Game Development Reference
In-Depth Information
Figure 1.3. Projection of one vector onto another.
1.2.3 Dot Product
The dot product of two vectors a and b is defined as
a
·
b =
a
b
cos θ,
(1.1)
where θ is the angle between a and b .
For two vectors using a standard Euclidean basis, this can be represented as
a · b = a x b x + a y b y + a z b z .
There are two uses of this that are of particular interest to game physics de-
velopers. First of all, it can be used to do simple tests of the angle between two
vectors. If a
·
b > 0,then θ<π/ 2;if a
·
b < 0,then θ>π/ 2;andif a
·
b =0,
then θ = π/ 2. In the latter case, we also say that the two vectors are orthogonal .
The other main use of the dot product is for projecting one vector onto another.
If we have two vectors a and b , we can break a into two pieces a || and a such
that a || + a = a and a || points along the same direction as, or is parallel to, b
(see Figure 1.3 ). The vector a ||
is also known as the scalar projection of a onto b .
cos θ ,whichwe
can see from Figure 1.3 is the length of the projection of a onto b . The projected
vector itself can be computed as
From Equation (1.1), if
b
=1,then a
·
b is simply
a
a || =( a · b ) b .
The remaining, or orthogonal portion of a can be computed as
a = a
a || .
1.2.4 Cross Product
The cross product of two vectors a and b is defined as
a
×
b =( a y b z
a z b y ,a z b x
a x b z ,a x b y
a y b x ) .
 
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