## 3 Results and Discussion
### Answering RQ3 to RQ5
This is because sensor-based tracking technically requires intensive tracking algorithms [11] and advanced deep learning methods such as CNN [161] to boost the tracking performance and reduce latency, thus achieving the real-time feature. Finally, a model-based tracking method is applied when a 3D CAD model of the tracked object's parts are available to extract, analyze and determine the pose and position for later recognition and tracking [8,73,139]. To conclude this subsection, similar to the selection of tracking method for a manufacturing context, when considering AR-based solutions for quality sector, different use cases and conditions of the working environment could be taken into account and evaluated to choose the most appropriate tracking method. This subsection provides solid references in terms of tracking methods for further utilization to develop AR-based solutions for the quality sector. * Software development platform Currently, there are lots of available Software Development Kits (SDKs) supporting AR application development in a fast and robust approach. The most well-known SDKs are Vtoria, ARToolKit, Vtoria, Metaio and Hololens. These SDKs provide detailed documentation and functionalities to develop AR applications while not requiring high coding skills and experience. One or more software development platforms can be utilized to develop an AR system. Table 16 lists in detail the different software development platforms mentioned across the selected articles in this SLR, which provides good references for the development of AR-based solutions in the quality sector later. The development process of software is not always specified in articles. Thus, it is difficult to demonstrate this information about which programming language is mostly used by data collection. Not considering all articles, the most mentioned and utilized languages are C# and C++, shown in Table 16. In addition, libraries of functions such as OpenCV (Open-Source Computer Vision), Matlab and OpenGL (for 2D/3D graphics rendering) are also applied for the development of AR solutions. Mid/low programming languages and libraries of functions allow an AR application's development from scratch, providing high flexibility. However, to be highly skilled in programming is required for developing those systems. The use of SDK was mentioned across selected articles. SDKs' utilization has increased recently because new devices on the market usually provide them (Hololens, HHD). However, in terms of developing a high functionality AR application, SDKs are not sufficient. More extensive software built with a game engine or mid/low-level programming language must be integrated to achieve a full functionality AR application. Unity3D and Unreal are the two most popular game engines utilized for AR application development. A game engine is technically a user-friendly platform allowing the building of AR applications with minimum programming knowledge, but skilled AR developers are required to use them. In order to create the 3D AR contents, other supporting development platforms are mentioned, such as Blender, Autodesk Inventor, Rhinoceros, SolidWorks, Catia and 3 ds-Max. Unity 3D engine, developed by Unity Technologies in 2005, is a commercial cross-platform game creation system, which supports C# scripting together with 3D/2D graphics and animations. This development platform consists of a graphical user interface (GUI) and five fundamental windows, which are ProjectWindow, HierarchyWindow, Inspection-Window, SceneView and Toolbar. It is one of the most effective platforms applied for AR applications to assist human-machine interaction (HCI). Unity is compatible with the Vuforia SDK plug-in which enhances the 3D object tracking and detection in AR applications [8]. In addition, Unity provides pre-defined functionalities to develop interactive 3D content for real scenarios.