(with thanks to Slakr)
Most people understand that games restrict a player’s actions. In soccer you are not permitted to handle the ball, for example, and in Halo you may only carry two weapons at once. The ways by which that a game restricts the player are called the game’s rules.
Some rules penalise or restrict, but others enable. In video games, some rules even allow players to do things that they could not ordinarily do. There are three different types of rule that operate in a game world, and they are constants, constraints, and conditions.
A constant is a rule that a game developer cannot change, and applies across all games. As a developer, you might try to overcome, fudge or get around constants but you will never be able to ignore them. Constants come from outside the game, from the physical ability of the player, the limitations of the platform and the psychology of play itself.
A board game usually allows a player to easily survey the whole board all the time, but a video game restricts a player’s perspective because that world is viewed through a screen. Often the player has to move either his doll, his camera or both in order to perceive another part of the world. That’s an example of the lensing constant in action.
A joypad has many buttons. However the player only has two fingers and one thumb on each hand that they can usefully use to activate buttons. Hand position factors restrict the complexity with which a game can reasonably be controlled, and is an example of the physical constant. All games are bound by several major constants.
A constraint is a rule that applies throughout a game mode. A constraint enables or restricts play in certain ways, but it does so universally and usually without variation. Constraints let the player know the boundaries of the game world, and the player develops an instinctive feel for them. They frame the actions that the player can take, and so the game world becomes a functioning self-enclosed system.
In many sports, constraints are more or less in line with actual physics. In soccer, the ball properties are constraints, for example, as is the use of a grass pitch. In most board games the limitation of turns is a constraint that globally restricts the entire game, for example, as is the set of available cards and pieces with which the game is played.
Video games get to have the most fun with constraints. In the virtual world, physical constraints like gravity and time can be mutable. The variability of them is a part of why actions in video games are often exhilarating, and why gamers sometimes think that games are better than life. In life you are restricted by your body, but in Pilotwings you can fly. That’s inherently powerful.
Players can sometimes affect constraints, depending on the game. When the player buys upgrades to his car in Forza, for example, he permanently alters the acceleration and steering of his car. He has changed the constraints, and so races will play out differently than previous attempts.
Constraints also play a role in AI behaviours. Enemies often have default or randomised behaviour patterns that they follow when not directly combatting against the player. The ability to recognise those patterns allows the player to play with a sense of strategy. Once he understands a constraint, it often becomes an advantage rather than a hindrance.
Conditions are rules which are triggered by player actions. The game (whether through referees or the game engine itself) monitors triggers, and when they are tripped the condition changes the game world.
Death and damage are common examples of conditions. In many games the player’s doll has a health rating, and when impacted by weapons from game characters, their health reduces according to a condition. If health reduces below a specific value (usually zero) then another condition invokes: the doll is dead, so the game world changes and the player’s position is reset.
Games use conditions to close open loops, whether as a part of the game system itself, or by permitting another player to take action. Some conditions are penalties, such as handballs in soccer or losing all of your health in Call of Duty. Others are win conditions like scoring goals or defeating Bowser.
Just as improvements to the game doll can affect constraints, so too they can affect conditions. If I acquire a tougher set of armour for my doll, then it will be more resistant to damage, and if I extend my actions by gaining a new power, sometimes I can avoid a condition altogether.
AI also responds to conditions. When I sneak up on an enemy in Metal Gear Solid, he is behaving according to his programmed constraints (wandering around, standing still etc). If I trip into his area of visibility, however, a condition changes and suddenly he is chasing after me.