Mirror bisect plane scaled in object space rather than mirror object space? #87592

Closed
opened 2021-04-17 23:57:46 +02:00 by Will · 15 comments

System Information
Operating system: Linux
Graphics card: Intel

Blender Version
Broken: 2.92

Short description of error

When an object which has a mirror modifier using use_bisect_axis and mirror_object also has some degree of rotation and anisotropic scale across the mirror axis, the bisection plane appears in the wrong place:

image.png

It looks like the bisection plane is being offset in the object's space instead of in the mirror object's space.

Exact steps for others to reproduce the error

  • Add a cube and an empty to an empty scene.

  • Add a mirror modifier to the cube.

  • Set mirror_object on the mirror modifier to the empty and enable "X" for use_bisect_axis.

  • Rotate the cube by any non-orthogonal angle around the global "Y" axis.

  • Scale the cube by any significant amount around its local "X" xor "Z" axis.

Or just download and open the below file:

ObjectBisectFailure.blend

**System Information** Operating system: Linux Graphics card: Intel **Blender Version** Broken: 2.92 **Short description of error** When an object which has a mirror modifier using `use_bisect_axis` and `mirror_object` also has some degree of rotation and anisotropic scale across the mirror axis, the bisection plane appears in the wrong place: ![image.png](https://archive.blender.org/developer/F9937878/image.png) It looks like the bisection plane is being offset in the object's space instead of in the mirror object's space. **Exact steps for others to reproduce the error** * Add a cube and an empty to an empty scene. * Add a mirror modifier to the cube. * Set `mirror_object` on the mirror modifier to the empty and enable "X" for `use_bisect_axis`. * Rotate the cube by any non-orthogonal angle around the global "Y" axis. * Scale the cube by any significant amount around its local "X" xor "Z" axis. Or just download and open the below file: [ObjectBisectFailure.blend](https://archive.blender.org/developer/F9937882/ObjectBisectFailure.blend)
Author

Added subscriber: @WCN

Added subscriber: @WCN
Member

Added subscriber: @filedescriptor

Added subscriber: @filedescriptor
Member

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

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

I can confirm this on the latest 2.93.0 Beta, branch: master, commit date: 2021-04-19 10:12, hash: c75b2019e1.

I can confirm this on the latest 2.93.0 Beta, branch: master, commit date: 2021-04-19 10:12, hash: `c75b2019e1`.
Member

Changed status from 'Confirmed' to: 'Archived'

Changed status from 'Confirmed' to: 'Archived'
Member

After taking a closer look, I found that the cube object had a non-uniform scale. Applying the scale solves the problem.
The bisecting could maybe take the scale into account, but this would not classify as a bug (unless it worked in a previous version).
Closing this for now.

After taking a closer look, I found that the cube object had a non-uniform scale. Applying the scale solves the problem. The bisecting could maybe take the scale into account, but this would not classify as a bug (unless it worked in a previous version). Closing this for now.
Author

@filedescriptor Anisotropic/non-uniform scale is explicitly a part of the description and reproduction steps. Applying the scale doesn't solve the problem so much as avoid it, and isn't an option for objects with shared data.

The bisecting already does take scale into account— Just not always correctly. If you scale the object uniformly in the axes that intersect with the mirror plane (so Z and X but not necessarily Y in the example), then the bisection plane stays at the world-relative center and angle of the mirror object. Likewise, if you rotate or translate the object, the bisection plane also stays at the correct place. It's only with non-uniform scale along axes which intersect with the mirror plane that the behaviour suddenly changes from how it behaves with all other transformations.

Note too that the actual mirror plane itself also stays at the world-relative center and angle of the mirror object. It's only the bisection plane that becomes separated from the mirror plane (which, in and of itself, is also a thing which should probably never happen).


The expected behaviour of the mirror modifier is for its mirror and bisection planes to be positioned at the same position and angle in world space as the mirror object.
A corollary of this is that the bisection plane should never be separated from the mirror plane.

This issue describes a reproducible scenario where that expected behaviour is clearly broken. Why would it not be classified as a bug?

@filedescriptor Anisotropic/non-uniform scale is explicitly a part of the description and reproduction steps. Applying the scale doesn't solve the problem so much as avoid it, and isn't an option for objects with shared data. The bisecting already **does** take scale into account— Just not always correctly. If you scale the object uniformly in the axes that intersect with the mirror plane (so Z and X but not necessarily Y in the example), then the bisection plane stays at the world-relative center and angle of the mirror object. Likewise, if you rotate or translate the object, the bisection plane also stays at the correct place. It's only with non-uniform scale along axes which intersect with the mirror plane that the behaviour suddenly changes from how it behaves with all other transformations. Note too that the actual mirror plane itself also stays at the world-relative center and angle of the mirror object. It's only the bisection plane that becomes separated from the mirror plane (which, in and of itself, is also a thing which should probably never happen). --- The expected behaviour of the mirror modifier is for its mirror and bisection planes to be positioned at the same position and angle in world space as the mirror object. A corollary of this is that the bisection plane should never be separated from the mirror plane. This issue describes a reproducible scenario where that expected behaviour is clearly broken. Why would it not be classified as a bug?
Member

Added subscriber: @ideasman42

Added subscriber: @ideasman42
Member

The bisecting could maybe take the scale into account, but this would not classify as a bug (unless it worked in a previous version).

I looked at the code as it currently is and this is working as intended. That's why I would say it is not a bug. The scale is just not taken into account. And the fact that uniform scale is working is just a by-product of how matrix multiplication works.
@ideasman42 Would you say we should classify this as a bug?

> The bisecting could maybe take the scale into account, but this would not classify as a bug (unless it worked in a previous version). I looked at the code as it currently is and this is working as intended. That's why I would say it is not a bug. The scale is just not taken into account. And the fact that uniform scale is working is just a by-product of how matrix multiplication works. @ideasman42 Would you say we should classify this as a bug?
Author

I know I'm just a user with an opinion.

But the tooltip for the "Bisect" options says "Cuts the mesh across the mirror plane". And this is a scenario where it does not cut the mesh across the mirror plane. Even if the behaviour was somehow originally intended, it still clearly defies both documentation and reasonable user expectation/utility.

I know I'm just a user with an opinion. But the tooltip for the "Bisect" options says "Cuts the mesh across the mirror plane". And this is a scenario where it does not cut the mesh across the mirror plane. Even if the behaviour was somehow originally intended, it still clearly defies both documentation and reasonable user expectation/utility.
Member

Changed status from 'Archived' to: 'Confirmed'

Changed status from 'Archived' to: 'Confirmed'
Member

@WCN I agree with you that this is confusing. I will reopen the report and mark it as a to-do. There is no good reason, why the bisection is affected by the scale of the object.

@WCN I agree with you that this is confusing. I will reopen the report and mark it as a to-do. There is no good reason, why the bisection is affected by the scale of the object.

This issue was referenced by 1e16662de8

This issue was referenced by 1e16662de8eaffecd95e42a1c14b16a4763c2ba7

This issue was referenced by 73af884df0

This issue was referenced by 73af884df004171bacffb82f3ff7f6e627a12690

Changed status from 'Confirmed' to: 'Resolved'

Changed status from 'Confirmed' to: 'Resolved'
Campbell Barton self-assigned this 2021-04-29 09:54:51 +02:00
Sign in to join this conversation.
No Label
Interest
Alembic
Interest
Animation & Rigging
Interest
Asset Browser
Interest
Asset Browser Project Overview
Interest
Audio
Interest
Automated Testing
Interest
Blender Asset Bundle
Interest
BlendFile
Interest
Collada
Interest
Compatibility
Interest
Compositing
Interest
Core
Interest
Cycles
Interest
Dependency Graph
Interest
Development Management
Interest
EEVEE
Interest
EEVEE & Viewport
Interest
Freestyle
Interest
Geometry Nodes
Interest
Grease Pencil
Interest
ID Management
Interest
Images & Movies
Interest
Import Export
Interest
Line Art
Interest
Masking
Interest
Metal
Interest
Modeling
Interest
Modifiers
Interest
Motion Tracking
Interest
Nodes & Physics
Interest
OpenGL
Interest
Overlay
Interest
Overrides
Interest
Performance
Interest
Physics
Interest
Pipeline, Assets & IO
Interest
Platforms, Builds & Tests
Interest
Python API
Interest
Render & Cycles
Interest
Render Pipeline
Interest
Sculpt, Paint & Texture
Interest
Text Editor
Interest
Translations
Interest
Triaging
Interest
Undo
Interest
USD
Interest
User Interface
Interest
UV Editing
Interest
VFX & Video
Interest
Video Sequencer
Interest
Virtual Reality
Interest
Vulkan
Interest
Wayland
Interest
Workbench
Interest: X11
Legacy
Blender 2.8 Project
Legacy
Milestone 1: Basic, Local Asset Browser
Legacy
OpenGL Error
Meta
Good First Issue
Meta
Papercut
Meta
Retrospective
Meta
Security
Module
Animation & Rigging
Module
Core
Module
Development Management
Module
EEVEE & Viewport
Module
Grease Pencil
Module
Modeling
Module
Nodes & Physics
Module
Pipeline, Assets & IO
Module
Platforms, Builds & Tests
Module
Python API
Module
Render & Cycles
Module
Sculpt, Paint & Texture
Module
Triaging
Module
User Interface
Module
VFX & Video
Platform
FreeBSD
Platform
Linux
Platform
macOS
Platform
Windows
Priority
High
Priority
Low
Priority
Normal
Priority
Unbreak Now!
Status
Archived
Status
Confirmed
Status
Duplicate
Status
Needs Info from Developers
Status
Needs Information from User
Status
Needs Triage
Status
Resolved
Type
Bug
Type
Design
Type
Known Issue
Type
Patch
Type
Report
Type
To Do
No Milestone
No project
No Assignees
4 Participants
Notifications
Due Date
The due date is invalid or out of range. Please use the format 'yyyy-mm-dd'.

No due date set.

Dependencies

No dependencies set.

Reference: blender/blender#87592
No description provided.