The Index Texture Extension

OpenGL 1.1 offers three texture environments: GL_MODULATE, GL_BLEND, and GL_ REPLACE (see "New Ways of Using Texture Environments"). The texture environments allow a program to use the values in the texture map to change (or replace) the color that would be painted without texturing.

The index texture extension, SGI_index_texture, supports two additional texture modes. It also allows you to use texturing in color index mode (which is not allowed in standard OpenGL). The extension provides these features:

• INDEX internal formats for texture images

• REPLACE, ADD, and MODULATE texture environments

You may specify a texture palette (the color table associated with a texture object) with a color index internal format. A texture palette is applied only to texture images if one of these conditions is met:

• The texture palette has a color index internal format and the context is a color index rendering context.

• The texture palette has a component internal format and the context is an RGBA rendering context.

If mipmapping is used, then all mipmap levels must be specified with the same internal format.

The code sample below illustrates how the extension is used. A complete example is provided in Appendix A, "Code Example for Index Texture Extension".

GLubyte texture[16] = {
     0xff, 0xff, 0x00, 0x00,
     0xff, 0xff, 0x00, 0x00,
     0x00, 0x00, 0xff, 0xff,
     0x00, 0x00, 0xff, 0xff,
};

glTexImage2d(GL_TEXTURE_2D, 0, GL_COLOR_INDEX8_EXT,
       4, 4, 0, GL_COLOR_INDEX, GL_UNSIGNED_BYTE, texture);

Note that in this example, GL_COLOR_INDEX8_EXT is used as a texture internal format. In addition, just as specified in "The Paletted Texture Extension," texels are loaded using the color index pixel transfer path when the texture internal format is a color index format.

When the extension is available, a new base internal format, COLOR_INDEX, is available for which three texture environments are defined as follows:

Texture Functions for Color Index Extension

Base Internal Format	REPLACE	MODULATE	DECAL	BLEND	ADD
...					undefined
COLOR_INDEX	Iv = It	Iv = If * It	undefined	undefined	Iv = If + It
...					undefined