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Convert notes into blockquotes

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James Gregory 2013-12-31 11:06:31 +11:00
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@ -40,9 +40,13 @@ vectors that lie within the surface. This means that we can generate the
screenspace normals we need by taking the cross product of two adjacent
polygon edges, as shown in Figure 61.4.
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![](images/i.jpg) *In fact, we can cull with only one-third the work needed to generate a full cross product; because we're interested only in the sign of the z component of the normal, we can skip entirely calculating the x and y components. The only caveat is to be careful that neither edge you choose is zero-length and that the edges aren't collinear, because the dot product can't produce a normal in those cases.*
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> ![](images/i.jpg)
> In fact, we can cull with only one-third the work needed to generate a
> full cross product; because we're interested only in the sign of the z
> component of the normal, we can skip entirely calculating the x and y
> components. The only caveat is to be careful that neither edge you
> choose is zero-length and that the edges aren't collinear, because the
> dot product can't produce a normal in those cases.
![**Figure 61.4**  *How the cross product of polygon edge vectors
generates a polygon normal.*](images/61-04.jpg)