Xiaohan (Roy) Shi

I'm a Ph.D. candidate at the State Key Lab. of CAD&CG, Zhejiang University. My advisor is Prof. Hujun Bao.

I took internship at the Internet Graphics Group, Microsoft Research Asia in Beijing during Oct. 2003 - May 2008, supervised by Kun Zhou. Later I took internship at the Live Search Machine Learning Team, Microsoft in Redmond during Feb. 2009 - Apr. 2009, supervised by Paul Viola.

My research interests include mesh deformation, animation, and simulation of elastic deformable objects. My research focuses on generating realistic deformation with great efficiency for real-time applications, and recently extends to out-of-core mesh processing.

Email: royitaqi@hotmail.com

Publications

Out-of-Core Multigrid Solver for Streaming Meshes

Xiaohan Shi, Hujun Bao, Kun Zhou.

ACM Transactions on Graphics (SIGGRAPH Asia 2009)

Abstract: We present an out-of-core multigrid for solving the Poisson equation defined over gigantic meshes. This enables gradient-domain operations on out-of-core meshes with irregular connectivity. Taking a streaming mesh and boundary constraints as input, our solver builds a multigrid hierarchy and refines the multigrid solution progressively by performing all operations as streaming computations. A set of rules are carefully designed to make neighboring multigrid nodes perform tasks cooperatively and efficiently. With a sublinear memory growth with respect to the number of mesh vertices, our approach handles meshes with 14M vertices using merely 84MB of memory, while an equivalent in-core multigrid implementation fails to fit into 2GB memory space.

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Sketching MLS Image Deformations on the GPU

Yanlin Weng, Xiaohan Shi, Hujun Bao, Jun Zhang.

Computer Graphics Forum (Pacific Graphics 2008)

Abstract: In this paper, we present an image editing tool that allows the user to deform images using a sketch-based interface. The user simply sketches a set of source curves in the input image, and also some target curves that the source curves should be deformed to. Then the moving least squares (MLS) deformation technique is adapted to produce realistic deformations while satisfying the curves’ positional constraints. We also propose a scheme to reduce image fold-overs in MLS deformations. Our system has a very intuitive user interface, generates physically plausible deformations, and can be easily implemented on the GPU for real-time performance.

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Example-based Dynamic Skinning in Real Time

Xiaohan Shi, Kun Zhou, Yiying Tong, Mathieu Desbrun, Hujun Bao, Baining Guo.

ACM Transactions on Graphics (SIGGRAPH 2008)

Abstract: In this paper we present an approach to enrich skeleton-driven animations with physically-based secondary deformation in real time. To achieve this goal, we propose a novel, surface-based deformable model that can interactively emulate the dynamics of both lowand high-frequency volumetric effects. Given a surface mesh and a few sample sequences of its physical behavior, a set of motion parameters of the material are learned during an off-line preprocessing step. The deformable model is then applicable to any given skeleton-driven animation of the surface mesh. Additionally, our dynamic skinning technique can be entirely implemented on GPUs and executed with great efficiency. Thus, with minimal changes to the conventional graphics pipeline, our approach can drastically enhance the visual experience of skeleton-driven animations by adding secondary deformation in real time.

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Mesh Puppetry: Cascading Optimization of Mesh Deformation with Inverse Kinematics

Xiaohan Shi, Kun Zhou, Yiying Tong, Mathieu Desbrun, Hujun Bao, Baining Guo.

ACM Transactions on Graphics (SIGGRAPH 2007)

Abstract: We present mesh puppetry, a variational framework for detail-preserving mesh manipulation through a set of high-level, intuitive, and interactive design tools. Our approach builds upon traditional rigging by optimizing skeleton position and vertex weights in an integrated manner. New poses and animations are created by specifying a few desired constraints on vertex positions, balance of the character, length and rigidity preservation, joint limits, and/or selfcollision avoidance. Our algorithm then adjusts the skeleton and solves for the deformed mesh simultaneously through a novel cascading optimization procedure, allowing realtime manipulation of meshes with 50K+ vertices for fast design of pleasing and realistic poses. We demonstrate the potential of our framework through an interactive deformation platform and various applications such as deformation transfer and motion retargeting.

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Subspace Gradient Domain Mesh Deformation

Jin Huang, Xiaohan Shi, Xinguo Liu, Kun Zhou, Liyi Wei, Shanghua Teng, Hujun Bao, Baining Guo, Heung-Yeung Shum.

ACM Transactions on Graphics (SIGGRAPH 2006)

Abstract: In this paper we present a general framework for performing constrained mesh deformation tasks with gradient domain techniques. We present a gradient domain technique that works well with a wide variety of linear and nonlinear constraints. The constraints we introduce include the nonlinear volume constraint for volume preservation, the nonlinear skeleton constraint for maintaining the rigidity of limb segments of articulated figures, and the projection constraint for easy manipulation of the mesh without having to frequently switch between multiple viewpoints. To handle nonlinear constraints, we cast mesh deformation as a nonlinear energy minimization problem and solve the problem using an iterative algorithm. The main challenges in solving this nonlinear problem are the slow convergence and numerical instability of the iterative solver. To address these issues, we develop a subspace technique that builds a coarse control mesh around the original mesh and projects the deformation energy and constraints onto the control mesh vertices using the mean value interpolation. The energy minimization is then carried out in the subspace formed by the control mesh vertices. Running in this subspace, our energy minimization solver is both fast and stable and it provides interactive responses. We demonstrate our deformation constraints and subspace deformation technique with a variety of constrained deformation examples.

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Geometrically Based Potential Energy for Simulating Deformable Objects

Jin Huang, Xiaohan Shi, Xinguo Liu, Kun Zhou, Hujun Bao, Baining Guo.

The Visual Computer (Pacific Graphics 2006)

Abstract: This paper presents a fast and stable technique for simulating deformable objects. Unlike in previous physically based methods, our potential energy of deformation is purely geometrically based. It is defined as the L2 norm of the change of the differential coordinates. A key feature of this energy formulation is that the corresponding stiffness matrix is approximately constant, which enables fast and stable implicit integration and large deformations. Our algorithm can simulate various effects including solid, thin shell and plasticity. We also adopt two schemes to accelerate the simulation process: dimensionality reduction in frequency domain and adaptive rotation computation in spatial domain.

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Interactive Mesh Deformation with Pseudo Material Effects

Jin Huang, Hongxin Zhang, Xiaohan Shi, Xinguo Liu, Hujun Bao.

Computer Animation and Virtual Worlds (CASA 2006)

Abstract: This paper presents a novel geometric solution for triangle mesh deformation. Our techniques can provide various plausible material effects, e.g. elastic membrane and cloth like effects, with interactive performance. Laplacian-like quantities, related to edge length and dihedral angles, are proposed to encode meshes in our framework. A distortion minimization in least square sense is then formulated to describe an appropriate deformation, concerning edge length preservation and shape bending. For more precise controls, deformation penalties for each edge can be guided by a specified vector field. We set up an iterative scheme to obtain deformation results by solving a sequence of least square problems. During the iteration procedure, deformation behavior can be efficiently adjusted on the fly and be affected by pseudo gravity. Our approach generates visually pleasant deformations in seconds with minimal user interaction.

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Mesh Editing with Poisson-Based Gradient Field Manipulation

Yizhou Yu, Kun Zhou, Dong Xu, Xiaohan Shi, Hujun Bao, Baining Guo, Heung-Yeung Shum.

ACM Transaction on Graphics (SIGGRAPH 2004)

Abstract: In this paper, we introduce a novel approach to mesh editing with the Poisson equation as the theoretical foundation. The most distinctive feature of this approach is that it modifies the original mesh geometry implicitly through gradient field manipulation. Our approach can produce desirable and pleasing results for both global and local editing operations, such as deformation, object merging, and smoothing. With the help from a few novel interactive tools, these operations can be performed conveniently with a small amount of user interaction. Our technique has three key components, a basic mesh solver based on the Poisson equation, a gradient field manipulation scheme using local transforms, and a generalized boundary condition representation based on local frames. Experimental results indicate that our framework can outperform previous related mesh editing techniques.

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