Motion capture is simply a way to track an object's action or processas it changes position relative to a fixed point in space. The This paperwill discuss what motion capture is, three types of motion capture, it'scurrent applications as well as a brief history of motion capture witha list of resources for further investigations. Motion capture is simplya way to track an objects action or process as it changes position relativeto a fixed point in space. The motion capture that we will be discussingis generally used for the mapping of motion onto a computer model withthe aid of software and various hardware inputs.
Motion capture is currently being used for a wide variety of applications,including virtual reality, simulation, entertainment (games, movies, andtelevision), medical, biomechanical, aeronautics, and robotics.
Magnetic motion capture uses sensors to measure the magnetic field createdby a source. The main components of magnetic motion capture systems areelectronic control units (into which the source and sensors are cabled)and host computer connected via a network or serial port. The computerhas a proprietary software that communicates with these devices. The sourceis usually placed either at the side or above the the scene elements tobe tracked. All the data captured is in relation to the source. These systemsanalyze data in real time and provide from 15 to 120 samples per second(depending on the model and number of sensors) The sensors are usuallypositioned at the joints(in the case of a human subject) and the feedbackincludes both position and orientation data (also called "6D"data or "6DOF" data.).
Atypical magnetic motion capture session has the same elements as a filmshoot. This includes a rehearsal to make the performers familiar with theconstraints of the cables and the available "active" space forcapture. The shot may be recorded several times along with an audio trackfor synchronization. And since the data is being captured in real time,it gives the director an opportunity to see it right away and make changes.Magnetic motion capture is ideally suited for situations in which the motionrange is limited and direct interaction between the actor, director, andcomputer character is important.interpolation after the motion has beencaptured. Another advantage is that all the data is in relation to a singleobject, the source.
This makes it easier when the captured data has to be transferred ontoa computer-generated object. Magnetic motion capture systems have beenin use in a variety of production environments, including on-air and liveperformances, for quite some time. In fact, one of the major companiesthat manufactures these systems, used these systems for military simulations25 years ago.Today, these systems are used in a variety of environments,mainly because of their ability to provide real time feedback.
This gives an opportunity for the talent, direction, and productionto all participate directly in the capture session.Since these systemsrely on magnetic data, one of the biggest disadvantages is their sensitivityto metal. This tends to make these sytems less friendly to office or productionenvironments. Therefore care must be taken that the stage, walls, and propsfor a motion capture session are non-metallic. Another drawback of thesesystems is that their effective range is substantially less than the maximumpossible for optical systems. Also, the use of cables attached to the body,causes a certain degree of limitation and encumbrance. This may not bea disadvantage for long due to the recent development of wireless systems.Magnetic motion capture is also not suitable for fast movements since thedata sampling rate, typically 30 to 60 Hz, is too slow. This problem isexacerbated when filtering is used to compensate for measurement jitter,effectively dropping the sampling rate down to 0 to 15 Hz.
Optical Motion Capture is a technique that detects motion visually.In most systems, proprietary software is used to translate the data fromthe hardware to a variety of popular modeling software packages. In additionto software, the main components of an optical capture system are reflectivemarkers (passive-sensors) and an array of cameras. The markers are smallspheres, about 1-5 cm in diameter, covered in a reflective material. Theyare attached to a performer (or object) at key points, such as joints,and the more markers that are used the more detailed the data produced.Typically anywhere from 20 to 70 markers are used. When capturing facialexpression only, sometimes only a single camera is used, but for full bodycapture at least 4 cameras should be used. As with the markers, the morecameras used the more accurate the data will be. The cameras are generallyequipped with infrared LED's and filters, to enhance the contrast of theimage so that only see the reflectors. Special care must be taken in thestudio setup and costuming to avoid false reflections. Before capture begins,the array of cameras must be calibrated using a carefully constructed sceneof markers in 3D space. Once capture has begun, the cameras track the XYposition of each marker, producing a stream of 2D data. The various proprietarysoftware programs then convert these into 3D XYZ position data sets forevery marker.
Oneof the distinct advantages of optical capture versus magnetic capture isa larger performance area, and therefore more freedom of movement for theactor. This advantage can be further enhanced by having more cameras. Inaddition, optical systems usually have a higher sampling rate: generally120mHz to 250MHz, which is sufficient to capture most human motion, withthe exception of impact-type and pitching (hitting/throwing) type movements.
Optical systems provide only position ("3D" or "3DOF")data. Optically captured data often requires tremendous editing to correctfor some of the inherent flaws in the system, such as marker occlusion,fusion and hidden-marker situations. Another common complaint is the inaccuratetranslation of movement to the virtual character when it's proportionsdo not match that of the performer. Recent improvements in translatingsoftware help automate these processes, and perhaps will eventually allowoptically captured motion to be used directly in character animation withno intervening effort. Casting, choreography and shot setup planning alsohelp solve these problems.
Although most optical systems can produce much more sophisticated animationsthan those produced magnetically, one of the distinct disadvantages isthat most optical capture systems do not produce data in real time. Therewill be some processing time of the data before it can be applied to ananimation, thereby reducing the interactivity of the motion capture session.
However,a company called FutureLight recently unveiled the first real-time opticalmotion capture system, in which a preliminary low-resolution version ofthe animation is produced instantly and later scaled to size. In theirsystem, the performer wears a specially designed suit and a VR headsetthrough which they can watch the animated character as it mimics theirmovements.
There are other systems currently in use which could be loosely groupedunder the description "Mechanical Motion Capture," the use ofmechanical means to derive the motion data. Different companies have widelydiffernet ways of utilizing the sort of input.Digital Image Design hasdeveloped a poseable human figured armature device. "The Monkey"has position encoders at each joint that are attached to control unitswhich tracks a movement on a host computer. This is familiar, yet moreefficient way for stop motion animators to interact with computer animation.
However, while the device can provide continuous data of the positionof the mannequin, it does not capture continuous motion. Lamb & Co.uses a data glove which senses the strain applied to mechanisms in the
fingerswhen a performer articulates his hand. This mechanical information is thentranslatednto motion of the hand model in the computer. Analogus Corporationhas introduced the Gypsy, a full body system using potentiometers placedat all major body joints. All possible movements of the joints are sensedas variations in potentiometer voltages. Gypsy captures motion in truereal-time, and is not range limited. With a portable data acquisition unit,it's possible to capture long-range activities such as skating or biking.
Puppet Works produces a configurable solution as a series of digitaljoints that can be quickly mapped to software elements. Custom hardwarearmatures can be created (like the Monkey), as can a full motion capturesuit, based on an external skeleton of telescoping rods and connectors.
Motion capture has spawned a whole new field, now called "performanceanimation."
But there is still much contention about the true usefullness of motioncapture in the entertainment / animation industry, which is where the primarydevelopment of the technology is taking place. Much of the arguments revolvearound the difficulty of using the proprietary software systems to translatethe raw data into the desired effect. Most companies are still using traditionalkeyframe animation, and the ones which are using motion capture are stillsuplementing it with keyframing. However, if advances in the field continueat the current rate, it's likely that it could soon become the primarymeans of character animation. This paper is by no means a definitive summaryof the technology in this constantly evolving field of data aquisition.