Developing the Mechanics of Plusminus: Designing for Emergence and Control in a Physics-Based Game

Abstract

“Plusminus” is a single-player action-puzzle-adventure game about magnetism, with mechanics developed to promote emergent gameplay. Agency, the ability to make choices, is an important factor in players’ enjoyment of games, and emergent gameplay can facilitate such agency. However, as emergence often arises in unexpected ways, it can also result in players feeling a lack of control and reduced agency. Furthermore, players expect physics in games to act consistently, according to the world around us, but their understanding and expectations of some physical phenomena like magentism may vary and be incomplete. This makes designing mechanics that promote emergence in a physics-based game challenging.

In “Plusminus”, we augmented a physics system with magnetism, and gave players meaningful control over it, to promote emergent gameplay and agency. The thesis contributes an approximate model of magnetic forces that ensures stable simulation, game design flexibility, and still conforms well enough to player expectations. More specifically, 1) to enable the player to turn objects into monopole magnets of positive or negative polarity, we simulate Coulomb forces between charged particles and shells, instead of actual magnetic fields. 2) To ensure stable simulation and allow the player to better anticipate simulation behaviour, each magnet has a maximum “field radius” visualised as a transparent bubble, and two magnets only attract or repel each other if their field bubbles intersect. This allows players and level designers to initiate and prevent interactions in a precise manner, and also prevents objects in separate game areas from affecting each other uncontrollably. 3) To ensure that forces produce stable and controllable interactions regardless of scale, the forces are computed such that the maximum possible accelerations produced between two magnets depends only on the mass ratio between them, as opposed to a combination of masses and magnetic charges. This reduces the number of variables that need balancing, making it easier to achieve a stable simulation. The findings improved player controllability while maintaining opportunities for emergence, in a way that matches player expectations of physics.