The RenderMan Interface - Paul Bourke

Coordinate System
”shader”
”current”
string
Description
The coordinate system in which the shader was defined. This
is the ”object” coordinate system when the shader is defined.
The coordinate system in which the shading calculations are
being performed. This is implementation-dependent, but is
usually either the ”camera” or ”world” coordinate system.
A named coordinate system established using RiCoordinateSystem.
Table 11.2: Point coordinate systems.
vector R;
normal Nf;
The initialization value may be any scalar or point-like expression. If a scalar expression is
used, the value is promoted to a point (or vector or normal) by duplicating its value into
each component.
Point, vector, and normal constants default to be in the ”current” coordinate system. Points,
vectors, and normals can be specified in any known coordinate system with:
point [space] (x,y,z)
vector [space] (x,y,z)
normal [space] (x,y,z)
where the space is a string literal containing the name of the coordinate system. For example,
point ”world” (0,0,0)
defines a point at the position (0,0,0) in world coordinates. This point is implicitly transformed
to the ”current” coordinate system. And similarly,
vector ”object” (0,0,1)
defines a vector pointing toward the +z axis in ”object” space, which is implicitly transformed
into the equivalent vector expressed in the ”current” coordinate system. Points,
vectors, and normals passed through the RenderMan Interface are interpreted to be in
”shader” or ”object” space, depending on whether the variable was set using a shader command
or a geometric primitive command, respectively. All points, vectors, and normals
passed through the RenderMan Interface are transformed to the ”current” coordinate system
before shader execution begins.
11.4 Transformation Matrices
The Shading Language has a matrix type that represents the transformation matrix required
to transform points and vectors from one coordinate system and another. Matrices are
represented internally by 16 floats (a 4 × 4 homogeneous transformation matrix).
116