At the end of the last tutorial, we prepared to discover how to create curved surfaces out of 3-D models composed only from a mesh of flat polygons.

Here's our coordinate system again.

Take a good look at the three axes in the picture above. Notice that they are illuminated by a single light, coming directly from the front and pointed down the z axis. The very front tip of the z axis is lit, as is the front side of the x and y axes. This simplest kind of lighting is purely directional. It does not spread out or emanate from a point. It is similar to sunlight. The sun is so far away from us that all its rays run parallel to each other for all practical purposes on earth.

Let's place a white plane at z = 0 so that it is crossed by the x and y axes. The z axis is Normal to the plane. In computer graphics, the important word "normal" is used both as a noun and an adjective. As an adjective, it is used as we just did, to mean that it is perpendicular to the plane (or polygon, which is just a segment of a plane). As a noun, it means the perpendicular line or ray itself. We speak of a "polygon and its normal." If a polygon is flat (if all its vertices are in one plane) it has a single normal that necessarily defines the direction the polygon faces.

The normal determines the effect of light on the surface of the polygon. In the picture above, the direction of the light is precisely the opposite of the direction of the normal of the polygon. This is the equivalent of high noon, and produces the greatest intensity of light on the surface.