Home

About   PT I

Product

Application

News Report

References

Site Map

Video

Motion capture is the process of sampling the posture and location information of a subject over time. The subject is usually a person, an animal or a machine. In case the subject is a person or animal, it is sometimes referred to as an 'actor'. Especially in the entertainment industry, motion capture is also frequently abbreviated as 'mocap'.

The technical goal of motion capture is to get the motion data of certain points of interest on the subject, so that either some parameters of the motion (e.g., speed, angle, distance, etc.) can be calculated or the data can be used to control or drive something else. In case parameters of the motion are calculated, the application may be motion analysis, sports analysis, biomechanics, biodynamics, etc. In case the data is used to drive a computer generated (CG) character or a scenery to mimic the motion, it is referred to as animation or visual special effects (VFX), etc. In case the data is used to control a machine, the application may be tele-surgery, tele-robotics, motion feedback control, etc. In case the data is used to control some displays or something else, the application may be virtual reality, interactive games, virtual training, virtual rehabilitation, motion directed music, etc.

The earliest form of motion capture was done with cameras taking a sequence of images (pictures) of the subject. Then the images are analyzed by people to figure out the 2D motions of certain points of interest of the subject. This technology is still being used today, except it has been advanced to become 3D in nature by using multiple cameras to simultaneously take multiple images of the subject from different directions, and software techniques have been developed to analyze the images by computer instead of by people.

The most difficult task of this motion capture approach is recognition of the points of interest, which seems easy in concept for human being but is highly difficult and time-consuming to do by computer software. The achievable accuracy of the recognition is also insufficient for many applications.

The difficulty of accurately recognizing the points of interest from marker-less images led to development of a new class of motion capture technologies which mark the points of interest explicitly with either sensors or markers. The electro-magnetic, mechanical, gyro, accelerometer and optical fibre based technologies mark the points of interest with sensors, while the optical technologies mark the points of interest with markers. The markers of the optical technologies are in turn distinguished as either 'passive' or 'active'. Passive markers do not generate light by themselves (e.g., reflective balls or checker-cross patches), while active markers do (e.g., LEDs).

Motion capture systems are also classified as 'self-contained' or not. Normally the mechanical, gyro, accelerometer and optical fibre systems are 'self-contained', meaning that they do not contain any other part which is separate from the capture subject. The electromagnetic, optical and some gyro-accelerometer systems are not self-contained and consist of parts which are separate from the subject which must be fixed relative to the ground.

The motion data captured by self-contained systems are normally referenced to the initial posture of the capture subject. This means that they can normally provide only relative posture information of the subject body. The absolute location of the subject, if provided, is derived by accumulating the previously sensed relative location movement information. Unfortunately the accumulation process ('integration') also accumulates any errors in the previous information, hence any absolute information provided by a self-contained system is prone to large errors which are referred to as 'drift'. To fix the problem some of these systems supplement their basic structures with additional sensing parts separate from the subject and lost their self-contained status.

The motion data captured by non-self-contained systems are referenced to the parts fixed relative to the ground. Unless the fixed parts moved, the absolute positions and location information of the subject will not drift. Should the fixed parts experienced any movement though (e.g., due to temperature variation or touching), a 'system calibration' process will need to be done in order to re-establish the data reference information (the 'coordinate reference frame'). This process involves collecting a relatively large amount of data from the capture space. Depending on the system being used, the process can be quite tedious, the user may be required to understand how it works, and it may need to be done frequently especially if the fixed parts are installed over a large and/or non-rigid frame.

Motion capture systems have been developed with at least seven distinct technologies as mentioned above. Depending on the application, deciding which system to use may be a brain teaser. Following are some of the questions one should consider before making a decision:

- How large capture space do you need?
- How much larger space do you have for setting up the system?
- Do you require high data accuracy or not?
- How complex motions do you need to capture?
- Do you need to capture motions from special angles?
- Do you need to use the captured data immediately (for 'real-time' applications)?
- In case of real-time application, can it sustain occasional system crashes
  due to the technology used?
- Do you mind having to learn well and do the system calibration manually?
- How much calibration work can you withstand?
- Will the system users leave regularly and new users need to re-learn the system all over again?
- How much can you withstand having to post-process the data before using it?
- Do you need the system to be portable and easily installed for use in different locations?

Motion capture system prices and features differ significantly. A wrong system can cost you much more during use than the extra price of another system. Many systems are being turned into junks because nobody has the time or interest or know-how to learn to calibrate them, to post-process their data, to fix their crash problems, or to re-configure them for capturing special motions. Therefore unless a large internal support resource will be available, the 'ease-of-use to yourself' should be placed high on your list of factors to consider before purchasing a system.