The RenderMan Interface - Paul Bourke

This example integrates over a hemisphere centered at the point on the surface with its
north pole in the direction of the normal. Since the integral extends only over the upper
hemisphere, it is not necessary to use the customary max(0,Ln.Nn) to exclude lights that
are locally occluded by the surface element.
The illuminate and solar statements are inverses of the illuminance statement. They control
the casting of light in different directions. The point variable L corresponding to a particular
light direction is available inside this block. This vector points outward from the light
source. The color variable Cl corresponds to the color in this direction and should be set.
Like the illuminance statements, illuminate and solar statements cannot be nested.
The illuminate statement is used to specify light cast by local light sources. The arguments
to the illuminate statement specify a three-dimensional solid cone. The general forms are:
illuminate( position ) stmt
illuminate( position, axis, angle ) stmt
The first form specifies that light is cast in all directions. The second form specifies that
light is cast only inside the given cone. The length of L inside an illuminate statement is
equal to the distance between the light source and the surface currently being shaded.
The solar statement is used to specify light cast by distant light sources. The arguments to
the solar statement specify a three-dimensional cone. Light is cast from distant directions
inside this cone. Since this cone specifies only directions, its apex need not be given. The
general forms of the solar statement are:
solar( ) stmt
solar( axis, angle ) stmt
The first form specifies that light is being cast from all points at infinity (e.g., an illumination
map). The second form specifies that light is being cast from only directions inside a
cone.
An example of the solar statement is the specification of a distant light source:
solar( D, 0 )
Cl = intensity * lightcolor;
This defines a light source at infinity that sends light in the direction D. Since the angle of
the cone is 0, all rays from this light are parallel.
An example of the illuminate statement is the specification of a standard point light source:
illuminate( P )
Cl = (intensity * lightcolor) / (L.L)
This defines a light source at position P that casts light in all directions. The 1/L.L term
indicates an inverse square law fall off.
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