Alex Wilkie

Tone Reproduction and Physically Based Spectral Rendering

The ultimate aim of realistic graphics is the creation of images that provoke the same responses that a viewer would have to a real scene. This STAR addresses two related key problem areas in this effort which are located at opposite ends of the rendering pipeline, namely the data structures used to describe light during the actual rendering process, and the issue of displaying such radiant intensities in a meaningful way. The interest in the first of these subproblems stems from the fact that it is common industry practice to use RGB colour values to describe light intensity and surface reflectancy. While viable in the context of methods that do not strive to achieve true realism, this approach has to be replaced by more physically accurate techniques if a prediction of nature is intended. The second subproblem is that while research into ways of rendering images provides us with better and faster methods, we do not necessarily see their full effect due to limitations of the display hardware. The low dynamic range of a standard computer monitor requires some form of mapping to produce images that are perceptually accurate. Tone reproduction operators attempt to replicate the effect of real-world luminance intensities. This STAR report will review the work to date on spectral rendering and tone reproduction techniques. It will include an investigation into the need for spectral imagery synthesis methods and accurate tone reproduction, and a discussion of major approaches to physically correct rendering and key tone mapping algorithms. The future of both spectral rendering and tone reproduction techniques will be considered, together with the implications of advances in display hardware.

Threshold devices: looking out from the home

Threshold devices present information gathered from the homes surroundings to give new views on the domestic situation. We built two prototypes of different threshold devices and studied them in field trials with participant households. The Local Barometer displays online text and images related to the homes locality depending on the local wind conditions to give an impression of the sociocultural surroundings. The Plane Tracker tracks aircraft passing overhead and imagines their flights onscreen to resource an understanding of the homes global links. Our studies indicated that the experiences they provided were compelling, that participants could and did interpret the devices in various ways, that their form designs were appropriate for domestic environments, that using ready-made information contributed to the richness of the experiences, and that situating the information they provided with respect to the home and its locality was important for the ways people engaged with them.

Combined rendering of polarization and fluorescence effects

We propose a practicable way to include both polarization and fluorescence effects in a rendering system at the same time. Previous research in this direction only demonstrated support for either one of these phenomena; using both effects simultaneously was so far not possible, mainly because the techniques for the treatment of polarized light were complicated and required rendering systems written specifically for this task. The key improvement over previous work is that we use a different, more easily handled formalism for the description of polarization state, which also enables us to include fluorescence effects in a natural fashion. Moreover, all of our proposals are straightforward extensions to a conventional spectral rendering system.

A reflectance model for diffuse fluorescent surfaces

Fluorescence is an interesting and visually prominent effect, which has not been fully covered by Computer Graphics research so far.While the physical phenomenon of fluorescence has been addressed in isolation, the actual reflection behaviour of real fluorescent surfaces has never been documented, and no analytical BRDF models for such surfaces have been published yet.This paper aims to illustrate the reflection properties typical for diffuse fluorescent surfaces, and provides a BRDF model based on a layered microfacet approach that mimics them.

Speculative method and Twitter: Bots, energy and three conceptual characters

This paper aims to contribute to recent innovations in social scientific methodology that aspire to address the complex, iterative and performative dimensions of method. In particular, we focus on the becoming-with character of social events, and propose a speculative method for engaging with the not-as-yet. This work, being part of a larger project that uses Speculative Design and ethnographic methods to explore energy-demand reduction, specifically considers the ways in which energy-demand reduction features in the Twitter-sphere. Developing and deploying three automated Bots whose function and communications are at best obscure, and not uncommonly nonsensical, we trace some of ways in which they intervene and provoke. Heuristically, we draw on the ‘conceptual characters’ of idiot, parasite and diplomat in order to grasp how the Bots act within Twitter to evoke the instability and emergent eventuations of energy-demand reduction, community and related practices. We conclude by drawing out some of the wider implications of this particular enactment of speculative method.

Realistic rendering of birefringency in uniaxial crystals

In this article we derive the complete set of formulas needed to generate physically plausible images of uniaxial crystals. So far no computer graphics publication contains all the formulas one needs to compute the interaction of light with such crystals in a form that is useable by a graphics application, especially if a polarization-aware rendering system is being used. This paper contains the complete derivation of the Fresnel coefficients for birefringent transparent materials, as well as for the direction cosines of the extraordinary ray and the Mueller matrices necessary to describe polarization effects. The formulas we derive can be directly used in a ray based renderer, and we demonstrate these capabilities in test scenes.

Verification of Physically Based Rendering Algorithms

Within computer graphics, the field of predictive rendering is concerned with those methods of image synthesis that yield results that do not only look real, but are also radiometrically correct renditions of nature, i.e. which are accurate predictions of what a real scene would look like under given lighting conditions.

An analytical model for skylight polarisation

Under certain circumstances the polarisation state of the illumination can have a significant influence on the appearance of scenes; outdoor scenes with specular surfaces - such as water bodies or windows - under clear, blue skies are good examples of such environments. In cases like that it can be essential to use a polarising renderer if a true prediction of nature is intended, but so far no polarising skylight models have been presented. This paper presents a plausible analytical model for the polarisation of the light emitted from a clear sky. Our approach is based on a suitable combination of several components with well-known characteristics, and yields acceptable results in considerably less time than an exhaustive simulation of the underlying atmospheric scattering phenomena would require.

A Hierarchical Subdivision Algorithm for Stochastic Radiosity Methods

The algorithm proposed in this paper uses a stochastic approach to incrementally calculate the illumination function over a surface. By tracking the illumination function at different levels of meshing resolution, it is possible to get a measure for the quality of the current representation, and to adaptively subdivide in places with inadequate accuracy. With this technique a hierarchical mesh that is based on the stochastic evaluation of global illumination is generated.

A multiresolution mesh generation approach for procedural definition of complex geometry

As a general approach to procedural mesh definition we propose two mechanisms for mesh modification: generalized subdivision and rule based mesh growing. In standard subdivision, a specific subdivision rule is applied to a mesh to get a succession of meshes converging to a limit surface. A generalized approach allows different subdivision rules at each level of the subdivision process. By limiting the variations introduced at each level, convergence can be ensured: however in a number of cases it may be of advantage to exploit the expressivity of different subdivision steps at each level, without imposing any limits. Rule based mesh growing is an extension of L-systems to not only work on symbols, but connected symbols, representing faces in a mesh. This mechanism allows the controlled introduction of more complex geometry in places where it is needed to model fine details. Using both these mechanisms in combination we demonstrate, that a great variety of complex objects can be easily modeled and compactly represented.