References:
§Green, Robin, Spherical Harmonic Lighting: The Gritty Details, 2003 Spring, SCEA
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§Peter-Pike, Sloan, Jan Kautz, and John Snyder, Precomputed Radiance Transfer for Real-Time Rendering in Dynamic, Low-Frequency Lighting Environments, SIGGRAPH 2002,July, 2002
 
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cosine-weighted integral of the corresponding hemisphere of incident illumination