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Item Context-based Space Filling Curves(Blackwell Publishers Ltd and the Eurographics Association, 2000) Dafner, Revital; Cohen-Or, Daniel; Matias, YossiA context-based scanning technique for images is presented. An image is scanned along a context-based space filling curve that is computed so as to exploit inherent coherence in the image. The resulting one-dimensional representation of the image has improved autocorrelation compared with universal scans such as the Peano-Hilbert space filling curve. An efficient algorithm for computing context-based space filling curves is presented. We also discuss the potential of improved autocorrelation of context-based space filling curves for image and video lossless compression.Item Tutorial 9 - Visibility(Eurographics Association, 1999) Chrysanthou, Yiorgos L.; Cohen-Or, Daniel; Fibich, Gadi; Halperin, Dan; Zadicario, Eyal; Lev-Yehudi, Shuly; Bartz, Dirk; Meißner, Michael; Hüttner, Tobias; Hüttner, Tobias; Bittner, Jiri; Havran, Vlastimil; Slavik, Pavel; Klosowski, James T.; Silva, Claudio T.The focus of this tutorial is a study of the techniques for solving visibility problems in virtual walkthroughs. The term walkthrough is referring not only to models of architectural models but rather to any large complex model, where the focus is on viewing and rendering the model. The tutorial touches on several applications such as fast visible surface determination, selection of relevant model sections to be transmitted on a client-server system, as well as improving image quality by shading algorithms.Item Selective Pixel Transmission for Navigating in Remote Virtual Environments(Blackwell Publishers Ltd and the Eurographics Association, 1997) Mann, Yair; Cohen-Or, DanielThis paper presents a technique to improve the performance of a walkthrough in remote virtual environments, where a scene is rendered jointly by the server and the client, in order to reduce the network requirements as much as possible. The client generates novel views by extrapolating a reference view based on the locally available geometric model, while the server transmits data necessary to prevent an accumulation of errors. Within this concept, we show that by transmitting only a selected subset of pixels, the quality of the extrapolated views can be improved while requiring less bandwidth. We focus on the selection process in which the visibility gaps between the reference view and novel view are detected, packed and transmitted compressed to the client.Item Conservative Visibility and Strong Occlusion for Viewspace Partitioning of Densely Occluded Scenes(Blackwell Publishers Ltd and the Eurographics Association, 1998) Cohen-Or, Daniel; Fibich, Gadi; Halperin, Dan; Zadicario, EyalComputing the visibility of out-door scenes is often much harder than of in-door scenes. A typical urban scene, for example, is densely occluded, and it is effective to precompute its visibility space, since from a given point only a small fraction of the scene is visible. The difficulty is that although the majority of objects are hidden, some parts might be visible at a distance in an arbitrary location, and it is not clear how to detect them quickly. In this paper we present a method to partition the viewspace into cells containing a conservative superset of the visible objects. For a given cell the method tests the visibility of all the objects in the scene. For each object it searches for a strong occluder which guarantees that the object is not visible from any point within the cell. We show analytically that in a densely occluded scene, the vast majority of objects are strongly occluded, and the overhead of using conservative visibility (rather than visibility) is small. These results are further supported by our experimental results. We also analyze the cost of the method and discuss its effectiveness.Item Selecting Effective Occluders for Visibility Culling(Eurographics Association, 2000) Koltun, Vladlen; Cohen-Or, DanielThis paper deals with the problem of identifying effective occluders for visibility culling. The solid-angle metric is commonly used for measuring the potential significance of occluders from a single viewpoint. In this paper, we show that it does not extend properly to from-region occlusion calculations. We propose to measure the effectiveness of an occluder by means of the size of its umbra. We first present an analytic object-space algorithm to accurately compute this measure. We then define an approximation which reflects the effectiveness of an occluder, and introduce a hardware-assisted algorithm to rapidly compute it.Item Automatic Camera Placement for Image-Based Modeling(Blackwell Publishers Ltd and the Eurographics Association, 2000) Fleishman, Shachar; Cohen-Or, Daniel; Lischinski, DaniWe present an automatic camera placement method for generating image-based models from scenes with known geometry. Our method first approximately determines the set of surfaces visible from a given viewing area and then selects a small set of appropriate camera positions to sample the scene from. We define a quality measure for a surface as seen, or covered, from the given viewing area. Along with each camera position, we store the set of surfaces which are best covered by this camera. Next, one reference view is generated from each camera position by rendering the scene. Pixels in each reference view that do not belong to the selected set of polygons are masked out.The image-based model generated by our method, covers every visible surface only once, associating it with a camera position from which it is covered with quality that exceeds a user-specified quality threshold. The result is a compact non-redundant image-based model with controlled quality.The problem of covering every visible surface with a minimum number of cameras (guards) can be regarded as an extension to the well-known Art Gallery Problem. However, since the 3D polygonal model is textured, the camera-polygon visibility relation is not binary; instead, it has a weight - the quality of the polygon's coverage.