Graphics Reference
In-Depth Information
One of the most fundamental operations used in rendering 3D graphics
is the application of textures to a surface. Textures allow for the
representation of additional detail not available just from the geometry of
a mesh. Textures in OpenGL ES 3.0 come in several forms: 2D textures, 2D
texture arrays, 3D textures, and cubemap textures.
Textures are typically applied to a surface by using texture coordinates,
which can be thought of as indices into texture array data. The following
sections introduce the different texture types in OpenGL ES and explain
how they are loaded and accessed.
A 2D texture is the most basic and common form of texture in OpenGL
ES. A 2D texture is—as you might guess—a two-dimensional array of
image data. The individual data elements of a texture are known as
texels
(short for “texture pixels”). Texture image data in OpenGL ES can be
represented in many different basic formats. The basic formats available
for texture data are shown in Table 9-1.
Each texel in the image is specified according to both its basic format
and its data type. Later, we describe in more detail the various data types
that can represent a texel. For now, the important point to understand
is that a 2D texture is a two-dimensional array of image data. When
rendering with a 2D texture, a texture coordinate is used as an index into
the texture image. Generally, a mesh will be authored in a 3D content
authoring program, with each vertex having a texture coordinate.
Texture coordinates for 2D textures are given by a 2D pair of coordinates
(
s
,
t
), sometimes also called (
u
,
v
) coordinates. These coordinates
represent normalized coordinates used to look up a texture map, as
shown in Figure 9-1.
The lower-left corner of the texture image is specified by the
st
-coordinates
(0.0, 0.0). The upper-right corner of the texture image is specified by
the
st
-coordinates (1.0, 1.0). Coordinates outside of the range [0.0, 1.0]
are allowed, and the behavior of texture fetches outside of that range is
defined by the texture wrapping mode (described in the section on texture
filtering and wrapping).
