We will now describe a set of interfaces and classes that allow us to incorporate texture into our material models. Recall that the materials in Chapter 9 are all based on various parameters that describe their characteristics (diffuse reflectance, glossiness, etc.). Because real-world material properties typically vary over surfaces, it is necessary to be able to represent this spatial variation. In pbrt, the texture abstractions serve this purpose. They are defined in a way that separates the pattern generation methods from the reflection model implementations, making it easy to combine them in arbitrary ways, thereby making it easier to create a wide variety of appearances.
In pbrt, a texture is a fairly general concept: it is a function that maps points in some domain (e.g., a surface’s parametric space or object space) to values in some other domain (e.g., spectra or the real numbers). A variety of implementations of texture classes are available in the system. For example, pbrt has textures that represent zero-dimensional functions that return a constant in order to accommodate surfaces that have the same parameter value everywhere. Image map textures are two-dimensional functions of parameter values that use a 2D array of pixel values to compute texture values at particular points (they are described in Section 10.4). There are even texture functions that compute values based on the values computed by other texture functions.
Textures may be a source of high-frequency variation in the final image. Figure 10.1 shows an image with severe aliasing due to a texture. Although the visual impact of this aliasing can be reduced with the nonuniform sampling techniques from Chapter 7, a better solution to this problem is to implement texture functions that adjust their frequency content based on the rate at which they are being sampled. For many texture functions, computing a reasonable approximation to the frequency content and antialiasing in this manner aren’t too difficult and are substantially more efficient than reducing aliasing by increasing the image sampling rate.
The first section of this chapter will discuss the problem of texture aliasing and general approaches to solving it. We will then describe the basic texture interface and illustrate its use with a few simple texture functions. Throughout the remainder of the chapter, we will present a variety of more complex texture implementations, demonstrating the use of a number of different texture antialiasing techniques along the way.