Applications. Systems. Motion capture pipeline. Biomechanical analysis. Graphics research

Transcription

1 Motion capture

2 Applications Systems Motion capture pipeline Biomechanical analysis Graphics research

3 Applications Computer animation Biomechanics Robotics Cinema Video games Anthropology

4 What is captured? Humans Objects Animals Celebrities

5 What is captured? Face Whole body Hands

6 Pros and cons Truthfully record all the fine details of the natural motion The captured motion is difficult to be generalized modified controlled

7 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

8 Applications Systems Motion capture pipeline Biomechanical analysis Graphics research

9 Types of Systems Optical systems Magnetic systems Motion tapes Vision-based systems

10 Optical systems Cameras High temporal resolution (120+ fps) Detect the locations of reflective markers Markers Sensitive to infrared

11 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

12 Motion tapes Contain optical fibers and sensors that can detect the bending and twisting Restriction of movement Another technology for detecting root translation Measure the shape of surface precisely

13 Vision-based mocap

14 Applications Systems Motion capture pipeline Biomechanical analysis Graphics research

15 Motion capture pipeline calibration capturing model building trajectory smoothing inverse kinematics marker labeling

16 Motion capture pipeline calibration capturing model building trajectory smoothing inverse kinematics marker labeling

17 Calibration Static calibration Figure out where the floor is Dynamic calibration Figure out the capture volume

18 Motion capture pipeline calibration capturing model building trajectory smoothing inverse kinematics marker labeling

19 Capturing Marker placement Markers should move rigidly with joints Asymmetric placement helps in post-processing T-pose and joint exercises Recording specific poses can help estimating bone lengths

20 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

21 Motion capture pipeline calibration capturing model building trajectory smoothing inverse kinematics marker labeling

22 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

23 Problem statement + generic skeleton rough handle positions specific pose used for calibration bone length handle offset

24 Motion capture pipeline calibration capturing model building trajectory smoothing inverse kinematics marker labeling

25 Marker labeling Ghost markers Missing markers Switching trajectories

26 Raw data 3D locations of markers

27 Motion capture pipeline calibration capturing model building trajectory smoothing inverse kinematics marker labeling

28 Inverse kinematics Input: articulated body with handles + desired handle positions Joint angles that move handles to desired positions

29 Motion capture pipeline calibration capturing model building trajectory smoothing inverse kinematics marker labeling

30 Trajectory smoothing Global optimization that minimizes the velocity of the joint angles while staying as close as possible to the desired handle positions

31 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

32 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

33 Applications Systems Motion capture pipeline Biomechanical analysis Graphics research

34 Biomechanical applications Understand and quantify the forces produced by muscles, ligaments, and bones via noninvasive instruments Synthesize realistic human locomotion

35 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

36 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

37 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

38 Measurement Inverse dynamics assumes there is no cocontraction of agonist and antagonist muscles

39 Joint kinetics Equal in joint forces and moments, but completely different in muscle activities

40 Model reduction Reduce complex anatomical structures F F F F M F F Foot with muscle force F Forces F* and -F* added at ankle center Couple F and -F* replaced by M F moment

41 Model reduction force from triceps surae ligament force bone-on-bone forces force from tibialis anterior F ankle M ankle ground contact force ground contact force gravity gravity

42 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

43 What s next?

44 Field trip to ANIMO lab meet at the front door of GCATT building (bring your Buzz Card) Need one volunteer Read: Layered acting for character animation by Dontcheva et. al.

45 4/9 Project 3 demo 4/4 Final project proposals

46 Final project proposal 5-10 minutes each group Problem statement Approach Expected results