Correct an STL face that is wound backwards

Neighboring triangles on a consistently wound surface traverse their shared edge in opposite directions. When they traverse it in the same direction, their orientations disagree. That disagreement is a topology finding; a viewer's shading alone can be misleading because rendering settings may display both sides of a face.

One reversed face

The supplied tetrahedron has four faces. Reversing one produces three inconsistent shared edges while leaving the coordinates and overall dimensions unchanged. The supported repair reverses that face back. The repaired file has zero inconsistent edges and the same 10 × 10 × 10 coordinate dimensions.

Preview before applying

1. Open the reversed-face sample and select Inconsistent winding.
2. Choose Correct winding where unambiguous, then Preview repair.
3. Compare the reported changes and inspect the preview.
4. Apply the preview and verify the export. Undo restores the last completed model.

The operation changes triangle orientation without moving vertices. The export then rereads the written geometry and checks dimensions, coordinates, triangle count, and topology. If that verification fails, it does not offer the result as a verified file.

When repair refuses

The component must be closed and manifold, with a consistent orientation solution and a unique majority among the existing face orientations. A tie would require a guess. Open, non-manifold, or non-orientable components are refused, and degenerate triangles must be addressed first.

Preserving a component's majority direction is not a determination of inside versus outside. A consistently inward shell is not automatically reversed, and nested shells are not interpreted as cavities. The tool also does not check self-intersections or wall thickness. Use the winding result for the property it measures, and inspect any remaining problems before handing off the model.