I am a Ph.D. student in Mechanical Engineering Department at Carnegie Mellon University under the supervision of Prof. Yaser Sheikh and work closely with Disney Research, Pittsburgh.

I am interested in geometry in computer vision, 3D reconstruction of dynamic scenes, motion capture, and image based rendering.

• T. Shiratori, H. S. Park, L. Sigal, Y. Sheikh, and J. Hodgins, Motion Capture from Body-Mounted Cameras, ACM Transactions on Graphics (Proc. ACM SIGGRAPH), 2011. (project page)

  Motion capture technology generally requires that recordings be performed in a laboratory or closed stage setting with controlled lighting. This restriction precludes the capture of motions that require an outdoor setting or the traversal of large areas. In this paper, we present the theory and practice of using body-mounted cameras to reconstruct the motion of a subject. Outward-looking cameras are attached to the limbs of the subject, and the joint angles and root pose are estimated through non-linear optimization. The optimization objective function incorporates terms for image matching error and temporal continuity of motion. Structure-from-motion is used to estimate the skeleton structure and to provide initialization for the non-linear optimization procedure. Global motion is estimated and drift is controlled by matching the captured set of videos to reference imagery. We show results in settings where capture would be difficult or impossible with traditional motion capture systems, including walking outside and swinging on monkey bars. The quality of the motion reconstruction is evaluated by comparing our results against motion capture data produced by a commercially available optical system.

• H. S. Park, S. Floyd , and M. Sitti, Roll and Pitch Motion Analysis of a Biologically Inspired Quadruped Water Runner Robot, International Journal of Robotics Research, December, 2009. (url)

  In this paper, the roll and pitch dynamics of a biologically inspired quadruped water runner robot are analyzed, and a stable robot design is proposed and tested. The robot’s foot-water interaction force is derived using drag equations. Roll direction instability is attributed to a small roll moment of inertia and large instantaneous roll moments generated by the foot-water interaction forces. Roll dynamics are modeled by approximating the water as a linear spring. Using this model, estimates on the roll moment of inertia that can endure moments generated by water interactions are derived. Instability in the pitch direction is caused by the thrust force the four feet exert on the water. To correct this, a circular tail which can negate the pitch moment around the center of mass is proposed. Both passive and active tail designs which can cope with disturbances are introduced. Based on these analyses, a stable water runner is designed, and built. Experimental high speed video footage demonstrates the stable roll and pitch motion of the robot. Simulations are used to estimate robustness against disturbances, waves, and leg running frequency variations. It is found that roll motion is more sensitive to disturbances when compared to the pitch direction.

- Conference Proceedings

• H. S. Park and Y. Sheikh, 3D Reconstruction of a Smooth Articulated Trajectory from a Monocular Image Sequence, International Conference on Computer Vision (ICCV), 2011 (project page)

An articulated trajectory is defined as a trajectory that remains at a fixed distance with respect to a parent trajectory. In this paper, we present a method to reconstruct an articulated trajectory in three dimensions given the two dimensional projection of the articulated trajectory, the 3D parent trajectory, and the camera pose at each time instant. This is a core challenge in reconstructing the 3D motion of articulated structures such as the human body. We simultaneously apply activity-independent spatial and temporal constraints, in the form of fixed 3D distance to the parent trajectory and smooth 3D motion. There exist two solutions that satisfy each instantaneous 2D projection and articulation constraint (a ray intersects a sphere at up to two locations) and we show that resolving this ambiguity by enforcing smoothness is equivalent to solving a binary quadratic programming problem. A geometric analysis of the reconstruction of articulated trajectories is also presented and a measure of the reconstructibility of an articulated trajectory is proposed.

• H. S. Park, T. Shiratori, I. Matthews, and Y. Sheikh, 3D Reconstruction of a Moving Point from a Series of 2D Projections, European Conference on Computer Vision (ECCV) (oral), Sept, 2010, (project page)

This paper presents a linear solution for reconstructing the 3D trajectory of a moving point from its correspondence in a collection of 2D perspective images, given the 3D spatial pose and time of capture of the cameras that produced each image. Triangulation-based solutions do not apply, as multiple views of the point may not exist at each instant in time. A geometric analysis of the problem is presented and a criterion, called reconstructibility, is defined to precisely characterize the cases when reconstruction is possible, and how accurate it can be. We apply the linear reconstruction algorithm to reconstruct the time evolving 3D structure of several real-world scenes, given a collection of non-coincidental 2D images.

• H. S. Park and M. Sitti, Compliant Footpad Design Analysis for a Bio-Inspired Quadruped Amphibious Robot, Proceedings of the 2009 IEEE/RSJ International Conference on Intelligent Robots and System (IROS), pp 645-651. (url)

  A quadrupedal water runner robot inspired by the basilisk lizard has previously demonstrated the capability of water surface locomotion. Since the robot is aimed for the amphibious locomotion, a compatible design on both ground and water surface is discussed in this paper. A compliant footpad which can transfer elastic energy to propulsive momentum is introduced and modeled using a pseudo-rigid-body model. Dynamic modeling of the footpad and the robot provides a criterion of effcient ground locomotion. For the water surface locomotion, drag force can be reduced by compliance of the footpad. The optimized design taking into account two locomotions is studied and analyzed for stability using the Poincar´e map.

• H. S. Park, S. Floyd, and M. Sitti, Dynamic Modeling and Analysis of Pitch Motion of a Basilisk Lizard Inspired Quadruped Robot Running on Water, Proceeding of the 2009 International Conference on Robotics and Automation (ICRA), pp 2655-2660. (url)

  A quadrupedal robot inspired by the basilisk lizard was developed and modeled with a 3-D real time simulation. Due to the robot’s geometry, leg motion, and water interactions, the net pitch moment at the center of mass is not zero making pitch motion unstable. This paper introduces two types of tails, passive and active, to stabilize pitch motion and analyzes the advantages and disadvantages of each. It is shown in simulation that a purely passive tail can stabilize pitch motion and lead to a steady state robot pitch angle in the absence of disturbances. It is further shown that an active tail can compensate for disturbances and correct any drift in the robot body pitch angle due to changes in robot running speed.

• H. S. Park, S. Floyd, and M. Sitti, Dynamic Modeling of a Basilisk Lizard Inspired Quadruped Robot Running on Water, Proceeding of the 2008 IEEE/RSJ International Conference on Intelligent Robots and System (IROS), pp 3101-3107. (url)

  This paper proposes a 3-D dynamics model of a previously developed basilisk lizard inspired quadruped robot, which is capable of locomotion on the surface of water. Analysis of lift force shows the robot is capable of running on water using viscous drag forces and provides a criterion for convergence to a steady state distance from the water. From the roll motion analysis, previous designs turn out to be unstable along the roll axis. The required roll moment of inertia is found as a function of the running frequency using a pendulum modeling analysis. It is determined that the roll moment of inertia needs to be approximately 1.3 E+3 kg m2 for the robot to be stable with respect to the roll axis. Further, it is determined that 7- 12 Hz is an appropriate running frequency range for the robot to lift its weight. Compliant footpads are found to be beneficial in reducing the force associated with pulling out of the water.

• H. S. Park, S. Floyd, and M. Sitti, 3-D Simulation of Bio-inspiredWater Running Robot, the 2008 International symposium on Adaptive Motion of Animals and Machines (AMAM).

- Presentations

• H. S. Park, Dynamic modeling of a basilisk lizard inspired quadruped robot running on water, Bennett Conference, MechE, CMU.
• H. S. Park and S. Kim, Horizontally Movable Vehicle without Inclination, 2nd POSTECH-Tokyo Tech.-KNU Joint Workshop on Mechanical Engineering.