Implementing Fast Global Illumination using Skyboxes

Based on Ramamoorthi & Hanrahan's 2001 paper An Efficiant Representation for Irradiance Enviornment Maps, I implemeted a global diffuse illimination model inside openGL. This produces a runtime diffuse lighting on any model based purely on the normal vector of the surface.

Summary

Ravi Ramamoorthi and Pat Hanrahan addressed an important problem of the time related to global illumination; through observations about diffuse lighting they came up with a model where the irradiance of a surface point is based purely on the point normal and a set of spherical harmonic coefficients. This greatly reduces both time and data needed to store this lighting as its does not require textures to save the illumination, and needs only one pass over the environment map′s pixels. To the left is an example from the paper of the coloration from each spherical harmonic mode.

My objective was to implement this with OpenGL. Ramamoorthi and Hanrahan used a prefabed irradiance map of spherical coordinates, so from Hichael F. Cohen and Donald P. Grenberg′s Radiosity Solution for Complex Enviornments I converted skybox cartesian coordiantes to spherical coordiantes. Once this was completed I did the following: before the program begins rendering to the screen, it itterates over trhe skybox and calculate each coefficient from Ramamoorthi's paper (9 in total) per color channel. these 9 coefficients go into a shader which is then added on top of the shader of the object, and done, global illumination during runtime! Here is a detailed explanation of the process.