FUNDAMENTALS OF GAME DESIGN, SECOND EDITION
Drains
A drain is a mechanic that determines the consumption of resources—that is, a rule specifying how resources permanently drop out of the game (not to be confused with a converter, which we'll look at next). In a shooter game, the player firing his weapon drains ammunition—that's what makes ammunition, a resource, disappear. Being hit by an enemy shot drains health points. Enemies drain out of the game by dying when their health points reach zero. The most common drain in a construction and management simulation is decay—ongoing damage to the objects the player constructed, which he must spend resources to reverse or repair. (Decay is also sometimes called entropy, although technically entropy refers to increasing disorder rather than loss of resources.) Typical decay mechanics look something like this: "Each section of road includes a numeric attribute indicating its level of decay as a percentage, with 0 (zero) indicating that the section is new and 100 indicating that the section is fully decayed and impassable. Sections of roads begin to decay 3 months after they are constructed, and 3 percent is added to their level of decay every year, plus an additional 1 percent for every 100,000 car trips over the section in the course of that year. When decay reaches or exceeds 100 percent, the road section becomes impassable and it must be replaced."
Because resources are valuable, the player wants to know why a resource disappears from the world and what benefit compensates for its loss. In Monopoly, players get money from the bank by passing "Go"—in effect, for no reason at all—but whenever a player has to give money back to the bank, the game provides a reason: The player owes income tax, incurs a fine, or something similar. Players don't mind getting money for free, but when they have to spend it, they want to know why. Explain your drains.
A converter is a mechanic—and usually an entity, too—that turns one or more resources into another type of resource. In designing a converter, you must specify its production rate and the input-to-output ratio that governs the relationship of resources consumed to resources produced. The Settlers offers several examples. The windmill converts grain into flour at a rate of one to one, so one bag of grain produces one bag of flour. It takes 20 real-time seconds to turn one bag of grain into one bag of flour, so the rate of production of flour works out to three bags per minute. The iron smelter turns one load of ore into one iron bar, consuming one load of coal in the process. However, if fed charcoal instead of coal, the smelter requires three loads of charcoal for each iron bar because charcoal is less efficient than coal.
A trader mechanic governs trades of goods, generally between the player and the game. In a stock-trading game, the trader may be a faceless financial construct; in a role-playing game, the trader usually comes in the form of a blacksmith who trades in swords or something similar.
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Traders cause no change in the game world other than reassignment of ownership. If you trade your old dirk and a gold coin for a new short sword, then in theory the game still contains that dirk, that coin, and that short sword, although all three articles have been assigned to new owners. The trader can, if your game permits it, sell the old dirk to the next player who comes along.
You can also build a bargaining feature into the mechanics of a trader, such that it sells at a high price but can, via a user interface mechanism designed for the purpose, lower its price after a little haggling. Your scheme might make some traders more flexible than others, thereby encouraging players to shop around for the best deal.
