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2.4.1 Figure 5(a)---Dot Product Test

This first test eliminates, using a simple, economical formula, up to half of the surfaces in an enclosure. The following criterion is used to invalidate the intersection (here, this means that the surface is not a viable candidate for intersection):

Equation (6) involves only two multiplications, one addition and one comparison. Note that, it is not required to calculate the intersection point to eliminate a surface according to the above criterion.