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Mirrored objects render black with Blender Internal
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System Information

Occurs on 2 systems (both Windows 8.1). One system has a Geforce GTX745 card, and the other has an Intel integrated HD 4600 graphics.

Blender Version

Broken: Bug occurs on 2.73a and 2.74RC2
Worked: works on 2.69


When a duplicated mesh object has been mirrored (with the object shading is set to smooth and auto-smooth enabled), the Blender Internal renderer renders the object completely black.

Exact steps for others to reproduce the error

  1. Add a UV Sphere and add a new material
  2. Set shading to smooth
  3. Enable auto smooth
  4. Create a linked duplicate (Alt-D) and place it beside the original sphere
  5. Mirror the duplicate about an axis (For example: Object > Mirror > X Global)
  6. Press F12 to render

The original object renders normally, but the duplicate renders as black. Even if the original object is deleted, and then the normals are recalculated on the second (remaining) object, it still renders as black. This bug appears to have similar symptoms as T40014 and T41079, which were previously fixed.

Event Timeline

Aaron Corey (aarondrew) updated the task description. (Show Details)
Aaron Corey (aarondrew) raised the priority of this task from to Needs Triage by Developer.
Aaron Corey (aarondrew) set Type to Bug.

I should also mention that I discovered a work-around for this issue. If you make the duplicate object single-user and then apply the scale (Object > Apply > Scale), the object will then render properly. But of course, you lose the benefit of linked duplicates if you do that. I assume that applying the scale works because mirroring the object is essentially the same as scaling it by -1 along an axis?

Can confirm this.

Turning off "Auto Smooth" or scaling by -1/mirroring again "solves" it. It also happens for every other object with a scale < 0 [Edit: and Auto Smooth enabled]. I bet Auto Smooth doesn't like negative scales... @Bastien Montagne (mont29), maybe you can have a look? Might even be related to you custom normals work.

Julian Eisel (Severin) triaged this task as Confirmed, Medium priority.Mar 23 2015, 7:40 PM

I did a little more testing to narrow down the version where the change occurred. 2.70a renders mirrored objects correctly, but 2.71 renders them black.