Final Project

Group Members

Nicolas Benitez

Michael Graeb

Goal

The goal of our project was to render a convincing image of the sky. More specifically, we were interested in modeling the sky in such a way that the rendered sky:

1) Had convincing coloration 2) Provided appropriate lighting for the other elements in the scene 3) Contained realistic clouds

The image that we used as a reference to try to create these effects was the following image of the New York city skyline, taken from http://wwp.new-york-usa.com/:

Sky Simulation

Nico was responsible for simulating the color and exitant luminance of the sky. To do so, we implemented a new type of light source as a pbrt plugin. We made several attempts at modeling the sky before settling on the current system.

The first version of our sky model was based on A Practical Analytic Model for Daylight by Preetham et al. In the paper, Preetham describes a method for rendering skies based on Perez's phenomenological model for sky color. Perez defines a function based on five parameters that determine the shape and intensity of the sky gradient. Preetham used data from an intricate simulation of the sky and attempted to fit the five parameters to this data.

We implemented this model, but found the results to be insufficient for our purposes. The main problem that we encountered with this model was that it was very difficult to "tweak" the colorization of the sky, since it depended only on solar angle and a single variable called "turbidity." In order to better capture the color of the original image, we decided to attempt a more phisically-based model.

The model that we ended up using was mainly inspired by two papers: Modeling and Rendering of the Atmosphere Using Mie-Scattering by D. Jackel and B. Walter, and Display Method of the Sky Color Taking into Account Multiple Scattering.

The main technical goal of our project will be to investigate and implement a time of day-dependant lighting system that simulates the different types of environmental lighting found in outdoor areas. We hope to create a lighting system that will approximate the changes in spectra that occur at different times of day and produce convincing lighting and sky color. We would like to create a series of images featuring the same scene at different times of day. We also believe that our lighting model will be allow us to reproduce the dramatic coloration that is seen on the reflective building surfaces in our referance images. In addition to helping reproduce our source images, such a lighting mechanism would be a very convenient resource to have when rendering any outdoor scene, since a time could be specified and appropriate lighting and sky coloration would be generated procedurally without needing to capture an environment map for the scene.

References

Jörg Haber, Marcus Magnor, Hans-Peter Seidel. "Physically based Simulation of Twilight Phenomena". http://www.mpi-inf.mpg.de/departments/irg3/dtd/.