero-weighted) vertex group membership, while new vertices in between them have no vertex group membership. An example of why this is problematic is screenshotted below, with the file attached: ![ZeroWeightSubdiv.png](https://archive.blender.org/developer/F9307822/ZeroWeightSubdiv.png) Each Subdivision'd sphere has a bumpy texture applied using a Displace modifier after a textured VertexWeightMix modifier. The rightmost sphere has its vertices weighted to 0.01, and interpolates and behaves correctly. The leftmost sphere has its vertices weighted to 0.00. It intuitively (and, numerically/extrapolatively) should be indistinguishable to the 0.01 sphere. Because its weights aren't correctly interpolated, it instead ends up with a bunch of spikes at each of its original vertices. The middle sphere has vertices weighted in a gradient from 0.00 to 0.50. It serves as an example of a feasible real-world application which would be broken by this kind of behaviour. Perhaps the original vertex weights act as a sort of base heightmap for a terrain model, and the changes made by the Subdivision and textured VertexWeightMix modifiers add an extra layer of detail. In this case, the clear and useful behaviour would be for behaviour to be continuous in the transition from 0.01 to 0.00. Instead, there is a sharp cutoff at 0.00, and inconsistent and discontinuous behaviour after that. [ZeroWeightSubdiv.blend](https://archive.blender.org/developer/F9307820/ZeroWeightSubdiv.blend) **Exact steps for others to reproduce the error** Apply a subdivision modifier to a mesh with zero vertex weights, then check the vertex group membership of its vertices afterwards. Or download and open the attached `.blend` file.

Will commented

2020-11-17 05:35:33 +01:00

Added subscriber: @WCN

Germano Cavalcante commented

2020-11-17 20:20:21 +01:00

Added subscriber: @mano-wii

Germano Cavalcante commented

2020-11-17 20:20:21 +01:00

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

Germano Cavalcante commented

2020-11-17 20:20:21 +01:00

Thanks for the report,
I confirm that vertices whose weight value is zero, do not interpolate.

Here is a simplified file:
zero_weight_vgroup_interp_bug.blend

To reproduce the problem in this file:

select all vertices of the cube
run the subdivide operator
deselect all vertices
Check which vertices are part of the Vertex Group (by the select option from the vertex group).

Only the "original" vertices remain.

Thanks for the report, I confirm that vertices whose weight value is zero, do not interpolate. Here is a simplified file: [zero_weight_vgroup_interp_bug.blend](https://archive.blender.org/developer/F9313240/zero_weight_vgroup_interp_bug.blend) To reproduce the problem in this file: - select all vertices of the cube - run the subdivide operator - deselect all vertices - Check which vertices are part of the Vertex Group (by the select option from the vertex group). Only the "original" vertices remain.

Campbell Barton commented

2020-11-18 10:14:50 +01:00

Added subscriber: @ideasman42

Campbell Barton commented

2020-11-18 10:14:50 +01:00

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

Campbell Barton commented

2020-11-18 10:14:50 +01:00

I don't think modifiers should be using vertex group membership. This is something that we avoided nearly everywhere.

Can you include a simple modifier setup that does this?

I don't think modifiers should be using vertex group membership. This is something that we avoided nearly everywhere. Can you include a simple modifier setup that does this?

Will commented

2020-11-19 00:52:23 +01:00

@ideasman42 I don't know if there are any modifiers that explicitly use vertex group membership, but the VertexWeightMix modifier definitely seems to implicitly use it in the sense that it will not assign new memberships to vertices that aren't already in a target/output group. As a result, being zero-weighted has a completely different effect compared to not being in a group, which in turn can affect any other modifiers that use its output weights further down the stack.

The file in my original post has this setup. Here's a screenshot, with the displacement modifier disabled:

The top sphere starts with zero weights across all its vertices. The bottom sphere starts with 0.010 weights across all its vertices. The middle sphere starts with a gradient from zero to 0.5 across the middle.

The Subdivision interpolates vertex weights and group membership only between non-zero vertices. As a result, the VertexWeightMix doesn't operate on the newly generated vertices, with no vertex group membership, in the areas that started with zero weights.

@ideasman42 I don't know if there are any modifiers that *explicitly* use vertex group membership, but the VertexWeightMix modifier definitely seems to implicitly use it in the sense that it will not assign new memberships to vertices that aren't already in a target/output group. As a result, being zero-weighted has a completely different effect compared to not being in a group, which in turn can affect any other modifiers that use its output weights further down the stack. The file in my original post has this setup. Here's a screenshot, with the displacement modifier disabled: ![image.png](https://archive.blender.org/developer/F9320182/image.png) The top sphere starts with zero weights across all its vertices. The bottom sphere starts with 0.010 weights across all its vertices. The middle sphere starts with a gradient from zero to 0.5 across the middle. The Subdivision interpolates vertex weights and group membership only between non-zero vertices. As a result, the VertexWeightMix doesn't operate on the newly generated vertices, with no vertex group membership, in the areas that started with zero weights.

Will commented

2020-11-21 00:46:00 +01:00

Also, IMHO, whether or not modifiers actually use vertex group membership is just an example, and not the main point of the issue. The behaviour of subdivision interpolating vertex weights for every value except 0.0 is itself inconsistent, and could thus also lead to other issues elsewhere.

Philipp Oeser commented

2020-12-11 18:39:59 +01:00

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

Philipp Oeser commented

2021-01-18 17:23:19 +01:00

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

Henrik D. commented

2021-08-13 12:18:23 +02:00

Added subscriber: @HDMaster84

Henrik D. commented

2021-08-13 12:18:23 +02:00

I don't know the details here, but I just want to add that this might be a performance optimization for the normal usecase of rigging, where there are lots of vgroups with zero weights in most places.

What I do know though is that this is a useful glitch! I don't think there is currently another way to get a vgroup on just the original geometry (is there?)

So I think this should be low prio, as it only happens for the "vertex weight mix/edit modifier" and the user can easily work around this issue with said modifiers.

I don't know the details here, but I just want to add that this might be a performance optimization for the normal usecase of rigging, where there are lots of vgroups with zero weights in most places. What I do know though is that this is a useful glitch! I don't think there is currently another way to get a vgroup on just the original geometry (is there?) So I think this should be low prio, as it only happens for the "vertex weight mix/edit modifier" and the user can easily work around this issue with said modifiers.

Philipp Oeser removed the

Download

What's New

Blender Studio

Manual

Developers Blog

Documentation

Benchmark

Blender Conference

Development Fund

One-time Donations

Subdivision modifier does not interpolate zero weights. #82771