Showing papers in "ACM Transactions on Graphics in 1999"

Reflectance and texture of real-world surfaces

Abstract: In this work, we investigate the visual appearance of real-world surfaces and the dependence of appearance on the geometry of imaging conditions. We discuss a new texture representation called the BTF (bidirectional texture function) which captures the variation in texture with illumination and viewing direction. We present a BTF database with image textures from over 60 different samples, each observed with over 200 different combinations of viewing and illumination directions. We describe the methods involved in collecting the database as well as the importqance and uniqueness of this database for computer graphics. A related quantity to the BTF is the familiar BRDF (bidirectional reflectance distribution function). The measurement methods involved in the BTF database are conducive to simultaneous measurement of the BRDF. Accordingly, we also present a BRDF database with reflectance measurements for over 60 different samples, each observed with over 200 different combinations of viewing and illumination directions. Both of these unique databases are publicly available and have important implications for computer graphics.

Two methods for display of high contrast images

Abstract: High contrast images are common in night scenes and other scenes that include dark shadows and bright light sources. These scenes are difficult to display because their contrasts greatly exceed the range of most display devices for images. As a result, the image constrasts are compressed or truncated, obscuring subtle textures and details. Humans view and understand high contrast scenes easily, “adapting” their visual response to avoid compression or truncation with no apparent loss of detail. By imitating some of these visual adaptation processes, we developed methods for the improved display of high-contrast images. The first builds a display image from several layers of lighting and surface properties. Only the lighting layers are compressed, drastically reducing contrast while preserving much of the image detail. This method is practical only for synthetic images where the layers can be retained from the rendering process. The second method interactively adjusts the displayed image to preserve local contrasts in a small “foveal” neighborhood. Unlike the first method, this technique is usable on any image and includes a new tone reproduction operator. Both methods use a sigmoid function for contrast compression. This function has no effect when applied to small signals but compresses large signals to fit within an asymptotic limit. We demonstrate the effectiveness of these approaches by comparing processed and unprocessed images.

Techniques for interactive design using the PDE method

Abstract: Interactive design of practical surfaces using the partial differential equation (PDE) method is considered. The PDE method treats surface design as a boundary value problem (ensuring that surfaces can be defined using a small set of design parameters). Owing to the elliptic nature of the PDE operator, the boundary conditions imposed around the edges of the surface control the internal shape of the surface. Moreover, surfaces obtained in this manner tend to be smooth and fair. The PDE chosen has a closed form solution allowing the interactive manipulation of the surfaces in real time. Thus we present efficient techniques by which we show how surfaces of practical significance can be constructed interactively in real time.

Anisotropic diffusion for Monte Carlo noise reduction

Abstract: Monte Carlo sampling can be used to estimate solutions to global light transport and other rendering problems. However, a large number of observations may be needed to reduce the variance to acceptable levels. Rather than computing more observations within each pixel, if spatial coherence exists in image space it can be used to reduce visual error by averaging estimators in adjacent pixels. Anisotropic diffusion is a space-variant noise reduction technique that can selectively preserve texture, edges, and other details using a map of image coherence. The coherence map can be estimated from depth and normal information as well as interpixel color distance. Incremental estimation of the reduction in variance, in conjunction with statistical normalization of interpixel color distances, yields an energy-preserving algorithm that converges to a spatially nonconstant steady state.

Radiance interpolants for accelerated bounded-error ray tracing

Abstract: Ray tracers, which sample radiance, are usually regarded as offline rendering algorithms that are too slow for interactive use. In this article we present a system that exploits object-space, ray-space, image-space, and temporal coherence to accelerate ray tracing. Our system uses per-surface interpolants to approximate radiance both interactive and batch ray tracers. Our approach explicity decouples the two primary operations of a ray tracer—shading and visibility determination—and accelerates each of them independently. Shading is accelerated by quadrilinearily interpolating lazily acquired radiance samples. Interpolation error does not exceed a user-specified bound, allowing the user to control performance/quality tradeoffs. Error is bounded by adaptive sampling at discontinuities and radiance nonlinearities. Visibility determination at pixels is accelerated by reprojecting interpolants as the user's viewpoint changes. A fast scan-line alogoithm then achieves high performance without sacrificing image quality. For a smoothly varying viewpoint, the combination of lazy interpolants and projection substantially accelerates the ray tracer. Additionally, an efficient cache management algorithm keeps the memory footprint of the system small with negilgible overhead.

The holodeck ray cache: an interactive rendering system for global illumination in nondiffuse environments

Abstract: We present a new method for rendering complex environments using interactive, progressive, view-independent, parallel ray tracing. A four-dimensional holodeck data structure serves as a rendering target and caching mechanism for interactive walk-throughs of nondiffuse environments with full global illumination. Ray sample density varies locally according to need, and on-demand ray computation is supported in a parallel implementation. The holodeck file is stored on disk and cached in memory by a server using a least-recently-used (LRU) beam-replacement strategy. The holodeck server coordinates separate ray evaluation and display processes, optimizing disk and memory usage. Different display systems are supported by specialized drivers, which handle display rendering, user interaction, and input. The display driver creates an image from ray samples sent by the server and permits the manipulation of local objects, which are rendered dynamically using approximate lighting computed from holodeck samples. The overall method overcomes many of the conventionl limits of interactive rendering in scenes with complex surface geometry and reflectance properties, through an effective combination of ray tracing, caching, and hardware rendering.

Combining constructive and equational geometric constraint-solving techniques

Abstract: In the past few years, there has been a strong trend towards developing parametric, computer-aided design systems based on geometric constraint solving. An effective way to capture the design intent in these systems is to define relationships between geometric and technological variables. In general, geometric constraint solving including functional relationships requires a general approach and appropriate techniques to achieve the expected functional capabilities. This work reports on a hybrid method that combines two geometric constraint solving techniques: constructive and equational. The hybrid solver has the capability of managing functional relationships between dimension variables and variables representing conditions external to the geometric problem. The hybrid solver is described as a rewriting system and is shown to be correct.

A unified approach for hierarchical adaptive tesselation of surfaces

Abstract: This paper introduces a unified and general tesselation algorithm for parametric and implicit surfaces. The algorithm produces a hierarchial mesh that is adapted to the surface geometry and has a multiresolution and progressive structure. The representation can be exploited with advantages in several applications.

Model and representation: the effect of visual feedback on human performance in a color picker interface

Abstract: User interfaces for color selection consist of a visible screen representation, an input method, and the underlying conceptual organization of the color model. We report a two-way factorial, between-subjects variable experiment that tested the effect of high and low visual feedback interfaces on speed and accuracy of color matching for RGB and HSV color models. The only significant effect was improved accuracy due to increased visual feedback. Using color groups as a within-subjects variable, we found differences in performance of both speed and accuracy. We recommend that experimental tests adopt a color test set that does not show bias toward a particular model, but is based instead on a range of colors that would be most likely matched in practice by people using color selection software. We recomment the Macbeth Color Checker naturals, primaries, and grays. As a follow-up study, a qualitative case analysis of the way users navigated through the color space indicates that feedback helps users with limited knowledge of the model, allowing them to refine their match to a higher degree of accuracy. Users with very little or a lot of knowledge of the color model do not appear to be aided by increased feedback. In conclusion, we suggest that visual feedback and design of the interface may be a more important factor in improving the usability of a color selection interface than the particular color model used.

Fast and accurate hierarchical radiosity using global visibility

Abstract: Recent hierarchical global illumination algorithms permit the generation of images with a high degree of realism. Nonetheless, appropriate refinement of light transfers, high quality meshing, and accurate visibility calculation can be challenging tasks. This is particularly true for scenes containing multiple light sources and scenes lit mainly by indirect light. We present solutions to these problems by extending a global visibility data structure, the Visibility Skeleton. This extension allows us to calculate exact point-to-polygon form-factors at vertices created by subdivision. The structures also provides visibility information for all light interactions, allowing intelligent refinement strategies. High-quality meshing is effected based on a perceptualy based ranking strategy which results in appropriate insertions of discontinuity curves into the meshes representing illumination. We introduce a hierarchy of triangulations that allows the generation of a hierarchical radiosity solution using accurate visibility and meshing. Results of our implementation show that our new algorithm produces high quality view-independent lighting solutions for direct illumination, for scenes with multiple lights and also scenes lit mainly by indirect illumination.

Searchlight and Doppler effects in the visualization of special relativity: a corrected derivation of the transformation of radiance

Abstract: We demonstrate that a photo-realistic image of a rapidly moving object is dominated by the searchlight and Doppler effects. Using a photon-counting technique, we derive expressions for the relativistic transformation of radiance. We show how to incorportate the Doppler and searchlight effects in the two common techniques of special relativistic visualization, namely ray tracing and polygon rendering. Most authors consider geometrical appearance only and neglect relativistic effects on the lighting model. Chang et al. [1996] present an incorrect derivation of the searchlight effect, which we compare to our results. Some examples are given to show the results of image synthesis with relativistic effects taken into account.

Analyzing bounding boxes for object intersection

Abstract: Heuristics that exploit bouning boxes are common in algorithms for rendering, modeling, and animation. While experience has shown that bounding boxes improve the performance of these algorithms in practice, the previous theoretical analysis has concluded that bounding boxes perform poorly in the worst case. This paper reconciles this discrepancy by analyzing intersections among n geometric objects in terms of two parameters: α an upper bound on the aspect ratio or elongatedness of each object; and σ an upper bound on the scale factor or size disparity between the largest and smallest objects. Letting Ko and Kb be the number of intersecting object pairs and bounding box pairs, respectively, we analyze a ratio measure of the bounding boxes' efficiency, ρ = Kb / (n + K0). The analysis proves that ρ = O(α√σlog2σ) and ρ = Ω(α√σ).One important consequence is that if α and σ are small constants (as is often the case in practice), then Kb= O(Ko)+ O(n, so an algorithm that uses bounding boxes has time complexity proportional to the number of actual object intersections. This theoretical result validates the efficiency that bounding boxes have demonstrated in practice. Another consequence of our analysis is a proof of the output-sensitivity of an algorithm for reporting all intersecting pairs in a set of n convex polyhedra with constant α and σ. The algorithm takes time O(nlogd-1n+ Kologd-1n) for dimension d = 2, 3. This running time improves on the performance of previous algorithms, which make no assumptions about α and σ.

Modeling generalized cylinders via Fourier morphing

Abstract: Generalized cylinders provide a compact representation for modeling many components of natural objects as well as a great variety of human-made industrial parts. This paper presents a new approach to modeling generalized cylinders based on cross-sectional curves defined using Fourier descriptors. This modeling is based on contour interpolation and is implemented using a subdivision technique. The definition of generalized cylinders uses a three-dimensional trajectory which provides an adequate control for the smoothness of bend with a small number of parameters and includes the orientation of each cross-section (i.e, the local coordinate system) in the interpolation framework. Fourier representations of cross-sectional curves are obtained from contours in digital images, and corresponding points are identified by considering angular and arc-length parametrizations. Changes in cross-section shape through the trajectory are performed using Fourier morphing. The technique proposed provides a comprehensive definition that allows the modeling of a wide variety of shapes, while maintaining a compact characterization to facilitate the description of shapes and displays.

A simple method for drawing a rational curve as two Bézier segments

Abstract: In this paper we give a simple method for drawing a closed rational curve specified in terms of control points as two Bezier segments. The main result is the following: For every affine frame (r,s) (where r