Automated Tweaking of Levels for Casual Creation of Mobile Games Edward J. Powley, Swen Gaudl, Simon Colton, Mark J. Nelson, Rob Saunders and Michael Cook The MetaMakers Institute, Games Academy, Falmouth University, UK metamakers.falmouth.ac.uk Abstract extensive tuning and playtesting of game levels, which can be tedious. To combat this, we have endeavoured to make Casual creator software lowers the technical barriers to cre- tuning and testing of levels an entertaining experience, and ative expression. Although casual creation of visual art, mu- to automate parts of the process where appropriate. We have sic, text and game levels is well established, few casual cre- ators allow users to create entire games: despite many tools sought to make this automation enjoyable, e.g. by allowing that aim to make the process easier, development of a game the user to watch the tuning process as it happens, rather from start to finish still requires no small amount of tech- than merely presenting them with a progress bar. nical ability. We are developing an iOS app called Gamika The work here represents a proof of concept for on- which seeks to change this, mainly through the use of AI and device casual game creation, including the invention of new computational creativity techniques to remove some of the game mechanics, without programming. Our contribution technical and creative burden from the user. In this paper we is the whole pipeline and the AI functionality embedded in describe an initial step towards this: a Gamika component Gamika which supports casual co-creation, rather than a fo- that takes a level designed by the user, and tweaks its pa- cus on studying and optimising one particular aspect. rameters to improve its playability. The AI techniques used In this paper we discuss our approach to provide support are straightforward: rule-based automated playtesting, ran- dom search, and decision trees learning. While there is room for level design through automated parameter adjustment. for improvement, as a proof of concept for this kind of mixed- We begin by surveying existing mobile apps which allow initiative creation, the system already shows great promise. users to create games or game content. We then describe the Gamika system, and present as a running example a partic- ular game that has been designed within the app by one of Introduction the present authors. The design of new levels for this game Liapis, Yannakakis, and Togelius (2014) describe automated presented us with a challenge, as levels often require fine- game design as a “killer application” for Computational tuning of parameters to yield a satisfying play experience. Creativity research, as it combines many disparate creative We present the results of our initial experimentation into disciplines into a cohesive whole. Casual games, i.e. games automated tweaking of parameters, using random search, with relatively shallow learning curves and relatively sim- learned classifiers (decision trees) and a simple automated ple game mechanics which demand only a relatively short playtester. We also discuss the nature of the search space, time investment for an engaging return, have broadened the and the difficulties we have encountered in trying to apply popularity of gaming as a pastime and have contributed to a more intelligent search methods. We conclude by looking shift in the overall demographic of those who play games forwards to our goals and plans for the Gamika project. regularly (Juul 2009). Similarly, casual creator software (Compton and Mateas 2015), which emphasises the enjoy- Game creation on mobile devices ment of creation over the necessity of completing a task, has The term casual creator (Compton and Mateas 2015) de- broadened the popularity of digital creation and begun to scribes a piece of software which allows users to quickly and blur the boundary between consumers and producers of cre- easily create artefacts such as musical compositions, filtered ative artefacts. However, the creative act of game design is digital photos, abstract artworks, stories, and poems. Some not yet fully supported by casual creation on mobile devices. games include features such as character creators or level A desire to further democratise game design has led us to editors which could be classed as casual creators. However, the building of the Gamika iOS application. We believe that there is a lack of such software that gives aspiring game de- this is the first tool which enables entirely new casual games signers fine-grained control over all aspects of a game, par- (containing multiple levels with novel aesthetics, game me- ticularly the underlying rules and mechanics of the game. chanics and player interactions) to be designed entirely on There are many software packages for novice game de- a mobile phone without programming. Gamika empowers signers to experiment and learn the craft, such as Scratch designers to experiment with new game mechanics and un- (scratch.mit.edu), Stencyl (stencyl.com) and AgentCubes tried gameplay. This naturally increases the requirement for (agentcubesonline.com). They can then move on to professional-grade tools such as GameMaker: Studio Gamika (yoyogames.com), Unity (unity3d.com) or Unreal (un- Gamika is an iOS application developed in the Swift pro- realengine.com). Many of these allow the designer to deploy gramming language and using Apple’s SpriteKit 2D game their game to mobile devices, however all require that the de- development library. SpriteKit includes a physics engine velopment takes place on a desktop or laptop computer. Also based on Box2D, which forms the basis of game object in- all require programming (using visual programming envi- teractions in Gamika. A game in Gamika is a list of levels, ronments and/or text-based programming languages) for all each of which comprises: 284 numerical parameter settings but the most trivial games. This significantly raises the bar- which control several aspects of gameplay; an optional vec- rier to entry for non-technical users. Practically, all casual tor graphic to be used as a controller; and an optional text games on the market today are made by professionals or string explaining the rules of the game to the player. By skilled hobbyists, and are created using these tools. analysing classic arcade games and by attempting to design Apps which enable creation of games or game assets on novel games within the app, we have grown parameter set the mobile device itself fall into three main categories: organically until satisfied that they admit game levels that are sufficiently diverse, interesting and engaging. • Apps which require programming skills. Many, such A game level features objects of several classes, with each as Scratch Jr (scratchjr.org) and HopScotch (gethop- object corresponding to a rigid-body in the physics simula- scotch.com), are primarily tools for teaching program- tion. The parameters that define a game level are split into ming skills to children, with game creation being a vehi- several categories: cle rather than an end in itself. These apps give the user a • Image. The level can optionally use an abstract art wide range of creative expression, but assume a degree of image generated by the ELVIRA evolutionary art sys- technical proficiency (or a desire to learn technical skills). tem (Colton, Cook, and Raad 2011) as an in-game object • Apps which enable only skinning existing game tem- or as a background image. plates, i.e. customisation of a game’s visual appearance • Look. The background image can be changed, as can the and audio, but little or no ability to change the gameplay. size, shape, colour and sprite images for the game objects. Examples here include Coda Game (codarica.com) and • Lights. Spotlight effects can optionally be added to the Playr (playr.us) which offer some degree of creative free- game, and the appearance of game objects under lighting dom, but only with respect to a game’s aesthetic qualities. (e.g. normal mapping) can be adjusted. • Apps which enable the authoring of levels for an exist- • Spawning. Objects of each class can be spawned at a ing game. Examples include Createrria 2 (incuvo.com) given rate, from a given position or range of positions, and Sketch Nation (sketchnation.com). Minecraft: Pocket optionally with an upper limit for the number of objects Edition (minecraft.net) can also be included in this cate- of each class allowed on screen at one time. gory, with advanced features such as redstone circuitry • Movement. The physical properties of the objects, such blurring the lines between level design and programming. as restitution, mass and linear damping, can be changed. Such apps empower creative expression, but limited to the Force-fields can be added, to attract or repel objects with characters, rules and game worlds provided. respect to a particular point, or apply a force in a given Gamika is our attempt to fill a gap in the market: a casual direction. Joints such as pins, springs and sliders can be creator app which empowers users to create entire games on added to objects. their mobile device, in particular enabling the invention of • Collisions. For each pair of object class, several actions entirely new game mechanics. One of our ultimate aims is can occur when objects collide. For example they may for Gamika to be able to automatically generate entire ca- bounce, pass through, stick, be destroyed, or change class. sual games, so that users can delegate as much (or as little) When objects stick together they form clusters; it is possi- of the creative responsibility as they like. Several authors ble to set a maximum cluster size so that larger clusters are (Lim and Harrell 2014; Khalifi and Fayek 2015; Nielsen et destroyed. Walls can be added to the edges of the screen.