Alexander Wilkie

Arbitrarily layered micro-facet surfaces

In this paper we present a method to combine several micro-facet based surface layers into a single unified, expressive BRDF model that is easy to use. The restriction to micro-facet based layers constitutes no loss of generality, since both perfectly specular and perfectly diffuse surfaces can be seen as limit cases of the micro-facet approach.n Such multi-layered surfaces can be used to re-create the appearance of a wide range of different materials, and yield good results without having to perform explicit sub-surface scattering computations.n This is achieved through suitable approximations and simplifications of the scattering within the simulated layered surface, while still taking absorption and total internal reflection into account. We also discuss the corresponding probability distribution function that is needed for sampling purposes, and investigate how the flexibility of this new approach is best put to use.

An analytic model for full spectral sky-dome radiance

We present a physically-based analytical model of the daytime sky. Based on the results of a first-principles brute force simulation of radiative transfer in the atmosphere, we use the same general approach of fitting basis function coefficients to radiance data as the Perez and Preetham models do. However, we make several modifications to this process, which together significantly improve the rendition of sunsets and high atmospheric turbidity setups -- known weak points of the Preetham model. Additionally, our model accounts for ground albedo, and handles each spectral component independently. The latter property makes it easily extensible to the near ultraviolet range of the spectrum, so that the daylight appearance of surfaces that include optical brighteners can be properly predicted. Due to its similar mathematical properties, the new model can be used as a drop-in replacement of the Preetham model.

Hero wavelength spectral sampling

We present a spectral rendering technique that offers a compelling set of advantages over existing approaches. The key idea is to propagate energy along paths for a small, constant number of changing wavelengths. The first of these, the hero wavelength, is randomly sampled for each path, and all directional sampling is solely based on it. The additional wavelengths are placed at equal distances from the hero wavelength, so that all path wavelengths together always evenly cover the visible range.

Procedural Modelling of Urban Road Networks

We present a model for growing procedural road networks in and close to cities. The main idea of our paper is that a city cannot be meaningfully simulated without taking its neighbourhood into account. A simple traffic simulation that considers this neighbourhood is then used to grow new major roads and to influence the locations of minor road growth. Waterways are introduced and used to help position the city nuclei on the map. The resulting cities are formed by allowing several smaller settlements to grow together and to form a rich road structure, much like in real world, and require only minimal per‐city input, allowing for batch generation.

Adding a Solar-Radiance Function to the Hošek-Wilkie Skylight Model

One prerequisite for realistic renderings of outdoor scenes is the proper capturing of the skys appearance. Currently, an explicit simulation of light scattering in the atmosphere isnt computationally feasible, and wont be in the foreseeable future. Captured luminance patterns have proven their usefulness in practice but cant meet all user needs. To fill this capability gap, computer graphics technology has employed analytical models of sky-dome luminance patterns for more than two decades. For technical reasons, such models deal with only the sky domes appearance, though, and exclude the solar disc. The widely used model proposed by Arcot Preetham and colleagues employed a separately derived analytical formula for adding a solar emitter of suitable radiant intensity. Although this yields reasonable results, the formula is derived in a manner that doesnt exactly match the conditions in their sky-dome model. But the more sophisticated a skylight model is and the more subtly it can represent different conditions, the more the solar radiance should exactly match the skylights conditions. Toward that end, researchers propose a solar-radiance function that exactly matches a recently published high-quality analytical skylight model.

Graphics Interaction: Interactive cloud rendering using temporally coherent photon mapping

This work presents a novel interactive algorithm for simulation of light transport in clouds. Exploiting the high temporal coherence of the typical illumination and morphology of clouds we build on volumetric photon mapping, which we modify to allow for interactive rendering speeds-instead of building a fresh irregular photon map for every scene state change we accumulate photon contributions in a regular grid structure. This is then continuously being refreshed by re-shooting only a fraction of the total amount of photons in each frame. To maintain its temporal coherence and low variance, a low-resolution grid is initially used, and is then upsampled to the density field resolution on a physical basis in each frame. We also present a technique to store and reconstruct the angular illumination information by exploiting properties of the standard Henyey-Greenstein function, namely its ability to express anisotropic angular distributions with a single dominating direction. The presented method is physically plausible, conceptually simple and comparatively easy to implement. Moreover, it operates only above the cloud density field, thus not requiring any precomputation, and handles all light sources typical for the given environment, i.e., where one of the light sources dominates.

Modeling and Verifying the Polarizing Reflectance of Real-World Metallic Surfaces

Using measurements of real-world samples of metals, the proposed approach verifies predictions of bidirectional reflectance distribution function (BRDF) models. It employs ellipsometry to verify both the actual polarizing effect and the overall reflectance behavior of the metallic surfaces.

A standardised polarisation visualisation for images

We discuss issues surrounding the visualisation of the polarisation properties of light stored in the pixels of an image. In order to facilitate comparisons between the work of different researchers, and in order to aid the development and debugging of polarisation-capable rendering systems, we propose a set of four standardised, easily comprehensible visualisations. These cover all aspects of polarisation that are relevant for rendering research, but also for optical design tasks. However, the so-called luminance scaled overlay form of the proposed visualisations is conceivably also useful and instructive to a wider, non-technical audience, and can be used for educational purposes.

Predictive rendering

This course intends to serve two closely related purposes: to provide an accurate definition of the term predictive rendering and to present the technological foundations for research in this area. The first goal of the course (a clear definition of the term) seems to be necessary due to the extreme prevalence of its antonym: believable rendering. Practically all contemporary production graphics, as well as most current graphics research efforts, fall into the latter category. The second (much larger and technical) part of the course presents the foundations of current predictive rendering. Unlike believable rendering, where any technology that delivers visually convincing results is acceptable for a given task, a predictive pipeline has the fundamental problem that all components have to be of a uniformly high quality to ensure a reliable result. The course describes an entire predictive pipeline, and for each stage it presents the graphics technologies (in some cases surprisingly few) that can be used in such a context. This course should enable anyone with a background in graphics to bootstrap a basic predictive rendering environment that can support further research.

A physically plausible model for light emission from glowing solid objects

The emissive properties of glowing solid objects appear to be something that the graphics community has not considered in depth before. While the volumetric emission of plasma, i.e. flames, has been discussed numerous times, and while the emission characteristics of entire luminaires can be handled via IESNA profiles, the exact appearance of glowing solid objects appears to have eluded detailed scrutiny so far. In this paper, we discuss the theoretical background to thermally induced light emission of objects, describe how one can handle this behaviour with very little effort in a physically based rendering system, and provide examples for the visual importance of handling this in a plausible fashion.