Inaccuracies in the subdivision surface modifier (vertex positions) #89090

New Issue

Florian · 2021-06-12T17:49:09+02:00

Florian commented

2021-06-12 17:49:09 +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: 2.92.0, branch: master, commit date: 2021-02-24 16:25, hash: 02948a2cab
Worked: maybe never ?

Short description of error

Edit: (I messed up my first posting and forgot to add the error description the second time)

I seems that the subdivision modifier creates numerical inaccuracies when subdividing planar faces. That is, the vertices of the subdivided plane are not necessarily co-planar anymore. This of course can lead to visible artifacts. I witnessed the issue in both the simple and Catmull-Clark mode.

subd_issue.blend

Exact steps for others to reproduce the error

In Object Mode: Add-> Mesh -> Plane
Go to Edit mode, select all, G->Z->100 (move all vertices 100 in the z-direction) [reason: the numerical inaccuracies are in the range of 1e-6, relative to the position of the vertices. In order to make them appear in the last digit displayed, we offset the initial position. If we leave the plane at zero, we would see nothing at all)
Go back to Object mode, G->Z->-100 (move object -100 in z-direction) [reason: we want to see something in the viewport]
Add a subdivision surface modifier to the object (3 levels, simple mode)
Apply the sub-d modifier
Object->Set Origin->Origin to Geometry [reason: all the vertex positions are at about 100 in z-direction. In order to have 6 digits to the right of the decimal point displayed, we need to subtract 100 again]
Go to edit mode and look at the z-position of the vertices, they should have differences at least in the 6th digit right of the decimal point

**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: 2.92.0, branch: master, commit date: 2021-02-24 16:25, hash: `02948a2cab` Worked: maybe never ? **Short description of error** Edit: (I messed up my first posting and forgot to add the error description the second time) I seems that the subdivision modifier creates numerical inaccuracies when subdividing planar faces. That is, the vertices of the subdivided plane are not necessarily co-planar anymore. This of course can lead to visible artifacts. I witnessed the issue in both the simple and Catmull-Clark mode. [subd_issue.blend](https://archive.blender.org/developer/F10168368/subd_issue.blend) **Exact steps for others to reproduce the error** - In Object Mode: Add-> Mesh -> Plane - Go to Edit mode, select all, G->Z->100 (move all vertices 100 in the z-direction) [reason: the numerical inaccuracies are in the range of 1e-6, relative to the position of the vertices. In order to make them appear in the last digit displayed, we offset the initial position. If we leave the plane at zero, we would see nothing at all) - Go back to Object mode, G->Z->-100 (move object -100 in z-direction) [reason: we want to see something in the viewport] - Add a subdivision surface modifier to the object (3 levels, simple mode) - Apply the sub-d modifier - Object->Set Origin->Origin to Geometry [reason: all the vertex positions are at about 100 in z-direction. In order to have 6 digits to the right of the decimal point displayed, we need to subtract 100 again] - Go to edit mode and look at the z-position of the vertices, they should have differences at least in the 6th digit right of the decimal point

Florian commented

2021-06-12 17:49:09 +02:00

Added subscriber: @LordoftheFleas

Pratik Borhade commented

2021-06-14 14:45:19 +02:00

Added subscriber: @PratikPB2123

Pratik Borhade commented

2021-06-14 14:45:19 +02:00

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

Pratik Borhade commented

2021-06-14 14:45:19 +02:00

@LordoftheFleas , can you please attach the screenshot of artifact visible to you ?

Graphics card: irrelevant

No, GPU and it's version is equally important. Do: Open Blender -> Help -> Report a Bug then copy system info and paste it to this task description.

@LordoftheFleas , can you please attach the screenshot of artifact visible to you ? >Graphics card: irrelevant No, GPU and it's version is equally important. Do: `Open Blender -> Help -> Report a Bug` then copy system info and paste it to this task description.

Florian commented

2021-06-14 18:09:12 +02:00

Added the requested information.

Pratik Borhade commented

2021-06-15 12:28:38 +02:00

Definitely I can notice artifacts but could not reproduce this from scratch.

simple quad offset by 100 in the z-direction.

Not sure I understand the step, can you please elaborate?

Definitely I can notice artifacts but could not reproduce this from scratch. > simple quad offset by 100 in the z-direction. Not sure I understand the step, can you please elaborate?

Florian commented

2021-06-15 18:33:05 +02:00

In #89090#1177074, @PratikPB2123 wrote:
Definitely I can notice artifacts but could not reproduce this from scratch.

simple quad offset by 100 in the z-direction.

Not sure I understand the step, can you please elaborate?

In Object Mode: Add-> Mesh -> Plane
Go to Edit mode, select all, G->Z->100 (move all vertices 100 in the z-direction) [reason: the numerical inaccuracies are in the range of 1e-6, relative to the position of the vertices. In order to make them appear in the last digit displayed, we offset the initial position. If we leave the plane at zero, we would see nothing at all)
Go back to Object mode, G->Z->-100 (move object -100 in z-direction) [reason: we want to see something in the viewport]
Add a subdivision surface modifier to the object (3 levels, simple mode)
Apply the sub-d modifier
Object->Set Origin->Origin to Geometry [reason: all the vertex positions are at about 100 in z-direction. In order to have 6 digits to the right of the decimal point displayed, we need to subtract 100 again]
Go to edit mode and look at the z-position of the vertices, they should have differences at least in the 6th digit right of the decimal point

As for the shading artifacts, I must admit that those were probably due to mistakes on my sides: when trying to reproduce them from scratch myself, they only appeared if a) there was an n-gon on the original mesh leading to overlapping faces after subdivision or b) I only had "smooth" turned on, but not autosmooth. So maybe the inaccuracies are not severe enough to cause shading problems in practice (when using the tools correctly).

> In #89090#1177074, @PratikPB2123 wrote: > Definitely I can notice artifacts but could not reproduce this from scratch. > >> simple quad offset by 100 in the z-direction. > Not sure I understand the step, can you please elaborate? In Object Mode: Add-> Mesh -> Plane Go to Edit mode, select all, G->Z->100 (move all vertices 100 in the z-direction) [reason: the numerical inaccuracies are in the range of 1e-6, relative to the position of the vertices. In order to make them appear in the last digit displayed, we offset the initial position. If we leave the plane at zero, we would see nothing at all) Go back to Object mode, G->Z->-100 (move object -100 in z-direction) [reason: we want to see something in the viewport] Add a subdivision surface modifier to the object (3 levels, simple mode) Apply the sub-d modifier Object->Set Origin->Origin to Geometry [reason: all the vertex positions are at about 100 in z-direction. In order to have 6 digits to the right of the decimal point displayed, we need to subtract 100 again] Go to edit mode and look at the z-position of the vertices, they should have differences at least in the 6th digit right of the decimal point As for the shading artifacts, I must admit that those were probably due to mistakes on my sides: when trying to reproduce them from scratch myself, they only appeared if a) there was an n-gon on the original mesh leading to overlapping faces after subdivision or b) I only had "smooth" turned on, but not autosmooth. So maybe the inaccuracies are not severe enough to cause shading problems in practice (when using the tools correctly).

Pratik Borhade commented

2021-06-28 10:38:22 +02:00

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

Philipp Oeser commented

2022-02-23 15:36:31 +01:00

Added subscriber: @lichtwerk

Philipp Oeser commented

2022-02-23 15:36:31 +01:00

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

Philipp Oeser closed this issue

2022-02-23 15:36:31 +01:00

Philipp Oeser commented

2022-02-23 15:36:31 +01:00

Dont think this is a bug.
Since you have 7 digits of precision in floats (doubles would have more precision here), you dont get the inaccuracies when doing this at the origin, but with an offset of 100 in local space, you are loosing 2 digits of precision, resulting in the inaccuraries after the 6th digit.

Thx reporting, but afraid we have to close this (but of course feel free to comment again if this is a misunderstanding or I am wrong on this).

Dont think this is a bug. Since you have 7 digits of precision in floats (doubles would have more precision here), you dont get the inaccuracies when doing this at the origin, but with an offset of 100 in local space, you are loosing 2 digits of precision, resulting in the inaccuraries after the 6th digit. Thx reporting, but afraid we have to close this (but of course feel free to comment again if this is a misunderstanding or I am wrong on this).

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Inaccuracies in the subdivision surface modifier (vertex positions) #89090