Yahoo web hosting - OpenGL Super Bible! Page 321 // value for
OpenGL Super Bible! Page 321 // value for vectors whose length may be calculated too close to zero. if(length == 0.0f) length = 1.0f; // Dividing each element by the length will result in a // unit normal vector. vector[0] /= length; vector[1] /= length; vector[2] /= length; } Finding a Normal Figure 9-13 presents another polygon that is not simply lying in one of the axis planes. The normal vector pointing away from this surface is more difficult to guess, so we need an easy way to calculate the normal for any arbitrary polygon in 3D coordinates. Figure 9-13 A nontrivial normal problem You can easily calculate the normal vector for any polygon consisting of at least three points that lie in a single plane (a flat polygon). Figure 9-14 shows three points, P1, P2, and P3, that you can use to define two vectors: vector V1 from P1 to P2, and vector V2 from P1 to P2. Mathematically, two vectors in three-dimensional space define a plane (your original polygon lies in this plane). If you take the cross product of those two vectors (written mathematically as V1 X V2, the resulting vector is perpendicular to that plane (or normal). Figure 9-15 shows the vector V3 derived by taking the cross product of V1 and V2.
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