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Bayesian Programming and Learning for Multi-Player Video Games ...

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Figure 4.3: Military moves from a StarCraft (PvT) game. The order of the screenshots goes<br />

from left to right <strong>and</strong> from top to bottom.<br />

4.2 RTS AI challenges<br />

In combinatorial game theory terms, competitive StarCraft (1 versus 1) is a zero sum, partialin<strong>for</strong>mation,<br />

deterministic 2 strategy game. StarCraft subsumes Wargus (Warcraft II open source<br />

clone), which has an estimated mean branching factor 1.5.10 3 [Aha et al., 2005] (Chess: ≈ 35,<br />

Go: < 360): Weber [2012] finds a branching factor greater than 1.10 6 <strong>for</strong> StarCraft. In a given<br />

game (with maximum map size) the number of possible positions is roughly of 10 11500 , versus the<br />

Shannon number <strong>for</strong> Chess (10 43 ) [Shannon, 1950]. Also, we believe strategies are much more<br />

balanced in StarCraft than in most other games. Otherwise, how could it be that more than a<br />

decade of professional gaming on StarCraft did not converge to a finite set of fixed (imbalanced)<br />

strategies?<br />

Humans deal with this complexity by abstracting away strategy from low level actions: there<br />

are some highly restrictive constraints on where it is efficient (“optimal”) to place economical<br />

main buildings (Protoss Nexus, Terran Comm<strong>and</strong> Center, Zerg Hatchery/Lair/Hive) close to<br />

minerals spots <strong>and</strong> gas geysers. Low-level micro-management* decisions have high horizontal<br />

continuity <strong>and</strong> humans see these tasks more like playing a musical instrument skillfully. Finally,<br />

2 The only stochasticity is in attacks failures (miss rate) from lower grounds to higher grounds, which is easily<br />

averaged.<br />

63

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