Oscar Anson

Technical Section: Simulation of atmospheric phenomena

This paper presents a physically based simulation of atmospheric phenomena. It takes into account the physics of non-homogeneous media in which the index of refraction varies continuously, creating curved light paths. As opposed to previous research on this area, we solve the physically based differential equation that describes the trajectory of light. We develop an accurate expression of the index of refraction in the atmosphere as a function of wavelength, based on real measured data. We also describe our atmosphere profile manager, which lets us mimic the initial conditions of real-world scenes for our simulations. The method is validated both visually (by comparing the images with the real pictures) and numerically (with the extensive literature from other areas of research such as optics or meteorology). The phenomena simulated include the inferior and superior mirages, the Fata Morgana, the Novaya-Zemlya, the Vikings end of the world, the distortions caused by heat waves and the green flash.

Visualizing Underwater Ocean Optics

Simulating the in‐water ocean light field is a daunting task. Ocean waters are one of the richest participating media, where light interacts not only with water molecules, but with suspended particles and organic matter as well. The concentration of each constituent greatly affects these interactions, resulting in very different hues. Inelastic scattering events such as fluorescence or Raman scattering imply energy transfers that are usually neglected in the simulations. Our contributions in this paper are a bio‐optical model of ocean waters suitable for computer graphics simulations, along with an improved method to obtain an accurate solution of the in‐water light field based on radiative transfer theory. The method provides a link between the inherent optical properties that define the medium and its apparent optical properties, which describe how it looks. The bio‐optical model of the ocean uses published data from oceanography studies. For inelastic scattering we compute all frequency changes at higher and lower energy values, based on the spectral quantum efficiency function of the medium. The results shown prove the usability of the system as a predictive rendering algorithm. Areas of application for this research span from underwater imagery to remote sensing; the resolution method is general enough to be usable in any type of participating medium simulation.

Perceptual rendering of participating media

High-fidelity image synthesis is the process of computing images that are perceptually indistinguishable from the real world they are attempting to portray. Such a level of fidelity requires that the physical processes of materials and the behavior of light are accurately simulated. Most computer graphics algorithms assume that light passes freely between surfaces within an environment. However, in many applications, we also need to take into account how the light interacts with media, such as dust, smoke, fog, etc., between the surfaces. The computational requirements for calculating the interaction of light with such participating media are substantial. This process can take many hours and rendering effort is often spent on computing parts of the scene that may not be perceived by the viewer. In this paper, we present a novel perceptual strategy for physically based rendering of participating media. By using a combination of a saliency map with our new extinction map (X map), we can significantly reduce rendering times for inhomogeneous media. The visual quality of the resulting images is validated using two objective difference metrics and a subjective psychophysical experiment. Although the average pixel errors of these metric are all less than 1%, the subjective validation indicates that the degradation in quality still is noticeable for certain scenes. We thus introduce and validate a novel light map (L map) that accounts for salient features caused by multiple light scattering around light sources.

Chasing the green flash: a global illumination solution for inhomogeneous media

Several natural phenomena, such as mirages or the green flash, are owed to inhomogeneous media in which the index of refraction is not constant. This makes the light rays travel a curved path while going through those media. One way to simulate global illumination in inhomogeneous media is to use a curved ray tracing algorithm, but this approach presents some problems that still need to be solved. This paper introduces a full solution to the global illumination problem, based on what we have called curved photon mapping, that can be used to simulate several natural atmospheric phenomena. We also present a model of the Human Visual System (HVS) to display images in a more realistic way, taking into account how we perceive luminances in a real-world scene. This is of special interest in the green flash effect, where some of the perceived green is owed to bleaching of the photoreceptors in the human eye.

Perception-Based Rendering: Eyes Wide Bleached

Perception issues are a key factor in rendering Vision-Realistic images. Th is paper develops a novel spectral sensitive model of the Human Visual System (HVS) in order to simulate the bleach ing effect in retinal photopigments. First, the Stiles-Crawford effect is taken into account to determine the pupil s ize of the adapted eye and to compute the directional sensitivity of the photoreceptors in the retina. After that, the per centage of bleached pigment based on the incident retinal illuminance is calculated to simulate the loss of spectral s ensitivity of the human observer seeing the scene.

Efficient selective rendering of participating media

Realistic image synthesis is the process of computing photorealistic images which are perceptually and measurably indistinguishable from real-world images. In order to obtain high fidelity rendered images it is required that the physical processes of materials and the behavior of light are accurately modelled and simulated. Most computer graphics algorithms assume that light passes freely between surfaces within an environment. However, in many applications, ranging from evaluation of exit signs in smoke filled rooms to design of efficient headlamps for foggy driving, realistic modelling of light propagation and scattering is required. The computational requirements for calculating the interaction of light with such participating media are substantial. This process can take many minutes or even hours. Many times rendering efforts are spent on computing parts of the scene that will not be perceived by the viewer. In this paper we present a novel perceptual strategy for physically-based rendering of participating media. By using a combination of a saliency map with our new extinction map (X-map) we can significantly reduce rendering times for inhomogenous media. We also validate the visual quality of the resulting images using two objective difference metrics and a subjective psychophysical experiment. Although the average pixel errors of these metric are all less than 1%, the experiment using human observers indicate that these degradation in quality is still noticeable in certain scenes, unlike previous work has suggested.

NURBS-based Inverse Reflector Design

Commonly used direct rendering techniques simulate light t ransport for a complete scene, specified in terms of light sources, geometry, materials, participating media, etc. On the other hand, inverse rendering problems take as input a desired light distribution and try to find the unknown parts of the scene needed to get such light field. The latter kind, where inverse reflector design is included, is t raditionally solved by simulation optimization methods, due to the high complexity of the inverse problem. In this pap er we present an inverse reflector design method which handles surfaces as NURBS and simulates accurately the light transport by means of a mo dified photon mappingalgorithm. The proposed method is based on an optimization m ethod, calledpattern search , in order to compute the reflector needed to generate a target near light fi eld. Some assumptions are determined in order to reduce the complexity of the problem, such as a rotationally symmetric reflector or its perfectly specular reflective behavior. The optimization method specifies the reflector sh ape by handling a NURBS curve as a generatrix, sequentially modifying the position and weights of its cont rol points in order to obtain the reflector solution. Areas of applications of inverse reflector design span from archit ectural lighting design to car headlamps design.

Inelastic scattering in participating media using curved photon mapping

True global illumination algorithms in participating media are very costly, owed to the multiple interactions between light and the medium that need to be evaluated in order to achieve a correct result. Some common simplifications include considering only homogeneous media and single, isotropic scattering. We present Lucifer, a global illumination environment capable of handling inhomogeneous, participating media while taking into account multiple inelastic scattering. Additionally, light is bent through the media according to Fermat’s Principle. To the authors’ knowledge, it is the first time that all these conditions and effects are considered in a single algorithm. Several limitations and mechanisms of the Human Visual System (HVS) are also computed before mapping the image to the output device.

Efficient physically-based perceptual rendering of participating media

To significantly reduce physically-based rendering times for participating media, we propose a novel perceptual strategy based on the combination of a saliency map with our new physically-based extinction map (X-map), which stores in image-space the exponential decay of light in the medium. This combination is then used to guide a selective rendering of the scene, with more accurate estimates in the most perceptually important parts of the scene, without visible degradation. The novelties of this work can then be summarized as a) the introduction of the X-map concept and b) its combination with a saliency map to guide a perception-based renderer for inhomogeneous participating media. Results show a decrease of almost 60% in rendering times for our test scene.