Graphical Models and Image Processing最新文献

Binary pyramids in two and three dimensions can be used for multiresolution representation. The “standard” OR and AND pyramids have serious drawbacks, as they distort the shape significantly; therefore they can seldom be used effectively. Here we present alternative approaches to build binary pyramids, aimed at improving shape preservation (and, as far as possible, topology preservation) in lower resolutions. The algorithms are easy to implement and produce good results.

Using the same technique as for the C-B-splines, two other forms of C-Bézier curves and a reformed formula for the subdivisions are proposed. With these new forms, C-Bézier curves can unify the processes for both the normal cases, and the limiting case (α→0) with precise results. Like the C-B-splines, a C-Bézier curve can be approximated by its cubic Bézier curve in high accuracy. For any tensor product C-Bézier patch, a pair of its opposite sides could have different parameters of α. All this will make the C-Bézier curves and surfaces more efficient in algorithms, more flexible in assembling and representing arcs, and will satisfy the demands of high precision in engineering and fast calculation in computer display.

Continuum mechanics and the finite element method are used as basic tools to facilitate the animation of deformable objects. Two unique features in this paper are: (1) introduction of a framework for 3-D finite interface elements to deal with object interaction and (2) a simple approach to handle fracture propagation inside objects. Animation sequences have been constructed, and they indicate that the system is able to provide a realistic depiction of dynamic collision processes between deformable objects and a simple presentation of fracture propagation inside objects.

This paper deals with topological properties of sets of tetrahedra (“tetrahedral representations” of three-dimensional objects). Classes of such representations which we call normal and strongly normal are defined and some of their basic properties are established. Computationally efficient methods of counting the cavities and tunnels in the neighborhood of a tetrahedron are defined. A characterization of a simple tetrahedron is formulated, and an efficient approach is developed to identifying simple tetrahedra and computing measures of the local topological change when a tetrahedron is deleted.

In this paper, we present efficient algorithms for contact detection and accurate contact mechanics in granular flow simulations. The contact detection algorithms that we present are applicable to arbitrarily shaped rigid particles in a variety of environments including interactive as well as non-interactive. We present a very accurate force displacement model for characterizing the behavior of particles in contact. These algorithms are applied to granular flow simulations, in particular, soybean flow through an inclined chute. There are two phases of this research, namely, the development of contact detection algorithms and the development of accurate contact mechanics based force-displacement models. In the first phase, the algorithms consist of two stages. The first stage involves finding candidate particles for possible contacts. The second stage involves detecting precise contact between these candidates. In our application, the soybeans, which are approximately ellipsoidal in shape, are well approximated by a cluster of overlapping spheres. Detecting precise contact between spherical shapes is quite simple and hence, in this paper, we will focus only on the first stage of the first phase. The primary data structure used in the first stage is an octree. We build an octree for the enclosure/chute (containing thenparticles) and use it in the algorithm for detecting possible contacts. The average time complexity of this stage of the first phase isO(n)and the worst case isO(nlogn). In phase two, we develop a very accurate force-displacement model for dealing with the contact mechanics of soybean flow through an inclined chute. This model is a reasonably close approximation to the widely accepted Mindlin force-displacement model for elastic frictional contacts. Our force-displacement model accounts for normal and tangential direction contact forces acting between particles and/or particles and the chute walls. We also present examples of soybean flow simulations with fast contact detection and accurate contact mechanics.

This paper deals with sets consisting of digital curve segments which are presented on ann×ngrid. The main result is a general coding scheme which can be applied to the sets of digital curve segments, which may consist even of digital curve segments that result from digitization of curves of different kinds. Ifhis an upper bound for the number of intersection points of two digitized curves, thenh+3 integer parameters are sufficient for the coding. The proposed coding scheme preserves an asymptotically optimal coding (the minimum possible number of bits is used) whenhis assumed to be a constant. If it is allowed thathtends to infinity (whenntends to infinity, too), then the number of bits used for the coding is$\text{O}$(h²·logn). In addition, the authors show that the coding of digital curve segments by their least-squares polynomial fits is possible. It turns out that such a coding is a special case of the general coding scheme proposed here.

We present a new, interactive tool calledIntelligent Scissorswhich we use for image segmentation. Fully automated segmentation is an unsolved problem, while manual tracing is inaccurate and laboriously unacceptable. However, Intelligent Scissors allow objects within digital images to be extracted quickly and accurately using simple gesture motions with a mouse. When the gestured mouse position comes in proximity to an object edge, alive-wire boundary“snaps” to, and wraps around the object of interest. Live-wire boundary detection formulates boundary detection as an optimal path search in a weighted graph. Optimal graph searching provides mathematically piece-wise optimal boundaries while greatly reducing sensitivity to local noise or other intervening structures. Robustness is further enhanced withon-the-fly trainingwhich causes the boundary to adhere to the specific type of edge currently being followed, rather than simply the strongest edge in the neighborhood.Boundary coolingautomatically freezes unchanging segments and automates input of additional seed points. Cooling also allows the user to be much more free with the gesture path, thereby increasing the efficiency and finesse with which boundaries can be extracted.

The Robust Automatic Threshold Selection algorithm was introduced as a threshold selection based on a simple image statistic. The statistic is an average of the grey levels of the pixels in an image weighted by the response at each pixel of a specific edge detector. Other authors have suggested that many edge detectors may be used within the context of this method instead. A simple proof of this is given, including an extension to any number of image dimensions, and it is shown that in noiseless images with straight line edges these statistics all yield an optimum threshold. Biases caused by curvature of edges and by noise (uniform Gaussian and Poisson) are explored theoretically and on synthetic 2-D images. It is shown that curvature bias may be avoided by proper selection of the edge detector, and a comparison of two noise bias reduction schemes is given. Criteria for optimizing edge detectors are given and the performances of eight edge detectors are investigated in detail. The best results were obtained using two edge detectors which compute an approximation of the square of the gradient. It is shown that this conclusion can be extended to 3-D. Least sensitivity to noise was obtained when using 3 × 3 Sobel filter kernels to approximate partial derivatives inxandy.