Scripting Guide - SAS

Chapter 13 Three-Dimensional Scenes 457
Lighting and Normals
The normal vector is of length 1 and is perpendicular to the vertex. Typically, a vertex is shared between
several polygons and a smooth shaded effect is desired, so the perpendicular at the vertex is calculated as a
(possibly weighted) average of the polygon’s normals. It is important to calculate the “outward” normal for
polygons unless two-sided shading is enabled because only the outer face of the polygon is illuminated.
With a scaled polygon, the normal’s length is not 1 after scaling, and the lighting is wrong.
Normal vectors are set at the same time the surface is constructed, and are specified with the Normal
command. Use the Enable(NORMALIZE) command to have the normals re-normalized to 1 each time the
scene is drawn.
Shading Model
The shading model of a polygon is set using the Shade Model command.
Shade Model (mode)
where mode can be SMOOTH (the default) or FLAT. SMOOTH shading interpolates the colors of the primitive
from one vertex to the next. FLAT mode duplicates the color of one vertex across the entire primitive.
The following script changes the color at each of a triangle’s vertices. The FILL shade model interpolates the
color of the interior automatically.
// make a scene box...holds an OpenGL scene.
scene = SceneBox( 200, 200 );
// put the scene in a window.
NewWindow( "Shade Model", scene );
scene