scott wrote:
>> But now I'm curious... the cross product is only defined for 3D 
>> vectors, but you said you used a lot of cross products?
> 
> Treat your 2D working area as a plane in 3D space (with Z coordinate 
> zero if you like), then you can do cross products, and checking whether 
> the result points "in to" the plane or "out of" the plane can be pretty 
> useful (eg to find out which side of a line a point is on).

Oh, right.

...or you could just take the dot product instead, which requires only 2 
multiplies and one addition as opposed to four multiplies and two 
subtractions...

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