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УДК 130.2
РАССМОТРЕНИЕ ПРИНЦИПОВ И ПРИМЕНЕНИЯ ТЕХНОЛОГИИ ЗАХВАТА ДВИЖЕНИЯ
Ляо Шао-Бин
сотрудник отдела кино и телевизионного искусства Шанхайский издательско-полиграфического колледж Шанхай 200093, Китай
Аннотация. Технология захвата движения вступила в фазу практического применения, возможности этой технологии превышают анимационное производство, исследуются обширные возможности применения в таких областях, как виртуальная реальность, игры, эргономика, симуляторы, биомеханика и др.
Ключевые слова: технология захвата видео, трек движения, фотосфера, оптический тип.
DISCUSSION ON THE PRINCIPLES AND APPLICATION OF MOTION CAPTURE TECHNOLOGIES
Liao Xiaobing
Department of Film & Television Art Shanghai Publishing and Printing College Shanghai 200093, China
Abstract: Motion capture technology has entered the phase of practical application, of which the range has far exceeded animation production, and has explored vast application possibility in fields such as virtual reality, games, ergonomics, simulated training, biomechanics and etc.
Keywords: motion capture technology movement track photosphere optical type.
1. Introduction
The origin of motion capture technology dates back to the 1970s, when Disney tried to improve its animation production effect by capturing the motions of ac-tors.When computer technologies were initially applied in animation production, Rebecca Allen from New York Computer Graphics Technology Lab designed a type of optical device, which projected the acting postures of the actors on the computer screen as the reference for animation production.After that, starting from the 1980s, the US Biomechanics Lab, Simon Fraser University and MIT successively began their studies on computer capturing human motion.Since then, motion capture technology started drawing attentions from a growing number of researchers and developers and spearheaded practical application gradually from trial studies.In 1988, SGI developed a system which was able to capture the motion of human heads and expressions.With the rapid development of hardware technologies of computers and the rising of requirements on animation production, motion capture has entered into the phase of practical application in developed countries. Consequently, many manufacturers launched commercialized motion capture devices, such as Vicon, X-Ist, FilmBox, MotionAnalysis and etc., of which, the application scope far exceeded motion capture, successfully stepping into virtual reality, games, ergonomics, simulated training, biomechanics and etc.
2. Principles of motion capture technologies
As of today, general motion capture technologies are divided as mechanical, acoustic, electromagnetic and optical types.Devices of different principles may have their-respective advantages and defects, which are evaluated from the following aspects: the precision of positioning; timeliness; convenience of use; scope of movement which can be capture; cost; anti-interference; multi-target capture capability.Among which, optical motion capture technology is the most state-of-art technology currently and in the following paragraphs, I will give some more detailed statements of this technology.
An optical motion capture device generally consists of the following parts:
• Sensor fixed onto a certain location of the moving article, sending location information of the move-ment.Adopt normally photospheres made of special materials and stick such photospheres onto a certain location of the body.
• Signal capture device Responsible for capturing and identifying the signals of sensorsAs shown in Figure 1, the device is generally made of camera which can capture optical wave in special wave range, illuminator, lens and optical filters.
• Data transmission device Responsible for transmitting motion data fast and correctly from the signal capture device to the computer system.
Figure 1. Signal capture device
• Data processing device Responsible for processing the original signals captured by the system, calculating the movement track of the sensor, correcting and processing data and working together with 3D-role models.
Optical motion capture completes its task by monitoring and tracking specific spots on the objects. The common optical motion capture is mostly based on visual principles of computer. Theoretically, if a spot in a space can be seen simultaneously by two cameras, then the location of the point in the space at that moment can be located by the images snapped by two cameras at the same moment and the camera parameters. When the camera continuously snaps at a sufficiently high speed, then the motion track of such point can be obtained by image sequence.
Typical optical motion capture system generally employs 8-24 cameras deployed around the acting site. The overlapping area of the views of these cameras is the motion range of the actor (as shown in Figure 2).To facilitate processing, actors are generally asked to wear clothes of single color and stuck with some specially made signals or lighting spots (called as "Markers") on joints, hip, elbows, wrists and etc. The visual system will recognize and process such signals. After the system is calibrated, the camera will continuously snaps the motions of the actor and stores the images in orders for analysis and processing, which identify the signal points in the images and calculate the space location of each moment and then the movement track is gained. In order to acquire correct movement track, the camera is required to have relatively higher snapping speed, generally over 60 fps per second.
Expression capture may be realized by sticking small Markers on key expression points of the actors; however, the location of the cameras should be rearranged. Generally, use 3 to 6 cameras to face the face of the actor for the production of facial expressions.
The advantages of optical motion capture include large motion range for actors, no limitations on cables and mechanical devices, which allow actors to perform freely and conveniently. The sampling rate is high, which meets with the requirements of various high-speed motion measurements. Marker is cheap and easy for expansion.
The disadvantages of this method include requirement of a large amount of post-processing tasks (including identification of Marker, tracking and calculation of space coordinates) and certain requirements for lighting and reflection of the acting site. The calibration of the device is also a little complex. Especially, when the movement is complicated, Marker of different locations may become mixed and thus lead to mistakes, which need human interference of the post-processing procedure.
Figure 2. Motion range of actors
3. Application of motion capture technologies in animation production
Motion capture technology measures and records the motion track of the limbs of actors in 3D space for role animation production. It captures the movements of the actors and then transforms such movements into digital-ized "abstract movement" so the animation software may use such information to "drive" other role models and the model can finally perform the same motions as actors.
Motion capture technology may endow real living motions into the roles animated by the computer, thus the director may join the two parts seamlessly, making the animated motions more coordinated and vivid. Meanwhile, this technology may capture the motions of various actors. The director may use such information to drive hundreds even thousands of roles on the background and create splendid scenes. Hulk and Shrek all employed this technology. The precision of the motion capture technology is awesome, and the roles on the screen are extremely life-like.
In traditional animation production technologies, the motions of roles are adjusted by animators. In most cases, such as the subtle difference between walking postures of males and females, artist performance of top dancers, sports and changes in expression are very hard to achieve vivid and nature effects by manual adjustment. Very differently, the motion capture system directly captures the motions performed by the actors and drives role models by real motions and expressions and thus the final workscan be extreme real and natural. It is very hard for traditional animation production technologies to realize such effects.
Using motion capture system to collect the motion data and drive the model may render the design results for the animators in a very short time. Animators are now exempt from repeatedly adjusting the posture and the expressions of the models. By using this method, the animators get access to complete tasks easily by direct motion performance. This technology will greatly enhance the efficiency of animation production, shorten production time and cut production cost. Many successful application cases indicate that the production cost for employing performance animation technology is only less than 10% of the cost of traditional methods. Animation production process for methods using motion capture/analyzing system to collect motion data drive models is more direct and the effects are more vivid. Especially for complicated motion design, this technology can effectively save the cost of production. Motion capture system can provide actual motions meanwhile the performing motions and role models are separate. By changing the corresponding relationship between the motions and role models, we can get some incredible special effects, such as, using motions of people to drive 3D models of tigers.
In motion capture system, the actor is responsible for acting out various actions and expressions and the system will capture and record such actions and expressions. The initial product is 2D motion track, which is further subject to synthesis by professional software such as IQ to get the 3D moving track, that is, a series of moving light spots. Then animation software such as MOCAP will be utilized to drive role models by such moving tracks (MAYA or MAX may be employed for production), and the role models are now able to perform the same motions and expressions as the actors and the final animation sequence is created. The task for motion capture is to detect and record the moving track of the limbs of actors in the 3D space, capture the movements of the actors and then transform such movements into digitalized "abstract movement". Generally, it is not required to capture the motion of all points of an actor, but only of some key points. And then the final moving images may be generated on the basis of the physical and physiological limitations.
Employing motion capture technology in animation production significantly improves the level of animation production. It greatly speeds up the efficiency of animation production and the process of animation production becomes more direct and the effect becomes more vivid by virtue of this technology. With further maturity of technology, motion capture technology will be applied in a wider range. And as an integral and vital part of animation production, motion capture technology is bound to be more significant in the industry.
4. Motion capture technology is also applied widely in other industries
• Robot remotecontrol Robots send the information of dangerous environment to the controller and the controller performs various motions based on such information. The motion capture system will capture such motions and send them to the robot in real time for the robots to complete the same motions. Compared with traditional remote control, this system can
realize more direct, delicate, complex, and agile and fast motion control, greatly enhance robots' ability to respond to complex situations. In the context that the total automatic control of robots is not yet mature, this technology has an exceptional significance.
• Interactive games Employ motion capture technology to capture the motions of the player and thus drive the motions of the roles in the games, providing a brand new feeling of engagement in the games for the player and enhancing the sense of reality and interactivity.
• Sports training Motion capture technology may capture the motions of the athletes for quantitative analysis, figuring out the method of improvement by combining with the principles of human physiology and physicals. Sports training are then not only based on pure experience. It enters into a theorized and digitalized time. The motions of athletes of poor performance may also be captured to be compared with those of excellent athletes, by which to facilitate the training of the former ones.
• Medical rehabilitation Motion capture/analysis systems with high technical content are widely utilized in sport medicine studies and other study areas such as neuro medicine, plastic surgery, dentistry, clinical medicine, rehabilitation therapy, and biokinetics. By capture and measurement of human motions in real time, the 3D movement track of all joints of human body may be obtained. On the basis of the data of 3D movement track, the users of the technology may easily get the displacement, velocity, accelerating speed and other movement information. By processing and analyzing the movement data, auxiliary diagnosis such as 3D gait analysis, aid to artificial limb production and rectification, spinal curvature correction may be realized. Moreover, it can also facilitate clinical medicine since by analyzing the motions of the patients of dyskinesia due to cerebrovascular disease, neural paralysis, joint disease and so forth, the doctor may choose and determine the therapeutic and rehabilitation approaches.
It is highly expected that with the development of the technology itself as well as associated application fields, motion capture technology may be employed in an even wider range.
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