Texturing - Praise for OpenGL ES 30 Programming Guide - page 244

The three quads are rendered using the following setup code for the
texture wrap modes:
// Draw left quad with repeat wrap mode
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
glUniformlf(userData->offsetLoc, -0.7f);
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, indices);
// Draw middle quad with clamp to edge wrap mode
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S,
GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T,
GL_CLAMP_TO_EDGE);
glUniformlf(userData->offsetLoc, 0.0f);
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, indices);
// Draw right quad with mirrored repeat
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S,
GL_MIRRORED_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T,
GL_MIRRORED_REPEAT);
glUniformlf(userData->offsetLoc, 0.7f);
glDrawElements GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, indices);
In Figure 9-5, the quad on the far left is rendered using GL_REPEAT
mode. In this mode, the texture simply repeats outside of the range
[0, 1], resulting in a tiling pattern of the image. The quad in the center
is rendered with GL_CLAMP_TO_EDGE mode. As you can see, when the
texture coordinates go outside the range [0, 1], the texture coordinates are
clamped to sample from the edge of the texture. The quad on the right is
rendered with GL_MIRRORED_REPEAT , which mirrors and then repeats the
image when the texture coordinates are outside the range [0, 1].
Texture Swizzles
Texture swizzles control how color components in the input R, RG,
RGB, or RGBA texture map to components when fetched from in the
shader. For example, an application might want a GL_RED texture to
map to (0, 0, 0, R) or (R, R, R, 1) as opposed to the default mapping
of (R, 0, 0, 1). The texture component that each R, G, B, and A value
maps to can be independently controlled using texture swizzles set
using glTexParameter[i|f][v] . The component to control is set by
using GL_TEXTURE_SWIZZLE_R , GL_TEXTURE_SWIZZLE_G , GL_TEXTURE_
SWIZZLE_B , or GL_TEXTURE_SWIZZLE_A . The texture value that will be the