Spherical harmonic Shading







Tech Report, Computer Science department  Cliff Lindsay & Emmanuel Agu  Spring '04 WPI


Previously, graphics hardware and lighting algorithms for realtime rendering have focused on distant point light sources, which resulted in illumination that was less than optimal. To improve the quality of realtime graphics, more sophisticated offline rendering techniques, such as Global Illumination, need to be incorporated into realtime renders. Unfortunately, Global Illumination style lighting is impractical for realtime rendering because of the computational complexity involved in integrating over a hemisphere. Fortunately, several advances have been made in realtime graphics such as Spherical Harmonic shading that approximate hemispherical lighting and make Global Illumination possible in realtime.
There currently exists a plethora of lighting techniques that can generate images that look photorealistic to varying degrees. Some of these techniques include ray tracing, Radiosity, and multipass rendering. The disadvantage associated with these techniques is the lack of realtime rendering capability on modest graphics hardware (nonclustered rendering engines). Spherical Harmonics appears to be a panacea for the realtime Global Illumination problem by tractably reducing the light integration problem to a simple linear operation. Realtime Global Illumination like lighting can be achieved through several linear properties of Spherical Harmonics. These properties reduce integration over the hemisphere into a dot product for diffuse receivers and a linear transform for glossy receivers, caustics, interreflections, and soft and selfshadowing.
This paper will serve as a broad overview of the properties and compositions of Spherical Harmonics as well as provide the background on the notation. This paper will also discuss the numerous benefits of using Spherical Harmonics for approximating lighting and transfer of 3D models for realtime applications. In addition, this review will serve as a framework for learning more about the functional properties of Spherical Harmonic shading.



SH References:

[1] J. Kautz, P. Sloan, J. Snyder. Fast Arbitrary BRDF Shading for LowFrequency Lighting Using Spherical Harmonics. Proceedings of the 13th Eurographics workshop on Rendering, pages 291296, 2002.
[2] P. Sloan, J. Kautz, J. Snyder, Precomputed radiance transfer for realtime rendering in dynamic, lowfrequency lighting environments. ACM Transactions on Graphics (TOG), pages 527536, 2002.
[3] R. Ramamoorthi, P. Hanrahan, An Efficient Representation for Irradiance Environment Maps. Proceedings of ACM SIGGRAPH 2001, Computer Graphics Proceedings, Annual Conference Series, pages 497500, 2001.
[4] R. Ramamoorthi, P. Hanrahan, Frequency Space Environment Map Rendering. Proceedings of ACM SIGGRAPH 2002, pages 517526, 2002.
