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Flip Normals

In one sentence: Reverse the orientation of all triangles in the mesh, fixing a uniformly inside-out solid.

Overview

Workbench: Mesh
Menu: Mesh → Meshes → Flip Normals
Command: Mesh_FlipNormals
Shortcut: none

Reverses the winding direction of every triangle in the mesh. All normals that were pointing inward now point outward, and vice versa.

Intuition

If a mesh looks correct in wireframe but renders black (normals all pointing away from the camera) or behaves unexpectedly in Boolean operations (subtracting adds material, or vice versa), all normals are pointing the wrong way. Flip Normals corrects this in one step.

Flip Normals reverses all normals uniformly. If only some normals are wrong (inconsistent), use Harmonize Normals first to make them consistent, then Flip Normals if they ended up all pointing inward.

When to use it

  • The mesh renders black or very dark (normals pointing away from the viewer).
  • After Harmonize Normals, the result is uniformly inside-out.
  • A Boolean operation produces the inverse of the expected result.

When NOT to use it

  • Don't use it when only some normals are wrong — use Harmonize Normals to make them consistent first.

Python API

import FreeCAD as App

mesh_obj = App.ActiveDocument.getObject("Mesh")
mesh_obj.Mesh.flipNormals()
mesh_obj.touch()
App.ActiveDocument.recompute()

See also