# Extended Reality (XR) Engines for Developing Gamified Apps and Serious Games: A Scoping Review
## 2 Materials and Methods
### Study Selection and Eligibility  
At the conclusion of the search process, 553 relevant results were found. The results were as follows: 16 from IEEE Xplore Digital Library, 89 from ScienceDirect (Elsevier), 68 from SpringerLink, 174 from Wiley Online Library, and 206 from Google Scholar. Following the removing of duplicates and articles unrelated to the research, we relied on 107 works for the first analysis. After screening these records based on their title and abstract, we performed a full-text reading of 92 works, which were then assessed for eligibility based on a full-text reading analysis. We excluded 52 of these works since they did not pass the eligibility assessment per our established set of exclusion criteria, which are listed as follows:  
1. Studies on XR implementation in areas other than education
2. Studies on XR without gamification techniques
3. Studies on serious games without XR  
The remaining 40 records were included in the review (see Figure 1).  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Area** & **Keywords** & **Related Concepts** \\ \hline \multirow{4}{*}{Extended reality} & Gamification and virtual reality & Gamified apps \\  & Gamification and augmented reality & Serious games \\  & Gamification and extended reality & Mobile apps \\  & Serious games with virtual reality & Educational apps \\  & Serious games with augmented reality & Educational games \\  & Serious games with extended reality & Educational games \\ \hline \hline \end{tabular}
\end{table}
Table 2: Search keywords and related concepts.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Research Question ID** & **Question** & **Motivation** \\ \hline RQ1 & Which XR engines are reported in the literature? & Identify the main XR engines reported in the literature. \\ RQ2 & What are the main features of these XR game engines? & Identify the main features of XR engines. \\ RQ3 & What serious game attributes are supported in these XR game engines? & Identify the main game attributes supported by XR engines. \\  & What learning activities are supported in each serious game developed with the reviewed XR game engines? & Identify learning activities that support XR in serious games. \\ \hline \hline \end{tabular}
\end{table}
Table 1: Research questions and motivations.  
We downloaded the entire file of each of the 40 works collected for the review to ensure a proper analysis. We considered papers published between 2019 and 2023, with an upward trend in publishing. We found that most of the records (n = 24) were published in 2022 (see Figure 2). Figure 2 presents the distribution by year of primary studies about XR. As can be observed, XR is an emerging technology. The growth in the research about XR is very important, and it is a tendency that can be observed between the years 2019 and 2023. It is also important to mention that this study encompasses early 2023.  
Figure 3 displays the geographical distribution of the primary studies. The top 10 countries with major research on XR are located in Europe, the majority of them are from Germany, followed by the United States of America (USA), as can be observed in Figure 3. In Asia, China is the leader on studies about XR, AR, and virtual environments. Latin America is represented in the geographical distribution of XR studies by emerging countries like Mexico and Brazil. This geographical distribution demonstrates the global interest in technological development using innovative technologies on XR.  
Figure 1: Selection process for studies—PRISMA flow diagram.  
The primary studies were mainly retrieved from Wiley Online Library, followed by ScienceDirect and SpringerLink. However, records from IEEE and Google Scholar were less frequent.