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Tim Wendt (tim.wendt@daytonoh.ncr.com) wrote:
: What is the test to determine if a vector is is front or behind the partition
: vector (2D only ala Doom) when building a BSP tree?  

: I've read a few FAQs and articles but the only test I've seen in detail is the
: test for 3D polygons (which is the dot product between the normal of the plane
: a the polygon edge.)  One of the texts did say it was a simple dot product
: between the two 2D vectors.  I pulled out the old linear algebra textbook but
: I'm still confused.  The dot product was described as ||u|| ||v|| cos (angle
: between the vectors).  How do I find the angle between the two vectors?  Is
: this the easiest approach?  Thanks much!

: Tim

Well, the point usually is to calculate the cosine of the angle between the
vectors and not the other way round.
By definition: a = (a1, a2, a3) ; b = (b1, b2, b3)
Then a dot b = (a1*b1 + a2*b2 + a3*b3) = |a|*|b|*cos(angle)
You can get a and b using 3 3d points, ie
a = x3-x2; b = x1-x2; (where xn's are 3d-vectors)

That's about it.

--
ttfn
Frans van den Bergh (s9506131@cosine.cs.up.ac.za)
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