Texturing - Praise for OpenGL ES 30 Programming Guide - page 235

This application sets the unpack alignment to 1, meaning that each row
of pixels begins on a byte boundary (in other words, the data is tightly
packed). The full definition for glPixelStorei is given next.

void glPixelStorei (GLenum pname, GLint param )

specifies the pixel storage type to set. The following options
impact how data is unpacked from memory when calling
glTexImage2D , glTexImage3D , glTexSubImage2D , and
glTexSubImage3D :
GL_UNPACK_ROW_LENGTH , GL_UNPACK_IMAGE_HEIGHT ,
GL_UNPACK_SKIP_PIXELS , GL_UNPACK_SKIP_ROWS ,
GL_UNPACK_SKIP_IMAGES , GL_UNPACK_ALIGNMENT

pname

The following options impact how data is packed into memory
when calling glReadPixels :
GL_PACK_ROW_LENGTH , GL_PACK_IMAGE_HEIGHT ,
GL_PACK_SKIP_PIXELS , GL_PACK_SKIP_ROWS ,
GL_PACK_SKIP_IMAGES , GL_PACK_ALIGNMENT

All of these options are described in Table 9-2.

specifies the integer value for the pack or unpack option.

param

The GL_PACK_xxxxx arguments to glPixelStorei do not have any impact
on texture image uploading. The pack options are used by glReadPixels ,
which is described in Chapter 11, “Fragment Operations.” The pack
and unpack options set by glPixelStorei are global state and are not
stored or associated with a texture object. In practice, it is rare to use any
options other than GL_UNPACK_ALIGNMENT for specifying textures. For
completeness, the full list of pixel storage options is provided in Table 9-2.

Returning to the program in Example 9-1, after defining the image data,
a texture object is generated using glGenTextures and then that object
is bound to the GL_TEXTURE_2D target using glBindTexture . Finally, the
image data is loaded into the texture object using glTexImage2D . The
format is set as GL_RGB , which signifies that the image data is composed
of (R, G, B) triplets. The type is set as GL_UNSIGNED_BYTE , which signifies
that each channel of the data is stored in an 8-bit unsigned byte. There
are a number of other options for loading texture data, including the
different formats described in Table 9-1. All of the texture formats are
described later in this chapter in the Texture Formats section.