Chiew-Lan Tai

Skeleton extraction by mesh contraction

Extraction of curve-skeletons is a fundamental problem with many applications in computer graphics and visualization. In this paper, we present a simple and robust skeleton extraction method based on mesh contraction. The method works directly on the mesh domain, without pre-sampling the mesh model into a volumetric representation. The method first contracts the mesh geometry into zero-volume skeletal shape by applying implicit Laplacian smoothing with global positional constraints. The contraction does not alter the mesh connectivity and retains the key features of the original mesh. The contracted mesh is then converted into a 1D curve-skeleton through a connectivity surgery process to remove all the collapsed faces while preserving the shape of the contracted mesh and the original topology. The centeredness of the skeleton is refined by exploiting the induced skeleton-mesh mapping. In addition to producing a curve skeleton, the method generates other valuable information about the objects geometry, in particular, the skeleton-vertex correspondence and the local thickness, which are useful for various applications. We demonstrate its effectiveness in mesh segmentation and skinning animation.

MoXi: real-time ink dispersion in absorbent paper

This paper presents a physically-based method for simulating ink dispersion in absorbent paper for art creation purposes. We devise a novel fluid flow model based on the lattice Boltzmann equation suitable for simulating percolation in disordered media, like paper, in real time. Our model combines the simulations of spontaneous shape evolution and porous media flow under a unified framework. We also couple our physics simulation with simple implicit modeling and image-based methods to render high quality output. We demonstrate the effectiveness of our techniques in a digital paint system and achieve various realistic effects of ink dispersion, including complex flow patterns observed in real artwork, and other special effects.

Spatial relationship preserving character motion adaptation

This paper presents a new method for editing and retargeting motions that involve close interactions between body parts of single or multiple articulated characters, such as dancing, wrestling, and sword fighting, or between characters and a restricted environment, such as getting into a car. In such motions, the implicit spatial relationships between body parts/objects are important for capturing the scene semantics. We introduce a simple structure called an interaction mesh to represent such spatial relationships. By minimizing the local deformation of the interaction meshes of animation frames, such relationships are preserved during motion editing while reducing the number of inappropriate interpenetrations. The interaction mesh representation is general and applicable to various kinds of close interactions. It also works well for interactions involving contacts and tangles as well as those without any contacts. The method is computationally efficient, allowing real-time character control. We demonstrate its effectiveness and versatility in synthesizing a wide variety of motions with close interactions.

Dual Laplacian editing for meshes

Recently, differential information as local intrinsic feature descriptors has been used for mesh editing. Given certain user input as constraints, a deformed mesh is reconstructed by minimizing the changes in the differential information. Since the differential information is encoded in a global coordinate system, it must somehow be transformed to fit the orientations of details in the deformed surface, otherwise distortion will appear. We observe that visually pleasing deformed meshes should preserve both local parameterization and geometry details. We propose to encode these two types of information in the dual mesh domain due to the simplicity of the neighborhood structure of dual mesh vertices. Both sets of information are nondirectional and nonlinearly dependent on the vertex positions. Thus, we present a novel editing framework that iteratively updates both the primal vertex positions and the dual Laplacian coordinates to progressively reduce distortion in parametrization and geometry. Unlike previous related work, our method can produce visually pleasing deformations with simple user interaction, requiring only the handle positions, not local frames at the handles.

An object-oriented progressive-simplification-based vectorization system for engineering drawings: model, algorithm, and performance

Existing vectorization systems for engineering drawings usually take a two-phase workflow: convert a raster image to raw vectors and recognize graphic objects from the raw vectors. The first phase usually separates aground truth graphic object that intersects or touches other graphic objects into several parts, thus, the second phase faces the difficulty of searching for and merging raw vectors belonging to the same object. These operations slow down vectorization and degrade the recognition quality. Imitating the way humans read engineering drawings, we propose an efficient one-phase object-oriented vectorization model that recognizes each class of graphic objects from their natural characteristics. Each ground truth graphic object is recognized directly in its entirety at the pixel level. The raster image is progressively simplified by erasing recognized graphic objects to eliminate their interference with subsequent recognition. To evaluate the performance of the proposed model, we present experimental results on real-life drawings and quantitative analysis using third party protocols. The evaluation results show significant improvement in speed and recognition rate.

Bilateral Normal Filtering for Mesh Denoising

Decoupling local geometric features from the spatial location of a mesh is crucial for feature-preserving mesh denoising. This paper focuses on first order features, i.e., facet normals, and presents a simple yet effective anisotropic mesh denoising framework via normal field denoising. Unlike previous denoising methods based on normal filtering, which process normals defined on the Gauss sphere, our method considers normals as a surface signal defined over the original mesh. This allows the design of a novel bilateral normal filter that depends on both spatial distance and signal distance. Our bilateral filter is a more natural extension of the elegant bilateral filter for image denoising than those used in previous bilateral mesh denoising methods. Besides applying this bilateral normal filter in a local, iterative scheme, as common in most of previous works, we present for the first time a global, noniterative scheme for an isotropic denoising. We show that the former scheme is faster and more effective for denoising extremely noisy meshes while the latter scheme is more robust to irregular surface sampling. We demonstrate that both our feature-preserving schemes generally produce visually and numerically better denoising results than previous methods, especially at challenging regions with sharp features or irregular sampling.

Prototype Modeling from Sketched Silhouettes based on Convolution Surfaces

This paper presents a hybrid method for creating three‐dimensional shapes by sketching silhouette curves. Given a silhouette curve, we approximate its medial axis as a set of line segments, and convolve a linearly weighted kernel along each segment. By summing the fields of all segments, an analytical convolution surface is obtained. The resulting generic shape has circular cross‐section, but can be conveniently modified via sketched profile or shape parameters of a spatial transform. New components can be similarly designed by sketching on different projection planes. The convolution surface model lends itself to smooth merging between the overlapping components. Our method overcomes several limitations of previous sketched‐based systems, including designing objects of arbitrary genus, objects with semi‐sharp features, and the ability to easily generate variants of shapes.

Multitouch finger registration and its applications

We present a simple finger registration technique that can distinguish in real-time which hand and fingers of the user are touching the touchscreen. The finger registration process is activated whenever the user places a hand, in any orientation, anywhere on the touchscreen. Such a finger registration technique enables the design of intuitive multitouch interfaces that directly map different combinations of the users fingers to the interface operations. In this paper, we first study the effectiveness and robustness of the finger registration process. We then demonstrate the usability of our finger registration method for two new interfaces. Specifically, we describe the Palm Menu, which is an intuitive dynamic menu interface that minimizes hand and eye movement during operations, and a virtual mouse interface that enables user to perform mouse operations in multitouch environment. We conducted controlled experiments to compare the performance of the Palm Menu against common command selection interfaces and the virtual mouse against traditional pointing devices.

An efficient brush model for physically-based 3D painting

This paper presents a novel 3D brush model consisting of a skeleton and a surface, which is deformed through constrained energy minimization. The main advantage of our model over existing ones is in its ability to mimic brush flattening and bristle spreading due to brush bending and lateral friction exerted by the paper surface during the painting process. The ability to recreate such deformations is essential to realistic 3D digital painting simulations, especially in the case of Chinese brush painting and calligraphy. To further increase realism, we also model the plasticity of wetted brushes and the resistance exerted by pores on the paper surface onto the brush tip. Our implementation runs on a consumer-level PC in real-time and produces very realistic results.

Component-wise Controllers for Structure-Preserving Shape Manipulation

Recent shape editing techniques, especially for man‐made models, have gradually shifted focus from maintaining local, low‐level geometric features to preserving structural, high‐level characteristics like symmetry and parallelism. Such new editing goals typically require a pre‐processing shape analysis step to enable subsequent shape editing. Observing that most editing of shapes involves manipulating their constituent components, we introduce component‐wise controllers that are adapted to the component characteristics inferred from shape analysis. The controllers capture the natural degrees of freedom of individual components and thus provide an intuitive user interface for editing. A typical model usually results in a moderate number of controllers, allowing easy establishment of semantic relations among them by automatic shape analysis supplemented with user interaction. We propose a component‐wise propagation algorithm to automatically preserve the established inter‐relations while maintaining the defining characteristics of individual controllers and respecting the user‐specified modeling constraints. We extend these ideas to a hierarchical setup, allowing the user to adjust the tool complexity with respect to the desired modeling complexity. We demonstrate the effectiveness of our technique on a wide range of man‐made models with structural features, often containing multiple connected pieces.