Normal vectors are assumed to be of unit length, therefore, the equation should equal 1.0. The normal vector is transformed into eye coordinates using the inverse transpose of the current viewing matrix. It is then stored for use by the lighting equation.
New normals can occur as frequently as every new graphics position and as infrequently as desired.
The normal subroutine takes exactly the same parameter as the n3f subroutine. The only difference is that normal can be used in display lists and n3f cannot. The inverse transpose of this matrix is used to transform normals from model coordinates into eye coordinates. The projection matrix transforms eye coordinates into screen coordinates. The figure entitled Lighting Calculations shows how the various matrices operate on display list coordinates and normal vectors to produce screen coordinates and colors.