OpenGL Programming Guide

Chapter 9Texture Mapping

An Overview and an Example

Steps in Texture Mapping

To use texture mapping, you perform these steps:

1. Specify the texture.
   
   

2. Indicate how the texture is to be applied to each pixel.
   
   

3. Enable texture mapping.
   
   

4. Draw the scene, supplying both texture and geometric coordinates.

Keep in mind that texture mapping works only in RGB mode; the results in color-index mode are undefined.

Specify the Texture

In the simplest case, the texture is a single image. A texture is usually thought of as being two-dimensional, like most images, but it can also be one-dimensional. The data describing a texture can consist of one, two, three, or four elements per texel, representing anything from a modulation constant to an (R, G, B, A) quadruple.

Using an advanced technique, called mipmapping, you can specify a single texture at many different resolutions; this allows you to avoid mapping a full-resolution image of a brick wall on a wall that's so far away from the viewer that it appears on the screen as a single pixel, for example. Also, the specification of the map can include boundary values to use when the object's texture coordinates get outside the valid range. Boundary values allow you to paste together multiple texture maps smoothly, thereby increasing the effective size of the largest available texture.

Indicate How the Texture Is to Be Applied to Each Pixel

You can choose any of three possible functions for computing the final RGBA value from the fragment color and the texture-image data. One possibility is to simply use the texture color as the final color; this is the decal mode, in which the texture is painted on top of the fragment, just as a decal would be applied. Another method is to use the texture to modulate, or scale, the fragment's color; this technique is useful for combining the effects of lighting with texturing. Finally, a constant color can be blended with that of the fragment, based on the texture value.

Enable Texture Mapping

You need to enable texturing before drawing your scene. Texturing is enabled or disabled using glEnable() or glDisable() with the symbolic constant GL_TEXTURE_1D or GL_TEXTURE_2D for one- or two-dimensional texturing, respectively. (If both are enabled, *2D is preferred.)

Draw the Scene, Supplying Both Texture and Geometric Coordinates

You need to indicate how the texture should be aligned relative to the fragments to which it's to be applied before it's "glued on." That is, you need to specify both texture coordinates and geometric coordinates as you specify the objects in your scene. For a two-dimensional texture map, for example, the texture coordinates range from 0.0 to 1.0 in both directions, but the coordinates of the items being textured can be anything. For the brick-wall example, if the wall is square and meant to represent one copy of the texture, the code would probably assign texture coordinates (0, 0), (1, 0), (1, 1), and (0, 1) to the four corners of the wall. If the wall is large, you might want to paint several copies of the texture map on it. If you do so, the texture map must be designed so that the bricks on the left edge blend nicely with the bricks on the right edge, and similarly for the bricks on the top and those on the bottom.

You must also indicate how texture coordinates outside the range [0.0,1.0] should be treated. Do the textures wrap to cover the object, or are they clamped to a boundary value?

OpenGL Programming Guide