Introduction

Procedural shading is a proven rendering technique in which a short user-written procedure, called a shader, determines the shading and color variations across each surface. This gives great flexibility and control over the surface appearance.

The widest use of procedural shading is for production animation, where has been effectively used for years in commercials and feature films. These animations are rendered in software, taking from seconds to hours per frame. The resulting frames are typically replayed at 24-30 frames per second.

One important factor in procedural shading is the use of a shading language. A shading language is a high-level special-purpose language for writing shaders. The shading language provides a simple interface for the user to write new shaders. Pixar's RenderMan shading language [Upstill90] is the most popular, and several off-line renderers use it. A shader written in the RenderMan shading language can be used with any of these renderers.

Meanwhile, polygon-per-second performance has been the major focus for most interactive graphics hardware development. Only in the last few years has attention been given to surface shading quality for interactive graphics. Recently, great progress has been made on two fronts toward achieving real-time procedural shading. This course will cover progress on both. First, graphics hardware is capable of performing more of the computations necessary for shading. Second, new languages and machine abstractions have been developed that are better adapted for real-time use.

Interactive graphics machines are complex systems with relatively limited lifetimes. Just as the RenderMan shading language insulates the shading writer from the implementation details of the off-line renderer, a real-time shading system presents a simplified view of the interactive graphics hardware. This is done in two ways. First, we create an abstract model of the hardware. This abstract model gives the user a consistent high-level view of the graphics process that can be mapped onto the machine. Second, a special-purpose language allows a high-level description of each procedure. Given current hardware limitations, languages for real-time shading differ quite a bit from the one presented by RenderMan. Through these two, we can achieve device-independence, so procedures written for one graphics machine have the potential to work on other machines or other generations of the same machine.

1. Procedural techniques

We will show a shader that might be used for a brick wall (Figure 1.1). The wall is to be described as a single polygon with texture coordinates. These texture coordinates are not going to be used for image texturing: they are just a pair of numbers that parameterize the position on the surface.

The shader requires several additional parameters to describe the size, shape and color of the brick. These are the width and height of the brick, the width of the mortar between bricks, and the colors for the mortar and brick (see Figure 1.1). These parameters are used to fold the texture coordinates into brick coordinates for each brick. These are (0,0) at one corner of each brick, and can be used to easily tell whether to use brick or mortar color. A portion of the brick shader is shown in Figure 1.2 (this shader happens to be written in the pfman language, detailed in Chapter 3). In this figure, ss and tt are local variables used to construct the brick coordinates. The simple bricks that result are shown in Figure 1.3a.

Figure 1.1. Size and shape parameters for brick shader

// find row of bricks for this pixel (row is 8-bit integer)
fixed<8,0> row = tt/height;

// offset even rows by half a row
if (row % 2 == 0) ss += width/2;

// wrap texture coordinates to get "brick coordinates"
ss = ss % width;
tt = tt % height;

// pick a color for this pixel, brick or mortar
float surface_color[3] = brick_color;
if (ss < mortar  ||  tt < mortar)
    surface_color = mortar_color;

Figure 1.3. Evolution of a brick shader. a) simple version. b) with indented mortar. c) with added graininess. d) with variations in color from brick to brick. e) with color variations within each brick.

Figure 1.4. Examples of shaders. a+b) two frames of rippling mirror. c) yellow brick road. d+e) wood volume shader.

2. What's to come