• Clean >> Fix Bad Normals command searches and corrects all the reversed normals.

• To determine whether a normal vector is reversed or not, a face with a correct normal direction must be specified as the reference face. All the normals whose directions are out of accordance with the reference face's are reversed.

• There are three standards for judging the accordance. The first one uses the topology relationship of adjacent faces. This method works nicely unless the shell contains non-manifold faces. The second one detects reversed normals of a closed shell. The third one can fixed some twisted topology which cannot be fixed using the above two methods.

Step   

Execute Clean >> Fix Bad Normals command as follows:

1. Select Clean >> Fix Bad Normals from the menu or click the icon from the Clean toolbar.

2. Select desired shells. ( See Shell selection )

3. The following dialog box appears.

Check Consistent topology option if the shell has no non-manifold face.

If there exists a topological problem such as disjoint faces or non-manifold faces, check Consistent geometry option. In this case, the shell is assumed to be closed.

If there exists some twisted topology, check Fix Twisted Topology. This is used to correct the twisted topology that might be introduced when polygonize a point cloud not being divided into inside an outside; or when deal with a data from a bad quality scanner that produces noise evenly distributed in the whole data; or when deals with bad registered data.

These three options are exclusive, so only one of them can be selected at one time.

4. To complete the command, press OK button and select a reference face whose normal is correct.

• In case of Consistent Geometry, when the shell is not closed, correction may fail partially. In case of that, try again with Consistent topology option.

• If there are too many reversed normals, you may need  to execute this command several times.

• There are several possible causes for a twisted topology: the model is created by polygonizing a point cloud that was not divided into inside and outside; a data from a bad quality scanner which produces noise evenly distributed in the whole data; bad registered data, etc.

• This is used to correct the twisted topology that might be introduced when polygonize a point cloud not being divided into inside an outside; or when deal with a data from a bad quality scanner that produces noise evenly distributed in the whole data; or when deals with bad registered data.