Graphics Reference
In-Depth Information
The decision whether to use a procedural texture or a stored texture
should be based on careful analysis of the performance and memory
bandwidth requirements of each.
We now look at a simple example that demonstrates procedural textures.
We are familiar with how to use a checkerboard texture image to draw a
checkerboard pattern on an object. We now look at a procedural texture
implementation that renders a checkerboard pattern on an object. The
example we cover is the Checker.pod PVRShaman workspace in
Chapter_14/
PVR_ProceduralTextures
. Examples 14-18 and 14-19 describe the vertex
and fragment shaders that implement the checkerboard texture procedurally.
Example 14-18
Checker Vertex Shader
#version 300 es
uniform mat4 mvp_matrix; // combined model−view
// + projection matrix
in vec4 a_position; // input vertex position
in vec2 a_st; // input texture coordinate
out vec2 v_st; // output texture coordinate
void main()
{
v_st = a_st;
gl_Position = mvp_matrix * a_position;
}
The vertex shader code in Example 14-18 is really straightforward. It
transforms the position using the combined model-view and projection
matrix and passes the texture coordinate (
a_st
) to the fragment shader as
a varying variable (
v_st
).
The fragment shader code in Example 14-19 uses the
v_st
texture coordinate
to draw the texture pattern. Although easy to understand, the fragment
shader might yield poor performance because of the multiple conditional
checks done on values that can differ over fragments being executed in
parallel. This can diminish performance, as the number of vertices or
fragments executed in parallel by the GPU is reduced. Example 14-20 is a
version of the fragment shader that omits any conditional checks.
Figure 14-12 shows the checkerboard image rendered using the fragment
shader in Example 14-17 with
u_frequency
= 10.
