Investigation and Simulation of Diffraction on Rough Surfaces

Olaf Clausen, Yang Chen, Arnulph Fuhrmann and Ricardo Marroquim
In: Computer Graphics Forum, 42: 245-260.

The appearance of a rough aluminum sample varies drastically when illuminated from different directions (cf. Photo). It changes from a reddish appearance in the forward scattering to a greyish-bluish in the back-scattering. The BRDF spectra present an unusual linear shift towards long and short wavelengths. The speckle-like patterns indicate that this phenomenon originates from diffraction. The Cook-Torrance model cannot simulate such wave-optics phenomena (cf. Cook-Torrance GGX), whereas our BRDF model better approximates them at nearly no extra cost.

Abstract

Simulating light-matter interaction is a fundamental problem in computer graphics. A particular challenge is the simulation of light interaction with rough surfaces due to diffraction and multiple scattering phenomena. To properly model these phenomena, wave-optics have to be considered. Nevertheless, the most accurate BRDF models, including wave-optics, are computationally expensive, and the resulting renderings have not been systematically compared to real-world measurements.

This work sheds more light on reflectance variations due to surface roughness. More specifically, we look at wavelength shifts that lead to reddish and blueish appearances. These wavelength shifts have been scarcely reported in the literature, and, in this paper, we provide the first thorough analysis from precise measured data.

We measured the spectral in-plane BRDF of aluminum samples with varying roughness and further acquired the surface topography with a confocal microscope. The measurements show that the rough samples have, on average, a reddish and blueish appearance in the forward and back-scattering, respectively. Our investigations conclude that this is a diffraction-based effect that dominates the overall appearance of the samples. Simulations using a virtual gonioreflectometer further confirm our claims. We propose a linear model that can closely fit such phenomena, where the slope of the wavelength shifts depends on the incident and reflection direction. Based on these insights, we developed a simple BRDF model based on the Cook-Torrance model that considers such wavelength shifts.