AR is the term that's used to describe the technology that allows users to visualize part of the real world through the camera of a technological device (smartphone, tablet, or AR glasses) with virtual graphical information that's been added by this device. The device adds this virtual information to existing physical information. By doing this, tangible physical elements combine with virtual elements, thus creating augmented reality in real-time. The following image shows how AR works:

Now, we are going to look at the beginnings of AR and learn how AR can be divided according to its functionality.

AR is not a new technology. The beginnings of AR begin with the machine that was invented by Morton Heilig, a philosopher, visionary, and filmmaker, when, in 1957, he began to build a prototype with an appearance similar to the arcade video game machines that were very popular in the 90s. The following image shows a schema of how the prototype worked:

Morton called his invention Sensorama, an experience that projected 3D images, added a surround sound, made the seat vibrate, and created wind that was thrown as air at the viewer. The closest similar experience we can feel today is seeing a movie in a 4D cinema, but these experiences were created more than 60 years ago.

In 1968, Harvard Electrical Engineering professor Ivan Sutherland created a device that would be the key to the future of the AR technology known as the Human-Mounted Display (HMD). Far from the AR glasses that we know of today, this HMD, called the Sword of Damocles, was a huge machine that hung from the ceiling of a laboratory and worked when the user was placed in the right place. In the following image, you can see what this invention looked like:

In 1992, Boeing researcher Tom Caudell invented the term AR, and at the same time, AR technology was boosted from two other works. The first AR system, from L.B. Rosenberg, who works for the United States Air Force, is a device that gives advice to the user on how to perform certain tasks as they are presented, something like a virtual guide. This can be seen in the following image:

The other research in this area was led at Columbia University, where a team of scientists invented an HMD that interacted with a printer. The device, baptized as Karma (AR based on knowledge for maintenance assistance), projected a 3D image to tell the user how to recharge the printer, instead of going to the user manual.

The following diagram is a representation of the continuum of advanced computer interfaces, based on Milgram and Kishino (1994), where we can see the different subdivisions of the MIXED REALITY (MR) that go from the REAL ENVIRONMENT to the VIRTUAL REALITY. AR that's located nearer to the REAL ENVIRONMENT is divided between spatial AR and see-through AR. However, the appearance of mobile devices in the 21st century has allowed a different version of AR, where we can display it using the device screen and camera:

Now that we have introduced the beginnings of AR, let's learn how this technology can be classified depending on the trigger that's used to show virtual elements in the real world.

AR can be created in many ways; the main challenge is how to make the combination of the real and virtual worlds as seamless as possible. Based on what is used to trigger the virtual elements to appear in the real world, AR can be classified as follows:

• GPS coordinates: We use GPS coordinates, compasses, and accelerometers to locate the exact position of the user, including the cardinal point they are looking at. Depending on where the user is pointing to, they will see some virtual objects or others from the same position.
• Black and white markers: We use very simple images, similar to black and white QR codes, to project virtual objects on them. This was one of the first AR examples, although nowadays they are used less often as there are more realistic ways to create the AR experience.
• Image markers: We use the camera of the mobile device to locate predefined images (also called targets or markers) and then project virtual objects over them. This type of AR has substituted black and white markers.
• Real-time markers: The user creates and defines their own images with the mobile camera to project any virtual object in them.
• Facial recognition: Through the camera, we capture the movements of the face to execute certain actions in a request, for example, to give facial expressions to a virtual avatar.
• SLAM: Short for Simultaneous Localization And Mapping, this technology understands the physical world through feature points, thereby making it possible for AR applications to recognize 3D objects and scenes, as well as to instantly track the world, and overlay digital interactive augmentations.
• Beacons: eBeacons, RFID, and NFC are identification systems that use radio frequency or bluetooth, similar to GPS coordinates, to trigger the AR elements.

Now, you have a better grasp of what AR is and where it comes from. We have covered the basics of AR by looking at the first prototypes, and classified different types of AR according to the element that triggers the virtual images so that they appear on the screen. The next step is to see what is required to work with it.