# A Review on Mixed Reality: Current Trends, Challenges and Prospects
## 3 Concept of Mixed Reality
### Algorithm
#### 3.2.4 Object Tracking  
The interaction of virtual and real objects requires them to be positioned, which is achieved using appropriate tracking techniques. The MR tracking method is divided into three classes: sensor-based, vision-based, and hybrid. The hybrid method is a combination of the first two methods to improve the shortcomings associated with each of them. Sensor-based tracking uses sensing devices. This method is categorized into categories such as GPS, visual markers, acoustical tracking systems, magnetic and inertial sensors, and hybrid systems [5]. The sensing device estimates a value such as the position of a real object, for example, a game player for an MR application such as a race. Ref. [40] suggested using GPS to track multiplayer game play in outdoor spaces to determine players' relative positions. MR attributes such as motion properties requires the use of different sensors and fusion strategies to find the motion property. Ref. [41] proposed using a multimodal tracking model based on sensor fusion information for person tracking. Vision-based tracking is based on the use of a computer vision technique that utilizes image processing. This strategy may be feature-based or model-based. Ref. [42] suggested using image processing based on circle-based tracking in a virtual factory layout planning. Ref. [43] proposed using markerless tracking that utilizes feature-based tracking in a BIM-based markerless MR. Ref. [44] proposed markerless tracking driven by an ICP algorithm for an MR application for trying on shoes. Ref. [45] proposed three mechanisms containing feature-based tracking (using feature detection and SLAM), model-based tracking (comprising contour detection), and spatial-based tracking (using depth information) for self-inspection in the BIM model.  
Although the discussed methods are important for tracking, their practical use involves several challenges. In the sensor-based method, installing the device in an appropriate position, as well as power management, are important points. A good example in this regard is installing a local transmitter in the appropriate place to take advantage of existing satellites in the GPS method. Finding precise locations is necessary when using visual markers as well. The sensor-based method requires the use of a digital model of a real environment to map sensor data to a precise location. This is used to determine the location of the virtual object [5]. Removing the defect within an important method is a big challenge. For GPS data, noise removal using a gyroscope is a strategy for defect removal [40]. In cases that need sensor fusion, optimum allocation is necessary. The optimum allocation requires an appropriate analysis to obtain coherent data related to the object [41]. Vision-based tracking needs to consider new topics such as the use of multiple markers and multiple cameras to increase accuracy and coverage [43]. Practical use of the vision-based model needs to consider additional constraints such as different reference models for tracking. A good example in this regard is using one reference shoe model for a virtual shoe application. This renders the result of the system unsuitable for different users [44].