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Min Gyu Choi
Full Professor |
Short Biosketch I am a professor in the Department of Computer Science at Kwangwoon University. My research is in the area of computer graphics. I am particularly interested in character animation and physically-based modeling and simulation. (I received the BS, MS, and PhD degrees in computer science from Korea Advanced Institute of Science and Technology, Korea, in 1996, 1998, and 2003, respectively.) |
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Last update: August 28 2017 |
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We propose a new as-rigid-as-possible approach to the real-time simulation of physics-based
deformable models for interactive applications such as computer games. The key observation is
that the efficacy of an embedded oriented particle representation and the stability of a
variational implicit formulation of the projective dynamics are complementary to each other.
We reformulate the variational implicit formulation to deal with an embedded graph of oriented
particles. Our new formulation is extremely stable, and our alternating local/global optimization
solver is both easy to implement and computationally efficient. Our method can deal with
one-dimensional (cable and rod), two-dimensional (shell), and three-dimensional (solid) models
in a uniform manner. Experimental results demonstrate that hundreds of deformable models with an
extremely large number of polygons can be simulated robustly in real time using thousands of
particles.
Supplementary video: 
This paper proposes a practical approach for real-time simulation of large deformation and ductile
fracture. We adopt the oriented particles approach for robust and efficient simulation of large
deformation and develop a practical model for plastic flow and fracture. The proposed method finds
the optimal rotation and the optimal stretch in shape matching. The newly introduced optimal stretch
leads to a material strain that can be employed in the plastic flow and fracture criteria. We also
propose a GPU skinning method for real-time rendering of a deformed, fractured visual mesh.
Experimental results show that the proposed method can robustly simulate large, elastoplastic
deformation and ductile fracture of large visual meshes in real time.
Supplementary video: 
We present a physically based controller that simulates the flapping behavior of a bird in flight.
We recorded the motion of a dove using marker-based optical motion capture and high-speed video
cameras. The bird flight data thus acquired allow us to parameterize natural wingbeat cycles and
provide the simulated bird with reference trajectories to track in physics simulation.
Our controller simulates articulated rigid bodies of a bird's skeleton and deformable feathers
to reproduce the aerodynamics of bird flight. Motion capture from live birds is not as easy as
human motion capture because of the lack of coorperation from subjects. Therefore, the flight data
we could acquire were limited. We developed a new method to learn wingbeat controllers even from
sparse, biased observations of real bird flight. Our simulated bird imitates life-like flapping
of a flying bird while actively maintaining its balance. The bird flight is interactively
controllable and resilient to external disturbances.
Supplementary video:
This paper proposes a real-time simulation technique for thin rods undergoing large rotational
deformation. Rods are thin objects such as ropes and hairs that can be abstracted as one-dimensional
structures. Development of a real-time physical model that can produce visually convincing animation
of thin rods has been a challenging problem in computer graphics. We adopt continuum mechanics to
formulate the governing equation, and develop a modal warping technique for rods to integrate the
governing equation in real-time; This is a novel extension of the previous modal warping techniques
developed for solids and shells. Experimental results show that the proposed method runs in real-time
even for large meshes and it can simulate large bending and/or twisting deformations.
Supplementary video: 
This paper proposes a real-time simulation technique for thin shells undergoing large deformation.
Shells are thin objects such as leaves and papers that can be abstracted as 2D structures.
Development of a satisfactory physical model that runs in real-time but produces visually
convincing animation of thin shells has been remaining a challenge in computer graphics.
Rather than resorting to shell theory which involves the most complex formulations in continuum
mechanics, we adopt the energy functions from the discrete shells proposed by
Grinspun et al. [GHDS03]. For real-time integration of the governing equation, we develop a modal
warping technique for shells. This new simulation framework results from making extensions
to the original modal warping technique [CK05] which was developed for the simulation of 3D solids.
We report experimental results, which show that the proposed method runs in real-time even for large
meshes, and that it can simulate large bending and/or twisting deformations with acceptable realism.
Supplementary video:
Color quantization replaces the color of each pixel with the closest representative
color, and thus it makes the resulting image partitioned into uniformly-colored regions.
As a consequence, continuous, detailed variations of color over the corresponding regions in the
original image are lost through color quantization. In this paper, we present a novel blind scheme
for restoring such variations from a color-quantized input image without a priori knowledge of the
quantization method. Our scheme identifies which pairs of uniformly-colored regions in the input
image should have continuous variations of color in the resulting image. Then, such regions are
seamlessly stitched through optimization while preserving the closest representative colors.
The user can optionally indicate which regions should be separated or stitched by scribbling
constraint brushes across the regions. We demonstrate the effectiveness of our approach through
diverse examples, such as photographs, cartoons, and artistic illustrations.
Supplementary material: 
Video taken from a single monocular camera is the most common means of recording human motion.
In this article, we present a practical, semiautomatic method for synthesizing a human motion
that is guided by such video. After preprocessing an input video, we select a precaptured motion
clip called a reference motion from a motion library. We then compute the sequence of body
configurations of a virtual character by deforming this motion, according to spacetime constraints
derived from a sequence of 2D features in the input video. Experimental results show that our
method can synthesize highly dynamic motions, such as kicking and header motions of soccer players.
We also showed the potential of our scheme as a new paradigm for motion capture, that is, capturing
motions from videos taken with a monocular camera, even outside a motion capture studio.
Supplementary video: 
We present a new framework for simulating dynamic movements of complex hairstyles.
Assuming that hair consists of a number of wisps, we propose methods for simulating dynamic wisp
movements and handling wisp-body collisions and wisp-wisp interactions.
For simulation of wisps, we introduce a new hair dynamics model, a hybrid of the rigid multi-body
serial chain and mass-spring models, to formulate the simulation system using an implicit
integration method. Consequently, the simulator can impose collision/contact constraints
systematically, so that it can handle wisp-body collisions efficiently without the need for
backtracking or taking sub-timesteps. In addition, the simulator handles wisp-wisp collisions
based on the impulse while taking account of viscous damping and cohesive forces.
Experimental results show that the proposed technique can stably simulate hair with intricate
geometries while robustly handling wisp-body collisions and wisp-wisp interactions.
Supplementary video:
This paper proposes a real-time simulation technique for large deformations. Green's nonlinear
strain tensor accurately models large deformations; however, time stepping of the resulting
nonlinear system can be computationally expensive. Modal analysis based on a linear strain tensor
has been shown to be suitable for real-time simulation, but is accurate only for moderately
small deformations. In the present work, we identify the rotational component of an infinitesimal
deformation, and extend linear modal analysis to track that component. We then develop a procedure
to integrate the small rotations occurring at the nodal points. An interesting feature of our
formulation is that it can implement both position and orientation constraints in a straightforward
manner. These constraints can be used to interactively manipulate the shape of a deformable solid
by dragging/twisting a set of nodes. Experiments show that the proposed technique runs in real-time
even for a complex model, and that it can simulate large bending and/or twisting deformations with
acceptable realism.
Supplementary video:
