Kin-Chuen Hui

Geometric aspects of the mouldability of parts

The mouldability of an injection moulded or die-cast component depends on various factors. In particular, the shape of the part determines the existence of undercuts in the mould parting direction. Based on the concept of blockage in a given direction, a subdivision technique is developed to evaluate the geometry of an undercut. With the notion of internal and external undercut, the mouldability of a component is studied. External undercuts can be resolved with side cores while internal undercuts can be resolved by the use of split cores. However, the use of split cores requires considerations as to whether sufficient space is available for their movement. A local and a global blockage test are introduced to detect any interference between the moulded component and a side core or a split core. A search strategy is also developed for selecting a suitable combination of main parting, side core and split core directions.

Solid sweeping in image space: application in NC simulation

Displaying the result of sweeping a solid is usually accomplished by first evaluating the boundaries of the solid being swept out, and a standard shading method is then applied to obtain a realistic image of the solid. This paper present an algorithm for sweeping a 3D object in image space. The result obtained is another solid in image space. Boolean operations can then be performed on this model. By subtracting the swept-out solid from another solid, the proposed method can be used for simulating the material removal process in an NC machining operation.

Feature recognition by template matching

Abstract Most existing techniques in feature recognition are limited to the recognition of “regular” shape features such as hole, slot, pocket, etc. which are commonly used in mechanical CAD/CAM applications. This research tackles the problem of free-from features recognition. Free-from features are not only important in engineering and industrial design, but also in computer graphics application. A free-from feature is embedded within a single or a set of free-form surfaces. Its boundary is not clearly defined and thus the recognition process is more difficult. This research employs the template matching approach for the recognition process. An efficient algorithm has been developed which solves the problem in two phases. In the hunting phase, the rough location of a feature is identified. In the refinement, an exact location of the feature is found to a user-specified accuracy. An experimental program has been implemented to verify the feasibility of the method.

A feature-based shape blending technique for industrial design

Blending or averaging of two-dimensional shapes usually operates on sets of discrete points or polygons approximating the objects. This relies on a series of evenly distributed or properly positioned points on the boundary of the objects. Features or characteristics of the objects are not well considered. This paper presents a simple but effective technique for blending 2D shapes composed of curve segments. Features that are essential for shape blending are extracted. Correspondence between features of the objects are then established. The correspondence between points on a pair of corresponding features are finally established for the interpolation process. This allows characteristic features of the objects to be retained in the blending operation which is essential in industrial design. An experimental system was developed for blending 2D contours with curved boundary. Test results showed that the proposed approach produces results suitable for industrial design.

Free-form design using axial curve-pairs

Deformation of 3D shapes usually requires the use of a deformation tool. The freeform deformation technique requires the use of a lattice of control point for deforming an object. This may require a synchronized movement of the lattice control points in order to obtain the desired effects. The axial deformation technique allows an object to be deformed by manipulating an axial curve. However, unexpected twist of the object may be obtained. This is a result of the lack of control on the local coordinate frame of the curve. This paper presents a technique for deforming objects with a set of axial curve-pairs. The use of a curve-pair allows the local coordinate frame to be controlled intuitively. A curve-pair is composed of a primary and an orientation curve. The orientation curve is an approximate offset of the primary curve. A technique is proposed for maintaining the relation between the primary and the orientation curve when the curve-pair is adjusted. By associating a complex 3D object to a curve-pair, the object can be stretched, bended, and twisted intuitively through manipulating the curve-pair. This deformation technique is particularly suitable for manipulating complex shapes (e.g. decorative components) in industrial and aesthetic design, and is also suitable for modelling characters and animals with flexible bodies. Adjusting the curve-pair according to some motion constraints produces different postures of a character or animal model. This in turn can be used as decorative components for aesthetic design.

Mesh Composition on Models with Arbitrary Boundary Topology

This paper presents a new approach for the mesh composition on models with arbitrary boundary topology. After cutting the needed parts from existing mesh models and putting them into the right pose, an implicit surface is adopted to smoothly interpolate the boundaries of the models under composition. An interface is developed to control the shape of the implicit transient surface by using sketches to specify the expected silhouettes. After that, a localized Marching Cubes algorithm is investigated to tessellate the implicit transient surface so that the mesh surface of the composed model is generated. Different from existing approaches in which the models under composition are required to have pairwise merging boundaries, the framework developed based on our techniques have the new function to fuse models with arbitrary boundary topology.

Ellipsoid-tree construction for solid objects

As ellipsoids have been employed in the collision handling of many applications in physical simulation and robotics systems, we present a novel algorithm for generating a bounding volume hierarchy (BVH) from a given model with ellipsoids as primitives. Our algorithm approximates the given model by a hierarchical set of optimized bounding ellipsoids. The ellipsoid-tree is constructed by a top-down splitting. Starting from the root of hierarchy, the volume occupied by a given model is divided into k sub-volumes where each is approximated by a volume bounding ellipsoid. Recursively, each sub-volume is then subdivided into ellipsoids for the next level in the hierarchy. The k ellipsoids at each hierarchy level for a sub-volume bounding is generated by a bottom-up algorithm - simply, the sub-volume is initially approximated by m spheres (m » k), which will be iteratively merged into k volume bounding ellipsoids and globally optimized to minimize the approximation error. Benefited from the anisotropic shape of primitives, the ellipsoid-tree constructed in our approach gives tighter volume bound and higher shape fidelity than another widely used BVH, sphere-tree.

Bending-Invariant Correspondence Matching on 3-D Human Bodies for Feature Point Extraction

In this paper, we present an automatic approach to match correspondences on 3-D human bodies in various postures so that feature points can be automatically extracted. The feature points are very important to the establishment of volumetric parameterization around human bodies for the human-centered customization of soft-products (Trans. Autom. Sci. Eng., vol. 4, issue no. 1, pp. 11-21, 2007). For a given template human model with a set of predefined feature points, we first down-sample the input model into a set of sample points. Then, the corresponding points of these samples on the human model are identified by minimizing the distortion with the help of a series of transformations regardless of their differences in postures, scales or positions. The basic idea of our algorithm is to transform the template human body to the shape of the input model iteratively. To generate a bending invariant mapping, the initial correspondence/transformation is computed in a multidimensional scaling (MDS) embedding domain of 3-D human models, where the Euclidean distance between two samples on a 3-D model in the MDS domain corresponds to the geodesic distance between them in ℜ3 . As the posture change (i.e., the body bending) of a human model can be considered as approximately isometric in the intrinsic 3-D shape, the initial correspondences established in the MDS domain can greatly enhance the robustness of our approach in body bending. Once the correspondences between the surface samples on the template model and the input model are determined after iterative transformations, we have essentially found the corresponding feature points on the input model. Finally, the locations of the based local matching step.

Hands on a virtually elastic object

This paper reports on the development of a system for manipulating a virtually elastic object by integrating various techniques. A sensor glove is used for interacting with the virtual object, and a virtual hand is used for visualizing the interaction between the hand and the object. Motion of the virtual hand is controlled with the sensor glove. By using a simplified hand model and a virtual object with a hierarchical representation based on a sphere-tree, collision between the fingers and the object is detected. Contact points between the virtual hand and the object are used for establishing constraints for the estimation of deformation based on the finite-element method. Experiments show that the critical process for attaining interactive response is the inversion of the stiffness matrix in finite-element analysis. The method of condensation is adopted for reducing the sizes of the matrices to be inverted. Parallel processing is used for speeding up the matrix inversion process. Test results show that an interactive response can be attained for objectswith amatrix to be inverted being composed of less than 300 nodes, and the response time of the system is inversely proportional to the number of processors.

Human foot modeling towards footwear design

Comfort test of footwear is mainly based on subjective perception of the wearer and a large number of subjects are required to obtain a reliable result. Therefore, the subjective comfort test is expensive and time consuming. Although the foot size and shape of a subject can be obtained by using a three-dimensional (3D) foot scanner, it is still difficult to create foot motion animations of each subject suitable for computer simulation. In this paper, we propose a fast approach to model foot deformation and present its application in simulating interaction with footwear towards footwear design. The simulation determines deformation of foot and footwear models. It can also determine stress distribution in the footwear. Given an initial foot model and a captured foot motion, human foot animation is created first. Then, the footwear model is fitted to the foot to compute the deformation and stress in the footwear. In this article, the boundary element method (BEM) is adopted. We demonstrate the results by conducting simulation of a captured gait motion. Experimental results showed that the method can be used to simulate human gait motion, and can determine deformation of footwear.