# A Review on Mixed Reality: Current Trends, Challenges and Prospects
## 3 Concept of Mixed Reality
### Overview  
This section discusses the basics of MR space, covering its definition, characters, first MR implementation, MR benefits, and MR technologies. Figure 1: Research layers. **Definition 1**. : _The first definition of MR is presented by the reality-virtuality continuum, which is a scale modeling real-world representation of classes based on computing techniques. On the left side of this scale is real space and on the right side virtual space (i.e., an environment generated using computer graphical techniques). Figure 2 shows that MR is everything between augmented reality (AR) and augmented vitality (AV). The main goal of MR is the creation of a big space by merging real and virtual environments wherein real and virtual objects coexist and interact in real-time for user scenarios [14; 15; 16; 17; 18; 19; 20; 21]. Mixed Reality is a class of simulators that combines both virtual and real objects to create a hybrid of the virtual and real worlds [13]._  
**MR characters:** MR characters may be defined using three terms: immersion, information, and interaction. Immersion refers to the real-time processing and interpretation of the user's environment. User interaction with MR space is performed without any controller, using natural communication modes such as gestures, voice, and gaze. Information refers to virtual objects being registered in time and space in the user environment. This allows the user to interact with real and virtual objects in the user environment [22]. **First MR implementation**: The first MR prototype was implemented by the US Army MR in 1990. This implementation, a virtual fixture, refers to overlaying a registered virtual object on the real space of the user to increase the performance of implemented systems [23]. **MR benefits:** MR space provides many benefits in comparison to other computing ideas such as AR and VR (Figure 3 and Table 1) [22; 24]. **MR challenges:** Despite the importance of MR environments, there are two major challenges for creating such a platform: display technology and tracking. The MR platform needs to use appropriate  
Figure 3: Based on three main characters (Adopted from [22]). display technology to provide a reasonable output with appropriate resolution and contrast. In some instances, the system needs to consider the user's perspective. Interaction between virtual and real objects requires the use of precise methods in order to track both objects [5]. **MR technologies:** Optical see-through and video-see-through are the two types of technologies used for developing the MR space. In optical see-through technology, the real-world can be seen directly in tools such as transparent glass. In video-see-through technology, both virtual and real objects are present in an LCD (Liquid Crystal Display) [25]. The display tools in MR can be categorized into four classes: head-mounted display, handheld display devices, monitor-based, and projection-based displays [26]. To use a display tool in the MR space, it is necessary to pay attention to the two concepts, i.e., comfort (e.g., thermal management) and immersion (e.g., the field of view). These concepts provide ease of use that enables the execution of the scenario in an appropriate way for the user [27]. To create an MR environment using see-through technologies, some steps are required (Figure 4). In the device recognition step, the types of MR tools to be used, such as see-through or camera, are identified based on the project aims. Then, these devices are set using a calibration model. In the space recognition step, modeling space is performed to model some part of the real world in the virtual environment to run the user scenario. In the object and tracking recognition steps, the real-world object is identified and tracked. In mapping recognition, registration and mapping are performed to communicate between real and virtual objects. This provides interaction between the virtual and the real object.