OpenGL Programming Guide

Chapter 9Texture Mapping

Automatic Texture-Coordinate Generation

Environment Mapping

The goal of environment mapping is to render an object as if it were perfectly reflective, so that the colors on its surface are those reflected to the eye from its surroundings. In other words, if you look at a perfectly polished, perfectly reflective silver object in a room, you see the walls, floor, and other objects in the room reflected off the object. The objects whose reflections you see depend on the position of your eye and on the position and surface angles of the silver object. To perform environment mapping, all you have to do is create an appropriate texture map and then have OpenGL generate the texture coordinates for you.

Environment mapping is an approximation based on the assumption that the items in the environment are far away compared to the surfaces of the shiny object - that is, it's a small object in a large room. With this assumption, to find the color of a point on the surface, take the ray from the eye to the surface, and reflect the ray off the surface. The direction of the reflected ray completely determines the color to be painted there. Encoding a color for each direction on a flat texture map is equivalent to putting a polished perfect sphere in the middle of the environment and taking a picture of it with a camera that has a lens with a very long focal length placed far away. Mathematically, the lens has an infinite focal length and the camera is infinitely far away. The encoding therefore covers a circular region of the texture map, tangent to the top, bottom, left, and right edges of the map. The texture values outside the circle make no difference, as they are never accessed in environment mapping.

To make a perfectly correct environment texture map, you need to obtain a large silvered sphere, take a photograph of it in some environment with a camera located an infinite distance away and with a lens that has an infinite focal length, and scan in the photograph. To approximate this result, you can use a scanned-in photograph of an environment taken with an extremely wide-angle (or fish-eye) lens. Figure J-28 shows a photograph taken with a such a lens and the results when that image is used as an environment map.

Once you've created a texture designed for environment mapping, you need to invoke OpenGL's environment-mapping algorithm. This algorithm finds the point on the surface of the sphere with the same tangent surface as the point on the object being rendered, and it paints the object's point with the color visible on the sphere at the corresponding point. Programmatically, all you need to do is change these lines from Example 9-3 :

glTexGenfv(GL_S, GL_OBJECT_LINEAR, sgenparams); 
glEnable(GL_TEXTURE_GEN_S);

to:

glTexGenfv(GL_S, GL_SPHERE_MAP, 0); 
glTexGenfv(GL_T, GL_SPHERE_MAP, 0); 
glEnable(GL_TEXTURE_GEN_S); 
glEnable(GL_TEXTURE_GEN_T);

The GL_SPHERE_MAP constant is what invokes the environment mapping. As shown, you need to specify it for both the s and t directions. You don't have to specify any parameters for the texture-coordinate generation function, however.

OpenGL Programming Guide