The key word constant used in the Declare statement is called a class specifier . The next section introduces class specifiers . Class Specifiers Class specifiers identify how a primitive variable will be interpolated over the uv parameter space of a surface primitive. One value remains constant over the entire surface primitive. One value remains constant for each uv patch segment of the surface primitive.
Bilinear interpolation is used for interpolation between the four values.
Values are interpolated between each vertex in the surface primitive. The basis function of the surface is used for interpolation between vertices. A variable's class and the surface primitive determine how many samples must be included when a primitive variable is attached to a surface primitive. The following two tables identify the correct number of values that must be included for each combination of class specifier and surface primitive. This is NOT how uwrap , vwrap , and wrap are used in the RenderMan interface.