Screen Space Spherical Harmonics Filters for Instant Global Illumination

We present a general technique that allows for appling the instant global illumination (IGI) technique to scenes with glossy BRDFs and high-frequency geometric features like bump and normal maps. Like IGI, we use n×m interleaved sampling patterns to reduce the lighting computations by a factor of n×m, and filter incoming illumination in screen space. Unlike IGI, we filter (per-pixel) the incoming radiance field rather than the normal-dependent irradiance. To represent the incoming radiance field, we use spherical harmonics functions whose coefficients will then be filtered in screen space. This provides us with directional information on the incident illumination, which then allows to apply it to non-diffuse BRDFs, or on surfaces with strongly varying normals such as normal, bump, or displacement mapped surfaces.