Metagame Autobalancing for Competitive Multiplayer Games
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Metagame Autobalancing for Competitive Multiplayer Games Daniel Hernandez∗, Charles Takashi Toyin Gbadamosiy, James Goodmanz and James Alfred Walker∗, Senior Member, IEEE ∗Department of Computer Science, University of York, UK. fdh1135, [email protected] yDepartment of Computer Science, Queen Mary University of London, UK. fc.t.t.gbadamosi, [email protected] Abstract—Automated game balancing has often focused on make judgements about the state of the meta-game and provide single-agent scenarios. In this paper we present a tool for designers with insight into future adjustments, such as [2]. balancing multi-player games during game design. Our approach There are, however, several problems with this approach. requires a designer to construct an intuitive graphical represen- tation of their meta-game target, representing the relative scores Analytics can only discover balance issues in content that that high-level strategies (or decks, or character types) should is live, and by that point balance issues may have already experience. This permits more sophisticated balance targets to negatively impacted the player experience: this is a reactive be defined beyond a simple requirement of equal win chances. We approach and not a preventive one. Worse, games which do then find a parameterization of the game that meets this target not have access to large volumes of player data - less popular using simulation-based optimization to minimize the distance to the target graph. We show the capabilities of this tool on examples games - cannot use this technique at all. inheriting from Rock-Paper-Scissors, and on a more complex Furthermore, the process of data analytics itself is not asymmetric fighting game. typically within the skill-set of game designers. It is common for studios that run multiplayer games to hire data scientists I. INTRODUCTION to fill this need. This, in combination with the trial and Achieving game balance is a primary concern of the game error nature of the balance process, results in increased costs, designer, but balancing games is a largely manual process of becoming as a bottleneck for the development of new content. trial and error. This is especially problematic in asymmetric The importance of meta-game balance and the aforemen- multiplayer games where perceived fairness has a drastic tioned issues motivate alternate approaches to game balance. impact on the player experience. Changes to individual game This paper presents one such alternative - an automated elements or rules can have an impact on the balance between simulation-based approach to meta-game balance of mul- high-level strategies that depend on these, but this impact is tiplayer games. Our approach allows designers to directly unknown before changes are made and can only be guessed specify a meta-game balance state and have the game param- at by designers through experience and intuition. We term eters that would create the desired meta-game be discovered this balance between emergent high-level strategies the ‘Meta- automatically by a group of agents. game balance’. While in-house tools can be built for the adjustment and authoring of individual game elements. There are no tools for balancing and adjusting -game elements. II. PRELIMINARY NOTATION Game balancing takes a lot of time and resources, with Cursive lowercase letters represent scalars (n). Bold low- current trends indicating a systematic increase in the cost n ercase, vectors (π 2 R ). Bold uppercase, matrices (A 2 of game development [1]. It is reliant on human intuition n×n R ). and expert knowledge to estimate how changes in the game mechanics affect emergent gameplay. Human play testing as A. Game parameterization part of this process is time consuming, requiring many human Every video game presents a (potentially very large) number testers for long play-sessions, which grow longer with more of values that characterize the game experience, which we complex games. In short, human play testing does not scale. shall refer to as game parameters. These values can be An alternative approach to the discovery of meta-game numerical (such as gravitational strength, movement speed, changes that arise from game changes is through data ana- health) or categorical (whether friendly fire is activated, to lytics. Large scale multiplayer titles that have access to large which team a character belongs). As a designer, choosing quantities of player data can use a variety of techniques to a good set of parameters can be the difference between an excellent game and an unplayable one. We let E denote Thank you, Ozan Vardal, Nick Ballou and Sebastian Berns for your θ generous help in coding Workshop Warfare. This work was funded by a game environment, parameterized by an n-sized parameter the EPSRC Centre for Doctoral Training in Intelligent Games and Game vector θ 2 fΠi≤nΘig, where fΠi≤nΘig represents the joint Intelligence (IGGI) EP/L015846/1 and Digital Creativity Labs. parameter space, and Θi the individual space of possible values for the ith parameter in θ. 978-1-7281-4533-4/20/$31.00 c 2020 Crown B. Meta-games D. Empirical Response Graphs What a meta-game is can mean different things to dif- A directed weighted graph of v 2 N+ nodes can be ferent players. For example in deck-building games such as denoted by an adjacency matrix G 2 Rv×v. Each row i in Hearthstone, the ‘meta’ is usually interpreted to indicate which G signifies the weight of all of the directed edges stemming + decks are currently popular or especially strong; while in EVE from node i. Thus, gi;j 2 R corresponds to the weight of Online an important part of the ‘meta’ is player diplomatic the edge connecting node i to node j, where 1 ≤ i; j ≤ v. alliances, as well as which ship types are good against which Given an evaluation matrix Aπ computed from a set of others. See [3] for a good discussion of this notation. strategies (or agents) π, let its response graph [6] represent In this work we define a meta-game as a set of high-level the dynamics [4] between agents in π. That is, a representation strategies that are abstracted from the atomic game actions. of which strategies (or agents) perform favourably against Reasoning about a game involves thinking about how each which other strategies in π. In a competitive scenario, a individual action will affect the outcome of the game. In response graph shows which strategies win against which contrast, a meta-game considers more general terms, such as others. As a graph, each strategy i is represented by a node. An how an aggressive strategy will fare against a defensive one. edge connecting node i to node j indicates that i dominates In meta-games, high level strategies are considered instead j. The weight of the edge is a quantitative metric of how of primitive game actions. Take a card game like Poker. favourably strategy i performs against j. Figure 1a shows a Reasoning about a Poker meta-game can mean reasoning about response graph for the game of Rock-Paper-Scissors (RPS). how bluff oriented strategies will deal against risk adverse A response graph can be readily computed from an evalua- strategies. tion matrix. Each row i in an evaluation matrix A denotes The level of abstraction represented in a meta-game is de- which strategies i both wins and loses against, the former fined by the meta-game designer, and the same game can allow being indicated by positive entries and the latter by negative for a multitude of different levels of abstraction. For instance, ones. Therefore, generating a response graph G from an in the digital card game of Hearthstone, meta-strategies may evaluation matrix A is as simple as setting all negative entries 1 −2 correspond to playing different deck types, or whether to play of A to 0 such that, for instance, A = 2 −1 , becomes 1 0 more offensively or defensively within the same deck. A game G = ( 2 0 ). designer may want to ensure that no one deck type dominates, but be happy that a particular deck can only win if played E. Graph distance offensively. There is a rich literature on measuring distance between C. Empirical win-rate matrix meta-games graphs [7]. We concern ourselves here with a basic case. We are interested in measuring the distance between two graphs An interesting meta-game definition that has recently re- v×v which share the same number of nodes, G1; G2 2 R , ceived attention in multiagent system analysis [4] defines a and differ only in the weight of the edges connecting nodes. normal form game over a population of agents π, such that Because graphs can be represented as matrices, we look at the action set of each player corresponds to choosing an differences between matrices. We denote the distance between agent π 2 π from the population to play the game for 1 2 1 2 i two graphs G and G by d(G ; G ) 2 R. Equation them. How these agents were created is not relevant to us; (1) represents the average absolute edge difference and (2) these agents could use hand-crafted heuristics, be trained with represents the mean squared difference (MSE). reinforcement learning, evolutionary algorithms or any other P 1 2 P 1 2 2 method. i;j jgij − gijj i;j(gij − gij) n×n (1) (2) Let Wπ 2 R denote an empirical win-rate matrix. n n The entry wi;j for i; j 2 fng represents the win-rate of many head-to-head matches of policy πi when playing against policy Preliminary results showed no empirical difference between πj for a given game.