6.370 Home: Background: Primer: Units, Abilities, & Properties

Units are semi-autonomous state machines that can move around the map and interact with other units, buildings, and resources according to constraints called abilities. Local states that are directly affected by abilities are properties. Units are usually represented on a screen by animated sprites that move where you click and make cool sounds. They can perform some actions independently (e.g. attacking enemy units within range); this is why I call them semi-autonomous. However, initial commands (e.g. moving into attack range) must be given by a human player.

Reaver		Zealot		Probe
Units from the Protoss race in StarCraft.

All units of a type are identical, and a unit's interactions are constrained by the abilities and properties of its type. Abilities can often be enhanced (upgraded) by certain buildings. All types have the property of hit points (analogous to bodily health); when it is first produced, a unit possesses its maximum number of hit points. Combat interactions decrease hit points while repair interactions increase hit points. When a unit's hit points reach zero, the unit is removed from the map. Some special types have the property of energy which slowly recharges over time and is depleted by the use of special abilities (spells).

The properties of a High Templar. Hit points in grey, shield strength in green, spell energy in white.		The psi-storm is a special ability (spell) of the High Templar which costs 75 points of energy a pop.		Dark templar have the ability of remaining invisible to enemy units, such as this unsuspecting marine...

Common units include a worker (e.g. the probe), which gathers resources and constructs buildings, and various combat units (e.g. the scout and the corsair). Additional constraints appear in the form of combat restrictions; one type of unit is restricted to what other types of units it can attack based on terrain as shown below. These restrictions encourage you to deploy your units based on their abilities and form the "destroy" part of the model.

It only takes one probe to build an empire.		Scouts can harass zerglings with impunity, since these land critters can't attack targets in the air.		Since the Corsair can only attack other air targets, it is vulnerable even to a lone Marine on the ground.

A group of related units that can be controlled by a single player in any instance is called a race in StarCraft, a house in the Dune series, and similar names in other games. If each race has different units/abilities, there are more possible combat situations; races are just another layer of abstraction in adding constraints to games. Games in which both races possess the same units and abilities (like chess) are easier to balance than games with different races (e.g. StarCraft has released 9 patches so far to correct the advantages among its three warring races).

The powerful Protoss possess energy shields and psionic abilities.		The adaptable Terrans can repair their units and move their buildings.		The ravenous Zerg spawn quickly and can infect other races.