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January 17, 2019

Feedback loops in StarCraft

Feedback loops in StarCraft

I thought it would be interesting to identify feedback loops in StarCraft. Feedback occurs when a system takes outputs back as inputs. In each loop the input is either reinforced (positive feedback loop) or diminished (negative feedback loop).

Positive feedback loops

Phrases like "The rich get richer and the poor get poorer" or "When you are ahead, get more ahead" talk about positive feedback loops. With positive feedback loops success will lead to more success, and failures will lead to even more likely future failures.

Positive feedback loops are inherent to RTS games. They destabilize the game, put more emphasis on the early game, and lead to "snowballing" or "death spiral". However, they also help to finish the game faster without drawing it out. Few examples:

  • Good fight → you keep more army units alive → better future fights
  • Good fight → map control → better economy → better army → better fights
  • Bad fight →  lose your production/expansions → small army → worse fights
  • Resources → more workers & expansions → more resources
  • Abathur's Biomass and Dehaka's Essence are also strong positive feedback loops
    • kill enemy units → gain biomass/essence → kill enemy units faster
  • WarCraft 3 leveling
    • more experience → power advantage → better fight → more experience
    • (I think the fact that only heroes level up helps keeping this feedback loop in check)

Most of these examples are ingrained in the RTS genre. You would be surprised if you weren't ahead of your opponent after a decisive fight. Because positive feedback loops tend to destabilize the game, they are often accompanied by negative feedback loops.


How Games Use Feedback Loops | Game Maker’s Toolkit

Negative feedback loops

Negative feedback loops weaken the input, they introduce some downside to success or an upside to failure. Instead of snowballing they make it easier for a player to catch up. They tend to stabilize the game and make it longer. Negative feedback loops also reduce the emphasis on the early game introduced by positive feedback loops. If overused, they can make small victories inconsequential and drag out the game.

  • Various forms of defender's advantage – reinforcement travel time, Zerg's creep, static defenses, high ground, chokes...  (→ easier defending)
  • More bases → more spread out defenses → more open to attacks
  • More units → higher mechanical requirement to control them (especially significant in Brood War with the selection limit of twelve units)
  • More units → higher susceptibility to area of effect damage & abilities
  • More units → lower efficiency in chokepoints
  • Higher income → necessity to invest into more production first
  • Good fight → supply mechanics make it more expensive to build more army over the current supply limit, while it's less expensive to just rebuild.
  • Han and Horner's scrap mechanic:
    • lose units → collect scraps → rebuild few units back
  • WarCraft III's upkeep mechanic provides a strong negative feedback against building a big army.

In competitive multiplayer defender's advantage is the most impactful and gameplay stabilizing. That's why mechanics that reduce it are particularly problematic. Warpgate mechanic is the best example, and eventually the whole Protoss design had to bend around it. ZvZ matchups in both StarCraft II and Brood War are typical for being very aggressive thanks to limited defender's advantage. Creep benefits both players, chokes and high ground are less useful, and reinforcements are very fast.

LIMITS

Together with negative feedback loops, limits on certain things can help to stabilize the game by letting players to catch up.

  • Supply cap
  • Upgrade cap
  • Limited number of active workers on mineral patches & lower efficiency on the third worker
  • Limited resources can create a positive feedback if you starve out your opponent, but in split map scenarios it can create a negative feedback as the leading player will mine out faster.
  • Cooldown on Kerrigan's Assimilation Aura makes something that could be a strong positive feedback loop into only a small bonus

These limits were created intentionally. However, discrete negative feedback loops can degenerate into a limit if downsides are too strong.

Wrap up

Various feedback loops and their combinations are very important for shaping desired gameplay. They create a gameplay that's more engaging and with interesting choices. Uncontrolled positive loops can cause a lot of balancing problems—for example in XCOM with a death spiral on one side, and a lack of challenge on the other side. Negative feedback loops tend to stabilize the game. StarCraft II has a good amount of both. Varying strength of feedback loops in different matchups helps to make those matchups feel unique.


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