How to spot it
- Bambu StudioNo warning for one flipped face; it sliced like the clean blockTested: Bambu Studio 2.8 on macOS, tested 2026-10-27
- Bambu StudioError: reversed faces detected, which may affect renderingBambu Studio source
- PrusaSlicerAuto-repaired 4 errors: 4 facets reversedPrusaSlicer 2.9.6 source
- BlenderFace Orientation overlay shows the back of faces in redBlender manual
Why it happens
- Faces flipped while editing
- Converters and scripts that write the corners in the wrong order
In our tests
23/ 27
27 of the 198 public test files had faces that disagree with their neighbours. After FixSTL’s repair, 23 were free of them.
Every one of these files had other problems too, so few became fully repaired; the flipped faces themselves were usually turned back.
Public Thingi10K test set, FixSTL engine 0.6.0 (a7ea280), default repair. How we test
How to fix it
What FixSTL does
- Turns flipped faces so neighbours agree
- Turns an inside-out part outward; cavities inside a part keep facing in
What it leaves
- An inside-out part is found during the repair, not in the first check
- The basic check, used when your browser cannot run the full one, does not look for inside-out parts
Elsewhere
- Blender Edit Mode, select all, Mesh › Normals › Recalculate Outside