Graphics Reference
In-Depth Information
light volume, add a more natural appearance to a procedural texture, and
simulate water waves. Applying a 3D texture is a great way to economize
on performance, yet still achieve high-quality visual effects. It is unlikely
that you can expect handheld devices to compute noise functions in the
fragment shader and have enough performance to run at a high frame
rate. As such, having a precomputed noise volume will be a very valuable
trick to have in your toolkit for creating effects.
The next topic we cover is the generation of procedural textures. Textures
are typically described as a 2D image, a cubemap, or a 3D image. These
images store color or depth values. Built-in functions defined in the
OpenGL ES Shading Language take a texture coordinate, a texture object
referred to as a sampler, and return a color or depth value. Procedural
texturing refers to textures that are described as a procedure instead of
as an image. The procedure describes the algorithm that will generate a
texture color or depth value given a set of inputs.
The following are some of the benefits of procedural textures:
• They provide much more compact representation than a stored texture
image. All you need to store is the code that describes the procedural
texture, which will typically be much smaller in size than a stored image.
• Procedural textures, unlike stored images, have no fixed resolution.
As a consequence, they can be applied to the surface without loss of
detail. Thus we will not see problematic issues such as reduced detail
as we zoom onto a surface that uses a procedural texture. We will,
however, encounter these issues when using a stored texture image
because of its fixed resolution.
The disadvantages of procedural textures are as follows:
• Although the procedural texture might have a smaller footprint
than a stored texture, it might take a lot more cycles to execute the
procedural texture versus doing a lookup in the stored texture. With
procedural textures, you are dealing with instruction bandwidth,
versus memory bandwidth for stored textures. Both the instruction
and memory bandwidth are at a premium on handheld devices, and a
developer must carefully choose which approach to take.
• Procedural textures can lead to serious aliasing artifacts. Although
most of these artifacts can be resolved, they result in additional
instructions to the procedural texture code, which can impact the
performance of a shader.
