The Rift was one of the first major HMDs out on the market and was initially funded through Kickstarter. It had a few development kit versions that were released before the commercial version released. A later addition added two controllers and the ability for the software to track 6DoF. It uses outside-in tracking when using the "touch" kit they offer, using two cameras pointed at the user to track head and controller movement.
The Vive is a HMD and controller kit that uses outside-in tracking from two "lighthouses" placed in adjacent corners that give the HMD and controllers 6DoF. A new version of the headset called the Pro increases the resolution of the display and updates the design, adding built-in headphones and support for new controllers and accessories coming soon.
Microsoft is working with hardware manufacturers from HP, Acer, Asus, Dell and others to create "mixed reality" HMD kits that use inside-out tracking. The benefit to these devices is their low cost and setup, with no external cameras or sensors needed for 6DoF. The downside to them are that the controllers need to be in sight of the two cameras on the front of the HMD for them to be visible in the virtual environment, so holding one below or behind you may make the system lose track of them temporarily.
Despite the name, at the moment the "mixed reality" headsets only provide virtual reality environments.
The Playstation VR kit uses outside-in tracking, with a camera pointed at a seated user picking up the lights on the headset and from the wand controllers. While much more simple than other designs, the headset and controllers have 6DoF and match a lot of features of the PC equivalents. As can be expected for a game console peripheral, the focus of this device is gaming-oriented.
If you want to experience a VR environment right now, you probably have the means to get started in your pocket! Modern mobile phones with gyroscopes, accelerometers, magnetometers, and GPS make simple xR applications pretty simple to do. With a simple VR phone headset holder you can take video 360° field trips, explore neighborhoods on the other side of the world, and interact with simple VR environments.
This device was revolutionary for getting augmented reality into the public view. It works by displaying a heads up display (HUD) on a small reflective surface that reflects into the eye. This display can show directions, reminders, notes, maps, and other information overlaid on the real world. It is being used in healthcare and manufacturing, among other uses.
There are apps withtin the Google Play Store and Apple App Store that demonstrate augmented reality use.
Pokémon Go was a phenomenon starting in 2016 with its ability to travel in the real world to places to catch virtual creatures. The AR use in this app was simplistic, showing the pokémon in a certain direction when the camera was pointed at it, but it floated in space and could appear on a wall or in other odd situations, not adhering to real life physics. Regardless, this was many people's introduction to AR environments and was very successful.
Google Sky Map is one of the oldest AR apps but very useful in showing constellations, planets, and other objects in the sky just by pointing your camera at the sky. It overlays information over the camera view, identifying and giving information on the area of the sky you are viewing.
Microsoft has a developer kit out called the Hololens that is one of the only examples of MR devices available today. Since it is a developer kit, not too many people have used it yet, but its use of room scanning, and accurately anchoring digital objects in the physical space is remarkable and a sign of what systems will be able to do in the future.
Apple is working on technology that allows augmented (mixed) reality apps to work on their new devices. Examples of this technology would be 3D models and stickers that when placed digitally on a physical surface will stick to that surface and persist even the camera view is turned away from the scene.
When thinking about working in a xR environment, it is helpful to remember how things move in a 3D space, whether virtual or physical. When using xR technology, your actions and view are tracked so the motions and display are synced up to create the illusion of being in the environment. There are two tracking styles that allow for degrees of interactivity.
When you are watching a VR 360º video or playing a game with your phone in a head mounted viewer, you are experiencing three degrees of freedom (3DoF), with the view changing depending on your head's (and the device's) yaw, pitch, and roll, or the rotation along the x, y, and z axis. You can think about this as a camera that is stationary that can film above and below itself, to the left and right, and tilted clockwise and counter-clockwise. This allows for an xR experience while sitting or standing still and is sometimes called "sitting scale" xR experiences. This allows for some limited immersion and interaction with the environment.
Six degrees of freedom (6DoF) expands upon yaw, pitch, and roll and adds surge, heave, and sway, which just means that the view can additionally move back and forward, up and down, and left and right. Now our camera is not stuck in one spot spinning around, but can move around a 3D space while spinning. This is sometimes called "room scale" xR experiences, since you can explore a virtual room with 6DoF.
In xR environments, the hardware needs to track the movements of the head (and in more sophisticated systems, each hand) in order to work correctly. There are two main methods for tracking in xR: Outside-In tracking and Inside-Out tracking.
This tracking involves sensors that locate devices that stand out against the background. This usually involves using infrared LED sources (think flashlight or lamp that shines infrared light) on the devices that have sensors over the surfaces to catch this light (this is why some headsets and controllers look funny; the design is usually more for function than form in this case). This type of tracking has been used for awhile and is the more simple of the two methods. The Nintendo Wii used this type of tracking on their controllers to track their position for pointing and aiming on the screen.
This method of tracking (marketed by Microsoft as "World Scale") keeps track of the position of the head and controllers through internal sensors and cameras, negating the need for external fixed light sources. Cameras on the device are used to view and map the surroundings and track where the device is positioned. This type of tracking is more difficult to accomplish, but offers the ability to use the technology without being tied to a specific area. So far the only major products utilizing the inside-out tracking technology are the Microsoft Hololens developer head mounted display (HMD) and the Microsoft-partners' "Mixed Reality" HMD kits.