# A Review on Mixed Reality: Current Trends, Challenges and Prospects
## 3 Concept of Mixed Reality
### Framework  
After becoming acquainted with the MR components, it is important to be familiar with the types of such systems and their functionality. This section follows two general concepts. The first part introduces the features of MR systems, and the second provides examples of MR frameworks that have \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Software Toolkit** & **Detection or Tracking** & **Platform** & **Development Language** & **Licensing** & **Current Version** \\ \hline AR Toolkit & 2D objects & Android, iOS, Linux, macOS, Unity3D & C, C\#, Java & Free open source & ARToolKit6 (2017) \\ \hline Layer & 2D objects & Android, iOS, Blackberry & HTTP(BSTful), JSON & Paid & Layar v7.0 \\ \hline Vuforia & 3D objects, marker based visual SLAM & Android, iOS, Windows (selected devices), Unity 3D & C\#+, C\#, Java & Free (with paid versions) & Vuforia 6.5 \\ \hline Zapbox & 3D objects, marker based visual SLAM & Android, iOS & JavaScript & Paid (with 30-day free trial) & Zapworks Studio (2017) \\ \hline EasyAR & 3D objects, visual SLAM & Android, iOS, macOS, Windows, Unity3D & C, C\#, Java & Free (with paid version) & EasyAR SDK 2.0 \\ \hline kudan & 3D object, visual SLAM & Android, iOS, Unity 3D & Objective-C, Java & Free (with paid versions) & kudan v1.0 \\ \hline Wikitude & 3D objects, visual SLAM & Android, iOS, Smart Glasses (EpsonMoverio, ODG, Vuzix), Unity3D & Objective-C, Java, JavaScript & Paid & wikitude SDK 7.0 \\ \hline ARkit & 3D objects, visual and depth SLAM & iOS & Objective-C & Free & ARKit (2017) \\ \hline OSVR & 2D objects, orientation-based SLAM & Vuzix, OSVR, HTC, C\#, JSON for & C, C\#, JSON for (opensource) & CSVR Hacker Development Kit (HDK) 2.0 \\ \hline Meta 2 SDKs & 3D objects, visual and depth SLAM & Meta 2 glasses & C\#, C\#+, JavaScript & Free & Meta 2 SDKs \\ \hline Windows Mixed Reality & visual, depth, orientation-based SLAM & Windows MR, HoloLens, Unity & C\#, C\#, C\#, JavaScript & Free & Windows MR API \\ \hline \hline \end{tabular}
\end{table} 
Table 2: MR software, toolkits, and application programming interface [6]. been practically implemented. Ref. [69] describes the characteristics of MR systems as follows: (1) real and digital content is integrated into an application; (2) an MR application is interactive in real-time; and (3) an MR application is registered in three dimensions. Regarding these three features, various types of MR systems have been presented in various studies such as, multimodal MR [70], visuohaptic MR [71], tangible MR [72], context-aware real-time MR [37], markerless MR [43], adaptive MR [55], and multiscale MR [73]. This framework provides many benefits for users. For example, in multimodal MR, the user can switch between several display modes (such as AR, VR, and cyberspace) to select the appropriate method for a scenario. In a visuohaptic MR, real-virtual interaction is created. For example, a real scene can involve touching a virtual object with a haptic tool. Tangible MR provides the user with the ability to manipulate virtual objects. For example, a user can move, tilt, and rotate the 3D design model in front of their eyes. In adaptive MR, the system sets the level of detail and lighting of the virtual model (such as ruins or tourist sites) according to the user's location. Multiscale MR creates a multiscale interaction between AR and virtual content as an element of the MR environment. One of the best examples of this is the Snow Dome app. In this application, if a virtual object such as a virtual user enters a virtual dome, the user shrinks to miniature size and enlarges when exiting the app. In this application, the user can retrieve a virtual miniature tree. When this tree comes out, it enlarges and the environment is filled with virtual objects. However, despite the importance of the proposed frameworks, attention to the following matters is important. Visuohaptic MR needs an appropriate strategy to handle dynamic scene problems when combined with virtual object haptic devices [71].