Texturing - Praise for OpenGL ES 30 Programming Guide - page 257

shader code then uses the built-in function texture to fetch from the
texture map. The texture built-in function takes the form shown here:

vec4 texture (sampler2D sampler , vec2 coord [ ,
float bias ])

sampler a sampler bound to a texture unit specifying the texture from
which to fetch.
coord a 2D texture coordinate used to fetch from the texture map.
bias

an optional parameter that provides a mipmap bias used for
the texture fetch. This allows the shader to explicitly bias the
computed LOD value used for mipmap selection.

The texture function returns a vec4 representing the color fetched from
the texture map. The way the texture data is mapped into the channels
of this color depends on the base format of the texture. Table 9-11 shows
the way in which texture formats are mapped to vec4 colors. The texture
swizzle (described in the Texture Swizzles section earlier in this chapter)
determines how the values from each of these components map to
components in the shader.

Table 9-11 Mapping of Texture Formats to Colors

Base Format Texel Data Description

GL_RED

(R, 0.0, 0.0, 1.0)

GL_RG

(R, G, 0.0, 1.0)

GL_RGB

(R, G, B, 1.0)

GL_RGBA (R, G, B, A)

GL_LUMINANCE

(L, L, L, 1.0)

GL_LUMINANCE_ALPHA

(L, L, L, A)

GL_ALPHA

(0.0, 0.0, 0.0, A)

In the case of the Simple_Texture2D example, the texture was loaded
as GL_RGB and the texture swizzles were left at the default values, so the
result of the texture fetch will be a vec4 with values (R, G, B, 1.0).