Brian E. Smits

A practical analytic model for daylight

Sunlight and skylight are rarely rendered correctly in computer graphics. A major reason for this is high computational expense. Another is that precise atmospheric data is rarely available. We present an inexpensive analytic model that approximates full spectrum daylight for various atmospheric conditions. These conditions are parameterized using terms that users can either measure or estimate. We also present an inexpensive analytic model that approximates the effects of atmosphere (aerial perspective). These models are fielded in a number of conditions and intermediate results verified against standard literature from atmospheric science. Our goal is to achieve as much accuracy as possible without sacrificing usability.

A clustering algorithm for radiosity in complex environments

We present an approach for accelerating hierarchical radiosity by clustering objects. Previous approaches constructed effective hierarchies by subdividing surfaces, but could not exploit a hierarchical grouping on existing surfaces. This limitation resulted in an excessive number of initial links in complex environments. Initial linking is potentially the most expensive portion of hierarchical radiosity algorithms, and constrains the complexity of the environments that can be simulated. The clustering algorithm presented here operates by estimating energy transfer between collections of objects while maintaining reliable error bounds on each transfer. Two methods of bounding the transfers are employed with different tradeoffs between accuracy and time. In contrast with the O(s2) time and space complexity of the initial linking in previous hierarchical radiosity algorithms, the new methods have complexities of O(slogs) and O(s) for both time and space. Using these methods we have obtained speedups of two orders of magnitude for environments of moderate complexity while maintaining comparable accuracy.

Painting with light

We present a new approach to lighting design for image synthesis. It is based on the inverse problem of determining light settings for an environment from a description of the desired solution. The method is useful for determining light intensities to achieve a desired effect in a computer simulation and can be used in conjunction with any rendering algorithm. Given a set of lights with fixed positions, we determine the light intensities and colors that most closely match the target image painted by the designer using a constrained least squares approach. We describe an interactive system that allows flexible input and display of the solution.

Interactive ray tracing

We examine a rendering system that interactively ray traces n image on a conventional multiprocessor. The implementation i s “brute force” in that it explicitly traces rays through every scree n pixel, yet pays careful attention to system resources for acceleratio n. The design of the system is described, along with issues related to ma erial models, lighting and shadows, and frameless rendering. The syst m is demonstrated for several different types of input scenes . CR Categories: I.3.0 [Computer Graphics]: General; I.3.6 [Computer Graphics]: Methodology and Techniques.

An importance-driven radiosity algorithm

We present a new radiosi[y algorithm for efficiently computing global solutions with respect to a constrained set of views. Radiosi ties of directly visible surfaces are computed to high accuracy, while those ot’ surfaces having only an indirect effect are computed to an accuracy commensurate with their contribution. The algorithm uses an adaptive subdivision scheme that is guided by the interplay between two closely related transport processes: one propagating power from the light sources, and the other propagating imporrarwc from the visible surfaces. By simultaneously refining approximate solutions to the dud transport equations, computation is significantly reduced in areas that contribute little to the region of interest. This approach is very effective for complex environments in which only a small fraction is visible at any time. Our statistics show dramatic speedups over the fastest previous radiosity algorithms for diffuse environments with details at a wide range of scales. CR

A framework for the analysis of error in global illumination algorithms

In this paper we identify sources of error in global illumination algorithms and derive bounds for each distinct category. Errors arise from three sources: inaccuracies in the boundary data, discretization, and computation. Boundary data consists of surface geometry, reflectance functions, and emission functions, all of which may be perturbed by errors in measurement or simulation, or by simplifications made for computational efficiency. Discretization error is introduced by replacing the continuous radiative transfer equation with a finite-dimensional linear system, usually by means of boundary elements and a corresponding projection method. Finally, computational errors perturb the finite-dimensional linear system through imprecise form factors, inner products, visibility, etc., as well as by halting iterative solvers after a finite number of steps. Using the error taxonomy introduced in the paper we examine existing global illumination algorithms and suggest new avenues of research.

Bounds and error estimates for radiosity

We present a method for determining a posteriori bounds and estimates for local and total errors in radiosity solutions. The ability to obtain bounds and estimates for the total error is crucial fro reliably judging the acceptability of a solution. Realistic estimates of the local error improve the efficiency of adaptive radiosity algorithms, such as hierarchical radiosity, by indicating where adaptive refinement is necessary. First, we describe a hierarchical radiosity algorithm that computes conservative lower and upper bounds on the exact radiosity function, as well as on the approximate solution. These bounds account for the propagation of errors due to interreflections, and provide a conservative upper bound on the error. We also describe a non-conservative version of the same algorithm that is capable of computing tighter bounds, from which more realistic error estimates can be obtained. Finally, we derive an expression for the effect of a particular interaction on the total error. This yields a new error-driven refinement strategy for hierarchical radiosity, which is shown to be superior to brightness-weighted refinement.

Dynamic Acceleration Structures for Interactive Ray Tracing

Acceleration structures used for ray tracing have been designed and optimized for efficient traversal of static scenes. As it becomes feasible to do interactive ray tracing of moving objects, new requirements are posed upon the acceleration structures. Dynamic environments require rapid updates to the acceleration structures. In this paper we propose spatial subdivisions which allow insertion and deletion of objects in constant time at an arbitrary position, allowing scenes to be interactively animated and modified.

Visual cues for imminent object contact in realistic virtual environments

Distance judgments are difficult in current virtual environments, limiting their effectiveness in conveying spatial information. This problem is apparent when contact occurs while a user is manipulating objects. In particular, the computer graphics used to support current-generation immersive interfaces does a poor job of providing the visual cues necessary to perceive when contact between objects is about to occur. This perception of imminent contact is important in human motor control. Its absence prevents a sense of naturalness in interactive displays which allow for object manipulation. This paper reports results from an experiment evaluating the effectiveness of binocular disparity, cast shadows and diffuse inter-reflections in signaling imminent contact in a manipulation task.

An RGB-to-spectrum conversion for reflectances

This paper presents a solution to the problem of using RGB data created by most modeling systems in a spectrally-based renderer. It differs from previous methods in that it attempts to create physically plausible spectra for reflectances. The method searches the metamer space for a spectrum that best fits a set of criteria. The results are used in an algorithm that is simple and efficient enough to be used within the rendering process for both importing models and for texture mapping.