In this document, shading includes the entire process of computing the color of a point on a surface. The shading process requires the specification of light sources, surface material properties, and volume or atmospheric effects. 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 three 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.

• 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.

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 that solid; otherwise, the exterior shader of the spawning primitive is used. Whenever an incident ray intersects a surface, the surface shader attached to that geometric primitive is invoked to control the spawning of new rays and to determine the color and intensity of the incoming or incident ray from the color and intensity of the outgoing rays and the material properties of the surface. Finally, whenever a ray is cast to a light source, the light source shader associated with that light source is evaluated to determine the color and intensity of the light emitted. The shader evaluation pipeline is illustrated in Figure 9.2.

Figure 9.1 The ray tracing paradigm

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Figure 9.2 Shader evaluation pipeline

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This description of the shading process in terms of ray tracing is done because ray tracing provides a good metaphor for describing the optics of image formation and the properties of physical materials. However, the Shading Language is designed to work with any rendering algorithm, including scanline and z-buffer renderers, as well as radiosity programs.

The Shading Language is also used to program two other processes:

• Displacement and transformation shaders. These shaders change the position and normals of points on the surface. Displacements are used to place bumps on surfaces. Transformations are used to bend and twist objects, as well as to specify special camera projections.

• Imager shader. Imager shaders are used to program pixel operations that are done before the image is quantized and output.