This work was conducted in the context of perceptually realistic image simulations of paint materials. These simulations aim at producing the same visual response in an observer as real objects given certain illumination and observation conditions. The colorimetric validation ensures that the colors in the simulated images differ from the real scene below a given threshold and human observers cannot discern between them. Most existing validations use the direct approach of comparing renderings with real scenes and are usually briefly described. We propose a colorimetric validation using radiometrically calibrated photographs as valid perceptual references of the real scene, which avoids taking spectral measurements of the scene. This paper details important methodological aspects, providing objective validation results for a Macbeth color chart and a sample paint coating. Our results show that we can achieve xy chromaticity differences under 3%, relative luminance differences under 5%, and CIEDE2000 differences under 10% for the Macbeth chart. In the case of a sample paint coating, achieved chromaticity differences are under 11% and average luminance and CIEDE2000 differences are under 1%. Validation by visual inspection is not addressed here.
Predictive rendering of material appearance means going deep into the understanding of the physical interaction between light and matter and how these interactions are perceived by the human brain. In this paper we describe our approach to predict the appearance of composite materials by relying on the multi-scale nature of the involved phenomena. Using recent works on physical modeling of complex materials, we show how to predict the aspect of a composite material based on its composition and its morphology. Specifically, we focus on the materials whose morphological structures are defined at several embedded scales. We rely on the assumption that when the inclusions in a composite material are smaller than the considered wavelength, the optical constants of the corresponding effective media can be computed by a homogenization process (or analytically for special cases) to be used into the Fresnel formulas.
In the field of image synthesis, the simulation of material's appearance requires a rigorous resolution of the light transport equation. This implies taking into account all the elements that may have an influence on the spectral radiance, and that are perceived by the human eye. Obviously, the reflectance properties of the materials have a major impact in the calculations, but other significant properties of light such as spectral distribution and polarization must also be taken into account, in order to expect correct results. Unfortunately real maps of the polarized or spectral environment corresponding to a real sky do not exist.
Therefore, it seemed necessary to focus our work on capturing such data, in order to have a system that qualifies all the properties of light and capable of powering simulations in a renderer software. As a consequence, in this work, we develop and characterize a device designed to capture the entire light environment, by taking into account both the dynamic range of the spectral distribution and the polarization states, in a measurement time of less than two minutes. We propose a data format inspired by polarimetric imaging and fitted for a spectral rendering engine, which exploits the "Stokes-Mueller formalism."