Physically-Based Real-Time Diffraction Using Spherical Harmonics

Abstract

Diffraction, interference, dispersive refraction and scattering are four wavelength-dependent mechanisms that produce iridescent colors. Wavelength-dependent functions need to be sampled at discrete wavelengths in the visible spectrum, which increases the computational intensity of rendering iridescence. Furthermore, diffraction requires careful sampling since its response function varies at a higher frequency variation with sharper peaks than interference or dispersive refraction. Consequently, rendering physically accurate diffraction has previously either been approximated using simplified color curves, or been limited to offline rendering techniques such as ray tracing. We propose a technique for real-time rendering of physically accurate diffraction on programmable hardware. Our technique adaptively samples the diffraction BRDF and precomputes it to Spherical Harmonic (SH) basis that preserves the peak intensity of the reflected light. While previous work on diffraction used low dynamic range lights, we preserve the full dynamic range of the incident illumination and the diffractive response over the entire hemisphere of incoming light directions. We defer conversion from a wavelength representation to a tone mapped RGB triplet until display.