Blender Trouble Shooting: Mesh

Reversed Normals

How to recognize and fix flipped normals
Reversed normals an a digital mesh.

Faces of a digital mesh have an inside and an outside. Blender does a reasonably good job of making certain that the faces are pointing the the correct direction, but occasionally, some faces become reversed. When this happens, an otherwise smooth mesh can appear to have a nasty scar on it, like the image on the left.

Ugly though they may be, flipped normals are easy to verify and fix. Tab into edit mode. Verify the flipped normals by pressing the N menu and clicking view vertex or face normals. Hairs will appear to radiate outwards on most of the mesh, and into those faces which are reversed, or vise versa.  

Once you have verified that this is the issue, head to the properties menu on the left. You have two options, you can recalculate normals or flip normals. In both cases, select the region that you want adjusted and click the button. To recalculate normals on a simple mesh, I usually just select all with A, then recalc. This fixed the issue in the image. For stubborn faces, you can select the face and individually flip it to the direction you need. The region with corrected vertices is shown below, right.

Posted on 12/18/11 _________________________________________________________________________________________________