   How to judge two vector direction
  given two vector that point the same direction or total opposition.How to know they the same or the opposition?use dot product ,cos(0)= cos(180);   
  Use the dot product for 2 vectors. If they are going in the same general direction, the dot product should be positive, else the dot product should be negative.   ;  cos(0) = 1cos(180) = -1if you normalize the 2 vectors and then dot them, you will get 1 if they point exactly the same direction, or -1 if they point exactly the opposite direction.Also, as Endar said, if they point in the same hemisphere, the value will be > 0, and if they point in opposite hemispheres, the value will be < 0.   