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Figure 10.45. Various ways of encoding irradiance. From left to right: the environment map and diffuse lighting computed via Monte Carlo integration for the irradiance; irradiance encoded with an ambient cube; spherical harmonics; spherical Gaussians; and H-basis (which can represent only a hemisphere of directions, so backfacing normals are not shaded). (Images computed via the Probulator open-source software by Yuriy O’Donnell and David Neubelt.)
Although cube maps and spherical harmonics are the most popular representations for irradiance environment maps, other representations are possible. See Figure ...