Points > Repair Normals manipulates oriented and un-oriented normals on unordered point objects.
An un-oriented normal is a vector that is perpendicular to the surface and which points to either the outside or inside of the object (because no determination of inside and outside has been made).
An oriented normal is a vector that is perpendicular to the surface and points to the outside of the object, and all such normals point to the outside. This definition implies that a single object cannot have both oriented and un-oriented normals.
Several other functions rely on well-oriented normals, such as Points > Reduce Noise, Points > Wrap, and Points > Merge Point Objects.
Manipulations apply to selected portions of the point object, or to the whole object if no points are selected. Selections can be made before or after starting this function.
If no normals exist when the function starts, the user is prompted whether to create un-oriented normals. A negative response terminates the function.
Use Ctrl-Z to undo the most recent operation, or Ctrl-Y to re-do the most recently undone operation.
Recompute button - computes a normal for every point in the object, or for selected points, such that all normals are perpendicular to their surface. If existing normals are oriented, the new ones will be similarly oriented.
Flip button (only when normals are oriented) - reverses the normals of all points, or of selected points. The result is that each point is considered to “face” in the opposite direction.
Orient to View button - orients the normals of all points, or of selected points (whether un-oriented or already oriented) to point toward the side that is visible in the Viewing Area. Thus, the visible side becomes the "outside".
Auto-Orient button - creates oriented normals (all pointing outward) using a best-guess algorithm. This operation is relatively slow, and works least optimally on objects with thin walls (because the inside or outside of a thin wall is harder to ascertain). If existing normals are un-oriented, this operation applies to whole objects only, not to selected portions, because oriented and un-oriented normals cannot exist on the same object.
OK button - saves changes to the point object in the Model Manager and closes the dialog.
Cancel button - terminates the dialog without modifying normals.
If this function is recorded in a macro, then a) selections of regions are not possible and therefore should be done before starting the function, b) only one of the four operations can be performed per instance of the function, and c) there is no way to rotate an object to ensure that the outer side is defined as the visible side, so this should be done before starting the function such as by switching to a pre-defined view.