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 Features
Collision Determination
The basic
goal of any collision determination system is to find out if two units
have collided. For the time being, we'll represent all collisions as two-entity
collisions. We'll cover compound collisions (collisions involving three
or more entities) next month. Once a collision is found, each entity needs
to know about the collision in order to make appropriate movement decisions.
Basic collision determination for most strategy games consists of treating
all units as spheres (circles in 2D) and doing a simple spherical collision
check. Whether or not such a system is sufficient depends on the specific
requirements of a game. Even if a game implements more complex collision
- such as oriented bounding boxes or even low-level polygon to polygon
intersection tests - maintaining a total bounding sphere for quick potential
collision elimination will usually improve performance.
There are three distinct entity types to take into account when designing
a collision system: the single unit, a group of units, and a formation
(see Figure 2 below). Each of these types can work well using a single
sphere for quick collision culling (elimination of further collision checks).
In fact, the single unit simply uses a sphere for all of its collision
checking. The group and the formation require a bit more work, though.
Figure 2. Collision entities.
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For
a group of units, the acceptable minimum is to check each unit in the
group for a collision. By itself, this method will allow a non-grouped
unit to sit happily in the middle of your group. For our purposes, we
can overlook this discrepancy, because formations will provide the additional,
more rigid collision checking. Groups also have the ability to be reshaped
at any time to accommodate tight quarters, so it's actually a good idea
to keep group collision checking as simple as possible.
A formation requires the same checks as a group, but these check must
further ensure that there are no internal collisions within the formation.
If a formation has space between some of its units, it is unacceptable
for a non-formed unit to occupy that space. Additionally, formations generally
don't have the option to reshape or break. However, it's probably a good
idea to implement some game rules that allow formations to break and reform
on the other side of an obstacle if no path around the obstacle can be
found.
For our system, we'll also keep track of the timing of the collision.
Immediate collisions represent collisions currently existing between two
objects. Future collisions will happen at a specified point in the future
(assuming neither of the objects changes its predicted movement behavior).
In all cases, immediate collisions have a higher resolution priority than
future collisions. We'll also track the state of each collision as unresolved,
resolving, or resolved.
 Discrete
vs. Continuous Simulation
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