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 sky 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 skies 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 ozone, which is ground, not taking ground albedo into account, not working typically not included in sky models, though it has a for twilight, 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 wavelengths of visible light, as well as ultraviolet 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 polarization, diffrac- formulates and refines his theory of the scattering of light tion, dispersion, and atmospheric refraction)—instead, I by air molecules, now known as Rayleigh scattering. have focused on the effects that have the most visual im- pact, such as Rayleigh and Mie scattering from air, water vapor, dust, and pollutants. [AKP74] examined the effect of ozone absorption and aerosols on the color of the twilight sky using computer This project makes the following contributions: simulations. • I have developed a new sky rendering system that pro- duces physically accurate and aesthetically pleasing im- ages. [McC76] describes turbidity, the scattering of light by haze • I have learned about a variety of subjects that I will need in the atmosphere. Turbidity is an important aspect of to understand to do further research, and subjects that will atmospheric scattering. be highly relevant and useful in the movie and game indus- tries. In 1982, Jim Blinn developed methods for simulating light scattering in volumes in [Bli82]. He brought science from 1.1 Design Goals physics literature to computer graphics. The target audience for my project was anyone who wants to be able to utilize realistic, computer-generated In [Kla87], Klassen simulated the color of the sky using a skies and atmospheric effects. The user is able to create planar model of the atmosphere and single scattering. realistic and pretty pictures with this technology. Kaneda et al. simulated the color of the sky using a spherical model of the atmosphere and atmospheric density 1.2 Projects Proposed Features and Functionality that decreases exponentially with altitude. I have implemented the following features and function- ality for my design project: Later, [NDKY96] extended the previous paper to take - Simulated atmospheric scattering using Monte Carlo multiple scattering into account. path tracing, unbiased distance sampling (in place of ray marching), direct sun sampling, various other © SIG Center for Computer Graphics 2012 N. I. Badler & A. Normoyle - CIS 497 Preetham et al. presented an analytic sky model in 1999, will save HDR images, along with deep alpha maps and which has been widely used ever since. The paper contains other information. many descriptions, data, and references. I will sample the sun directly, otherwise almost all rays [LL01] is a well-reviewed book that covers a large range of would get lost in space and images would take forever to light and color phenomenon. It contains both high-level, converge. In order to sample the sun directly, I will likely intuitive descriptions of these phenomenon, as well as have to ignore atmosphere refraction, which is aesthetically technical details. It also contains many photographs and not very significant, except when the sun is very close to diagrams. the horizon. However, I will think about ways that atmo- spheric refraction could be simulated efficiently, and add it if I come up with a good idea. I will use the sun’s actual At SIGGRAPH 2012, Hosek and Wilkie introduced a new emission spectrum for the light that the sun emits, poten- and improved analytic sky model [HL12] which is more tially using it for importance sampling wavelengths when accurate than the Preetham model. They derived their ray-casting from the camera. analytic model from a series of brute force simulations. The model and their simulations have limitations though. If I have time leftover after simulating the atmosphere (which I likely will not), I will add clouds. Storing volu- 3. PROJECT PROPOSAL metric data for a sky full of clouds with sufficient detail would use a lot of memory. To avoid doing this, I will use Below are my plans for the project, written months ago at proceduralism where applicable. To generate plausible the beginning of the project. To summarize, I planned to clouds, I will investigate various techniques and technolo- build a spectral renderer, model the Earth’s atmosphere, gies including metaballs, advanced data structures (non- and simulate atmospheric scattering. I also planned to add uniform grids, frustum-aligned grids, or no grids at all), clouds if time permitted. and Perlin noise. I will try to mimic the appearance of real- life clouds, however they will not be dynamic, physic- ally-based, or the result of a simulation. They should never- 3.1 Anticipated Approach theless add to the realism of the resultant images, and seamlessly integrate with the physically-based atmosphere The atmosphere simulation will work from any location simulation. The structures of these clouds will be more ad- and altitude (including from space), with various input vanced and fine-tuned than those of the volumetric renderer parameters. written in CIS 460, and the clouds will be lit much more realistically by taking into account multiple scattering and realistic phase functions. For the simulation I will use Monte Carlo path tracing and ray marching. Ray marching will be necessary because the Earth’s atmosphere varies as a function of location—alti- 3.2 Target Platforms tude in particular. I will use real-life data for the composi- tion of the Earth's atmosphere, using interpolation to create I will write the simulation in C++. As a starting frame- a continuous model of atmospheric composition between work, I will use some pieces of my own 3D renderer data points. I will keep the system as general as possible so (Photorealizer, which I wrote from scratch): a basic Qt that it can also be used for other planets. GUI, some image processing features (including anti-ali- asing, gamma correction, and writing bitmap image files to disk), and code for ray-casting from the camera, and code I will use real-life physics for the scattering properties of for generating sampling points on the sun. I will consider the atmosphere and clouds (under a geometric optics ap- using GLM as a linear algebra library. proximation), including Rayleigh scattering for light scat- tering from air molecules, which is what gives the sky its blue color during the day under normal conditions. 3.3 Evaluation Criteria I will model the Earth as a sphere.