Game Development Reference
Fig 2.3 A pirate facing (1,0) and the
vector to the treasure.
In computer graphics, a positive value for an angle always indicates an
anticlockwise turn. An anticlockwise turn in this case would have the pirate facing
away from the treasure. When calculating the angle between vectors using the
dot product, the angle is always positive. Therefore, you need another method to
determine the turn direction. This is where the cross product comes into play.
The cross product of two vectors results in another vector. The resulting vector
is perpendicular (at 90°) to both the initial vectors. This sounds odd working in
2D as a vector at right angles to two vectors in 2D would come right out of the
page. For this reason, the cross product is only defined for 3D. The formula to
work out the cross product is a little obtuse and requires further knowledge of
vector mathematics, but we will try to make it as painless as possible.
The cross product of two vectors, v and w , denoted v × w is shown in
Equation (2.3) :
v w vw vw
The equation is defined in terms of standard 3D unit vectors. These vectors are
three unit length vectors orientated in the directions of the x , y , and z axes. If
you examine Equation (2.3) you will notice that there are three parts added
together. The first part determines the value of the x coordinate of the vector,
as the unit vector (1,0,0) only has a value for the x coordinate. The same occurs
in the other two parts for the y and z coordinates.