Light field constancy within natural scenes

Abstract

The structure of light fields of natural scenes is highly complex due to high frequencies in the radiance distribution function. However it is the low-order properties of light that determine the appearance of common matte materials. We describe the local light field in terms of spherical harmonics and analyze the qualitative properties and physical meaning of the low-order components. We take a first step in the further development of Gershun's classical work on the light field by extending his description beyond the 3D vector field, toward a more complete description of the illumination using tensors. We show that the three first components, namely, the monopole (density of light), the dipole (light vector), and the quadrupole (squash tensor) suffice to describe a wide range of qualitatively different light fields. In this paper we address a related issue, namely, the spatial properties of light fields within natural scenes. We want to find out to what extent local light fields change from point to point and how different orders behave. We found experimentally that the low-order components of the light field are rather constant over the scenes whereas high-order components are not. Using very simple models, we found a strong relationship between the low-order components and the geometrical layouts of the scenes.

Representing the light field in finite three-dimensional spaces from sparse discrete samples

“…Our custom-made device, called a plenopter, is capable of measuring local light fields up to the second order in terms of spherical harmonics. This extends the conventional description by a new parameter-the squash tensor (the second-order harmonic [7]). Taking into account that the low-order components of light fields in natural scenes typically vary slowly and rather systematically with location [7], the second-order approximation of the radiance distribution function can be estimated reasonably well for all points of the scene using interpolation between a limited number of observations.…”

“…This extends the conventional description by a new parameter-the squash tensor (the second-order harmonic [7]). Taking into account that the low-order components of light fields in natural scenes typically vary slowly and rather systematically with location [7], the second-order approximation of the radiance distribution function can be estimated reasonably well for all points of the scene using interpolation between a limited number of observations. In order to describe the global structure of the light field of a scene, we utilize the concept of light tubes, which provides an easy and intuitive understanding of the propagation of radiant flux throughout the scene.…”

“…The second-order contribution is more complicated and can correspond to different shapes of brightness-distribution solids from a "light clamp" to a "light ring." It is referred to as a "squash tensor" [7]. In natural scenes, the low-order contributions to light fields show very systematic and smooth behavior varying very slowly from one point of the scene to another [7,18].…”

Ecological optics of natural materials and light fields

“…How do natural scene statistics (e.g. of light fields 5,6 ) constrain these sets? The only formal description of how such combinations result in the same image concerns the bas-relief ambiguity:…”

“…7240 724009-7 very simple geometrical models. 5 High angular frequencies can be described statistically, and hardly influence the appearance of most matte materials. Furthermore, in different scenes of similar geometries we found structurally similar light fields, which suggests that in some way the light field can be thought of as a property of the geometry.…”