How VR Headsets Actually Work

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How VR Headsets Actually Work
October 11, 2016

HOW VR HEADSETS ACTUALLY WORK
 
How a lens, some sensors, and more can transport you to awesome new worlds.
 
Virtual reality might seem like black magic, but the truth is each headset is a carefully engineered product, blending cutting edge tech with more than a few clever tricks designed to make you forget about everything except for what's on the screen. We're breaking down how these headsets work and taking a look at the individual components that make virtual reality such a convincing experience.
 
Lens
 
The lens is one of the most crucial elements of what makes virtual reality so convincing. These lenses fool your eyes into thinking you're looking at a vast depth of space rather than two flat displays inches away. The lens achieves this by focusing the light to make it appear as if the displays were an infinite distance away.
 
Many headsets have opted to use special Fresnel lenses, which achieve the same effect as bulkier curved lenses with much less material by using thin, circular arrays of prisms. These lenses also magnify the headset's built-in display so that you don’t notice the edges of the screens, making the image encompass your entire field of view.
 
Display
 
High performance displays are another important factor in convincing virtual reality. They must have a density of pixels to display clear images, but also display them at a fast enough rate to make movement in VR smooth.
 
"High-end headsets use dual screens to create a stereoscopic 3D."
 
Both the HTC Vive and Oculus have opted for two 1080x1200 displays, one for each eye, that are intended to display images at 90 frames per second — affording users smooth movement and a wide 110 degree field of view that encompasses much of your range of vision.
 
High-end headsets also use dual screens to create a stereoscopic 3D effect similar to the Nintendo 3DS. Each screen displays a slightly offset image to each eye that our brain then joins together into one image, creating an illusion of depth in the process.

The Samsung Gear VR provides a low-cost entry point by using a smartphone as the display, sacrificing field of view and graphical fidelity for a truly wireless headset. Its two replaceable lenses are what creates the stereoscopic image in this instance.
 
Focus Adjustments
 
Because each person has a different distance between the center of their pupils (called the interpupillary distance), headsets must include options for adjusting the lenses in order to complement the spacing of our own eyes to achieve the proper stereoscopic 3D effect.
 
The Oculus Rift also sports hybrid Fresnel lenses that are shaped to have variable levels of focus across the lens, letting you move the headset up or down to adjust focus and find that sweet spot.
 
Positional Sensors
 
In order to display accurate images, headsets must track the movement of your head as you look around to submillimeter accuracy. This is achieved through a variety of built-in sensors. With the data of all these sensors, headsets can achieve true "six degrees of freedom," allowing for tracking of every conceivable movement the headset could make.
 
"Gyroscopes provide a much finer measurement of the rotation of an object."
 
Magnetometers measure Earth's magnetic fields in order to always know where "magnetic North" is. By detecting this, it can always make sure it's pointed in the proper direction, preventing "drifting" errors where the headset thinks it's facing one direction when it's facing another.
 
Accelerometers have a few uses, like detecting gravity to know which direction up is. This is most commonly seen when rotating your smartphone and the screen adjusts to reflect the new orientation. Like their name implies, accelerometers can also measure acceleration along an axis, providing useful data for how fast an object is moving.

Gyroscopes provide a much finer measurement of the rotation of an object by tracking subtle shifts in orientation along an axis, like tilting your head slightly or nodding.
 
The Samsung Gear VR forgoes more advanced infrared tracking methods to use an inertial measurement unit (IMU), which is somewhat of an all-in-one device using magnetometers, accelerometers, and gyroscopes. Unlike in most smartphones, this IMU is created specifically to reduce lag and improve head tracking performance.
 
Infrared Tracking
 
Both the Oculus Rift and HTC Vive use infrared lasers to track the movement of the headset, but each one has their own method.

"All of this is done nearly instantly, meaning there's almost zero lag."
 
Oculus uses the "Constellation" IR camera that sits on your desk and tracks blinking IR lights placed on both the front and back of the Oculus Rift. If using Oculus Touch controllers, a second camera is needed to prevent confusion when tracking the lights on both the headset and controllers. Each sensor is tracked separately, and your computer gathers all of that information to render images that are positionally appropriate for where you are looking at any moment. All of this is done nearly instantly, meaning there's almost zero lag between when the coordinates of each IR sensor is captured, processed, and the image is displayed.
 
The HTC Vive uses "Lighthouse" IR emitters, which are placed in the corners of a play space and rapidly fire sweeping lasers across the room that IR sensors on the Vive pick up and use to triangulate its position within a space. The system works similarly to the Oculus, but essentially switches the roles of each device by having the Lighthouses act as emitters and the headset the camera.

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