I learned it from a paper "Spherical Harmonic Lighting: The Gritty Details" (link below), where the author uses a different form of the equation.<p>I like the 3D in this one.<p><a href="http://silviojemma.com/public/papers/lighting/spherical-harmonic-lighting.pdf" rel="nofollow">http://silviojemma.com/public/papers/lighting/spherical-harm...</a>