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collection instance rotates 90 degrees automatically when being instantiated by another object with mirror modifier
Confirmed, NormalPublicBUG


System Information
Operating system: mac os 10.14.6
Graphics card: Intel Iris Plus Graphics 655

Blender Version
Broken: version: 2.82a, 2.30.0 release, 2.90 (sub 3), branch: master, commit date: 2020-05-24 21:18, hash: rB5db2d9c82b7f
Worked: Unsure.

Short description of error
as shown in the following screenshot, to use modifiers on a collection instance I parent the collection instance to a plane and change the instancing of the plane to faces, so that the collection instance will follow what modifiers will affect on the plane, but when using the mirror modifier with mirror object set to an empty at the center, the collection instance on the other side rotates 90 degrees on z axis automatically, which is not expected

Exact steps for others to reproduce the error
open the attached .blend file and you'll see the problem

Steps if you want to recreate from scratch:

  1. Create a new collection, I will call it Suzzane.
  2. In the collection Suzzane create a Suzzane (Monkey Head) then disable the collection in the outliner.
  3. In the new collection, create an instance of the collection Suzzane by pressing Shift+A -> Collection Instance -> Suzzane. From now onwards, we will be working in this new collection.
  4. Create a plane. Select the instanced Suzzane then the plane and parent the Suzzane to the plane with Ctrl+P -> Object
  5. In the outliner, select the instanced Suzzane and hid it with the little eye icon.
  6. Select the plane and in the properties editor in the Object tab select Instancing -> Faces.
  7. Give the plane a mirror modifier and notice how to newly instanced Suzzane is rotated 90 degrees rather than mirrored.

Event Timeline

Alaska (Alaska) changed the task status from Needs Triage to Needs Information from User.Jun 8 2020, 6:43 AM
Alaska (Alaska) added a project: Modeling.
Alaska (Alaska) added a subscriber: Alaska (Alaska).

open the attached .blend file and you'll see the problem

Are you able to upload the .blend file or provide the steps required to reproduce the issue?

I'm having difficultly trying to reproduce this issue in my own files.

zyc17 (zyc17) updated the task description. (Show Details)Jun 8 2020, 7:10 AM

open the attached .blend file and you'll see the problem

Are you able to upload the .blend file or provide the steps required to reproduce the issue?

I'm having difficultly trying to reproduce this issue in my own files.

sorry, I accidentally deleted the file when I edit the task and I didn't notice, the file is now uploaded

Alaska (Alaska) changed the task status from Needs Information from User to Confirmed.Jun 8 2020, 7:25 AM
Alaska (Alaska) changed the subtype of this task from "Report" to "Bug".

Can confirm. Seems to also affect Blender 2.83 and 2.82.

Will add some steps to the original task just in case someone wants to recreate this from scratch.

I encountered this same problem today and did a little investigation. It appears this is a side-effect of how face instancing works:

When calculating the basis for the instance it takes the Z axis to be the face normal and the X axis to point in the direction of the first edge of the polygon. Mirroring a polygon requires its vertex order to be reversed for the normal to be consistent, which means X will now point in the direction of the last edge. For a quad, this results in a 90 degree rotation.

Here's an illustration of the problem with a pentagon: only the two indicated vertices affect the orientation of the axis marker.

To solve this, Blender would need to either:

  1. rotate the vertices by one spot any time it reverses the vertex order; i.e. [0,1,2,3] would become [1,0,3,2] (instead of [0,3,2,1]), or
  2. change how the coordinate space is calculated (and probably offer options to select how to decide the X axis)

Hope this helps.

This comment was removed by David Swift (rococo).

What's the status of this bug? Is there some ETA for fix? I had the same issue as well, parented Collection Instance to Plane that had Array + Curve + Mirror. Instances on mirrored side had incorrect rotations sadly. I've attached an simplified .blend if it helps anyone

In my case, I was modeling an audio mixing console and found this problem.

With Faces Instancing, you only need to bind one object that array multiple planes to an armature.
It's very efficient because you only need one source mesh and one instance object.
A more complete modeling workflow would be possible if the opposite side flipped by the mirror modifier didn't rotate 90 degrees.

I created a potential fix for this some time ago but it never felt like the "right" solution. As far as I know this is (one of?) the only features in Blender that cares about vertex order. There aren't tools for viewing or reordering face vertices and based on the little bit of research I've done it seems that going down that path is discouraged (which I think I agree with).

In my opinion the real fix is improving the instancing toolset to provide some options for the instancing itself. This could better leverage features Blender already provides. For example:

  • Texture coordinates:
    • Use the tangent space of the UV map to define the local x/y space
    • z is calculated to be perpendicular (same as current behaviour)
    • Mirroring geometry will produce predicable results
    • UV tools can be used to adjust instancing

I don't know whether this fits better with the roadmap for instancing in general, however.