The vector from the eye point to the vertex is denoted as ,
normalized to . Since the computation is performed in eye
coordinates, the eye is located at the origin and is equal to the
location of the vertex in eye-space.
The current normal is transformed to eye
coordinates by the inverse transpose of the modelview matrix and
is normalized, becoming . The reflected vector can be
The above equations look intimidating, but they are not as mystifying as they first appear. Equation 7 is the standard equation for computing the reflection vector given a surface normal and incident vector. Most introductory graphics textbooks have a short proof and a good explanation of this equation.9
Because and are normalized, is normalized as well. You can think of as a 3D point on the unit sphere. Our task is to make this 3D point into a 2D texture coordinate. Reducing to two dimensions is accomplished by projecting onto the unit circle in the plane. This is the reason for dividing by .
Because we know is normalized, the result of and must be within the range . But to index into a 2D texture map, we need coordinates in the range. In Equations 8 and 9, the scale by and bias by serve to map values in the range the to the range .
When constructing sphere maps, the reverse mapping from to is often needed. The reverse mapping is:
The image in a sphere map can be thought of as a circle of radius centered at . Plugging into the reverse mapping equations above results in a reflection vector of . OpenGL assumes the eye looks down the negative Z axis. So the vector is reflected straight back at the eye. If we think of a sphere map as what we see when looking directly at a chrome sphere, then dead in the center of the sphere we expect to see our own reflection.
Try plugging coordinates on the edge of the sphere map's circle such as and into the reverse mapping equations. For any point on the circle edge, the result is always . Again, if we think of a sphere map as what we see when looking at a chrome sphere, the silhouette edge of the sphere is seen as an extreme grazing reflection of whatever is directly behind the sphere. If this does not make sense, look back at Figure 66 to convince yourself of this fact. This means that every point on the circle's edge of a properly generated sphere map should the same color.
Finally try plugging coordinates for points outside the sphere map's circle into the reverse mapping equations. Try points such as and . The operand of the square root in the equations for both and become negative. This should not be too surprising because properly normalized reflection vectors are guaranteed to fall within the sphere map's circle.