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

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. 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.