Implementation and Analysis of an Image-Based Global Illumination Framework for Animated Environments
Jeffry Nimeroff, Julie Dorsey, and Holly Rushmeier
Abstract-We describe a new framework for efficiently computing and storing global illumination effects for complex, animated environments. The new framework allows the rapid generation of sequences representing any arbitrary path in a "view space" within an environment in which both the viewer and objects move. The global illumination is stored as time sequences of range-images at base locations that span the view space. We present algorithms for determining locations for these base images, and the time steps required to adequately capture the effects of object motion. We also present algorithms for computing the global illumination in the base images that exploit spatial and temporal coherence by considering direct and indirect illumination separately. We discuss an initial implementation using the new framework. Results and analysis of our implementation demonstrate the effectiveness of the individual phases of the approach; we conclude with an application of the complete framework to a complex environment that includes object motion.
Index Terms -Animation, global illumination, image-based rendering, radiosity, ray tracing, walk-throughs .
Structured Penumbral Irradiance Computation
George Drettakis and Eugene L. Fiume
Abstract-A definitive understanding of irradiance behavior in penumbral regions has been hard to come by, mainly due to the computational expense of determining the visible parts of an area light source. Consequently, sampling strategies have been mostly ad hoc, and evaluation of the resulting approximations has been difficult. In this paper, the structure of penumbral irradiance is investigated empirically and numerically. This study has been made feasible by the use of the discontinuity mesh and the backprojection, an efficient data structure representing visibility in regions of partial occlusion. Regions of penumbral in which irradiance varies nonmonotonically are characterized empirically, and numerical tests are performed to determine the frequency of their occurrence. This study inspired the development of two algorithms for the construction of interpolating approximations to irradiance: One algorithm reduces the number of edges in the mesh defining the interpolant domain, and the other algorithm chooses among linear, quadratic, and mixed interpolants based on irradiance monotonicity. Results from numerical tests and images are presented that demonstrate good performance of the new algorithms for various realistic test configurations.
Index Terms -Rendering, primary and global illumination, sampling, interpolation, structure, penumbra, experimental study, irradiance, radiosity, discontinuity meshing, backprojection, mesh simplification, interpolant degree reduction.
Fractal Volume Compression
Wayne O. Cochran, John C. Hart, and Patrick J. Flynn
Abstract-This research explores the principles, implementation, and optimization of a competitive volume compression system based on fractal image compression. The extension of fractal image compression to volumetric data is trivial in theory. However, the simple addition of a dimension to existing fractal image compression algorithms results in infeasible compression times and noncompetitive volume compression results. This paper extends several fractal image compression enhancements to perform properly and efficiently on volumetric data, and introduces a new 3D edge classification scheme based on principal component analysis. Numerous experiments over the many parameters of fractal volume compression suggest aggressive settings of its system para meters. At this peak efficiency, fractal volume compression surpasses vector quantization and approaches within 1 dB PSNR of the discrete cosine tranform. When compared to the DCT, fractal volume compression represents surfaces in volumes exceptionally well at high compression rates, and the artifacts of its compression error appear as noise instead of deceptive smoothing or distracting ringing.
Index Terms -Data compression, fractal, iterated function system, volume visualization.
Interactive Display of Large NURBS Models
Subodh Kumar, Dinesh Manocha, and Anselmo Lastra
Abstract-We present algorithms for interactive rendering of large-scale NURBS models. The algorithms convert the NURBS surface to Bezier surfaces, tessellate each Bezier surface into triangles, and render them using the triangle-rendering capabilities common to current graphics systems. This paper presents algorithms for computing tight bounds on surface properties in order to generate high quality tessellatioin of Bezier surfaces. We introduce enhanced visibility determination techniques and present methods to make efficient use of coherence between successive frames. In addition, we also discuss issues in parallelization of these techniques. The algorithm also avoids polygonization anomalies like cracks. Our algorithms work well in practice and, on high-end graphics systems, are able to display models described using thousands of Bezier surfaces at interactive frame rates.
Index Terms -NURBS, tessellation, triangulation, visibility, interactive display, CAD, parallel algorithm.
Image Metamorphosis with Scattered Feature Constraints
Seungyong Lee, George Wolberg, Kyung-Yong Chwa, and Sung Yong Shin.
Abstract-This paper describes an image metamorphosis technique to handle scattered feature constraints specified with points, polylines, and splines. Solutions to the following three problems are presented: feature specification, warp generation, and transition control. We demonstrate the use of snakes to reduce the burden of feature specification. Next, we propose the use of multilevel free-form deformations (MFFD) to compute C2-continuous and one-to-one mapping functions among the specificed features. The resulting technique, based on B-spline approximation, is simpler and faster than previous warp generation methods. Furthermore, it produces smooth image transformations without undesirable ripples and foldovers. Finally, we simplify the MFFD algorithm to derive transition functions to control geometry and color blending. Implementation details are furnished and comparisons among various metamorphosis techniques are presented.
Index Terms-Image metamorphosis, morphing, snakes, multilevel free-form deformation, multilevel B-spline interpolation.
Visualizing Unstructured Flow Data Using Dual Stream Functions
David Knight and Gordon Mallinson
Abstract-One of the most important ways of visualizing fluid flow is the construction of streamlines, which are lines that are everywhere tangential to the local fluid velocity. Stream surfaces are defined as surfaces through which no fluid penetrates. Streamlines can therefore be computed from the intersection of two nonparallel stream surfaces. This paper presents new algorithms for the computation of dual stream functions from Computational Fluid Dynamics data that is defined on an unstructured tetrahedral mesh. These algorithms are compared with standard numerical rountines for computing streamlines, and are shown to be quicker and mroe accurate than techniques involving numerical integration along the streamline.
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Reconstruction Error Characterization and Control: A Sampling Theory Approach
Raghu Machiraju and Roni Yagel
Abstract-Reconstruction is prerequisite whenever a discrete signal needs to be resampled as a result of transformation such as texture mapping, image manipulation, volume slicing, and rendering. We present a new method for the characterization and measurement fo reconstruction error in spatial domain. Our method uses the Classical Shannon's Sampling Theorem as a basis to develop error bounds. We use this formulation to provide, for the first time, an efficient way to guarantee an error bound at every point by varying the size of the reconstruction filter. We go further to support position-adaptive reconstruction and data- adaptive reconstruction which adjust filter size to the location of reconstruction point and to the data values in its vicinity. We demonstrate the effectiveness of our methods with 1D signals, 2D signals (images), and 3D signals (volumes).
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