Image analysis – Applications – Motion or velocity measuring
Reexamination Certificate
1997-12-04
2001-11-27
Boudreau, Leo (Department: 2621)
Image analysis
Applications
Motion or velocity measuring
C382S103000, C382S106000, C382S154000, C382S291000, C382S304000, C250S203300, C250S559320, C356S620000, C356S623000, C702S152000
Reexamination Certificate
active
06324296
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is directed toward a system and method for determining the positions of plural objects over time, i.e., to capturing or tracking motion of objects. More particularly, the invention is directed toward an optical motion capture (MC) system and method.
2. Description of the Related Art
Motion capture is one of the hottest topics in computer graphics animation today. What is motion capture? Motion capture involves measuring at least one object's position and orientation in physical space, then recording that information over time in a computer-usable form. Objects of interest include human and non-human bodies, facial expressions, hand gestures, camera or light positions, and other elements in a scene.
Once data is recorded in computer-usable form, animators can use it to control elements in a computer generated scene. Such a scene can be used in a biomechanical analysis, e.g., in a reproduction of a golfer's swing for slow motion analysis from a variety of computer-generated viewing angles by an instructor. Such a scene might also form the basis of an animated sequence in a science fiction movie.
Animation which is based purely on motion capture uses the recorded positions and orientations of real objects to generate the paths taken by synthetic objects within the computer-generated scene. However, because of constraints on mismatched geometry, quality constraints of motion capture data, and creative requirements, animation rarely is purely motion capture-based.
Data from real-time motion capture devices can be used interactively (assuming minimal transport delay) to provide real-time feedback regarding the character and quality of the captured data. Non-real-time motion capture devices either provide data that requires additional post-processing before it can be used in an animation or computer graphic display or provide data that is merely a snapshot of the measured objects.
Motion capture systems reflect a balancing of a number of competing considerations, or tradeoffs. Those tradeoffs are: the number of points that can be tracked, resolution, ease of use, affordability, convenience and flexibility in terms of how readily the system can adapt to different motion capture tasks. The typical motion capture systems are either magnetically or optically based.
MAGNETIC MOTION CAPTURE SYSTEMS
Magnetic motion capture systems use sensors to accurately measure the magnetic field created by a source. Such systems are real-time, in that they can provide from 15 to 120 samples per second (depending on the model and number of sensors) of
6
degree-of-freedom data (position and orientation) with minimal transport delay.
A typical magnetic motion capture system has one or more electronic control units into which the source(s) and 10 to 20 sensors are cabled. The electronic control units are, in turn, attached to a host computer through a network or serial port. The motion capture or animation software communicates with these devices via a driver program. The sensors are attached to the scene elements being tracked. The source is set either above or to the side of the active area. There can be no metal in the active area, because it can interfere with the motion capture.
The ideal approach for magnetic motion capture is to place one sensor at each joint of a body. However, the physical limitations of the human body (the arms must connect to the shoulder, etc.) allow an exact solution with significantly fewer sensors. Because a magnetic system provides both position and orientation data, it is possible to infer joint positions by knowing the limb lengths of the motion-capture subject.
The typical magnetic motion capture session is run much like a film shoot. Careful rehearsal ensures that the performers are familiar with the constraints of the tethers and the available “active” space for capture. Rehearsal often includes the people handling the cables to ensure that their motion aligns to the motion of the performers. The script is broken down into manageable shot lengths and is often story boarded prior to motion capture. Each shot may be recorded several times, and an audio track is often used as a synchronizing element.
Because the magnetic systems provide data in real-time, the director and actors can observe the results of the motion capture both during the actual take and immediately after, with audio playback and unlimited ability to adjust the camera for a better view. This tight feedback loop makes magnetic motion capture ideally suited for situations in which the motion range is limited and direct interaction between the actor, director, and computer character is important.
ADVANTAGES OF TYPICAL MAGNETIC MOTION CAPTURE
A magnetic motion capture system has several advantages. It provides position and orientation information, and so requires fewer sampling locations and less inferred information. Distances and rotations are measured in relation to a single object, the source, so there is less device calibration. Registration with other data requires only a knowledge of the source location (and obviously the measurement accuracy). Real-time interactive display and verification of the captured data is made possible, providing a closed loop model where the actor(s), director and production staff can all participate directly in the capture session. The cost of a typical magnetic system is less than ⅓ to ⅙ of the cost of a typical optical system.
DISADVANTAGES OF TYPICAL MAGNETIC MOTION CAPTURE
A magnetic motion capture system has several disadvantages.
The commercially available magnetic motion capture systems are so sensitive to metal that they cannot be considered office or production environment friendly devices. Care must be taken that the stage, walls, and props for a motion capture session are non-metallic. The maximum effective range of these devices is substantially less than the maximum possible for optical systems, although for longer ranges optical system accuracy decreases linearly (or nearly so).
The subject of a magnetic system is encumbered by cables. The sensors (rather than the sources) are located on the subject(s) and are connected to control units via fairly thick cables to a human subject. The sampling rate is too low for many sports motions. For body tracking applications, magnetic systems tend to have 30 to 60 Hz effective sampling rates. A fastball pitcher's hand moves at roughly 40 meters per second, approximately a meter per sample. Also, filtering is typically used to compensate for measurement jitter, reducing the effective frequency range to 0 to 15 Hz.
OPTICAL MOTION CAPTURE SYSTEMS
Typical optical motion capture systems are based on high contrast video imaging of 20 to 32 markers which are attached to the object whose motion is being recorded. The typical passive markers are retroreflective, e.g., small spheres covered with reflective material. The typical active markers are light emitting diodes (LEDs). A typical active optical system only permits one marker at a time to provide light so as to make it trivial to identify the marker.
The markers of an optical system are typically imaged by standard or HDTV high speed, black and white digital cameras. At a mere 30 frames per second, or 30 Hz sampling rate, the typical system must process between 307200 pixels, for a 640×480 camera, and 1,024,000 for a 1280×800 camera. Some optical systems increase the frame rate, i.e., the sampling rate, by decreasing the resolution, but that is an undesirable compromise.
The number of cameras used in a typical optical system depends on the type of motion to be captured. Facial motion capture usually uses one camera, sometimes two. Full body motion capture may use four to six (or more) cameras to provide full coverage of the active area. To enhance contrast, each camera is equipped with infrared (IR) emitting LEDs and IR (pass) filters are placed over the camera lens. The cameras are attached to controller cards, typically in a PC chassis.
Depending on the system
Black John R.
Jivan Vinay C.
Johnson Jack L.
McSheery Tracy D.
Nollet Scott R.
Boudreau Leo
Phasespace, Inc.
Werner Brian P.
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