# Extended Reality (XR) Engines for Developing Gamified Apps and Serious Games: A Scoping Review
## 4 Discussion
### RQ1. What Game Engines for XR Are Reported in the Literature?
Vuforia is a proprietary XR engine, but it only offers a basic free plan, which users need to register for a Vuforia developer account to use. It also offers a premium plan with a higher cost and more functionalities like production support, among others [78]. * Unreal Engine 5: The most recent version of Unreal Engine includes XR features, a technology that combines real-world experiences with 3D virtual interfaces for wearable technologies and remote controls to interact with machines. The three types of XR are VR, AR, and mixed reality [79; 80]. Unreal Engine is a source-available software released through a source code distribution model that includes arrangements where the source can be viewed, and in some cases modified, but without necessarily meeting the criteria to be called open-source. Also, Unreal Engine has an enterprise program and custom license with premium support, options for private training, and additional procurement options [81]. * Nuke: This engine is a node-based digital compositing and visual effects app. Nuke offers a flexible node-based composite approach with native multi-pass workflows and a powerful integrated 3D environment to meet the needs of developers [82]. Nuke is a proprietary XR engine with some versions having a cost and a free 30-day trial [83]. * HeroMirror: This engine is a standalone AR engine that provides visitors and guests to any space with an immersive and personal AR experience featuring animated, green-screen, or volumetric AR content and characters [84; 85]. HeroMirror uses a proprietary license [86]. * Godot: This engine is a modern game engine that provides features such as 2D and 3D rendering, platform support, and a common development environment. It is free and open source, released under the permissive MIT License [87; 88]. Godot engine is free and open-source software released under the permissive MIT license [89]. * Verge3D: This engine is an instant renderer and toolkit for building interactive 3D web experiences. It allows users to convert content from 3D modeling tools to view in a web browser [90; 91]. Verge3D uses a trialware license, during which Verge3D is a fully functional software that is made available for a limited time or a limited number of executions as a marketing tool [92]. * Amazon Sumerian (AS): Offered by Amazon Web Services (AWS), AS allows users to create and share AR and VR scenes using URLs that are reachable from the most recent Chrome or Firefox versions or other compatible browsers. It supports the most popular virtual reality (VR) headsets available today, such as the now-open-source Google Cardboard and HTC Vive headsets [93; 94]. Amazon Sumerian uses the Apache License Version 2.0 [95]. JavaScript-based engines for developing XR gamified apps and serious games are listed as follows:  
* A-Frame: This engine is an HTML-based web framework for building VR experiences [96]. A-Frame uses the MIT License. The MIT License is a permissive free software license originating at the Massachusetts Institute of Technology (MIT). As a permissive license, it puts only very limited restrictions on reuse and it, therefore, has a high license compatibility [97]. * Three.JS: This engine is a JavaScript library for creating 3D graphics intuitively. Users can create and animate 3D scenes directly on the browser, taking full advantage of WebGL and modern browsers. To create Three.Js WebGL apps, users merely need a text editor and one of the supported browsers to render the results [65; 98; 99]. Three.JS uses the MIT License [100]. * AR.JS: This engine is a lightweight library for augmented reality on the web, which includes features like image tracking, location based AR and marker tracking [99]. AR.JS uses the MIT License [101]. * PlayCanvas: PlayCanvas is a game engine developed using web-based graphics processing with real-time shaders [8]. Contents created by PlayCanvas are readily available and perform excellently [102]. PlayCanvas uses the MIT License [103].