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Motion capture techniques have been used is started from the late of 1970s, this techniques is used for recording the human actions or body movement. Motion capture is tracking the point of the human arms, joints of the body parts. After tracking the movement actions, they will implement the body movement to the animation characters.
Goniometers(Simon Fraser University) from 1980 - 1983
Within this period of time, biomechanics labs were started to use computers to analyze and identify the human movement. Computer Science and Professor of kinesiology at Simon Fraser University - Tom Calvert, using potentiometers to a body and combine with the computer animated characters for clinical assessment of movement abnormalities and choreographic studies. (David J.Sturman, 2007)
Graphical Marionette (MIT) from 1982 - 1983
New York Institute of Technology Computer Graphics Lab and MIT Architecture Machine Group experimented with optical tracking of the human body. The optical tracking is pick out the reflecting dots or the LED flashing points. (David J.Sturman, 2007)
Mike the Talking Head (deGraf/Wahrman) from 1988
deGraf/Wahrman using his new 4D machines to developed "Mike the Talking Head" for Silicon Graphics to display the real-time capabilities. The Silicon Graphics hardware can provide real-time combination between head geometry and facial expressions. (David J.Sturman, 2007)
Waldo C. Graphic (Pacific Data Images) from 1988
Due to the new Silicon Graphic 4D series workstation and the professional in Pacific Data Images they found the solution to solve the capability of the technology problem that faced Jim Henson Productions. The new Silicon Graphic 4D series workstation can custom tracking 8 degree of freedom input device through the SGI. They are able to control the points or positions of the mouth movements in real-time. (David J.Sturman, 2007)
Dozo (Kleiser - Walczak) from 1989
Kleiser Walczak produced Dozo, he used multiple cameras to triangulate the images of the reflective tape that placed on the actors body. The output will be based on the three dimensional trajectory of each reflector. (David J.Sturman, 2007)
Mario (SimGraphics) from 1992
SimGraphics developed a facial tracking system name as Face Waldo. They separate the face into four parts, which is chin, eyebrows, cheeks and each part is attached by the mechanical sensors. The advantages of this system is one actors can manipulate all the facial expressions of a character and the actor only act like as himself/herself the only needed is the facial expression. (David J.Sturman, 2007)
Alive! (Brad deGraf) from 1992
He developed a hand device which is having five plungers actuated by the puppeteer's finger. This device is used to control the facial expressions. (David J.Sturman, 2007)
Acclaim from 1993
Acclaim had developed a power and high performance optical motion tracking system that much likely used for Dozo and Graphical Marionette. The different of this optical motion tracking system is, this system able to track up to hundred points simultaneously in real-time. The purpose for Acclaim developed this power system is because this system can helped in generate character motion for video games. (David J.Sturman, 2007)
Motion capture or motion tracking are define as the process of track or recording the body movement or action of the actors and convert to movement or actions into digital model. The motion tracking system is used for merge with the computer animated characters; to do this is because make used of the reality actions that same with the real human. The motions capture system with track our human arms, legs, body movement and others, to translate the motions into digital model and implement to the computer animated character to perform a smooth actions.
Motion capture system is sampled many times per second, so the movement of the actor will be record point by point. The motion capture system is also referring to the performance capture, because high performance and fast sampled rate can avoid the missing movement of the actors.
After getting the entire movement digital model, the motion model will mapped to the animation data. So the 3D model character will perform the same actions as the real actors with the smooth and nice motion.
Motion capture systems there are basically separate into three types which is electromagnetic, mechanical and optical based system. Even separate into three but the basic process are still the same.
First step is the input, recording the actors' movement.
Second step is the processed to identify the corresponding marker of the actors and transfer into digital model.
Lastly, the result is converting into 3D trajectory data (mocap file).
Figure 1.0.1: The process of motion capture (K.C.Beh 2011)
Figure 1.0.2 : The examples of Motion Capture(Lord of the ring & Polar Express, 2008)
Problem statement of motion capture
Motion capture is helpful for apply into many fields for creating the physically perfect and smooth motion, but this system still has few significant weaknesses. This motion capture has low flexibility, secondly the recording data is based on the motion capture system; so different motion capture system have different data format.
Motion capture mostly used in animation and gaming industry, normally they will stored the motion clips that already capture or will used for a particular animation or games project. For example in sport science the motion capture data is used for analyzing and to make the actions or movement prefect they need to repeating the actions to get the better samples. This mean the motion capture library are accumulated with the vast of motion collections, and this is not good in stored to the database and retrieved the data files as database.
Due to the grow of motion capture system, many organizations is started to developed their own motion capture system which has their own calculate format and the file formats also design in their own specific format. So there are no standard motion file formats, it is hard for exchanging the motion capture data and different motion capture system has their own format mean it is hard to predict how different software will react with the different motion capture files.
Systems & Techniques
Motion capture or motion tracking was started from 70's to 80's, that time was a biomechanics research. They implement into training, education, sports and currently for computer animation. The actors will wear the markers to fit each joint, because they want to identify the motion by angles and positions between the markers. LED, inertial, acoustic, magnetic equipment or reflective markers, all of this are used to tracking the motion, the rate of tracking is very fast can be many frames in one second, because want to sampled out the desired motion high tracking rate is needed even there is only sub millimeter positions(lis3353 2010).
3.1 Optical Systems
Optical system is used to utilize the data that is capture from the image sensors and using this data to triangulate the 3D positions. They are using the technique of one or more cameras to capture to provide overlapping projections. This system need to using the special marker and attached to the human actor. These system produce data (X, Y, Z) 3 degrees of position for each marker, so it is able for recording the angle of elbow, shoulder and wrist.
3.1.1 Passive markers
Passive optical system is using retroreflective material as the markers, because the reflective material can reflect the light back to the cameras lens. The reflective light and marker will be sampled by ignore the fabric and skin, of course the camera threshold need to be adjusted and only will sampled out the reflection markers.
To find out the center point of the marker is, they estimated the position between the 2 dimensional images that camera capture just now which is in grayscale value, because black and white can easily different out the reflective light and calculate out the sub-pixel which is accurate.
3.1.2 Active marker
Active optical system triangulates the positions by tracking the one or multiple LED at a time and using the special software to identify the relative positions. This is the reverse method of using the reflective light, the marker themselves are having the power to emit their own light. Since the inverse of square law provides one quarter the power at two times the distance, this can enhanced the volume and distances for capture.
There are two movie example are using this technique for the motion capture which is Van Helsing and the TV series Stargate SG1. They using this technique is because, it is useful in real-time application to identify the marker.
Figure 188.8.131.52 : The Example of Active Marker(Voice of the crew 2011)
3.1.3 Time modulated active marker
Time modulated active markers are refined by tracking one or multiple markers over a time and modulate the pulse width or amplitude to provide marker ID. The unique ID can avoid stack and providing much consistent data.
3.1.4 Semi-passive imperceptible marker
One method that reverses the traditional approach that based on performance cameras. The system uses the way of photosensitive marker tags to decode the optional signals, rather than using the active or retroreflective light.
The marker is tagging by the photo sensors, the tags can compute out the reflectance, orientation and incident illumination.
This tracking tags is work well in outdoor which having the better natural lighting conditions. The system also can support unlimited tagging in a scene, because each tag is unique to a identify marker.
Figure 184.108.40.206 : The Example of Semi- passive Marker
(Animation & Fashion Design 2010)
3.2 Non-optical systems
3.2.1 Inertial systems
Inertial motion capture system is based on the biomechanical models, sensor fusion algorithms and miniature inertial sensors. Most of the inertial system is using gyroscopes to calculate or measure the rotational rates. The rotation data rate will translate to a skeleton model. Using inertial systems no marker or external cameras are needed for the relative motions.
This Inertial system are similar to the Wii controller, it is very portability and no need large capture areas. The weakness for this inertial system is the accuracy problem.
Figure 220.127.116.11 : The Inertial Systems(Rec-Motion 2010)
3.2.2 Mechanical motion
Mechanical motion capture, as the name describe, the actor need to wear the mechanical equipment which directly track the body joint angles, the method is similar to the exo-skeleton motion capture system. When the actor make an action, the mechanical parts stretch and through this method the system can measured or calculate out the angle and degrees. Mechanical motion capture system are cheaper than others, real-time based and wireless systems.
Figure 18.104.22.168 : The Mechanical Motion(Ignition 2010)
3.2.3 Magnetic systems
Magnetic system calculate position and orientation is depend on the both transmitter and each receiver that having magnetic flux of three orthogonal coils inside. (lisha3353 2010) The sensor output is six degree of freedom (6DOF), which mean it can giving a consistent and useful result to obtained the 2/3 of the number of markers that required from the optical systems. Magnetic system is having problem on the magnetic and electrical interference, because something like metal of wiring will affect or influence the magnetic field. So with the magnetic system, there are separate into two power system which is AC and DC systems; one using sine wave another one used square pulses.
Figure 22.214.171.124 : The Magnetic System(Ignition 2010)
4.0 Related techniques
4.1 Facial motion capture
The facial motions capture also a type of motion capture system, but as the name we know that this facial motion capture system is focus on the face expressions. Normally the motion capture system also got provide the low grade facial motion capture system which can provide low resolution facial data capture that can only utilize 32 to 300 markers.
So want a high fidelity facial motion capture, the technology is needed, because facial motion capture is needed a high performance computer to utilize the record and the complex movement of face expression.
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Figure 4.1.1 : The Facial Motion Capture(cnetaustralia 2010 & Fxguide 2010)
4.2 Radio frequency positioning
Radio frequency positioning system are becoming as a higher frequency radio frequency devices that allow better precision. The speed of light is 30 cm per billionth of a second, so 10 billion cycles per second radio frequency signal is enables an accuracy about 3 cm.
4.3 Non-traditional systems
This is giving the unlimited walking area by using the rotating sphere, for example like the hamster ball for the hamster keep running and the sphere will keep rotating. For this systems the "hamster ball" is apply with the internal sensors to record the angular movements, using this method can reduce the needs of external cameras and the external sensors equipment.
5.0 Advantages & Disadvantages of Motion Capture
5.1 Advantages of Motion Capture
The advantage of motion capture is the different between live actors and computer animated character. Because in the real world, one people act for one character; but motion capture technology is allows one actor to act multiple characters in a single film. The actor only needs to stay in front of a greenscreen or bluescreen with wearing the motion capture equipments and act for the movement or actions.
The second advantage for motion capture is easy for the fitting screen compared to the real video shooting. To create a large amount of CGI movie using computer animated characters sometimes is less problematic, because all the characters is in computer animated world, so the environment will become same visual look and look naturally.
The third advantage is the director can choose the unusual angle or any angle as he desire for a scene, because in the real world catching the video shooting angle is not easy. Using motion capture is allowing the rotating effect, so the director can rotate until the angle that he want.
The fourth advantage is using motion capture is more rapid. The result can be process even in real-time, they only need to implement the tracking motion to the computer animated character will do.
The fifth advantage is the real world actor need to have make-up, costumes, pretty face for the actor, body size and etc. With motion capture, the listing job above can be avoided, even the lighting, colors and filters for film shooting also no need, because all the job can be done with digitally later.
5.2 Disadvantages of Motion Capture
The disadvantage of motion capture is this system needed the special programs to calculate and specific hardware to obtain and make used of record or process the data. The cost of the software and hardware equipment is very costly.
The second disadvantage of motion capture is, even you getting the motion capture system you still need to find a large empty space, because the motion capture system have the own specific space requirements to make it working.
The third disadvantage of motion capture is, when the tracking data is having problems it is easy for you to reshoot the scene rather than finding out the errors of the motion capture data. And only have a few motion capture systems is allow for real-time viewing of the data.
The fourth disadvantage of motion capture is, the tracking digital model is hard to apply the motion into quadruped characters. Because the height and width is not same as human and is hard to let an animal to wear the equipment and follow the action instructions to do the movement that you prefer.
The fifth disadvantage of motion capture is, the movement or actions that does not follow laws of physics normally is cannot be represented. Even sometime the real life performance also not really can be translated to the computer model as they expected.
The sixth disadvantage of motion capture is, if the computer animated character model has different proportion or ratio, for example the character has over-sized or big hands, so the tracking point may intersect with the body part when the hand point is near to the body point. So the model will become very weird or stack together, the motion will be failed.
The seventh disadvantage of motion capture is, the motion capture system is follow the technology trend, one system maybe will used for only few film or games product. After few years there will be come out another new motion capture system with better software and techniques are implemented or more advanced. Money wasted.
6.0 Trend Motion Capture
6.1 Current Trend
6.1.1 Motion Capture for Games
Mocap is a great way and tools that is helpful in create animation for several types of games. The most useful thing for mocap is using in the 3D game with character animation. Currently few of the games company are using these types of techniques, for example like Namco, Square, RARE, Gremlin, Electronic Arts and Konami(META Motion 2010).
Because this few games company are like to create the sports games for example like football, American football, basketball, wrestling and others. All of the example above are in characters based, let take Konami and Electronic Arts as topic. These two games companies are create many types of sports game which is famous in soccer or football and basketball or NBA. In the real world, some of the sport player are having the own styles, so the games company need to used the motion capture system to model out the motion and implement to the animated characters in the games.
6.1.2 Motion Capture in Animation
Motion capture in animation is become widely used in movie industry right now. Motion capture is getting famous in animation industry, is because there are few great mocap animation is produce and get a good response from the audiences.
There are a few example of the movie industry that are using motion capture method to create the animation movies. Beowulf, Polar Express and Monster House which is create by the Sony Pictures company. Inside the Beowulf, Angelina Jolie is act the medusa in the animation. For the Polar Express, Tom Hank are act as the train driver. Lastly, is the famous motion capture animation - Avatar is the milestones of the real-time motion capture systems, and it is very successful.
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Figure 126.96.36.199 : The examples of Animation Movies(MovieShop 2007& dvdactive 2008)
6.2 Future trend
Motion capture system should improve in the Markerless techniques which is now doing research by Stanford University. Using this technique, the actors no need wear any magnetic or sensor equipment for the motion tracking. This system is using the special algorithm that design to analyze multiple streams optical input. The system will break human into several part for tracking, as a good example that I can provide is the Microsoft Kinect system. Microsoft are plane to do the XBOX 360 which is using markerless motion capture for the players by using their own body as the controller.
In the future, maybe should improve in the games algorithm. For example, we can using the camera to identify or face and implement to the RPG game characters, rather than every times before start playing the game we need to create the character and choose the face, hairs, skin colors and others. It make the players more convenient and can make the games standard become higher, because there is now haven come out with this type of high fidelity facial capture that implement to the game.
Through this report, I understand that the technique of the motion capture and the how many types of the system are been using in the motion capture system. Motion capture systems basically are separate into two major system which is optical system and non-optical system. Within this two systems, there are still having many sub techniques inside the system, for optical system is passive markers, active marker, time modulated active marker, and semi-passive imperceptible marker; for the non- optical system is inertial systems, mechanical motion, and magnetic systems.
So now the motion capture is getting widely used, I hope that the price of the system will be decrease and the standard of the motion capture system should be standardize to all the motion capture system. Because the mocap file data is a major problem for all the motion capture system, each of the motion capture system is using their own type of file format.
The more advanced of the motion capture can provide the more accuracy model data, and this is how we can watch the nice and better quality of the animation movies and the games character are getting high fidelity. It is a good way of how motion capture have been used, and hope that in the future, the future trend I write as above will become true and available.