# Extended Reality (XR) Engines for Developing Gamified Apps and Serious Games: A Scoping Review
## 1 Introduction  
Extended Reality (XR), gamification, and serious games are all concepts that involve the integration of technology into interactive experiences. XR is an emerging technology that enables seamless interaction between virtual and real environments. XR includes Virtual Reality (VR) and Augmented Reality (AR). XR technologies have multiple applications, such as site design with virtual walkthroughs, rapid prototyping, dynamic operation simulation, detailed design coordination, and marketing presentations to customers [1]. In the educational domain, XR is an innovative factor in the learning process that allows learners to interact with virtual objects and models within virtual environments to build knowledge and generate a meaningful learning experience while simultaneously reducing cognitive load [2, 3].  
Gamification incorporates game design elements into non-game contexts, such as business, marketing, work, and education [4]. On the other hand, serious games are digital games that exist for the purpose of learning. Game-based learning, simulated environments, and serious games can promote knowledge acquisition, skill development, and awareness of specific phenomena [5]. Serious gaming uses games to fill needs other than entertainment [6, 7]. XR techniques can be exploited in gaming to build knowledge in virtual environments and multimedia content. Gamification for education and training uses game elements in non-game settings, such as apps and systems [8, 9], to help learnerslearn. In this sense, XR-based serious games can improve learning experiences with the help of virtual models from the real world [10].  
Current studies exploring gamification and serious games have reported favorable results on the effectiveness of using game attributes and learning activities for learning purposes in software apps. Given these varied findings, it becomes important to identify and study how XR can impact the development of serious games and gamified apps.  
This review of the XR literature is fueled by the advancement in display technology and computing, which has resulted in new devices to overlay digital representations of the real world, integrating all aspects of it into virtual environments. These reviews address the impact of XR-enriched mobile apps in medicine, education, and manufacturing. In their work, [PERSON] et al. [11] provided an overview of emerging multimodal technologies by summarizing the latest advances in multimodal interaction technology based on head-mounted display (HMD) XR systems. On the other hand, [PERSON] et al. [12] analyzed concerns common to XR, such as safety and hardware variability, in a survey on the drawbacks and opportunities of XR technologies. Furthermore, other studies [13; 14; 15; 16] have explored how XR technologies, such as realistic 3D visualization and touch-free interfaces, impact learning in medical apps.  
In this scoping review, our primary objective is to describe the current state of XR engines used for developing gamified apps and serious games. To this end, we analyze four key aspects: (1) existing XR game engines, (2) their main features, (3) supported serious game attributes, and (4) supported learning activities. To ensure comprehensiveness, we examined a body of scientific literature published between 2019 and March 2023, which is when XR-based educational environments started to emerge. The remainder of this research is structured as follows: Section 2 describes the methods and materials used to perform the review. In Section 3, we summarize our findings, whereas in Section 4 we discuss such findings with respect to our research questions. In Section 5, we discuss current challenges in XR with respect to our findings, and in Section 6, our conclusions are summarized.