OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Lighting with Spherical Harmonics

In 2001, Ravi Ramamoorthi and Pat Hanrahan presented a method that uses spherical harmonics for computing the diffuse lighting term. This method reproduces accurate diffuse reflection, based on the content of a light probe image, without accessing the light probe image at runtime. The light probe image is pre-processed to produce coefficients that are used in a mathematical representation of the image at runtime. The mathematics behind this approach is beyond the scope of this book. Instead, we lay the necessary groundwork for this shader by describing the underlying mathematics in an intuitive fashion. The result is remarkably simple, accurate, and realistic, and it can easily be codified in an OpenGL shader. ...