Graphics Reference
In-Depth Information
from a shadow map. Similarly, rendering reflections with projective
texturing requires that you transform the position into the projective
view space of the reflection camera. You would do the same thing we
have done here, but substitute the light matrices for the reflection camera
matrices. Projective texturing is a very powerful tool in creating advanced
effects, and you should now understand the basics of how to use it.
The next rendering technique we cover is using a 3D texture for noise. In
Chapter 9, “Texturing,” we introduced the basics of 3D textures. As you will
recall, a 3D texture is essentially a stack of 2D texture slices representing
a 3D volume. 3D textures have many possible uses, one of which is the
representation of noise. In this section, we show an example of using a 3D
volume of noise to create a wispy fog effect. This example builds on the linear
fog example from Chapter 10, “Fragment Shaders.” The example is found in
Chapter_14/Noise3D
, the results of which are shown in Figure 14-10.
Figure 14-10
Fog Distorted by 3D Noise Texture
The application of noise is a very common technique that plays a
role in a large variety of 3D effects. The OpenGL Shading Language
(
not
OpenGL ES Shading Language) included functions for computing
noise in one, two, three, and four dimensions. These functions return a
pseudorandom continuous noise value that is repeatable based on the
input value. Unfortunately, the functions are expensive to implement.
Most programmable GPUs did not implement noise functions natively in
hardware, which meant the noise computations had to be implemented
using shader instructions (or worse, in software on the CPU). It takes
a lot of shader instructions to implement these noise functions, so the
performance was too slow to be used in most real-time fragment shaders.
Recognizing this problem, the OpenGL ES working group decided to drop
noise from the OpenGL ES Shading Language (although vendors are still
free to expose it through an extension).
