CIS 497 Senior Capstone Design Project Final Report Instructors: Norman I. Badler and Aline Normoyle Physically-Based Atmosphere Rendering

Peter Z. Kutz

Advisor: Norman I. Badler

University of Pennsylvania

ABSTRACT

In computer generated imagery, as in photography, the is very ubiquitous and important―as a source of light, as a backdrop, and as a subject in and of itself. However, realistic and aesthetically appealing are also very difficult to render. Models exist to approximate the appearance of the sky, but they make many assumptions and have many limitations. To create truly realistic images a full physical simulation of atmospheric scattering, including clouds, is necessary. For this project, I have simulated and produced photorealistic and aesthetically pleasing renders of atmospheres, lit by the sun.

Project Blog: http://skyrenderer.blogspot.com/ For the clearest and most detailed description of this project, and for many more pictures, please see my blog.

CIS 497 Senior Design Project N. I. Badler & A. Normoyle - CIS 497

forms of importance sampling, and various optimiz- 1. INTRODUCTION ations. Realistic and dynamic skies are very difficult to render. - Used real-life data for the composition of the Earth's Analytic sky models can be used to model the appearance atmosphere, and real-life physics for the scattering of the sky, but they make assumptions and have many lim- properties of the atmosphere. itations, such as only working from the point of view of - Included preferential absorption by , which is ground, not taking ground albedo into account, not working typically not included in sky models, though it has a for , not modeling clouds, etc. Image-based lighting very significant effect on the color of the sky, espe- in the form of HDR environment maps can also replicate cially when the sun is low or below the horizon. real-life lighting, however they are static—only valid for a - Wrote a spectral rendering system, that works for all very specific time and location. Some researchers have of visible light, as well as tried simulating the atmosphere, which can be very accur- and infrared. ate but very slow. - Employed colorimetry to convert spectral data to im- ages that can be displayed on computer displays. In real life, the sky is very ubiquitous and important. It serves as a source of light. It acts as a backdrop for - Generated and saved panoramic HDR images that can everything we do. It indicates the time of day, the season, be used as light sources in other renderers. and the weather. It affects our moods. And it serves as a subject of contemplation, admiration, and artwork. The sky can also serve these same purposes in computer graphics. 2. RELATED WORK In particular, realistic sky light can make the difference between a flat, unrealistic image and a photorealistic im- People in various disciplines have been interested in char- age. acterizing the appearance of the sky for many many years. Summarized below are some of the significant past works that I have drawn on while developing my sky renderer. I have written an atmosphere simulation that balances physical accuracy, aesthetic beauty, flexibility, and per- formance. The simulation works from any location and alti- Over a period of decades in the late 1800s, Lord Rayleigh tude (including from space), with various input parameters, published a series of papers that explained his theories and, if time permits, with clouds. I will use real-life data about atmospheric scattering and how it caused the color and physics where applicable. I have not simulated all of and appearance of the sky. In [Ray71] and [Ray99] he the complex effects of light (including , diffrac-