The smooth shading element is a logic switch. If it is on, then the surface color of the individual faces changes gradually between the colors of the faces' individual vertices. If it is off, then the color of one of the vertices will be used as the face color. It cannot be determined which vertex will be used. The OpenGL typically uses different vertex than the DirectX.

 

In objects having rounded surface (sphere, rotational objects) the points of concurrence of two neighboring faces contain, as a rule, the same color (the same diffuse color and the same normal). Thanks to this fact a smooth object surface appearance can be reached, as opposed to the situation with angular objects (cube) where the points of concurrence of the neighboring faces are lighted in a different way (different surface normal). In such case the edges are visible on the object surface even when the smooth shading is on. Nevertheless, in angular objects the smooth shading will manifest itself, too, as a smooth shade change at an uneven object lighting and as a smooth fog change in large objects.

 

The smooth shading can be switched off in each object separately by the object smooth shading command (for an active object). If the smooth shading switch is off, then the smooth shading switch is off in all objects, independently of the position of switches for smooth shading of the individual objects. The default setting of all switches is ON.