What is Motion Capture?

What is Motion Capture

What is motion capture? The term can be intuitively understood as recording the movements of the observed subject (whether human, object, or animal) through various technological means and processing them effectively. From a professional standpoint, motion capture is an advanced technology capable of measuring and recording the motion trajectories and postures of moving objects in real 3D space in real-time and reconstructing these movements in virtual 3D space at each moment.

Motion Capture Devices Based on Different Principles

Since it's a technology, there are various methods to implement it. Motion capture technology can currently be divided into the following types: optical, inertial, mechanical, acoustic, and electromagnetic.

Optical motion capture, as the name suggests, uses optical principles to capture and locate objects. It captures the position information of markers fixed on the human body or objects through optical cameras to complete the capture of motion postures. Optical motion capture relies on a set of precise and complex optical cameras, which use computer vision principles and multiple high-speed cameras to track target feature points from different angles to complete the capture of full-body actions. Optical motion capture can be divided into passive and active types. This classification is based on the markers. Active markers emit light actively and can even carry ID codes, allowing the cameras to observe them in the field of view and record their motion trajectories. Passive optical motion capture uses specific wavelength infrared light emitted by the camera itself, which is reflected by specially treated reflective markers, allowing the cameras to capture and record the marker's motion trajectory within the field of view.

Inertial motion capture involves the use of inertial navigation sensors such as AHRS (Attitude and Heading Reference Systems) and IMUs (Inertial Measurement Units) to measure the acceleration, orientation, and tilt angles of the subject or object being captured. Inertial motion capture requires various accessories such as wireless devices, battery packs, and sensors. It is similar to wearing a full suit with sensors attached to various parts of the body to capture the data of human or object movements.

Mechanical motion capture systems rely on mechanical devices to track and measure motion trajectories. A typical system consists of multiple joints and rigid rods, with angle sensors installed in the rotatable joints to detect changes in joint rotation angles. When the device moves, the position and motion trajectory of the rod ends in space can be determined based on the measured angle changes and the lengths of the rods.

An acoustic motion capture system generally consists of a transmitter, a receiver system, and a processing system. The transmitter is typically an ultrasonic generator, and the receiving system is usually composed of three or more ultrasonic probes. By measuring the time or phase difference of the sound waves from a transmitter to the sensors, the distance to the receiving sensors is determined, and the position and orientation of the ultrasonic generator relative to the receivers are calculated based on the distance information from the three triangularly arranged receiving sensors.

An electromagnetic motion capture system generally consists of a transmitter, receiving sensors, and a data processing unit. The transmitter generates an electromagnetic field in space with a certain spatiotemporal distribution. Receiving sensors are placed at key positions on the performer's body and move within the electromagnetic field according to the performer's movements. The sensors transmit the received signals to the processing unit via cable or wireless means, and these signals are used to compute the spatial position and orientation of each sensor.

Current State of Motion Capture Device Applications

Mechanical motion capture, while low in cost and high in precision, is inconvenient due to the size and weight of the mechanical equipment. Acoustic motion capture suffers from significant latency and low accuracy, making it unsuitable for most applications. Electromagnetic motion capture equipment demands stringent environmental conditions, as nearby metallic objects can distort the electromagnetic field, affecting precision. Consequently, mechanical, acoustic, and electromagnetic motion capture systems are less commonly used in modern applications.

The mainstream motion capture technologies are inertial motion capture and optical motion capture. In optical motion capture, because active markers require power, the accessories and wiring needed to fix the markers can affect usability. Therefore, passive optical motion capture is now the mainstream. Compared to the sub-millimeter accuracy of passive optical motion capture, the error of inertial motion capture accumulates over time, and its accuracy is not as good as that of passive optical motion capture. Regarding the environment of use, sensors in inertial motion capture may become magnetized over time if exposed to magnetic fields, so it is necessary to avoid magnetic fields (including but not limited to computers, keyboards, televisions, etc.) during use. In fields such as automation control, motion analysis, gait analysis, virtual reality, ergonomics, film and animation, passive optical motion capture often has advantages. Considering the cost advantage of inertial motion capture over passive optical motion capture, inertial motion capture is often chosen in fields where high precision is not required (such as crowd motion capture in some movies and television).

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