Game Development Reference
In-Depth Information
F IGURE 9.1
The angle that an object is facing.
In two dimensions we can represent an object by its two-dimensional position
and an angle that shows how it is oriented. Just as the position is specified relative to
some fixed origin point, the angle is also given relative to a predetermined direction.
Figure 9.1 illustrates this.
If the object is rotating, its orientation will change over time. Just as velocity is
the first derivative of position (see chapter 2), angular velocity is the first derivative of
orientation.
I will use the word orientation throughout this topic to refer to the direction in
which an object is facing. The word rotation has many meanings in different contexts,
and while most people feel they know what it means, it is one of those terms that can
be a chameleon, causing subtle confusion.
To be specific, I'll try to use rotation only to mean a change in orientation (the
exception being that when everybody and their dog calls something “rotation,” I'll
avoid the temptation to make up a new name). If something is rotated, it is natural to
mean that its orientation has changed.
If an object is spinning, however, I'll use the term angular velocity to mean the
rate of change of orientation.
9.1.1
T HE M ATHEMATICS OF A NGLES
If we do any mathematics with orientations, we need to be careful: many different
orientation values can represent the same orientation. If we measure orientation in
radians (there are 2 π radians in the 360 of a circle), then the orientation of 2 π is the
same as 0. Developers normally set a fixed range of orientation values, say (
π,π ]
(the square bracket indicates that π is included in the range, and the curved bracket
that
π is not). If an orientation falls outside this range, it is brought back into the
range. The mathematical routines that deal with this kind of orientation scalar can
look messy, with lots of adjustments and checks.
An alternative approach is to use vectors to represent orientation. We take a two-
element vector representing the direction in which the object is pointing. The vector
is related to the scalar value according to the equation
cos θ
sin θ
θ
=
[9.1]