The visible object side element is a numeric variable deciding, which object face side is supposed to be visible. To accelerate the 3D graphics painting only the front (outer) object face side is painted, the back (inner) side remaining transparent. To distinguish, which face side is the front and which the back the order of the face vertices is determining. The Peter system uses as a default value the vertices' clockwise succession. The other side of the face (counter-clockwise) is deemed to be the back one, and consequently is not painted.

 

0 both face sides are visible

1 only front side of face is visible (clockwise)

2 only back side of face is visible (counter-clockwise)

 

The default setting of the individual objects can be changed by the visible object side element. This element is exercisable for an active object. The setting cannot be changed globally for all objects. The default value is 1, which means that the face front side is visible. By setting this value to 2 the face back side becomes visible. This feature can be utilized e.g. when reading objects from a file, if an object uses the opposed face orientation. By setting the value equal 0 both face sides will become visible. This setting may be useful in objects' wire models (the filling surfaces switch) or in non-spatial objects (as e.g. a wall). The face back side remains lighted the same way as the face front side was lighted. For this reason the wall remains dark, if its back side is lighted.

 

To light objects, normals are used. They are vectors that are perpendicular to the face and pointing out of the object. The normals determine how an object face will be lighted depending on the face orientation toward the light source. If the inner space of an object must be used, it is not sufficient to change only the object faces visible side. The normals remain unchanged, the object faces are lighted from the outer side and the object surface appears as if its faces were transparent, being lighted from the outer side. For this purpose it is more convenient to use the object inversion command, which provides both the face orientation change and the normal direction change.