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VR Interfaces: Head Mounted Display


Overview of Head Mounted Display
The human brain perceives depth only because it has two eyes for visual input. Each eye sees a slightly different angle of the same scene (as evidenced when you hold your finger in front of your nose, then look at it with one eye closed, both eyes and the other eye closed ? the image shifts).

These two separate views are combined in the brain to form a single, 3D image, with parts of the data from each eye used to work out relative distances.

To replicate this effect in VR, you require a device that can do the same thing ? give each eye a separate view. Head Mounted Displays ? HMDs ? are one method of doing this.

The head mounted display is the oldest of the stereoscopic displays. Originally not even head mounted, but held on a floor mounted boom due to their immense weight, they consist of a unit which is worn over the head, the first HMDs resembling crash helmets with massive protrusions on the front, and power packs strapped on top. Wearing one for any significant length of time resulted in neck strain, and the heat was comparable to sticking your head in a sauna.

Things have come a long, long way since then. The latest models resemble goggles, and are lighter than the average pair of spectacles.

The entire point of a HMD is to surround the user in the experience, to the fullest extent possible. This means most HMDs are stereoscopic, with one dedicated monitor for each eye, allowing you to see in stereo. Monoscopic HMDs do exist, their selling point being twice the resolution of a stereoscopic display. However, these systems are becoming rarer as hardware technologies occlude this benefit.

More advanced HMDs also offer integrated headphones, and gyroscopes so that neck movements can be tracked and treated like joystick movements, for that extra, immersive kick.

Because HMDs work by completely occluding the outside world, unlike HUDs or shutter glasses, they are ideal for pure VR experiences. However, this produced a problem with latency delays between the senses that resulted in a new condition: Simulator Sickness.

Simulator sickness occurs when the senses are completely submerged in a simulation, as happens with a HMD device.

When the user moves their head in the physical world, the visual feedback they receive changes immediately, as their eyes track across different objects. With a HMD however, the user moves their head, the computer picks this up, works out the view they should now see, and sends it back to be displayed. With the original HMDs, this took maybe half a second, resulting in the user moving their head, and the display scrambling to catch up. Worse, when the user stopped moving their head, the display kept right on moving for a fraction of a second, usually promoting a nauseous reaction. Repeated over ten, twenty minutes, the end result for many users is to physically empty the contents of their stomachs.

An outbreak of HMD-related simulator sickness in the late 1990s was directly responsible for the near-disappearance of VR from the public view at that time.

More modern HMDs are significantly improved. Computer processing speeds are radically different from those in the late 90s, and response delay times are in the milliseconds. However, the stigma of old simulator sickness remains like a cloud over this VR interface method, limiting the frequency of its use.

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