Extended Reality (XR) refers to all real-and-virtual combined environments and human-machine interactions generated by computer technology and wearables. XR is an umbrella term for different types of realities (see TR 26.918 and TR 26.928):

• Virtual reality (VR) is a rendered version of a delivered visual and audio scene. The rendering is designed to mimic the visual and audio sensory stimuli of the real world as naturally as possible to an observer or user as they move within the limits defined by the application. Virtual reality usually, but not necessarily, requires a user to wear a head mounted display (HMD), to completely replace the user's field of view with a simulated visual component, and to wear headphones, to provide the user with the accompanying audio. Some form of head and motion tracking of the user in VR is usually also necessary to allow the simulated visual and audio components to be updated in order to ensure that, from the user's perspective, items and sound sources remain consistent with the user's movements.
• Augmented reality (AR) is when a user is provided with additional information or artificially generated items or content overlaid upon their current environment. Such additional information or content will usually be visual and/or audible and their observation of their current environment may be direct, with no intermediate sensing, processing and rendering, or indirect, where their perception of their environment is relayed via sensors and may be enhanced or processed.
• Mixed reality (MR) is an advanced form of AR where some virtual elements are inserted into the physical scene with the intent to provide the illusion that these elements are part of the real scene.

Other terms used in the context of XR are Immersion as the sense of being surrounded by the virtual environment as well as Presence providing the feeling of being physically and spatially located in the virtual environment. The sense of presence provides significant minimum performance requirements for different technologies such as tracking, latency, persistency, resolution and optics. This document uses the acronym XR throughout to refer to equipment, applications and functions used for VR, AR and MR. Examples include, but are not limited to HMDs for VR, optical see-through glasses and camera see-through HMDs for AR and MR and mobile devices with positional tracking and camera. They all offer some degree of spatial tracking and the spatial tracking results in an interaction to view some form of virtual content.

For providing XR experiences that make the user feel immersed and present, several relevant quality of experience factors have been collected (see TR 26.926). Presence is the feeling of being physically and spatially located in an environment. Presence is divided into 2 types: Cognitive Presence and Perceptive Presence. Cognitive Presence is the presence of one's mind. It can be achieved by watching a compelling film or reading an engaging book. Cognitive Presence is important for an immersive experience of any kind. Perceptive Presence is the presence of one's senses. To accomplish perceptive presence, one's senses, sights, sound, touch and smell, have to be tricked. To create perceptive presence, the XR Device has to fool the user's senses, most notably the audio-visual system. XR Devices achieve this through positional tracking based on the movement. The goal of the system is to maintain your sense of presence and avoid breaking it. Perceptive Presence is the objective to be achieved by XR applications. The Human field of view (FoV) is defined as the area of vision at a given moment (with a fixed head). It is the angle of visible field expressed in degrees measured from the focal point. The monocular FoV is the angle of the visible field of one eye whereas the binocular FoV is the combination of the two eyes fields (see TR 26.918). The binocular horizontal FoV is around 200-220°, while the vertical one around 135°. The central vision, which is about 60°, is also called the comfort zone where sensibility to details is the most important. Although less sensitive to definition, the peripheral vision is more receptive to movements. In XR, actions and interactions involve movements and gestures. Thereby, the Degrees of Freedom (DoF) describe the number of independent parameters used to define movement in the 3D space (see TR 26.928):

• 3DoF: three rotational and un-limited movements around the X, Y and Z axes (respectively pitch, yaw and roll). A typical use case is a user sitting in a chair looking at 3D 360 VR content on an HMD.

• 6DoF: 3DoF with full translational movements along X, Y and Z axes. Beyond the 3DoF experience, it adds (i) moving up and down (elevating/heaving); (ii) moving left and right (strafing/swaying); and (iii) moving forward and backward (walking/surging). A typical use case is a user freely walking through 3D 360 VR content (physically or via dedicated user input means) displayed on an HMD.

An XR View describes a single view into an XR scene for a given time. Each view corresponds to a display or portion of a display used by an XR device to present the portion of the scene to the user. An XR Viewport describes a viewport, or a rectangular region, of a graphics surface. The XR viewport corresponds to the projection of the XR View onto a target display. An XR viewport is predominantly defined by the width and height of the rectangular dimensions of the viewport. An XR Pose describes a position and orientation in space relative to an XR Space. An essential element of XR is the spatial tracking of the viewer pose.

XR engines provide a middleware that abstracts hardware and software functionalities for developers of XR applications (see TR 26.928). Typical components include a rendering engine for graphics, an audio engine for sound, and a physics engine for emulating the laws of physics. In the remainder of this Technical Report, the term XR engine is used to provide any type of typical XR functionalities as mentioned above. The processing of an XR engine is not exclusively carried out in the device GPU. In power and resource constrained devices, it can be assisted or split across the network through edge computing (see TR 22.842): the UE sends the sensor data in uplink direction to the cloud side in a real time manner and when the cloud side receives the sensor data, it performs rendering computing and produces the multimedia data and then sends back to the user devices for display. This is where NR can play an essential role.