Light diffuseness metric Part 1: Theory

Abstract

The light density, direction and diffuseness are important indicators of the spatial and form-giving character of light. Mury presented a method to describe, measure and visualise the light field's structure in terms of light density and direction variations in three-dimensional spaces. We extend this work with a theoretical and empirical review of four diffuseness metrics leading to a novel metric proposal D Xia. In particular, the relationships between these diffuseness metrics were studied using a model named 'probe in a sphere'. Diffuseness metric D Xia re-frames the diffuseness metric of Cuttle in an integral description of the light field. It fulfils all diffuseness criteria and has the advantage that it can be used in a global, integrated description of the light flow and diffuseness throughout three-dimensional spaces.

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

“…From USC's high-resolution re-creations of Debevec's light probe images (http://gl.ict.usc.edu/ Data/HighResProbes/), we selected three light maps (Glacier, Ennis, and Grace-new) that best represent our canonical lighting modes (ambient, focus, and brilliance, respectively). The selection was made by using a combination of a diffuseness metric (Xia, Pont, & Heynderickx, 2017) and a brilliance metric (Zhang et al, 2019), both based on the relative power of their SH decomposition components, and both metrics range from 0 to 1. Specifically, the Glacier map represents the ambient lighting (D Xia = 1) the best, as it scores highest for Xia's diffuseness metric (D Xia = 0.83; B = 0.42); the Ennis map represents the focus lighting (D Xia = 0) the best, as it scores lowest on Xia's diffuseness metric (D Xia = 0.17; B = 0.71); the Grace-new map represents the brilliance lighting (B = 1) the best, as it scores highest on our brilliance metric (D Xia = 0.40; B = 0.79).…”

A systematic approach to testing and predicting light-material interactions

“…Using Xia's diffuseness metric (Xia, Pont, & Heynderickx, 2017) we could calculate the diffuseness of all high-resolution high dynamic range (HDR) maps, via the ratio of the power of the first order d l¼1 to the power of the zeroth order d l¼0 of the SH decomposition. By normalization, diffuseness scores range between 0 (zero), which corresponds to extremely directed lighting (i.e., focus lighting) to 1, which corresponds to fully diffuse lighting (i.e., ambient lighting), as in the equation below:…”

“…Metrics using the spherical harmonics decomposition for the USC high-resolution HDR maps. Left: results of the diffuseness metric(Xia et al, 2017) used to select the Glacier map for the ambient lighting and the Ennis map for the focus lighting. Right: results of the brilliance metric sampled up to the 10th order.…”

A toolbox for volumetric visualization of light properties

“…Based on the above-mentioned considerations, we chose to visualize the (variations of) values of mean illuminance, direction and diffuseness of the light, introduced in this combination by Xia et al 32,53 We did this via shapes and variation of their proportions. In order to obtain the values of the light properties, we use Cuttle's 17,42 approach of cubic illuminance adopted for multiple measurements, physical 19,32,53 as well as virtual. 54…”

“…The left column shows the light flow for the sunny scene, the right for the rainy scene Figure 13 Measurement devices. Left to right: cubic meter based on the Konica Minolta system, 32,53 luxmeter and smartphone with Luxi with a white diffusion cap (Luxi) and the corresponding app (see measurements evaluation study by Gutierrez-Martinez et al 57 ). We used the Luxi for cubic measurements in the same manner as the luxmeter.…”