Jihad El-Sana

Adaptive real-time level-of-detail based rendering for polygonal models

We present an algorithm for performing adaptive real-time level-of-detail-based rendering for triangulated polygonal models. The simplifications are dependent on viewing direction, lighting, and visibility and are performed by taking advantage of image-space, object-space, and frame-to-frame coherences. In contrast to the traditional approaches of precomputing a fixed number of level-of-detail representations for a given object, our approach involves statically generating a continuous level-of-detail representation for the object. This representation is then used at run time to guide the selection of appropriate triangles for display. The list of displayed triangles is updated incrementally from one frame to the next. Our approach is more effective than the current level-of-detail-based rendering approaches for most scientific visualization applications, where there are a limited number of highly complex objects that stay relatively close to the viewer. Our approach is applicable for scalar (such as distance from the viewer) as well as vector (such as normal direction) attributes.

Generalized View-Dependent Simplification

We propose a technique for performing view‐dependent geometry and topology simplifications for level‐of‐detail‐based renderings of large models. The algorithm proceeds by preprocessing the input dataset into a binary tree, the view‐dependence tree of general vertex‐pair collapses. A subset of the Delaunay edges is used to limit the number of vertex pairs considered for topology simplification. Dependencies to avoid mesh foldovers in manifold regions of the input object are stored in the view‐dependence tree in an implicit fashion. We have observed that this not only reduces the space requirements by a factor of two, it also highly localizes the memory accesses at run time. The view‐dependence tree is used at run time to generate the triangles for display. We also propose a cubic‐spline‐based distance metric that can be used to unify the geometry and topology simplifications by considering the vertex positions and normals in an integrated manner.

Automatic reconstruction of tree skeletal structures from point clouds

Trees, bushes, and other plants are ubiquitous in urban environments, and realistic models of trees can add a great deal of realism to a digital urban scene. There has been much research on modeling tree structures, but limited work on reconstructing the geometry of real-world trees -- even then, most works have focused on reconstruction from photographs aided by significant user interaction. In this paper, we perform active laser scanning of real-world vegetation and present an automatic approach that robustly reconstructs skeletal structures of trees, from which full geometry can be generated. The core of our method is a series of global optimizations that fit skeletal structures to the often sparse, incomplete, and noisy point data. A significant benefit of our approach is its ability to reconstruct multiple overlapping trees simultaneously without segmentation. We demonstrate the effectiveness and robustness of our approach on many raw scans of different tree varieties.

External Memory View-Dependent Simplification

In this paper, we propose a novel external‐memory algorithm to support view‐dependent simplification for datasets much larger than main memory. In the preprocessing phase, we use a new spanned sub‐meshes simplification technique to build view‐dependence trees I/O‐efficiently, which preserves the correct edge collapsing order and thus assures the run‐time image quality. We further process the resulting view‐dependence trees to build the meta‐node trees, which can facilitate the run‐time level‐of‐detail rendering and is kept in disk. During run‐time navigation, we keep in main memory only the portions of the meta‐node trees that are necessary to render the current level of details, plus some prefetched portions that are likely to be needed in the near future. The prefetching prediction takes advantage of the nature of the run‐time traversal of the meta‐node trees, and is both simple and accurate. We also employ the implicit dependencies for preventing incorrect foldovers, as well as main‐memory buffer management and parallel processes scheme to separate the disk accesses from the navigation operations, all in an integrated manner. The experiments show that our approach scales well with respect to the main memory size available, with encouraging preprocessing and run‐time rendering speeds and without sacrificing the image quality.

Haptic sculpting of dynamic surfaces

Abstract Conventional free-form surface design usually require tedious control-point manipulation and/or painstaking constraint specifica- tion via unnatural mouse-based interfaces. This paper presents a novel haptic approach for the direct manipulation of physics-based B-spline surfaces. Our method permits users to interactively sculpt virtual yet real material with a standard haptic device, and feel the physically realistic presence of virtual B-spline objects with force feedback throughout the design process. We aim to develop var- ious haptic sculpting tools to expedite the direct manipulation of B-spline surfaces with haptic feedback and constraints. One signif- icant contribution of this paper is that point, normal, and curvature constraints can be specified interactively and modified naturally us- ing forces. We propose and formulate a dual representation for B- spline surfaces in both physical and mathematical space. This mass- spring model is mathematically constrained by the B-spline surface throughout the sculpting session. The equations of motion control- ling the physical behavior of the B-spline surface are solved using a tractable numerical solver in real-time. The integration of haptics with traditional geometric modeling will increase the bandwidth of human-computer interaction, and thus shorten the time-consuming design cycle. We envision that this integrated approach promises a much greater potential in computer-integrated design and manufac- turing, haptic interface, interactive graphics, medical applications, and virtual environments. CR Categories: 1.3.5 [Computer Graphics]: Physics-based mod- eling; 1.3.3 [Computer Graphics]: Modeling packages; 1.3.6 [Com- puter Graphics]: Interaction techniques; 1.3.7 [Computer Graph- ics]: Virtual reality; 1.3.7 [Computer Graphics]: Animation;

Seamless patches for GPU-based terrain rendering

In this paper we present a novel approach for interactive rendering of large terrain datasets. Our approach is based on subdividing a terrain into rectangular patches at different resolutions. Each patch is represented by four triangular tiles that are selected form different resolutions, and four strips which are used to stitch the four tiles in a seamless manner. Such a scheme maintains resolution changes within patches through the stitching strips, and not across patches. At runtime, these patches are used to construct a level-of-detail representation of the input terrain based on view-parameters. A selected level of detail only includes the layout of the patches and their boundary edges resolutions. The layout includes the location and dimension of each patch. Within the graphics hardware, the GPU generates the meshes of the patches by using scaled instances of cached tiles and assigns elevation for each vertex from cached textures. Since adjacent rectangular patches agree on the resolution of the common edges, the resulted mesh does not include cracks or degenerate triangles. Our algorithm manages to achieve quality images at high frame rates while providing seamless transition between different levels of detail.

Online Arabic Handwriting Recognition Using Hidden Markov Models

Online handwriting recognition of Arabic script is a difficult problem since it is naturally both cursive and unconstrained. The analysis of Arabic script is further complicated in comparison to Latin script due to obligatory dots/stokes that are placed above or below most letters. This paper introduces a Hidden Markov Model (HMM) based system to provide solutions for most of the difficulties inherent in recognizing Arabic script including: letter connectivity, position-dependent letter shaping, and delayed strokes. This is the first HMM-based solution to online Arabic handwriting recognition. We report successful results for writerdependent and writer-independent word recognition.

Integrating occlusion culling with view-dependent rendering

We present an approach that integrates occlusion culling within the view-dependent rendering framework. View-dependent rendering provides the ability to change level of detail over the surface seamlessly and smoothly in real-time. The exclusive use of view-parameters to perform level-of-detail selection causes even occluded regions to be rendered in high level of detail. To overcome this serious drawback we have integrated occlusion culling into the level selection mechanism. Because computing exact visibility is expensive and it is currently not possible to perform this computation in real time, we use a visibility estimation technique instead. Our approach reduces dramatically the resolution at occluded regions.

Skip strips: maintaining triangle strips for view-dependent rendering

View-dependent simplification has emerged as a powerful tool for graphics acceleration in visualization of complex environments. However, view-dependent simplification techniques have not been able to take full advantage of the underlying graphics hardware. Specifically, triangle strips are a widely used hardware-supported mechanism to compactly represent and efficiently render static triangle meshes. However, in a view-dependent framework, the triangle mesh connectivity changes at every frame, making it difficult to use triangle strips. We present a novel data structure, Skip Strip, that efficiently maintains triangle strips during such view-dependent changes. A Skip Strip stores the vertex hierarchy nodes in a skip-list-like manner with path compression. We anticipate that Skip Strips will provide a road map to combine rendering acceleration techniques for static datasets, typical of retained-mode graphics applications, with those for dynamic datasets found in immediate-mode applications.

Shape recognition and pose estimation for mobile augmented reality

In this paper we present Nestor, a system for real-time recognition and camera pose estimation from planar shapes. The system allows shapes that carry contextual meanings for humans to be used as Augmented Reality (AR) tracking fiducials. The user can teach the system new shapes at runtime by showing them to the camera. The learned shapes are then maintained by the system in a shape library. Nestor performs shape recognition by analyzing contour structures and generating projective invariant signatures from their concavities. The concavities are further used to extract features for pose estimation and tracking. Pose refinement is carried out by minimizing the reprojection error between sample points on each image contour and its library counterpart. Sample points are matched by evolving an active contour in real time. Our experiments show that the system provides stable and accurate registration, and runs at interactive frame rates on a Nokia N95 mobile phone.