Head-mounted display

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Head-mounted display or HMD is a device worn over your head. It features a display in front of one or both of your eyes. The display streams data, images and other information in front of the wearer's eye(s). Certain HMDs such as Oculus Rift or HTC Vive have displays over both of their users' eyes. Others like Google Glass only have a display over one of the users' eyes.

The majority of Virtual Reality (VR) and Augmented Reality (AR) devices are head-mounted displays. In AR, the display is usually transparent and digital information is superimposed onto real life objects. These HMDs are called Optical head-mounted displays or OHMDs. In VR, the display is not transparent and only virtual information and images are displayed in front of wearer's eyes.

Virtual Reality HMDs

See also: Virtual Reality Devices

Head movement → Tracker → CPU → GPU → Display → Photons → Optics → Eyes

Features

Large Field of View

VR HMDs have displays with large field of view (FOV) that comprise the entirety of the user's vision. With both eyes, humans have about 180 degrees FOV when looking in front of them. The display of a VR device should cover as much of the vision range as possible. A large FOV is important to create immersion for the wearer.

Stereoscopic 3D Imagery

The HMD has to create the illusion of a 3D world. To accomplish this, the display shows both of our eyes two different images of the same scene viewed from slightly different angles. This process creates depth perception by mimicking how our eyes perceive the world in real life.

Rotational Tracking

A key feature of an HMD is the ability to track the wearer's head rotations. The images shown on the display change according to the wearer's head movements. Head-tracking is an essential aspect of the HMD that allows the user to become immersed and feel presence.

Rotational Tracking is achieved through sensors such as the Gyroscope, Accelerometer and Magnetometer within the HMD.

Positional Tracking

Positional Tracking is often performed with sensors and cameras external to the HMD. These peripherals can track the position of the user's head, body and hands anywhere within the range of the devices. They can not only track the rotational movements like the inboard sensors but also translational movements.

HMDs in the future will be able to track translational motion and perform positional tracking.

Augmented Reality HMDs

Main article: Optical head-mounted display

Augmented Reality HMDs also known as Optical head-mounted displays or OHMDs. The displays of these devices are transparent. They allow users to see through them while projecting images and information in front of the users' eyes.

Types of HMDs

Slide-on HMD is the most primitive of all HMD types. These devices rely on the inserted smartphones to do all of the leg work. Discrete HMD contains a display and some electronics for tracking and other functions. While they are more advanced than the Slide-ons, Discrete HMDs still require connections to PCs for processing. Integrated HMD is the most sophisticated of them all. From display to tracking to processing, it has everything needed for VR or AR within the HMD.

Slide-on HMD

Slide-on HMD, also known as Smartphone mount, is the cheapest and most accessible form of VR. The Slide-on consists of a smartphone holder, lenses and some form of basic input. A smartphone is inserted into the HMD and the entire device is played upon the eyes of the user to create the VR experience. The Slide-on utilizes the smartphone for display, processing and rotational tracking. Some Slide-on HMDs such as the Gear VR use their own built-in IMUs instead of relying on that of the smartphones.

Popular Slide-on HMDs:

Discrete HMD

Discrete HMD also known as Tethered HMD supplies everything required for an immersive VR experience aside from the processing. It contains display, lenses, rotational tracking, positional tracking, audio and advanced input. For processing, Discrete HMDs are connected to PCs through cables. While this type of HMD is less mobile than its 2 counter parts, it is able to deliver the best VR experience.

Popular Discrete HMDs:

Integrated HMD

Integrated HMD is the most sophisticated and expensive type of HMD and OHMD. This independent computing device is able to deliver VR and/or AR experiences without any external hardware such as PC or smartphone. It contains everything from display to processors to camera and is able to display stereoscopic 3D images, perform complex tracking and utilize advanced input methods. Currently, most of Integrated HMDs are AR OHMDs with high price point and target only business professionals.

Popular Integrated HMDs:

Components of HMDs

Display

Lenses

Sensors

Cameras

Processors


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