Decimate modifier doesn't decimate evenly #112025

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opened 2023-09-06 14:01:43 +02:00 by Robert S · 5 comments

Robert S commented

2023-09-06 14:01:43 +02:00

System Information
Operating system: Windows-10-10.0.19045-SP0 64 Bits
Graphics card: NVIDIA GeForce RTX 3090/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 536.23

Blender Version
Broken: version: 3.6.2, branch: blender-v3.6-release, commit date: 2023-08-14 14:05, hash: ffe93138751e
Worked: (newest version of Blender that worked as expected)

Short description of error
Decimate modifier result:

Decimate modifier vertex group content:

In other parts of the map, the decimate doesn't do much at all, while in some parts it reduces too much.

Exact steps for others to reproduce the error

Download this example .blend file.
Look at the Decimate modifier with vertex group results.
Compare it to the actual vertex group contents.

Expected: Decimation should be "even" across the entire vertex group content.

Actual: Decimation is extremely unevenly distributed, seemingly doing massive decimations in a corner of the model while ignoring the rest.

**System Information** Operating system: Windows-10-10.0.19045-SP0 64 Bits Graphics card: NVIDIA GeForce RTX 3090/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 536.23 **Blender Version** Broken: version: 3.6.2, branch: blender-v3.6-release, commit date: 2023-08-14 14:05, hash: `ffe93138751e` Worked: (newest version of Blender that worked as expected) **Short description of error** Decimate modifier result: ![image](/attachments/d6cdec10-a842-458e-bcea-ee30e26699fe) Decimate modifier vertex group content: ![image](/attachments/b85def6d-4ab9-4dbe-ab30-58896f9b110d) In other parts of the map, the decimate doesn't do much at all, while in some parts it reduces too much. ![image](/attachments/f2cd8080-4c98-4235-b3cc-eb85ecb08cbc) **Exact steps for others to reproduce the error** 1. Download [this example .blend](https://we.tl/t-GpN3UHnIVW) file. 2. Look at the Decimate modifier with vertex group results. 3. Compare it to the actual vertex group contents. Expected: Decimation should be "even" across the entire vertex group content. Actual: Decimation is extremely unevenly distributed, seemingly doing massive decimations in a corner of the model while ignoring the rest.

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Robert S added the

labels 2023-09-06 14:01:44 +02:00

Philipp Oeser commented

2023-09-08 12:25:55 +02:00

Member

Can confirm, will check

Can confirm, will check

Philipp Oeser added

and removed

labels 2023-09-08 12:26:33 +02:00

Philipp Oeser commented

2023-09-08 13:20:49 +02:00

Member

Hm, upon having a first look at BM_mesh_decimate_collapse it turns out that this is extremely sensitive to normals (slight bumps hills in the map)

This is also the case with not using vertex groups at all

If you make the whole Ukraine planar, it collapses quite evenly

Without digging (much) deeper, I cannot tell why it is so "drastic", @ideasman42 might know better (#37121 might be related).

It might not be considered a bug, will still leave open since it is rather unexpected behavior.

That said, you could alternatively use the Weld modifier, this does not try to preserve the original exact shape so much and is probably more in line of what you would expect here? (instead of tweaking the Ratio, you would need to adjust the weld Distance)

Hm, upon having a first look at `BM_mesh_decimate_collapse` it turns out that this is extremely sensitive to normals (slight bumps hills in the map) This is also the case with not using vertex groups at all ![image](/attachments/0ec6073f-7148-47d8-b567-112f75256122) ![image](/attachments/bee9d126-4986-43ff-a743-fc17e14e448a) If you make the whole Ukraine planar, it collapses quite evenly ![image](/attachments/fe990925-ec84-4577-a773-f2a946ebc1e2) Without digging (much) deeper, I cannot tell why it is so "drastic", @ideasman42 might know better (#37121 might be related). It might not be considered a bug, will still leave open since it is rather unexpected behavior. That said, you could alternatively use the Weld modifier, this does not try to preserve the original exact shape so much and is probably more in line of what you would expect here? (instead of tweaking the `Ratio`, you would need to adjust the weld `Distance`) ![image](/attachments/bfd1b16b-c863-4c50-a545-1e747bf864c7)

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decimate-group_repro_weld.blend

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Philipp Oeser changed title from ~~Decimate modifier using Vertex Group doesn't decimate evenly~~ to Decimate modifier doesn't decimate evenly

2023-09-08 13:22:19 +02:00

Paul Kotelevets commented

2023-09-11 12:16:21 +02:00

What has always confused me is that the decimate modifier count percentage is always calculated across all vertices of the model number, not the given group number.
As a result, percentage amount is very sensitive if the model is heavy and group is relatively small.
This may have some impact here as well.
What will happen if to detach vertex group and apply the modifier to separated part?

Also, is the scale applied?

What has always confused me is that the decimate modifier count percentage is always calculated across all vertices of the model number, not the given group number. As a result, percentage amount is very sensitive if the model is heavy and group is relatively small. This may have some impact here as well. What will happen if to detach vertex group and apply the modifier to separated part? Also, is the scale applied?

Philipp Oeser commented

2023-09-11 12:26:56 +02:00

Member

What will happen if to detach vertex group and apply the modifier to separated part?

Dont think that makes a difference (wrt the "uneven" result), this is still influenced most by the shape (not so much by the percentage -- your suggestion might make sense though, having the ratio for just the parts that are part of the vertex group)

> What will happen if to detach vertex group and apply the modifier to separated part? Dont think that makes a difference (wrt the "uneven" result), this is still influenced most by the shape (not so much by the percentage -- your suggestion might make sense though, having the ratio for just the parts that are part of the vertex group)

Campbell Barton commented

2023-09-12 15:18:20 +02:00

Owner

Checked on this file and I don't consider this is a bug.

Entirely flat surfaces will decimate before other slightly flat surfaces.
This results in some regions not being decimated when they have some subtle curvature.

What you might be after is a decimate that prioritizes uniform geometry density (at the expense of quality), but this is a feature, not an error in the current logic.

Checked on this file and I don't consider this is a bug. - Entirely flat surfaces will decimate before other slightly flat surfaces. - This results in some regions not being decimated when they have some subtle curvature. What you might be after is a decimate that prioritizes uniform geometry density (at the expense of quality), but this is a feature, not an error in the current logic.

Campbell Barton closed this issue

2023-09-12 15:18:23 +02:00

Blender Bot added

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labels 2023-09-12 15:18:24 +02:00

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