Section: New Results

Analysis and Simulation

Figure 8. We study how the approximations made by layered material models impact their accuracy, and ultimately material appearance. Here we compare four models side by side with our reference simulation on a frosted metal – one of the 60 material configurations we have considered in our study. This specific choice is particularly problematic for the model of Weidlich and Wilkie [WW09], which creates oddly-colored reflections away from normal incidence. The variant of Elek [Ele10] is devoid of these artefacts, but clearly overestimates the intensity of the metallic base. Belcour's models [Bel18] (forward and symmetric) produce more accurate results, even though the intensity of the metallic base remains slightly higher. They still deviate from the reference simulation, especially at grazing angles as seen for instance at the bottom of the spheres. Our analysis in BRDF (and BTDF) space provides explanations for such departures from the reference.

Numerical Analysis of Layered Materials Models [12], [14]

Publications: [12], [14]

Most real-world materials are composed of multiple layers, whose physical properties impact the appearance of objects. The accurate reproduction of layered material properties is thus an important part of physically-based rendering applications. Since no exact analytical model exists for arbitrary configurations of layer stacks, available models make a number of approximations. In this technical report, we propose to evaluate these approximations with a numerical approach: we simulate BRDFs and BTDFs for layered materials in order to compare existing models against a common reference. More specifically, we consider 60 layered material configurations organized in three categories: plastics, metals and transparent slabs. Our results (see Figure 8) show that: (1) no single model systematically outperforms the others on all categories; and (2) significant discrepancies remain between simulated and modeled materials. We analyse the reasons for these discrepancies and introduce immediate corrections that improve models accuracy with little effort. Finally, we provide a few challenging cases for future layered material models.

A systematic approach to testing and predicting light-material interactions [11]

Publication: [11]

Photographers and lighting designers set up lighting environments that best depict objects and human figures to convey key aspects of the visual appearance of various materials, following rules drawn from experience. Understanding which lighting environment is best adapted to convey which key aspects of materials is an important question in the field of human vision. The endless range of natural materials and lighting environments poses a major problem in this respect. Here we present a systematic approach to make this problem tractable for lighting–material interactions, using optics-based models composed of canonical lighting and material modes. In two psychophysical experiments, different groups of inexperienced observers judged the material qualities of the objects depicted in the stimulus images. In the first experiment, we took photographs of real objects as stimuli under canonical lightings. In a second experiment, we selected three generic natural lighting environments on the basis of their predicted lighting effects and made computer renderings of the objects. The selected natural lighting environments have characteristics similar to the canonical lightings, as computed using a spherical harmonic analysis. Results from the two experiments correlate strongly, showing (a) how canonical material and lighting modes associate with perceived material qualities; and (b) which lighting is best adapted to evoke perceived material qualities, such as softness, smoothness, and glossiness. Our results demonstrate that a system of canonical modes spanning the natural range of lighting and materials provides a good basis to study lighting–material interactions in their full natural ecology.