The RenderMan Interface - Paul Bourke

Section 12
SHADER EXECUTION ENVIRONMENT
When a shader is attached to a geometric primitive it inherits a set of varying variables
that completely defines the environment in the neighborhood of the surface element being
shaded. These state variables are predefined and should not be declared in a Shading
Language program. It is the responsibility of the rendering program to properly initialize
these variables before a shader is executed.
All the predefined variables which are available to each type of shader are shown in Table
12.1, Predefined Surface Shader Variables, through Table 12.5, Predefined Imager Shader Variables.
In these tables the top section describes state variables that can be read by the shader.
The bottom section describes the state variables that are the expected results of the shader.
By convention, capitalized variables refer to points and colors, while lower-case variables
are floats. If the first character of a variable’s name is a C or O, the variable refers to a
color or opacity, respectively. Colors and opacities are normally attached to light rays; this
is indicated by appending a lowercase subscript. A lowercase d prefixing a variable name
indicates a derivative.
All predefined variables are considered to be read-only, with the exception of the result
variables, which are read-write in the appropriate shader type, and Cs, Os, N, s and t,
which are read-write in any shader in which they are readable. Vectors are not normalized
by default.
12.1 Surface Shaders
The geometry is characterized by the surface position P which is a function of the surface
parameters (u,v). The rate of change of surface parameters are available as (du,dv). The
parametric derivatives of position are also available as dPdu and dPdv. The actual change
in position between points on the surface is given by P(u+du)=P+dPdu*du and P(v+dv)=P+dPdv*dv.
The calculated geometric normal perpendicular to the tangent plane at P is Ng. The shading
normal N is initially set equal to Ng unless normals are explicitly provided with the
geometric primitive. The shading normal can be changed freely; the geometric normal
is automatically recalculated by the renderer when P changes, and cannot be changed by
shaders. The texture coordinates are available as (s,t). Figure 12.1 shows a small surface
element and its associated state.
The optical environment in the neighborhood of a surface is described by the incident ray
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