The RenderMan Interface - Paul Bourke

13.3 Illuminance and Illuminate Statements
The Shading Language contains three new block statement constructs: illuminance, illuminate,
and solar. illuminance is used to control the integration of incoming light over a
hemisphere centered on a surface in a surface shader. illuminate and solar are used to specify
the directional properties of light sources in light shaders. If a light source does not
have an illuminate or solar statement, it is a non-directional ambient light source.
Unlike other control statements, illuminance, illuminate, and solar statements cannot be
nested. However, multiple illuminance, illuminate, or solar statements may be given sequentially
within a single shader.
The illuminance statement controls integration of a procedural reflectance model over the
incoming light. Inside the illuminance block two additional variables are defined: Cl or
light color, and L or light direction. The vector L points towards the light source, but may
not be normalized (see Figure 12.2). The arguments to the illuminance statement specify a
three-dimensional solid cone. The two forms of an illuminance statement are:
illuminance( [string category,] point position )
statements
illuminance( [string category,] point position, vector axis, float angle )
statements
The first form specifies that the integral extends over the entire sphere centered at position.
The second form integrates over a cone whose apex is on the surface at position. This cone
is specified by giving its centerline, and the angle between the side and the axis in radians.
If angle is PI, the cone extends to cover the entire sphere and these forms are the same as
the first form. If angle is PI/2, the cone subtends a hemisphere with the north pole in the
direction axis. Finally, if angle is 0, the cone reduces to an infinitesimally thin ray.
Light shaders can specify “categories” to which they belong by declaring a string parameter
named category (this name has two underscores), whose value is a comma-separated
list of categories into which the light shader falls. When the illuminance statement contains
an optional string parameter category, the loop will only consider lights for which
the category is among those listed in its comma-separated category list. If the illuminance
category begins with a - character, then only lights not containing that category will
be considered.
When the optional category string is omitted, an illuminance loop will execute its body for
every nonambient light source.
A Lambertian shading model is expressed simply using the illuminance statement:
Nn = normalize(N);
illuminance( P, Nn, PI/2 ) {
Ln = normalize(L);
Ci += Cs * Cl * Ln.Nn;
}
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