Page 320 OpenGL Super Bible! Polygon Winding: Take
Page 320 OpenGL Super Bible! Polygon Winding: Take special note of the order of the vertices in the jet s triangle. If you viewed this triangle being drawn from the direction in which the normal vector points, the corners would appear counterclockwise around the triangle. This is called polygon winding. By default, the front of a polygon is defined as the side from which the vertices appear to be wound in a counterclockwise fashion. Unit Normals As OpenGL does its magic, all surface normals must eventually be converted to unit normals. A unit normal is just a normal vector that has a length of 1. The normal in Figure 9-12 has a length of 9. You can find the length of any normal by squaring each component, adding them together, and taking the square root. Divide each component of the normal by the length and you get a vector pointed in exactly the same direction, but only 1 unit long. In this case, our new normal vector would be specified as (0,1,0). This is called normalization. Thus, for lighting calculations, all normal vectors must be normalized. Talk about jargon! You can tell OpenGL to convert your normals to unit normals automatically, by enabling normalization with glEnable and a parameter of GL_NORMALIZE: glEnable(GL_NORMALIZE); This does, however, have performance penalties. It s far better to calculate your normals ahead of time as unit normals instead of relying on OpenGL to do this for you. Given any normal vector specified by a coordinate triplet that indicates the direction from the origin, you can easily find the equivalent unit normal vector with the function in Listing 9-2. Listing 9-2 A function that reduces any normal vector to a unit normal vector // Reduces a normal vector specified as a set of three coordinates, // to a unit normal vector of length 1. void ReduceToUnit(float vector[3]) { float length; // Calculate the length of the vector length = (float)sqrt((vector[0]*vector[0]) + (vector[1]*vector[1]) + (vector[2]*vector[2])); // Keep the program from blowing up by providing an acceptable
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