Physically-Based Real-Time Diffraction Using Spherical Harmonics

Diffraction, interference, dispersive refraction and scattering are four wavelength-dependent mechanisms that produce iridescent colors. The per-wavelength sampling required to render iridescence significantly increases the computation required to render iridescence in real time. Furthermore, diffraction exhibits much higher frequencies and sharper peak responses than interference and dispersive refraction. The rendering of physically based diffraction has previously been limited to offline global illumination techniques such as ray tracing, employed simple color ramps to approximate diffraction physics or sampled slower varying interference functions. We propose a technique to render physically accurate diffraction in real-time on programmable hardware. Our technique represents the diffractive surface response in terms of a Spherical Harmonic (SH) basis that preserves the entire peak intensity of the reflected light while maintaining a compact representation. Previous diffraction rendering techniques have also employed simplistic point or small area lights with bounded low dynamic range lighting and reflections, representing color in terms of a trichromatic color. Our technique uses the Composite Color model to preserve the full dynamic range of the direct illumination, diffraction, and their interaction over the complete lighting hemisphere of incoming light directions. We defer conversion from the Composite Color representation to a tone mapped RGB triplet until its use in display. Direct lighting and diffraction reflection are both represented in terms of Spherical Harmonic basis, making the lighting integral a dot product of the SH coefficients. CR Categories: I.3.7 [Computer Graphics]: Color, shading, texture, physically-based modeling