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Rossignac,
Jarek
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Jarek
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ItemSAM: Steady Affine Motions(Georgia Institute of Technology, 20091123) Rossignac, Jarek ; Vinacua, ÀlvarAn affine motion is a continuous map from time value t to an affinity A subscript t. It is a SAM (Steady Affine Motion), when A subscript t = A superscript t. Although the beauty of a motion is subjective, the above equation provides one mathematical characterization and includes the screw ("universal instantaneous") motion and the golden ("mirabilis") spiral. Although a real matrix, A superscript t, may not exist, we show that it does for a dense set of affinities A covering a significant range of rotations and shears around the identity and that it may be computed efficiently and robustly in two and three dimensions using closed form expressions. SAMs have remarkable properties. For example, the velocity of any point remains constant, both in the global (fixed) and local (moving) frames, which facilitates the exact computation of derived entities, such as the envelope surfaces used to define the boundary of a swept volume. We say that a pattern of features F subscript i is steady when there exists an affinity M such that F subscript i = M superscript i F subscript 0. Each M superscript i is a frame of a SAM and may be computed as A superscript (i/n), where A is the afiine relation F subscript n = A F subscript 0 between the first and the last feature. This option makes it possible to edit directly the feature count n or the cumulative transformation A.

ItemITR/PE+SY digital clay for shape input and display(Georgia Institute of Technology, 20071130) Book, Wayne J. ; Rossignac, Jarek ; Mynatt, Elizabeth D. ; Allen, Mark G. ; Goldthwaite, John Randall ; Rosen, David W. ; Glezer, Ari

ItemOptimized Blist Form (OBF)(Georgia Institute of Technology, 20070523) Rossignac, JarekAny Boolean expressions may be converted into positiveform, which has only union and intersection operators. Let E be a positiveform expression with n literals. Assume that the truthvalues of the literals are read one at a time. The numbers s(n) of steps (operations) and b(n) of working memory bits (footprint) needed to evaluate E depend on E and on the evaluation technique. A recursive evaluation performs s(n)=n–1 steps but requires b(n)=log(n)+1 bits. Evaluating the disjunctive form of E uses only b(n)=2 bits, but may lead to an exponential growth of s(n). We propose a new Optimized Blist Form (OBF) that requires only s(n)=n steps and b(n)=⌈log2j⌉ bits, where j=⌈log2(2n/3+2)⌉. We provide a simple and linear cost algorithm for converting positiveform expressions to their OBF. We discuss three applications: (1) Direct CSG rendering, where a candidate surfel stored at a pixel is classified against an arbitrarily complex Boolean expression using a footprint of only 6 stencil bits; (2) the new Logic Matrix (LM), which evaluates any positive form logical expression of n literals in a single cycle and uses a matrix of at most n×j wire/line connections; and (3) the new Logic Pipe (LP), which uses n gates that are connected by a pipe of ⌈log2j⌉ lines and when receiving a staggered stream of input vectors produces a value of a logical expression at each cycle.

ItemMultiple Object Selection in Pattern Hierarchies(Georgia Institute of Technology, 2007) Jang, Justin ; Rossignac, JarekHierarchies of patterns of features, of subassemblies, or of CSG subexpressions are used in architectural and mechanical CAD to eliminate laborious repetitions from the design process. Yet, often the placement, shape, or even existence of a selection of the repeated occurrences in the pattern must be adjusted. The specification of a desired selection of occurrences in a hierarchy of patterns is often tedious (involving repetitive steps) or difficult (requiring interaction with an abstract representation of the hierarchy graph). The OCTOR system introduced here addresses these two drawbacks simultaneously, offering an effective and intuitive solution, which requires only two mouseclicks to specify any one of a wide range of possible selections. It does not require expanding the graph or storing an explicit list of the selected occurrences and is simple to compute. It is hence well suited for a variety of CAD applications, including CSG, featurebased design, assembly mockup, and animation. We discuss a novel representation of a selection, a technology that makes it possible to use only two mouseclicks for each selection, and the persistence of these selections when the hierarchy of patterns is edited.

ItemRinging Js refinements(Georgia Institute of Technology, 2007) Schaefer, Scott ; Rossignac, JarekBoth the fourpoint and the uniform cubic Bspline refinement (i.e. subdivision) schemes double the number of vertices of a closedloop polygonal curve jP and respectively produce sequences of vertices fk and bk. The Js refinement proposed here produces vertices vk=(1s)fk+sbk. Iterative applications of Js yield a family of curves parameterized by s. It includes the fourpoint curve (J0), the uniform cubic Bspline (J8/8), and the quintic Bspline (J12/8). Iterating Js converges to a C2 curve for 0[less than]s[less than]1, to a C3 curve for 1[less than or equal to]s[less than]3/2, and to a C4 curve for s=S3/2. J3/8 tends to reduce the error between consecutive refinements and is useful to reduce popping when switching levelsofdetail in multiresolution rendering. J4/8 produces the Jarek curve, which, in 2D, encloses a surface area that is usually very close to the area enclosed by the original control polygon 0P. We propose modeldependent and modelindependent optimizations for these parameter values. As other refinement schemes, the Js approach extends trivially to open curves, animations, and surfaces. To reduce memory requirements when evaluating the final refined curve, surface, or animation, we introduce a new evaluation technique, called Ringing. It requires a footprint of only 5 points per subdivision level for each curve and does not introduce any redundant calculations.

ItemSimulation of Bubbles in Foam With The Volume Control Method(Georgia Institute of Technology, 2007) Kim, Byungmoon ; Liu, Yingjie ; Llamas, Ignacio ; Jiao, Xiangmin ; Rossignac, JarekLiquid and gas interactions often produce bubbles that stay for a long time without bursting on the surface, making a dry foam structure. Such long lasting bubbles simulated by the level set method can suffer from a small but steady volume error that accumulates to a visible amount of volume change. We propose to address this problem by using the volume control method. We track the volume change of each connected region, and apply a carefully computed divergence that compensates undesired volume changes. To compute the divergence, we construct a mathematical model of the volume change, choose control strategies that regulate the modeled volume error, and establish methods to compute the control gains that provide robust and fast reduction of the volume error, and (if desired) the control of how the volume changes over time.

ItemPressing: Smooth Isosurfaces with Flats from Binary Grids(Georgia Institute of Technology, 2007) Chica, Antonio ; Williams, Jason ; Andujar, Carlos ; Brunet, Pere ; Navazo, Isabel ; Rossignac, Jarek ; Vinacua, AlvarWe explore the automatic recovery of solids from their binary volumetric discretizations. In particular, we propose an approach, called Pressing, for smoothing isosurfaces extracted from binary volumes while recovering their large planar regions (flats). Pressing yields a surface that is guaranteed to contain the samples of the volume classified as interior and exclude those classified as exterior. It uses global optimization to identify flats and constrained bilaplacian smoothing to eliminate sharp features and highfrequencies from the rest of the isosurface. It recovers sharp edges between flat regions and between flat and smooth regions. Hence, the resulting isosurface is usually a very accurate approximation of the original solid. Furthermore, the segmentation of the isosurface into flat and curved faces and the sharp/smooth labelling of their edges may be valuable for shape recognition, simplification, compression, and various reverse engineering and manufacturing applications.

ItemSimulation of Bubbles and Liquid Films(Georgia Institute of Technology, 2006) Kim, Byungmoon ; Liu, Yingjie ; Llamas, Ignacio ; Rossignac, JarekLiquid and gas interactions often contain bubbles surrounded by thin liquid films. Simulation of these liquid films is challenging since they quickly become thinner than the grid resolution, which leads to premature bursting or merging of the bubbles. We prevent this thinning process by applying a disjoining force to the film, obtaining bubbles that last much longer without bursting or merging. The surface tension on the liquid film is the next diffuculty. Since the level set is not differentiable at the center of the thin liquid film, the curvature computed from the level set gradient is noisy, and the thin liquid film ruptures quickly. To prevent this, we compute the surface tension from the local isosurface, obtaining longlasting liquid films. However, since bubbles stay longer without bursting or merging, the volume loss of each bubble is noticeable. To solve this problem, we modify the pressure projection to produce a velocity field whose divergence is controlled by the proportional and integral feedback. This allows us to preserve the volume or, if desired, to inflate or deflate the bubbles. In addition to premature bursting and volume change, another difficulty is the complicated liquid surface, which increases memory and computational costs. To reduce storage requirement, we collocate the velocity and pressure to simplify the octree mesh. To reduce the computational complexity of the pressure projection, we use a multigrid method.

ItemBoundary of the Volume Swept By a Freeform Solid in Screw Motion(Georgia Institute of Technology, 2006) Rossignac, Jarek ; Kim, J. J. ; Song, S. C. ; Suh, K. C. ; Joung, C. B.The swept volume of a moving solid provides an excellent aid for path and accessibility planning in robotics and for simulating various manufacturing operations. To compute the boundary of the swept volume, we approximate the motion by a polyscrew (continuous, piecewisescrew), generate candidate faces, compute the twocells of their arrangement, and use a new pointinsweep test to select the correct cells whose union forms the boundary of the swept volume.

ItemAdvections with Significantly Reduced Dissipation and Diffusion(Georgia Institute of Technology, 2006) Kim, Byungmoon ; Liu, Yingjie ; Llamas, Ignacio ; Rossignac, JarekBack and Forth Error Compensation and Correction (BFECC) can be applied to reduce dissipation and diffusion in advection steps, such as velocity, smoke density, and image advections. It can be implemented trivially as a small modification of the firstorder upwind or semiLagrangian integration of advection equations. It provides secondorder accuracy in both space and time and reduces volume loss significantly. We demonstrate its benefits on the simulation of smoke, bubbles, and interaction between water, a solid, and air.