OpenGL Programming Guide

Chapter 9Texture Mapping

Advanced Features

The Texture Matrix Stack

Just as your model coordinates are transformed by a matrix before being rendered, texture coordinates are multiplied by a 4 × 4 matrix before any texture mapping occurs. By default, the texture matrix is the identity, so the texture coordinates you explicitly assign or those that are automatically generated remain unchanged. By modifying the texture matrix while redrawing an object, however, you can make the texture slide over the surface, rotate around it, stretch and shrink, or any combination of the three. In fact, since the texture matrix is a completely general 4 × 4 matrix, effects such as perspective can be achieved.

When the four texture coordinates (s, t, r, q) are multiplied by the texture matrix, the resulting vector (s' t' r' q') is interpreted as homogeneous texture coordinates. In other words, the texture map is indexed by s'/q' and t'/q' . (Remember that r'/q' is currently ignored, but it might be used in the future.) The texture matrix is actually the top matrix on a stack whose depth must be at least two. All the standard matrix-manipulation commands such as glPushMatrix(), glPopMatrix(), glMultMatrix(), and glRotate() can be applied to the texture matrix. To modify the current texture matrix, you need to set the matrix mode to GL_TEXTURE, as follows:

glMatrixMode(GL_TEXTURE); /* enter texture matrix mode */
glRotate(...); 
/* ... other matrix manipulations ... */
glMatrixMode(GL_MODELVIEW); /* back to modelview mode */

OpenGL Programming Guide