CG Basics - An Introduction to Computer Graphics for Artists - page 24

Fig. 2.18 A cube with all normals displayed

2.4.1.5 Normals

Vectors connected to object components for the purpose of identifying their orientation
are called normals . Normals are displayed as starting from the center of the compo-
nent they are attached to, but their direction is defi ned in the same manner as any
other vector. Normals are automatically generated for you, but some programs
allow you to modify them. The origin of a normal is the center of your polygon.
Edges and vertices inherit normals from the face they belong to. For faces that share
an edge, normals are averaged between the values for either face.
Normals are used to calculate how a ray of light affects your object. Does it skim
off to the side? Or does it hit it straight on, causing a dazzling brightness? This
affects the rendering calculation dramatically, so it is important for these to be set
correctly. By default, normals are set to be perpendicular to the faces they belong to;
with edge and vertex normals borrowed from the faces they are a part of (Fig. 2.18 ).
An incorrect normal will cause light to pass through a polygon, like a ball through
a straight pipe, without returning a color value to the camera (Fig. 2.19 ). The result is
that back-facing polygons, polygons that should face away from the camera, instead
face towards it and are rendered instead of polygons in front that should block them.
The effect is like looking at the inside of a mold used to cast another object.

2.4.1.6 Nodes

Nodes are not geometry, but containers for geometry and other things. In that sense,
a node can be considered part of the geometry it is connected to. Nodes are used
to organize a fi le, or to separate the parts of an object so they may be treated differ-
ently. For instance, if you had an animation that included three objects, and you