Chapter 11. Procedural Texture Shaders

The fact that we have a full-featured, high-level programming language to express the processing at each fragment means that we can algorithmically compute a pattern on an object's surface. We can use this new freedom to create a wide variety of rendering effects that wouldn't be possible otherwise.

In the previous chapter, we discussed shaders that achieve their primary effect by reading values from texture memory. This chapter focuses on shaders that do interesting things primarily by means of an algorithm defined by the shader. The results from such a shader are synthesized according to the algorithm rather than being based primarily on precomputed values such as a digitized painting or photograph. This type of shader is sometimes called a PROCEDURAL TEXTURE SHADER, and the process of applying such a shader is called PROCEDURAL TEXTURING. Often the texture coordinate or the object coordinate position at each point on the object is the only piece of information needed to shade the object with a shader that is entirely procedural.

In principle, procedural texture shaders can accomplish many of the same tasks as shaders that access stored textures. In practice, there are times when it is more convenient or feasible to use a procedural texture shader and times when it is more convenient or feasible to use a stored texture shader. When deciding whether to write a procedural texture shader or one that uses stored textures, keep in mind some of the main advantages of procedural texture shaders.

• Textures generated procedurally have very low memory requirements compared with stored textures. The only representation of the texture is in the algorithm defined by the code in the procedural texture shader. This representation is extremely compact compared with the size of stored 2D textures. Typically, it is a couple of orders of magnitude smaller (e.g., a few kilobytes for the code in a procedural shader versus a few hundred kilobytes or more for a high-quality 2D texture). This means procedural texture shaders require far less memory on the graphics accelerator. Procedural texture shaders have an even greater advantage when the desire is to have a 3D (solid) texture applied to an object (a few kilobytes versus tens of megabytes or more for a stored 3D texture).

• Textures generated by procedural texture shaders have no fixed area or resolution. They can be applied to objects of any scale with precise results because they are defined algorithmically rather than with sampled data, as in the case of stored textures. There are no decisions to be made about how to map a 2D image onto a 3D surface patch that is larger or smaller than the texture, and there are no seams or unwanted replication. As your viewpoint gets closer and closer to a surface rendered with a procedural texture shader, you won't see reduced detail or sampling artifacts like you might with a shader that uses a stored texture.

• Procedural texture shaders can be written to parameterize key aspects of the algorithm. These parameters can easily be changed, allowing a single shader to produce an interesting variety of effects. Very little can be done to alter the effect from a stored texture after it has been created.

Some of the disadvantages of using procedural shaders rather than stored textures are as follows.

• Procedural texture shaders require the algorithm to be encoded in a program. Not everyone has the technical skills needed to write such a program, whereas it is fairly straightforward to create a 2D or 3D texture with limited technical skills.

• Performing the algorithm embodied by a procedural texture shader at each location on an object can be a lot slower than accessing a stored texture.

• Procedural texture shaders can have serious aliasing artifacts that can be difficult to overcome. Today's graphics hardware has built-in capabilities for antialiasing stored textures (e.g., filtering methods and mipmaps).

• Because of differences in arithmetic precision and differences in implementations of built-in functions such as noise, procedural texture shaders could produce somewhat different results on different platforms.

The ultimate choice of whether to use a procedural shader or a stored texture shader should be made pragmatically. Things that would be artwork in the real world (paintings, billboards, anything with writing, etc.) are good candidates for rendering with stored textures. Objects that are extremely important to the final "look" of the image (character faces, costumes, important props) can also be rendered with stored textures because this presents the easiest route for an artist to be involved. Things that are relatively unimportant to the final image and yet cover a lot of area are good candidates for rendering with a procedural shader (walls, floors, ground).

Often, a hybrid approach is the right answer. A golf ball might be rendered with a base color, a hand-painted texture map that contains scuff marks, a texture map containing a logo, and a procedurally generated dimple pattern. Stored textures can also control or constrain procedural effects. If our golf ball needs grass stains on certain parts of its surface and it is important to achieve and reproduce just the right look, an artist could paint a gray scale map that would direct the shader to locations where grass smudges should be applied on the surface (for instance, black portions of the grayscale map) and where they should not be applied (white portions of the grayscale map). The shader can read this CONTROL TEXTURE and use it to blend between a grass-smudged representation of the surface and a pristine surface.

All that said, let's turn our attention to a few examples of shaders that are entirely procedural.