Estimating Specular Roughness and Anisotropy from Second Order Spherical Gradient Illumination

Abstract

This paper presents a novel method for estimating specular roughness and tangent vectors, per surface point, from polarized second order spherical gradient illumination patterns. We demonstrate that for isotropic BRDFs, only three second order spherical gradients are sufficient to robustly estimate spatially varying specular roughness. For anisotropic BRDFs, an additional two measurements yield specular roughness and tangent vectors per surface point. We verify our approach with different illumination configurations which project both discrete and continuous fields of gradient illumination. Our technique provides a direct estimate of the per-pixel specular roughness and thus does not require off-line numerical optimization that is typical for the measure-and-fit approach to classical BRDF modeling.

Polarization imaging reflectometry in the wild

“…We also note that while the Brewster angle can vary over large surfaces, we only found a 5 − 6 • variations over our largest sample (canvas print/~0.75 m wide), which is still within the acceptable range as per Figure 11. Figure 12 presents a comparison between reflectance maps of the red book cover acquired with our approach under uncontrolled outdoor illumination (Figure 7) and those acquired with controlled LCD panel illumination using the approach of [Ghosh et al 2009]. Overall, the maps acquired with our proposed approach are qualitatively very similar to those obtained with controlled measurements.…”

“…12. Comparison between the reflectance and normal maps of red book estimated with our proposed approach (top row) vs. those obtain with controlled measurements using an LCD panel [Ghosh et al 2009] (center row). Error is visualized in false color (bottom row).…”

“…Ours [Ghosh et al 2009] [Aittala et al 2015] Surf. We also conducted a statistical analysis of the surface normals estimated for the red book under various outdoor illumination conditions including cloudy sky, and sunny sky conditions at various times of day (Table 1).…”

“…An alternative to linear light sources, to help further reduce the amount of data and time spent in measurements, is to employ an LCD panel as an extended source of illumination. Ghosh et al [2009] proposed to project second order spherical gradient patterns from an LCD screen to estimate per-pixel, spatially varying reflectance maps of planar samples and exploited the inherent polarization of LCD illumination for diffuse-specular separation. Francken et al [2009] instead proposed employing Gray codes for this purpose, and more recently Aittala et al [2013] have proposed to capture a sample's response to band-limited illumination patterns in the frequency domain using 2D Fourier patterns.…”

Gloss and Normal Map Acquisition of Mesostructures Using Gray Codes

“…As we focus on increasing the wide applicabily rather than improving the quality of recent BRDF methods, an approximate glossiness acquisition suffices for our purposes. The most closely related approach was presented by Ghosh et al [30]. They estimate roughness as well as anisotropy from second order spherical gradient illumination.…”