# A Review on Mixed Reality: Current Trends, Challenges and Prospects
## 3 Concept of Mixed Reality
### Algorithm
#### 3.2.6 Visualization and Rendering  
Displaying MR content requires the use of appropriate analytical algorithms that handle different challenges, e.g., adding a virtual file to video images to enrich it, close-to-reality representation of the virtual object using lighting and shadow, managing the level of details regarding user position, solving occlusion and screen off for an invisible object, using standard schema for faster display, managing bulky 3D large data such as point cloud, and synthetizing different display information sources. Ref. [50] proposed using chroma-keying with PCA for the immersion of virtual objects in video images. This method was useful for matting, which is a process used for foreground identification with separation images to the back and foreground. Ref. [51] used the spherical harmonics light probe estimator for light estimation based on deep learning. This method processes a raw image without the need for previous knowledge about the scene. Ref. [52] proposed a lighting model including a refractive object in the instant radiosity (DIR) model. Ref. [53] recommended using the fuzzy logic model to add soft shadows to a virtual object during embedding in a real scene. Ref. [54] proposed the use of a model containing precomputed radiance transfer, approximate bidirectional texture function, and filtering with adaptive tone mapping to render materials with shading and noise. Ref. [55] suggested using the level of detail (LOD) model for the visualization of a virtual historical site. Ref. [56] proposed a method based on holographic rendering using window MR to place geospatial raster-based data in real terrain. This method uses large-scale data from different sources and places it in the real world. In this model, the real world is simulated with a virtual globe, and the data is collected at different LOD from different sources and is then positioned on a virtual globe based on hierarchical LOD rendering with respect to the camera. This model contains the following steps: checking the view through the intersection with the bounding box of the interest area, using transformation to project data on the virtual globe, and performing rendering by running the virtual coarser tile. Ref. [57] proposed a combination of visibility-based blending and semantic segmentation to handle the occlusion problem in space MR. Ref. [58] recommend a model that uses space-distorting visualizations for handling screen-off and occlusion while displaying POI. Ref. [59] suggested taxonomy containing schema-related data, visualization, and views to retrieve the best display model and display information for medical applications. Ref. [60] used the Octree model to handle large volumetric data such as those obtained from point cloud during rendering. A spatial data structure model based on Voronoi-diagrams was proposed by [33]. This model was used for fast-path editing and fast rendering in the user's place. The model process stopped rendering models that were not in real visibility. Ref. [61] recommended using 3D web synthetic visualizations with HDOM to combine information such as the QRcode-AR-Marker during embedding into real space. Although the discussed methods have great benefits for close-to-reality visualizations in MR applications, there are important challenges impeding their use. In the chroma key, the use of statically-based techniques for temporal matting and Holoing artifacts is needed [50]. Improvement of the light and shadow models requires temporal and dynamic effects modeling (such as abrupt changes in lighting, dynamic scene, and local light source) in addition to specifying classes of different algorithms for different light paths [51; 54]. The LOD model requires study of the appropriate architecture to determine the volume of the process in the server and client [55]. The occlusion model needs to model temporal effects, as well as to consider complex scenes (such as complex plant effects) [57].