Graphics Reference
In-Depth Information
Using Shaders
Now that we have given an overview of fragment shaders, we will
demonstrate how to implement several fixed-function techniques using
shaders. The fixed-function pipeline in OpenGL ES l.x and desktop
OpenGL provided APIs to perform multitexturing, fog, alpha test, and
user clip planes. Although none of these techniques is provided explicitly
in OpenGL ES 3.0, all of them can still be implemented using shaders.
This section reviews each of these fixed-function processes and provides
example fragment shaders that demonstrate each technique.
We start with multitexturing, which is a very common operation in
fragment shaders used for combining multiple texture maps. For example,
a technique that has been used in many games, such as Quake III, is to store
precomputed lighting from radiosity calculations in a texture map. That
map is then combined with the base texture map in the fragment shader to
represent static lighting. Many other examples of using multiple textures
exist, some of which we cover in Chapter 14, “Advanced Programming
with OpenGL ES 3.0.” For example, often a texture map is used to store
a specular exponent and mask to attenuate and mask specular lighting
contributions. Many games also use normal maps, which are textures that
store normal information at a higher level of detail than per-vertex normals
so that lighting can be computed in the fragment shader.
The point of mentioning this information here is to highlight that you have
now learned about all of the parts of the API that are needed to accomplish
multitexturing techniques. In Chapter 9, “Texturing,” you learned how to
load textures on various texture units and fetch from them in the fragment
shader. Combining the textures in various ways in the fragment shader is
simply a matter of employing the many operators and built-in functions
that exist in the shading language. Using these techniques, you can easily
achieve all of the effects that were made possible with the fixed-function
fragment pipeline in previous versions of OpenGL ES.
An example of using multiple textures is provided in the
Chapter_10/
MultiTexture
example, which renders the image in Figure 10-3.
This example loads a base texture map and light map texture and
combines them in the fragment shader on a single quad. The fragment
shader for the sample program is provided in Example 10-1.
