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Generative modifiers (solidify, mirror) draw face normals from original face (wrong direction)
Needs Information from Developers, NormalPublicKNOWN ISSUE

Description

System Information
@Karol Szczerba (Baszczer) information
Operating system: Ubuntu 20.04
Graphics card: Radeon Pro WX 7100

@Alaska (Alaska) information
Operating system: Linux-5.4.0-7629-generic-x86_64-with-debian-bullseye
Graphics card: GTX 1050Ti 440.82

Blender Version
Broken: 2.83 (2020-05-26 22:42) rBf772a4b8fa87, 2.90 (2020-05-26 20:02) rBeb5422828ac1
Worked: 2.82a

Short description of error:
The viewpoint overlay for displaying faces normals in edit mode for faces that are part of a solidify modifier will always point in the direction of the face normal being solidified with Blender 2.83 and 2.90.


2.82a


2.83/2.90

Exact steps for others to reproduce the error:

  1. Open Blender.
  2. Add a plane.
  3. Give the plane a solidify modifier.
  4. In the modifier panel, enable the "On cage" option for the solidify modifier.
  5. Enter edit mode on the plane.
  6. In the viewport overlays enable the "Display normals" option for faces.

In 2.82a, this will be have as expected. The normal lines will be perpendicular to the faces of the solidifies plane. In 2.83, all the normal lines align with the normal line of the face being solidified.

Video because I feel like I'm having a hard time explaining this in words:

Event Timeline

Alaska (Alaska) changed the task status from Needs Triage to Confirmed.May 27 2020, 12:43 PM
Alaska (Alaska) added a subscriber: Alaska (Alaska).

I will update the description of the task with details on how to reproduce the issue.

Alaska (Alaska) changed the subtype of this task from "Report" to "Bug".
Clément Foucault (fclem) changed the subtype of this task from "Bug" to "Known Issue".May 28 2020, 2:24 PM

Ok this was a design choice to avoid this bug present in 2.82:

Put simply we use the original face normal to avoid conflicting normals for multiple faces sharing the same original face (since there will only be 1 face dot/normal displayed).

So I would reconsider this as known issue.

If current behavior is intended, there is no reason to keep this open. Closing.

Just noting that this also breaks the mirror modifier as noted in T94826: Face Normals display as NOT mirrored when Mirror Modifier is used

Ok this was a design choice to avoid this bug present in 2.82:

Put simply we use the original face normal to avoid conflicting normals for multiple faces sharing the same original face (since there will only be 1 face dot/normal displayed).

So I would reconsider this as known issue.

For the subd modifier, this might make sense, but solidify and mirror really suffer from this.
Which commit was that, rBa2d19c1f7811: NormalOverlay: Center Dot Normal Drawing With Modifiers?

Philipp Oeser (lichtwerk) renamed this task from Different face normals behaviour with solidify modifier to Generative modifiers (solidify, mirror) draw face normals from original face (wrong direction).Wed, Jan 12, 3:11 PM

Cant we be more distinct and only do this if we know they will be drawn in the same place?

@Philipp Oeser (lichtwerk) Thanks Philipp! Glad the devs are aware of this issue. I admit it's quite confusing to see inverted normals when modelling with the mirror modifier. A temporary workaround is to either turn off the "Cage" display on the modifier or enable "Face Orientation" in the Viewport Overlays so one can tell what normals are truly flipped.

A possible solution: would it be feasible to have the face normals draw on generated faces when "Cage" is enabled and only draw on original faces when "Cage" is disabled? This should also solve the original issue when a subsurf modifier is applied, with the added benefit of making more visual sense to the user (see images below):