Gregory J. Ward

Measuring and modeling anisotropic reflection

A new device for measuring the spatial reflectancedistributionsof surfaces is introduced, along with a new mathematical model of sniaorropic reflectance. The reflectance model presented is both simple and accurate, permitting efficient reflectance data reduction rasdreproduction. Tire validity of the model is substantiated with comparisons to complete meaarsremems of surface reflectance functions gathered with the novel retlectometry device. This new device uses imaging technology to capture the entire hemisphem of reflected directions simttkarreously, which greatly accelerates the reflectance data gathering process, making it pssible to measure dozens of surfaces in the time that it used to take to do one. Example measurements and simulations are shown. and a table of fitted parameters for several surfaces is presented. General Terms: algorithms, measurement, theory, verification. CR Categories and Descriptors: 1.3.7 Three-dimensionalgraphics and rw#ism, 1.6.4 Model validation and analysis. Additional

The RADIANCE lighting simulation and rendering system

This paper describes a physically-based rendering system tailored to the demands of lighting design and architecture. The simulation uses a light-backwards ray-tracing method with extensions to efficiently solve the rendering equation under most conditions. This includes specular, diffuse and directional-diffuse reflection and transmission in any combination to any level in any environment, including complicated, curved geometries. The simulation blends deterministic and stochastic ray-tracing techniques to achieve the best balance between speed and accuracy in its local and global illumination methods. Some of the more interesting techniques are outlined, with references to more detailed descriptions elsewhere. Finally, examples are given of successful applications of this free software by others.

Comparing Real and Synthetic Images: Some Ideas About Metrics

This paper explores numerical techniques for comparing real and synthetic luminance images. We introduce components of a perceptually based metric using ideas from the image compression literature. We apply a series of metrics to a set of real and synthetic images, and discuss their performance. Finally, we conclude with suggestions for future work in formulating image metrics and incorporating them into new image synthesis methods.

Adaptive Shadow Testing for Ray Tracing

We present a simple technique for improving the efficiency of ray tracing in scenes with many light sources. The sources are sorted according to their potential contribution, and only those sources whose shadows are above a specified threshold are tested. The remainder are added into the result in proportion to a statistical estimate of their visibility. The algorithm requires very littie storage, and produces no visible artifacts.

Improving the Performance of Photo-Electrically Controlled Lighting Systems

The ability of a photo-electrically controlled lighting system to maintain a constant total light level on a task surface by responding to changing daylight levels is affected by the control algorithm used to relate the photosensor signal to the supplied electric light level and by the placement and geometry of the photosensor. We describe the major components of a typical control system, discuss the operation of three different control algorithms, and derive expressions for each algorithm that express the total illuminance at the task as a function of the control photosensor signal. Using a specially-designed scale model, we measured the relationship between the signal generated by various ceiling-mounted control photosensors and workplane illuminance for two room geometries under real sky conditions. The measured data were used to determine the performance of systems obeying the three control algorithms under varying daylight conditions. Control systems employing the commonly-used integral reset algorithm supplied less electric light than required, failing to satisfy the control objective regardless of the control photosensor used. Systems employing an alternative, closed-loop proportional control algorithm achieved the control objective under virtually all tested conditions when operated by a ceiling-mounted photosensor shielded from direct window light.

Energy preserving non-linear filters

Monte Carlo techniques for image synthesis are simple and powerful, but they are prone to noise from inadequate sampling. This paper describes a class of non-linear filters that remove sampling noise in synthetic images without removing salient features. This is achieved by spreading real input sample values into the output image via variable-width filter kernels, rather than gathering samples into each output pixel via a constant-width kernel. The technique is nonlinear because kernel widths are based on sample magnitudes, and this local redistribution of values cannot generally be mapped to a linear function. Nevertheless, the technique preserves energy because the kernels are normalized, and all input samples have the same average influence on the output. To demonstrate its effectiveness, the new filtering method is applied to two rendering techniques. The first is a Monte Carlo path tracing technique with the conflicting goals of keeping pixel variance below a specified limit and finishing in a finite amount of time; this application shows how the filter may be used to “clean up” areas where it is not practical to sample adequately. The second is a hybrid deterministic and Monte Carlo ray-tracing program; this application shows how the filter can be effective even when the pixel variance is not known.

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

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.

A perceptually validated model for surface depth hallucination

Capturing detailed surface geometry currently requires specialized equipment such as laser range scanners, which despite their high accuracy, leave gaps in the surfaces that must be reconciled with photographic capture for relighting applications. Using only a standard digital camera and a single view, we present a method for recovering models of predominantly diffuse textured surfaces that can be plausibly relit and viewed from any angle under any illumination. Our multiscale shape-from-shading technique uses diffuse-lit/flash-lit image pairs to produce an albedo map and textured height field. Using two lighting conditions enables us to subtract one from the other to estimate albedo. In the absence of a flash-lit image of a surface for which we already have a similar exemplar pair, we approximate both albedo and diffuse shading images using histogram matching. Our depth estimation is based on local visibility. Unlike other depth-from-shading approaches, all operations are performed on the diffuse shading image in image space, and we impose no constant albedo restrictions. An experimental validation shows our method works for a broad range of textured surfaces, and viewers are frequently unable to identify our results as synthetic in a randomized presentation. Furthermore, in side-by-side comparisons, subjects found a rendering of our depth map equally plausible to one generated from a laser range scan. We see this method as a significant advance in acquiring surface detail for texturing using a standard digital camera, with applications in architecture, archaeological reconstruction, games and special effects.

Relightable buildings from images

We propose a complete image-based process that facilitates recovery of both gross-scale geometry and local surface structure to create highly detailed 3D models of building facades from photographs. We approximate both albedo and sufficient local geometric structure to compute complex self-shadowing effects, and fuse this with a gross-scale 3D model. Our approach yields a perceptually high-quality model, imparting the illusion of measured reflectance. The requirements of our approach are that image capture must be performed under diffuse lighting and surfaces in the images must be predominantly Lambertian. Exemplars of materials are obtained through surface depth hallucination, and our novel method matches these with multi-view image sequences that are also used to automatically recover 3D geometry. In this paper we illustrate the effictiveness of our approach through visually realistic reconstructions of historic buildings in two test cases, together with a table showing the breakdown of effort required to reconstruct each.

A case study evaluation: perceptually accurate textured surface models

This paper evaluates a new method for capturing surfaces with variations in albedo, height, and local orientation using a standard digital camera with three flash units. Similar to other approaches, captured areas are assumed to be globally flat and largely diffuse. Fortunately, this encompasses a wide array of interesting surfaces, including most materials found in the built environment, e.g., masonry, fabrics, floor coverings, and textured paints. We present a case study of naïve subjects who found that surfaces captured with our method, when rendered under novel lighting and view conditions, were statistically indistinguishable from photographs. This is a significant improvement over previous methods, to which our results are also compared.