Upload
buikiet
View
218
Download
3
Embed Size (px)
Citation preview
Is a good dancer more sexually attractive?
(with Dr. Cronk and Dr. Trivers from Anthropology, Rutgers)
Pros and cons
• Truthfully record all the fine details of the natural motion
• The captured motion is difficult to be
• generalized
• modified
• controlled
How to use the data?• Off-line
• Motion libraries
• Motion graphs
• Training examples
• On-line
• Drive characters based on the movement of the actors in real time
Types of Systems
• Optical systems
• Magnetic systems
• Motion tapes
• Vision-based systems
• Inertial and ultrasonic systems
Optical systems
• Cameras
• High temporal resolution (120+ fps)
• Detect the locations of reflective markers
• Markers
• passive: sensitive to infrared
• active: emit LED light
Magnetic system
• Cumbersome sensors (heavier and also wired)
• Smaller workspace
• Record both position and orientation
• Lower resolution (80 fps max)
• Sensitive to EMI/metal in the environment
Motion tapes
Contain optical fibers and sensors that can detect the bending and twisting
Restriction of movement
Need another technology for detecting root translation
Measure the shape of surface precisely
Markerless mocap
• http://www.organicmotion.com/
• Kinect
Ultrasonic + Inertial
• A wearable self-contained system
• Inertial information is provided by gyroscopes and accelerometers
• Microphones are used to record the distance between each pair of sensors
Motion capture pipeline
calibration capturingmodel
building
marker labelinginverse
kinematicstrajectory
smoothing
Motion capture pipeline
calibration capturingmodel
building
marker labelinginverse
kinematicstrajectory
smoothing
Calibration
• Static calibration
• Figure out where the floor is
• Dynamic calibration
• Figure out the capture volume
Motion capture pipeline
calibration capturingmodel
building
marker labelinginverse
kinematicstrajectory
smoothing
Capturing• Marker placement
• Markers should move rigidly with joints
• Asymmetric placement helps in post-processing
• T-pose and range of motion
• Recording specific poses can help estimating bone lengths
3D marker position
• In principle, two cameras are sufficient to reconstruct the 3D location of a marker
• In practice, more cameras can
• reduce occlusion
• increase precision
Motion capture pipeline
calibration capturingmodel
building
marker labelinginverse
kinematicstrajectory
smoothing
Model building
• Given recored marker positions, estimate the dimension of each body part
• Optimize both bone length and handle positions at the same time
• Templates and heuristics help
Problem statement
generic skeletonrough handle positions
specific pose used for calibration
bone lengthhandle offset
+
Motion capture pipeline
calibration capturingmodel
building
marker labelinginverse
kinematicstrajectory
smoothing
Motion capture pipeline
calibration capturingmodel
building
marker labelinginverse
kinematicstrajectory
smoothing
Inverse kinematics
• Input: articulated body with handles + desired handle positions
• Joint angles that move handles to desired positions
Motion capture pipeline
calibration capturingmodel
building
marker labelinginverse
kinematicstrajectory
smoothing
Trajectory smoothing
• Global optimization that minimizes the velocity of the joint angles while staying as close as possible to the desired handle positions
Issues
The main problem with motion capture associated with characters has to do
with mass distribution, weight and exaggeration. It is impossible for a performer to produce the kind of motion exaggeration that a cartoon character needs,
and the mass and weight of the performer almost never looks good when applied to a character
of different proportions.
Eric Darnell, codirector of Antz
Issues
The mapping of human motion to a character with non-human proportions doesn’t
work, because the most important things you get out of motion capture are the weight shifts and the
subtleties and that balancing act of the human body. If the proportions change, you throw all that out
the door, so you might as well animate it.
Richard Chuang, VP at PDI
Biomechanical applications
• Understand and quantify the forces produced by muscles, ligaments, and tendons via noninvasive instruments
• Synthesize realistic human locomotion
Measurement
• Need to record accurate kinematic properties of the motion
• video or infrared based motion analysis systems
• Need to measure the external forces precisely
• force platforms that measures the ground reaction forces
Motion analysis• Interaction of muscle
contractions across several joints is extremely complex
• Most invasive devices can only measure forces in single tissues
• surgical stables
• buckle force transducers
Motion analysis
• Inverse dynamics can only measure the net effect of the internal forces and torques across several joints
• Inverse dynamics can compute total load on a system, but can not determine the distribution of the load
Model reductionReduce complex anatomical structures
F F
F!
F!
!F!
MF
Foot with muscle force F Forces F* and -F* added at ankle center
Couple F and -F* replaced by MF moment
Model reduction
force from triceps surae
ligament forcebone-on-bone forces
force from tibialis anterior
gravity gravity
ground contact forceground contact force
Fankle
Mankle
Limitations
• ID relies on assumption that are not always valid
• joint friction and air friction
• non-uniform distribution of mass
• movement of joint center of rotation
• approximation of body segment parameters
• Measurement error and numerical error propagation