What are the different types of motion capture?

In this article we will take a brief but comprehensive look at each type of tracking technology that Target3D utilise across verticals and projects. These include (but are not limited to) Optical, Inertial, Magnetic and Markerless. Within each of these technology categories are numerous manufacturers but we'll be focusing on what makes each approach unique and where they might best fit.

1. Optical

Optical motion capture requires cameras positioned around the desired volume, and reflective sensors placed on the body at the major joints and segments of interest. The cameras emit an infrared light that is reflected off of the markers.  The reflections are then seen by the cameras.  When more than one camera sees a reflection from a marker, the position of that marker in 3D space is determined to sub-millimeter accuracy.  Therefore, the more cameras, the better, since body parts can block or shadow other markers during movement (occlusion).

Objects that need to be tracked are equipped with retro-reflective passive markers, which reflect the incoming IR light back to the cameras, or active IR LEDs. The IR reflections are detected by the cameras and then internally processed by the optical tracking system. This system calculates the 2D marker position in image coordinates with high precision. Using multiple cameras, the 3D position of each marker can be derived.

The 3D position can be measured by using a single marker in the measurement space. To be able to also measure the orientation of an object or to track multiple objects simultaneously, multiple markers are placed on each object. Such a configuration is easily created by simply sticking markers randomly onto the object, making sure multiple markers can be seen from each angle. By using a model of this configuration of each object, the optical tracking system is able to distinguish between objects and determine the 3D position and orientation of each object.

2. Inertial

Inertial motion capture uses IMUs (inertial measurement units) with built in sensors to detect position and movement.  These typically include gyroscopes, accelerometers and sometimes magnetometers. The gyroscope measures angular rate. It is used to determine the rotational orientation of the IMU. The accelerometer detects acceleration and gravitational force.  This is then used to calculate change in position relative to gravity (tilt) as well as movement of the IMU through acceleration in any direction. The magnetometer measures the Earth’s magnetic field or an artificially created magnetic field.  This is used to orient the IMU.

Each sensor requires a base position to move from.  This allows the detected movements to be translated to meaningful data (movement relative to a position in space). This poses problems when it comes to accuracy.

3. Magnetic

Magnetic tracking uses an electromagnetic base and a probe that interacts with the magnetic field it creates.  The person or object's position is tracked within X, Y and Z coordinates of a space, as well as the object's orientation, otherwise known as yaw, pitch and roll. This technology delivers reliable data, with repeatable results perfect for situations that require discreet embedded tracking in small environments with very high accuracy.

4. Markerless

Markerless tracking is a method of positional tracking - the determination of position and orientation of an object within its environment. This is a very important feature in virtual reality (VR) and augmented reality (AR), making it possible to know the field-of-view and perspective of the user - allowing for the virtual environment to react accordingly or the placement of augmented reality content in accordance with real objects. For a complete motion tracking experience, the tracking system needs to measure movement in six degrees-of-freedom.
While marker-based methods of motion tracking use specific optical markers, markerless positional tracking does not require them, making it a more flexible method. It also avoids the need for a prepared environment in which fiducial markers are placed beforehand, for example. Contrary to marked-based tracking, a markerless approach allows the user to walk freely in a room or a new environment and still receive positional feedback, expanding the applicability range.
Markerless tracking only uses what the sensors can observe in the environment to calculate the position and orientation of the camera. The method depends on natural features instead of specific markers, and it can use a model-based approach or do image processing in order to detect features which provide data to determine position and orientation.