Head-mounted display
A head-mounted display (HMD) is a display that is fixed to a user's head. It shares a coordinate system with the user's head. It is either monoscopic or two-view. A head-mounted display is a head-fixed display that rests its mass on the user's head.
The use of a headmounted display is to display a 3D view of which one has control over the contents of the scene using a 3D control peripheral.
A common use is a VR headset.
An HMD is largely useful if it is binocular, and as such, shows two images, one for each eye. Other HMDs like the Google Glass only have a small display over one eye that is partially transparent.
The majority of virtual reality (VR) and augmented reality (AR) gadgets are head-mounted displays. In VR, the display is not transparent and only virtual information and images are displayed in front of wearer's eyes. In AR, the display is usually transparent and digital information is superimposed onto real life objects. These HMDs are called optical see through head mounted displays (OST-HMD). Video passthrough can also be done.
A display is placed very close to the users' eyes, covering a large portion of their field of view. Most headsets are flat focus binocular, meaning two images that are very similar but have different perspectives are channeled into the eyes, one for the left and one for the right, to allow the human perceptual system to perceive binocular disparity and see some depth.
The first was Ivan Sutherland's head-mounted three dimensional display.
VR headsets with flat focus have lenses that are placed between the display and the user's eyes. The lenses allow our eyes to focus on the images on the display, even though the display is only a few inches in front of our faces. Without lenses, our entire VR world would become blurry because human eyes have trouble focusing on things that are very close.[1]
Virtual reality[edit]
VR headsets input head movement, which is taken in by a tracking system, fed to a CPU, then a scene is rendered according to the head position by a GPU, which is sent to a display system, which converts electrical signals to photons (light). Then the light optionally passes through optics and into the user's eyes.
Features[edit]
Large Field of View[edit]
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 gadget should cover as much of the vision range as possible. A large FOV is important to create immersion for the wearer.
Binocular stereoscopic imagery[edit]
A VR HMD has to create the illusion of a 3D world. To accomplish this, the display shows each eye a different image of the same scene. The images are 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. It is called binocular disparity.
Rotational Tracking[edit]
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[edit]
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 gadgets. 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[edit]
Augmented reality HMDs are either optical see through gadgets or video passthrough gadgets.
In an optical see through device, the displays of these gadgets are transparent. They allow users to see through them while projecting images and information.
Types[edit]
Slide-on HMD is the most primitive of all HMD types. These gadgets 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[edit]
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 gadget 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[edit]
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:
New
Integrated HMD[edit]
A standalone HMD, also known as an Integrated HMD, is the most sophisticated and expensive type of HMD. This independent computing gadget 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. Examples are the Meta Quest Pro and the Meta Quest 3.