Texturing maps a portion of a specified image onto each primitive for which
texturing is enabled.
This mapping is accomplished by using the color of an image at the location
indicated by a fragment's
coordinates to modify the fragment's RGBA color
(r is currently ignored).
Texturing is specified only for RGBA mode;
its use in color index mode is undefined.
The GL provides a means to specify the details of how texturing of a primitive is effected. These details include specification of the image to be texture mapped, the means by which the image is filtered when applied to the primitive, and the function that determines what RGBA value is produced given a fragment color and an image value.