Motion capture

Motion Capture (often just abbreviated MoCap) is a technology that can be used to digitally record the movements of people, animal and things. Nowadays, this technology is mainly used by film and game studios. Recently, it has also found its use in science, especially in the health sector for the analysis of movement or assessment of the development of rehabilitation of patients with severe damage to the nervous motor system.

Motion detection systems are divided into two basic groups:

• online systems – for motion sensing that can produce data in real time.
• offline systems – they first record motion data, and only after the end of the recording, information about the position and rotation of the scanned objects is calculated from them.

Breakdown of Motion Sensing Systems

Body reference points

Use for 3D animation

1.Motion Capture|Optical sensing systems – These are among the more accurate and flexible sensors. Optical sensing systems use a camera system to determine the position and location of an object. Currently, most commercially available systems use CCD („Charge–Coupled Device“) cameras for positioning.

Optical sensing systems are divided into

• Optical systems using markers – In order to be detected by the camera system, the object being scanned must have retroreflective markers on it. From these marks, the light from the camera is reflected back to its source, thereby determining the position of the object in relation to the camera. The position of the markers is captured by infrared or red cameras.
• Optical system without markers – New techniques and research in the field of motion capture lead to the invention of a technique where the object will not need to use special markers for detection in space. Newly designed computer algorithms analyze the optical inputs and can thus identify the object.
  

2. Magnetic sensing systems – Motion detection sensors are placed on the object and connected to the control unit using cables. The control unit the determines the relative position.

3. Mechanical Sensing Systems – An exoskeleton is attached to the body we are sensing to sense movement. An exoskeleton is a structure made up of solid parts that correspond to individual limbs, and which are connected to each other by joints. Only the human body can be scanned using the mechanical system, not animals or objects.

4. Inertial Sensing Systems – Use two types of sensors to process movement – gyroscopes and accelerometers.

Features of Motion Sensing Systems

Latency – Means the delay between a change in the motion or rotation of an object and the recording of this change by the sensing device. Systems, that work in real time must have a latency that is invisible to the eye.
Accuracy – Indicates the maximum error that can occur when sensing position or rotation. For most devices, this value is not constant, but depends on the distance between the sensors and the scanned objects.
Refresh rate – Determines, how many times in one second the position or rotation of the scanned object is measured. The higher the refresh rate, the smoother the recorded movements.
Variation – Indicates the deviation by which the values at the output of the sensing device fluctuate for an object that is not moving. High dispersion during scanning causes jitter and not very smooth movements.

Links

• Optitrack
  
• Motion Capture Technology
  

References

• BŘEZINA, Pavel. Motion Capture [online]. Pavel Březina, The last revision 2012, [cit. 2012-11-27]. <http://pavka.shotzone.cz/motioncapture/motioncapture.html#motioncapture-zdroje>.

• VINKLER, Michal. Capture and reconstruction of character movement [online]. Masaryk University, ©2009. The last revision 2012, [cit. 2012-11-27]. <https://is.muni.cz/th/n4i18/?so=nx. Bachelor thesis>.