The RenderMan Interface - Paul Bourke

Section 9
OVERVIEW OF THE SHADING PROCESS
In this document, shading includes the entire process of computing the color of a point on a
surface or at a pixel. The shading process requires the specification of light sources, surface
material properties, volume or atmospheric effects, and pixel operations. The interpolation of
color across a primitive, in the sense of Gouraud or Phong interpolation, is not considered
part of the shading process. Each part of the shading process is controlled by giving a
function which mathematically describes that part of the shading process. Throughout this
document the term shader refers to a procedure that implements one of these processes.
There are thus five major types of shaders:
• Light source shaders. Lights may exist alone or be attached to geometric primitives. A
light source shader calculates the color of the light emitted from a point on the light
source towards a point on the surface being illuminated. A light will typically have a
color or spectrum, an intensity, a directional dependency and a fall-off with distance.
• Displacement shaders. These shaders change the position and normals of points on the
surface, in order to place bumps on surfaces.
• Surface shaders. Surface shaders are attached to all geometric primitives and are
used to model the optical properties of materials from which the primitive was constructed.
A surface shader computes the light reflected in a particular direction by
summing over the incoming light and considering the properties of the surface
• Volume shaders. Volume shaders modulate the color of a light ray as it travels through
a volume. Volumes are defined as the insides of solid objects. The atmosphere is the
initial volume defined before any objects are created.
• Imager shader. Imager shaders are used to program pixel operations that are done
before the image is quantized and output.
Conceptually, it is easiest to envision the shading process using ray tracing (see Figure 9.1).
In the classic recursive ray tracer, rays are cast from the eye through a point on the image
plane. Each ray intersects a surface which causes new rays to be spawned and traced
recursively. These rays are typically directed towards the light sources and in the directions
of maximum reflection and transmittance. Whenever a ray travels through space, its color
and intensity is modulated by the volume shader attached to that region of space. If that
region is inside a solid object, the volume shader is the one associated with the interior of
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