Game creation tools that minimize required resources and knowledge to use them have transformed the practice of learning and prototyping game design, especially in hobbyist and indie development contexts. However, little is known about how the different programming models found underlying these tools affect their expressiveness and usability. A recently proposed programming model allows creators to author the logic of the entire game using a single “game behaviour tree” with tile-grid rewrite rules at the leaves. We contribute to this body of knowledge by studying this recently proposed programming model. We have used it to make clones of popular games as case studies, from which we extracted a number of design patterns. To gain formative information about usability, we also conducted a small user study with people who have varying levels of experience authoring games with other tools. We find game behaviour trees capable of expressing a wide variety of 2D turn-based games. Study participants are quick to grasp the underlying concepts, but further research is needed to understand discrepancies with user intuitions that may arise from their familiarity with different programming models.

Research in procedural content generation (PCG) has recently heralded two major methodologies: machine learning (PCGML) and declarative programming. The former shows promise by automating the specification of quality criteria through latent patterns in data, while the latter offers significant advantages for authorial control. In this paper we propose the use of probabilistic logic as a unifying framework that combines the benefits of both methodologies. We propose a Bayesian formalization of content generators as probability distributions and show how common PCG tasks map naturally to operations on the distribution. Further, through a series of experiments with maze generation, we demonstrate how probabilistic logic semantics allows us to leverage the authorial control of declarative programming and the flexibility of learning from data.