# A Review on Mixed Reality: Current Trends, Challenges and Prospects
## 1 Introduction  
The development of new technologies has enabled the design of applications to create decision-making tools to solve the problems of daily life. For example, a mobile pathfinding application finds an optimum path and visualizes it for the user, which facilitates quick and accurate decision-making during an emergency situation [1]. In fact, in designing such an application, it is very important to design easy user interaction using appropriate visualization strategies. Many computing ideas have emerged to achieve these aims, such as virtual reality (VR), augmented reality (AR), and mixed reality (MR). VR provides a computer-generated environment wherein the user can enter a virtual environment with a VR headset and interact with it. Although a lack of relation with real space was a problem in VR, AR technology solved this problem and presented a new method of visualization to enable the addition of computer-generated content to the real world. This technology creates an augmented world that the user can interact with. Despite the importance of AR, the separation of the real and virtual world is a serious challenge. This problem decreases the user immersion level during AR scenarios. MR emerged to tackle this challenge with the creation of the MR environment. This environment merges the real and virtual worlds in such a manner that a window is created between them. As a result, a real-world object interacts with a virtual object to execute practicalscenarios for the user [2]. There are three important features of any MR system: (1) combining the real-world object and the virtual object; (2) interacting in real-time; and (3) mapping between the virtual object and the real object to create interactions between them [2]. Ref. [3] implemented an MR visualizing system to show blind spots for drivers. In this application, the driver uses MR see-through devices. This device shows blind spots to the driver to decrease the risk of traffic accidents. In fact, this application enhances the real environment by making invisible information visible to the user. MR-based applications are one of the top 10 ranked ICT technologies in 2020 [4]. Various studies have been conducted on MR technology, and different survey categories created in relation to the technology. Some surveys have separately included the MR component. Ref. [5] performed a comprehensive review of related MR applications and the technical limitations and solutions available to practically develop such applications. Ref. [6] discusses solutions to maintain privacy-related user data. Ref. [7] conducted a survey related to recent progress in MR interface design. Ref. [8] conducted an overview associated with issues for the evaluation of MR systems with respect to user view, and discussed challenges and barriers in MR system evaluation. Ref. [9] performed a survey to provide a guideline related to MR visualization methods such as an illumination technique. Other types of surveys have focused on a single domain of MR applications. Ref. [10] prepared an overview regarding the specialized use of MR applications for healthcare. Ref. [11] performed a review for education applications. Some surveys have discussed MR technology alongside other ideas such as AR and VR. Ref. [12] performed a survey on MR along with other computing ideas, such as virtual and augmented reality; that book provides useful information related to MR basics. Ref. [13] published a book that primarily focuses on basic concepts and general applications of MR. Despite the useful information provided in previous literature, a comprehensive study that presents a generic framework comprising all MR components needs to be undertaken. Proposing such a framework requires a comprehensive review of different domains of MR applications such as large-scale, small-scale, indoor, and outdoor applications, to make MR applications feasible for the practical use of stockholders.