Ecological optics of natural materials and light fields

Abstract

The appearance of objects in scenes is determined by their shape, material properties and by the light field, and, in contradistinction, the appearance of those objects provides us with cues about the shape, material properties and light field. The latter so-called inverse problem is underdetermined and therefore suffers from interesting ambiguities. Therefore, interactions in the perception of shape, material, and luminous environment are bound to occur.Textures of illuminated rough materials depend strongly on the illumination and viewing directions. Luminance histogram-based measures such as the average luminance, its variance, shadow and highlight modes, and the contrast provide robust estimates with regard to the surface structure and the light field. Human observers performance agrees well with predictions on the basis of such measures. If we also take into account the spatial structure of the texture it is possible to estimate the illumination orientation locally. Image analysis on the basis of second order statistics and human observers estimates correspond well and are both subject to the bas-relief and the convex-concave ambiguities. The systematic robust illuminance flow patterns of local illumination orientation estimates on rough 3D objects are an important entity for shape from shading and for light field estimates. Human observers are able to match and discriminate simple light field properties (e.g. average illumination direction and diffuseness) of objects and scenes, but they make systematic errors, which depend on material properties, object shapes and position in the scene. Moreover, our results show that perception of material and illumination are basically confounded. Detailed analysis of these confounds suggests that observers primarily attend to the low-pass structure of the light field. We measured and visualized this structure, which was found to vary smoothly in natural scenes in-and outdoors.

Effects of light map orientation and shape on the visual perception of canonical materials

“…These modes represent properties of light that human observers can distinguish ( Doerschner, Boyaci, & Maloney, 2007 ; Kartashova et al, 2016 ; Morgenstern, Geisler, & Murray, 2014 ; Schirillo, 2013 ; Xia, Pont, & Heynderickx, 2017 ). Moreover, they are known in perception-based lighting design as the basic components of an integral lighting plan ( Ganslandt & Hofmann, 1992 ; Kelly, 1952 ; Pont, 2009 , 2013 ; Pont, & de Ridder, 2018 ). They thus span the space of natural light conditions.…”

A systematic approach to testing and predicting light-material interactions

“…Specifically, for the lighting we consider an ambient light mode consisting of a spherically diffuse light environment, a focus light mode represented by a collimated light source, and a brilliance light mode in the form of a large number of small light sources. These modes represent the zero order, first order, and higher order contributions of a spherical harmonic decomposition of the local light field (Mury, Pont, & Koenderink, 2007), have a physical and perceptual meaning (Pont, 2009), and correspond to the basic layers that are used in perception-based lighting design (Kelly, 1952;Ganslandt & Hofmann, 1992;Pont, 2009). For the materials, we covered smooth objects with four different types of finishes: matte paint, glossy paint, a velvet-like (or flocked) layer, and a glittery layer, representing, respectively, a constant BRDF (diffuse scattering), a peaked BRDF in the forward (mirror) direction, a BRDF that ''explodes'' along the surface (asperity scattering), and a broadened noisy BRDF (specular multifacet scattering).…”

Soft like velvet and shiny like satin: Perceptual material signatures of fabrics depicted in 17th century paintings

“…Figure 10 A shows the luminance histogram of the crop shown in Figure 10 B. The highlight mode, one of the three general modes for a histogram-based measure of the surface structure proposed by Pont (2009) , is indicated by a black bar (note that here the width and height of the bar have no other meaning beside providing a clear visual indication of the threshold value used to binarize the image, whereas in Pont (2009) these parameters were related to the width and the height of the mode). To binarize the images, we manually selected the threshold at the minimum value of the highlight mode (indicated by the red line in Figure 10 A).…”