Simultaneous localization and mapping also known as S.L.A.M. is a technological form of mapping out an unknown environment around a device like a virtual reality headset or robot. The map created is constantly updated and all agents or objects within the map are tracked in real time. There are numerous ways of tracking this information with a variety of technologies that are meant to measure position or changes in position.
Stationary Tracking Systems
Mechanical, ultrasonic and electromagnetic tracking systems were all among the first forms of tracking systems to be used with augmented and virtual reality. Mechanical tracking uses mechanical arms attached to the user’s limbs to tracking their motions and position relative to the machine they are hooked up to. Ultrasonic tracking uses sound pulses to track position. By measuring the time it takes for the pulse to reach a sensor from a stationary source, the user’s relative position can be calculated. Electromagnetic tracking utilizes three or more magnetic field emitters that are detected by a device on the user. Position is calculated based on the strength of the magnetic signal picked from the device from each of the emitters.
Stationary tracking systems function well when there is not much movement opportunity on the user’s end. Mobile sensors are a better option for when movement needs to be tracked across larger environments. GPS is an example of this, since a car needs to be tracked across miles until it reached it’s destination it would be improbable to place stationary tracking systems everywhere along the way. Satellites track the location of a GPS receiver in real time and relays that information to a device. Devices like magnetometers, gyroscopes and accelerometers all aid in recording position. Magnetometers function like compasses that record position relative to magnetic poles. Gyroscopes measure rotational velocity and accelerometers measure position based on inertia.
S.L.A.M. In Virtual Reality
All these concepts thus far can be applied to VR and AR technologies in some way. Virtual reality headsets like the Valve Index or HTC Vive utilize ideas and technology from both stationary tracking systems and mobile sensors. They use base stations to hold and correct data picked up by devices like gyroscopes from within the headsets. Even popular apps like Pokemon Go or Google Maps use GPS technology to track the user’s position using existing infrastructure. Each form of tracking has it’s own drawbacks to worry about like what kind of interference they face. Ideas like latency and update rates are necessary to understand how to craft a more quality XR experience. High latency can confuse a user by staggering what happens in the virtual world relative to their real world movements. Update rates can help you understand the limitations of the technology you are working with and how accurate you can track movement.
Having an awareness of how positional tracking works can elevate AR and VR technologies. Our in-class projects can benefit from an understanding of S.L.A.M. by allowing us to conceptualize what we can create using technology present on smartphones.