## 4 Mixed Reality for Human-Robot Interaction
### Reality-Virtuality Continuum  
Extended reality (XR) encompasses the mutual embedding and fusion of physical and virtual scenes, along with the human-computer interaction occurring in the generated environment [80], where X represents the degree of interpenetration and integration of real scenes and virtual content in spatial or immersive computing technologies, involving the user's sense of presence and acquisition of perception [81]. XR spans the entire spectrum from entirely realistic to fully virtual, encompassing all representative forms and possible variations of AR, VR, AV, MR, and other interpolations between reality and virtuality [82; 83]. The reality-virtuality continuum presented by [PERSON] et al. [84] outlines the XR ecosystem and the relationship between AR/VR/AV/MR and fully real and virtual environments, which are schematically illustrated in Figure 3.  
Augmented reality (AR) overlays informative virtual 3D graphics onto the physical world and allows real-time interaction with the 3D graphics, enabling the user to reach into the augmented world and manipulate 3D objects directly as if they were real-world objects [85]. The advantage of AR is it superimposes important cues on physical objects or hovers the display in real space to indicate the user's potential or actual future actions, or the robot's motion planning, while eliminating distractions caused by typical display methods [86]. AR-HRI interfaces provide users with an exo-centric view of the robot and its surroundings and allow operators to maintain situational awareness of the robot and ensure intuitive interaction and communication between the human and robot using multimodal interfaces [87].  
Virtual reality (VR) uses fully immersive computer-generated graphics with scenes and objects that appear to be real to completely replace the physical surroundings in which the user is located [88]. In the virtual environment, the users perceive immersion in an identical or completely different scene from the physical surroundings and perceive an imaginary environment with muti-modalities of visual, acoustic, and haptic information [89]. VR allows users to move around in the scene and manipulate the virtual objects by using wireless controllers or haptic robotic structures as input sources.  
Mixed reality (MR) technology involves the merging of physical and imaginary spaces and does not occur exclusively in the physical or virtual world [90]. MR enables real-time visualization and interaction between physical and digital content [91] and takes full advantage of the visual information of real scenes and the three-dimensional immersion and interaction provided with virtual cues [92]. Head-mounted display (HMD)-based mixed reality can be achieved in two ways: either by displaying the camera captured video feedback of the real world in the HMD or by allowing the user some direct visibility of the real world in the HMD [93; 94].  
Augmented virtuality (AV) is a fully immersive computer-generated virtuality-dominated environment enhanced by sensory data from physical environments [95]. AV keeps the virtual environment central but superimposes the real-world elements on the virtual content [96]. AV is also classified as a subset of MR. The term AV is less commonly used in the literature for immersive telerobotic interfaces and is often generally referred to as MR.  
In MR, the hierarchy of reality spans from partial sensory input to a proximity infinitely close to immersive reality [97]. The reviewed MR-enhanced intuitive and immersive teleoperation schemes in this article fall into the category of partially superimposing real-world visual inputs onto computer-generated virtual content.