Publications (Full Publication List; ResearcherID; Google Scholar): Before 2006; 2006-2010; 2011-2015; 2016

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Recent Papers

Ran Yi, Chenming Wu, Yong-Jin Liu, Ying He, and Charlie C.L. Wang, "Delta DLP 3D printing of large models", IEEE Transactions on Automation Science and Engineering, conditionally accepted. [PDF] [Video@YouTube]

(This is an extended version of the paper - Delta DLP 3D printing with large size, which is published in 2016 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS 2016), Daejeon, Korea, October 9-14, 2016. [Video@YouTube])

This paper presents a 3D printing system that uses a low-cost off-the-shelf consumer projector to fabricate large models. Compared with traditional DLP 3D printers using a single vertical carriage, the platform of our DLP 3D printer using delta mechanism can also move horizontally in the plane. We show that this system can print 3D models much larger than traditional DLP 3D printers. The major challenge to realize 3D printing of large models in our system comes from how to cover a planar polygonal domain by a minimum number of rectangles with fixed size, which is NP-hard. We propose a simple yet efficient approximation algorithm to solve this problem. The key idea is to segment a polygonal domain by using its medial axis and afterwards merge small parts in the segmentation. Given an arbitrary polygon Q with n generators (i.e., line segments and reflex vertices in Q), we show that the time complexity of our algorithm is O(n2log2n) and the number of output rectangles covering Q is O(Kn), where K is an input-polygon-dependent constant. A physical prototype system is built and several large 3D models with complex geometric structures have been printed as examples to demonstrate the effectiveness.

Chenming Wu, Chengkai Dai, Guoxin Fang, Yong-Jin Liu, and Charlie C.L. Wang, "RoboFDM: a robotic system for support-free fabrication using FDM", IEEE International Conference on Robotics and Automation (ICRA 2017), Singapore, May 29 - June 3, 2017, accepted. [PDF] [Video@YouTube]

This paper presents a robotic system - RoboFDM that targets at printing 3D models without support-structures, which is considered as the major restriction to the flexibility of 3D printing. The hardware of RoboFDM consists of a robotic arm providing 6-DOF motion to the platform of material accumulation and an extruder forming molten filaments of polylactic acid (PLA). The fabrication of 3D models in this system follows the principle of fused decomposition modeling (FDM). Different from conventional FDM, an input model fabricated by RoboFDM is printed along different directions at different places. A new algorithm is developed to decompose models into support-free parts that can be printed one by one in a collision-free sequence. The printing directions of all parts are also determined during the computation of model decomposition. Experiments have been successfully taken on our RoboFDM system to print general freeform objects in a support-free manner.

Shuo Jin, Chengkai Dai, Yang Liu, and Charlie C.L. Wang, "Motion imitation based on sparsely sampled correspondence", ASME Journal of Computing and Information Science in Engineering, accepted. [PDF] [Video@YouTube]

Existing techniques for motion imitation often suffer a certain level of latency due to their computational overhead or a large set of correspondence samples to search. To achieve real-time imitation with small latency, we present a framework in this paper to reconstruct motion on humanoids based on sparsely sampled correspondence. The imitation problem is formulated as finding the projection of a point from the configuration space of a human's poses into the configuration space of a humanoid.

Gang Xu, Tsz-Ho Kwok, and Charlie C.L. Wang, "Isogeometric computation reuse method for complex objects with topology-consistent volumetric parameterization", Computer-Aided Design, vol.91, pp.1-13, October 2017. [PDF]

Volumetric spline parameterization and computational eciency are two main challenges in isogeometric analysis (IGA). To tackle this problem, we propose a framework of computation reuse in IGA on a set of three-dimensional models with similar semantic features. Given a template domain, B-spline based consistent volumetric parameterization is first constructed for a set of models with similar semantic features. An effcient quadrature-free method is investigated in our framework to compute the entries of stiffness matrix by Bézier extraction and polynomial approximation. In our approach, evaluation on the stiffness matrix and imposition of the boundary conditions can be pre-computed and reused during IGA on a set of CAD models. Examples with complex geometry are presented to show the effectiveness of our methods, and efficiency similar to the computation in linear finite element analysis can be achieved for IGA taken on a set of models.

Lianping Xing, Charlie C.L. Wang, and Kin-Chuen Hui, "Coherent spherical range-search for dynamic points on GPUs", Computer-Aided Design, vol.86, pp.12-25, May 2017. [PDF] [Project Page - with Code] [Video@YouTube for Particle Simulation]

We present an approach to accelerate spherical range-search (SRS) for dynamic points that employs the computational power of many-core GPUs. Unlike finding k approximate nearest neighbours (ANNs), exact SRS is needed in geometry processing and physical simulation to avoid missing small features. The spatial coherence of query points and the temporal coherence of dynamic points are exploited in our approach to achieve very efficient range-search on AABB-trees. We test our coherent SRS in several applications including point-point-set geometry processing, distance-field generation and particle-based simulation, which are best scenarios to present the spatial and the temporal coherence of spherical queries on dynamic points. On a PC with NVIDIA GTX 660 Ti GPUs, our approach can take 1M queries on 1M dynamic points at a rate of 1600 queries/ms, where 49 neighbours are found on average within the range of 1/100 of the bounding-box's diagonal length. We observe an increase of up to 4x compared with conventional voxel-based GPU searching approaches in the benchmark of particle-based fluid simulation. Moreover, the speedup can be scaled up to 150x when being applied to highly non-uniform distribution of particles in the simulation.

Wuyuan Xie, Ying Nie, Zhan Song, and Charlie C.L. Wang, "Mesh-based computation for solving photometric stereo with near point lighting", IEEE Computer Graphics and Applications, accepted. [PDF] [Data Set]

(This is an extended version of the paper - Photometric stereo with near point lighting: A solution by mesh deformation, which is published in 2015 IEEE CVPR Conference, Boston, Massachusetts, June 7-12, 2015. [Data-Set] [Video@YouTube] )

We tackle the problem of dense reconstruction with a practical system, in which near point lighting is employed. Different from the conventional formulation of photometric stereo that assumes parallel lighting, photometric stereo under the near point lighting (NPL) condition is a nonlinear problem as the local surface normals are coupled with its distance to the camera as well as the light sources. After obtaining the locations of point lights by a calibration process, we develop a new framework to solve this nonlinear reconstruction problem via mesh deformation, in which each facet is corresponding to a pixel in the image captured by the camera. In our framework, mesh deformation is decoupled into an iteration of interlaced steps of local projection and global blending. Experimental results verify that our method can generate accurate estimation of surface shape under NPL in a few iterations. Besides, this approach is robust to errors on the positions of light sources and is easy to be implemented.

Aamir Khan Jadoon, Chenming Wu, Yong-Jin Liu, Ying He, and Charlie C.L. Wang, "Interactive partitioning of 3D models into printable parts", IEEE Computer Graphics and Applications, accepted. [PDF] [Data Set]

In this paper, we present an easy, flexible and interactive tool for partitioning a 3D model, which is larger than 3D-printer's working volume, into printable parts in an intuitive way. Our presented tool is based on the elegant partitioning optimization framework Chopper. Our tool aims at improving Chopper by providing users three easy-to-use interactive operations: no-go region painting, cutting plane specification and components re-union. With these operations, we show that (1) exhaustive search in the BSP tree - the most time-consuming step in Chopper - can be avoided, (2) more flexible geometric configurations can be provided, (3) user's design intention is considered naturally and efficiently, and customized 3D partitioning results can be obtained. We test our tool on a wide range of 3D models and observe promising results. A preliminary user study also demonstrates its effectiveness and efficiency.

Chuhua Xian, Shuo Jin, and Charlie C.L. Wang, "Efficient C^2-weighting for image warping", IEEE Computer Graphics and Applications, accepted. [PDF] [More Results] [Video@YouTube]

Handle-driven image warping based on linear blending is widely used in many applications because of its merits on intuitiveness, efficiency and easiness of implementation. In this paper, we develop a method to compute high-quality weights within a closed domain for image warping. The property of C^2-continuity in weights is guaranteed by the carefully formulated basis functions. The efficiency of our algorithm is ensured by a closed-form formulation of the computation for weights. The cost of inserting a new handle is only the time to evaluate the distances from the new handle to all other sample points in the domain. A virtual handle insertion algorithm is developed to allow users to freely place handles within the domain while preserving the satisfaction of all expected criteria on weights for linear blending. Experimental examples for real-time applications are shown to demonstrate the effectiveness of this method.

Kai-Ming Yu, Yu Wang, and Charlie C.L. Wang, "Smooth geometry generation in additive manufacturing file format: problem study and new formulation”, Rapid Prototyping Journal, vol.23, no.1, 2017. [PDF]

In the newly released ASTM standard specification for Additive Manufacturing File (AMF) format - version 1.1, Hermite curve based interpolation is employed to refine input triangles to generate denser mesh with smoother geometry. This paper studies the problems of constructing smooth geometry based on Hermite interpolation on curves and proposes a solution to overcome these problems. A formulation using triangular Bezier patch is proposed in this paper to generate smooth geometry from input polygonal models. Different configurations on the boundary curves are analysed to further enrich this formulation. The proposed scheme has requirements for the input normals of a model, only C^0 interpolation can be generated on those cases with less-proper input. For these cases, the Boolean sum and the Nielson's point-opposite edge interpolation for triangular Coons patch are used to generate the smooth geometry as a C^0 interpolant.

Yunbo Zhang, Charlie C.L. Wang, and Karthik Ramani, "Optimal fitting of strain-controlled flattenable mesh surfaces", International Journal of Advanced Manufacturing Technology, vol.87, no.9, pp.2873-2887, December 2016. [PDF]

A flattenable mesh surface is a polygonal mesh surface that can be unfolded into a planar patch without stretching any polygon. This paper presents a new method for computing a slightly stretched flattenable mesh surface M from a piecewise-linear surface patch P in 3D, where the shape approximation error between M and P is minimised and the strain of stretching on M is controlled. Prior approaches result in either a flattenable surface that could be quite different from the input shape or a (discrete) developable surface has relative simple shape. The techniques investigated in this paper overcome these difficulties. First, we introduce a new surface modeling method to conduct a sequence of nearly isometric deformations to morph a flattenable mesh surface to a new shape which has a better approximation of the input surface. Second, in order to get better initial surfaces for fitting and overcome topological obstacles, a shape perturbation scheme is investigated to obtain the optimal surface fitting result. Last, to improve the scalability of our optimal surface fitting algorithm, a coarse-to-fine fitting framework is exploited so that very dense flattenable mesh surfaces can be modeled and boundaries of the input surfaces can be interpolated.

Jun Wu, Charlie C.L. Wang, Xiaoting Zhang, and Rüdiger Westermann, "Self-supporting rhombic infill structures for additive manufacturing", Computer-Aided Design, vol.80, pp.32-42, November 2016. [PDF] [Data Set]

Recent work has demonstrated that the interior material layout of a 3D model can be designed to make a fabricated replica satisfy application-specific demands on its physical properties, such as resistance to external loads. A widely used practice to fabricate such models is by layer-based additive manufacturing (AM), which however suffers from the problem of adding and removing interior supporting structures. In this paper, we present a novel method for generating application-specific infill structures on rhombic cells so that the resultant structures can automatically satisfy manufacturing requirements on overhang-angle and wall-thickness. Additional supporting structures can be avoided entirely in our framework. To achieve this, we introduce the usage of an adaptive rhombic grid, which is built from an input surface model. Starting from the initial sparse set of rhombic cells, via numerical optimization techniques an objective function can be improved by adaptively subdividing the rhombic grid and thus adding more walls in cells. We demonstrate the effectiveness of our method for generating interior designs in the applications of improving mechanical stiffness and static stability.

Chenming Wu, Ran Yi, Yong-Jin Liu, Ying He, and Charlie C.L. Wang, "Delta DLP 3D printing with large size", 2016 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS 2016), Daejeon, Korea, October 9-14, 2016. [PDF] [Video@YouTube]

We present a delta DLP 3D printer with large size in this paper. Compared with traditional DLP 3D printers that use a low-cost off-the-shelf consumer projector and a single vertical carriage, the platform of our delta DLP 3D printer can also move horizontally in the plane. We show that this structure allows the printer to have a larger printing area than the projection area of a projector. Our system can print 3D models much larger than traditional DLP 3D printers. The major challenge to realize delta 3D printing with large size comes from how to partition an arbitrary planar polygonal shape (possibly with holes or multiple disjoint polygons) into a minimum number of rectangles with fixed size, which is NP-hard. We propose a simple yet efficient approximation algorithm to solve this problem. The time complexity of our algorithm is O(n³ log n), where n is the number of edges in the polygonal shape. A physical prototype system is built and several large 3D models with complex geometric structures have been printed as examples to demonstrate the effectiveness of our approach.

Xiaoting Zhang, Xinyi Le, Zhihao Wu, Emily Whiting, and Charlie C.L. Wang, "Data-driven bending elasticity design by shell thickness", Computer Graphics Forum, Eurographics Symposium on Geometry Processing 2016, June 20-24, 2016, Berlin, Germany, vol.35, no.5, pp.157-166, 2016. [PDF] [Extended Technical Report] [Video@YouTube]

We present a method to design the deformation behavior of 3D printed models by an interactive tool, where the variation of bending elasticity at different regions of a model is realized by a change in shell thickness. Given a soft material to be used in 3D printing, we propose an experimental setup to acquire the bending behavior of this material on tubes with different diameters and thicknesses. The relationship between shell thickness and bending elasticity is stored in an echo state network using the acquired dataset. With the help of the network, an interactive design tool is developed to generate non-uniformly hollowed models to achieve desired bending behaviors. The effectiveness of this method is verified on models fabricated by different 3D printers by studying whether their physical deformation can match the designed target shape.

Shengjun Liu, Charlie C.L. Wang, Guido Brunnett, and Jun Wang, "A closed-form formulation of HRBF-based surface reconstruction by approximate solution", Computer-Aided Design, Special Issue of 2016 Symposium on Solid and Physical Modeling, June 20-24, 2016, Berlin, Germany, vol.78, pp.147-157, September 2016. [PDF] [Source Code] [Data Set]

The Hermite radial basis functions (HRBFs) implicits have been used to reconstruct surfaces from scattered Hermite data points. In this work, we propose a closed-form formulation to construct HRBF-based implicits by a quasi-solution to approximate the exact one. A scheme is developed to automatically adjust the support sizes of basis functions to hold the error bound of a quasi-solution. Our method can generate an implicit function from positions and normals of scattered points without taking any global operation. Robust and efficient reconstructions are observed in our experimental tests on real data captured from a variety of scenes.

(Supplementary Technical Report - "Error-bound, comparison and sub-sampling for closed-form HRBF surface reconstruction" [PDF])

Tsz Ho Kwok, Weiwei Wan, Jia Pan, Charlie C.L. Wang, Jianjun Yuan, Kensuke Harada, and Yong Chen, "Rope caging and grasping", IEEE International Conference on Robotics and Automation (ICRA 2016), pp.1980-1986, Stockholm, Sweden, May 16-21, 2016. [PDF] [Video@YouTube]

We present a novel method for caging grasps in this paper by stretching ropes on the surface of a 3D object. Both topology and shape of a model to be grasped has been considered in our approach. Our algorithm can guarantee generating local minimal rings on every topological branches of a given model with the help of a Reeb graph. Cages and grasps can then be computed from these rings, and physical experimental tests have been conducted to verify the robustness of our approach.

Qianwen Chao, Jiangfan Yu, Chengkai Dai, Tiantian Xu, Li Zhang, Charlie C.L. Wang, and Xiaogang Jin, "Steering micro-robotic swarm by dynamic actuating fields", IEEE International Conference on Robotics and Automation (ICRA 2016), pp.5230-5235, Stockholm, Sweden, May 16-21, 2016. [PDF] [Video@YouTube]

We present a general solution for steering micro-robotic swarm by dynamic actuating fields. In our approach, the motion of micro-robots is controlled by changing the actuating direction of a field applied to them. The time-series sequence of actuating field's directions can be computed automatically. Given a target position in the domain of swarm, a governing field is first constructed to provide optimal moving directions at every points. Following these directions, a robot can be driven to the target efficiently. However, when working with a crowd of micro-robots, the optimal moving directions on different agents can contradict with each other. To overcome this difficulty, we develop a novel steering algorithm to compute a statistically optimal actuating direction at each time frame. Following a sequence of these actuating directions, a crowd of micro-robots can be transported to the target region effectively. Our steering strategy of swarm has been verified on a platform that generates magnetic fields with unique actuating directions. Experimental tests taken on aggregated magnetic micro-particles are quite encouraging.

Rob B.N. Scharff, Eugeni L. Doubrovski, Wim A. Poelman, Pieter P. Jonker, Charlie C.L. Wang, and Jo M.P. Geraedts, "Towards behavior design of a 3D-printed soft robotic hand", Soft Robotics: Trends, Applications and Challenges, Proceedings of the Soft Robotics Week, pp.23-29, April 25-30, 2016, Livorno, Italy, Springer. [Video@YouTube]

This work presents an approach to integrate actuators, sensors, and structural components into a single product that is 3D printed using Selective Laser Sintering. The behavior of actuators, sensors, and structural components is customized to desired functions within the product. Our approach is demonstrated by the realization of human-like behavior in a 3D-printed soft robotic hand. This work describes the first steps towards creating the desired behavior by means of modeling specific volumes within the product using Additive Manufacturing. Our work shows that it is not necessary to limit the design of a soft robotic product to only integrating off-the-shelf components but instead we deeply embedded the design of the required behavior in the process of designing the actuators, sensors and structural components.

Tsz-Ho Kwok, Yanqiu Zhang, Charlie C.L. Wang, Yong-Jin Liu, and Kai Tang, "Styling evolution for tight-fitting garments", IEEE Transactions on Visualization and Computer Graphics, vol.22, no.5, pp.1580-1591, May 2016. [PDF] [Video@YouTube] [Project Page - with Data-Set]

We present an evolution method for designing the styling curves of garments. The procedure of evolution is driven by aesthetics-inspired scores to evaluate the quality of styling designs, where the aesthetic considerations are represented in the form of streamlines on human bodies. A dual representation is introduced in our platform to process the styling curves of designs, based on which robust methods for realizing the operations of evolution are developed. Starting from a given set of styling designs on human bodies, we demonstrate the effectiveness of set evolution inspired by aesthetic factors. The evolution is adaptive to the change of aesthetic inspirations. By this adaptation, our platform can automatically generate new designs fulfilling the demands of variations in different human bodies and poses.

Book Chapter

Tsz-Ho Kwok, Yong Chen, and Charlie C.L. Wang, "Geometric analysis and computation using Layered Depth-Normal Images for three-dimensional microfabrication", Chapter 5, Three-Dimensional Microfabrication Using Two-photon Polymerization, pp.119-147, 2016.

Additive manufacturing (AM) is a direct manufacturing process that provides the ability to fabricate parts with complex shape. Robust geometric computation is essential to deal with the complex geometry. Current geometric computation methods based on the boundary representation (B-rep) explicitly define and compute geometry. However, such approaches lack in simplicity and are prone to robustness problems. In this chapter, a point-based geometric computation method based on the Layered Depth-Normal Image (LDNI) is presented. A set of computation algorithms are developed for this new point-based method, including the conversions between the LDNI and B-rep models, the offsetting and the Boolean geometric operations, etc. A number of test cases has shown the robustness of the developed geometric operations, and a set of Computer-Aided Design and Manufacturing (CAD/CAM) applications related to the complex component design and manufacturing has also been explored.