A four-dimensional function that defines how light is reflected at an opaque surface and is a function of illumination geometry and viewing geometry. The function takes a negative incoming light direction, and outgoing direction, both defined with respect to the surface normal, and returns the ratio of reflected radiance exiting along outgoing direction to the irradiance incident on the surface from incoming light direction. Each direction is itself parameterized by azimuth angle and zenith angle; therefore the BRDF is a 4-dimensional function. The BRDF depends on wavelength and is determined by the structural and optical properties of the surface, such as shadow-casting, multiple scattering, mutual shadowing, transmission, reflection, absorption and emission by surface elements, facet orientation distribution and facet density. Specifically, the BRDF is an approximation of the BSSRDF, bi-directional sub-surface scattering reflectance distribution function. The BRDF ignores sub-surface scattering and assumes that the light striking the surface at some point will be reflected from that same point. The main characteristics of a physically plausible BRDF are the symmetry between incident and reflected directions (Helmholtz reciprocity) and that the total reflected power for a given direction of incident radiation is less than or equal to the energy of the incident light (Energy conservation). Some, but not all, BRDFs have a property called isotropy: they are unchanged if the incoming and outgoing vectors are rotated by the same amount about the surface normal. With isotropy, a useful simplification may be made: the BRDF is really a three-dimensional function in this case, and depends only on the difference between the azimuthal angles of incidence and exitance.
Published in Chapter:
Color Acquisition, Management, Rendering, and Assessment in 3D Reality-Based Models Construction
Marco Gaiani (Università di Bologna, Italy)
Copyright: © 2015
|Pages: 43
DOI: 10.4018/978-1-4666-8379-2.ch001
Abstract
In this chapter are presented a framework and some solutions for color acquisition, management, rendering and assessment in Architectural Heritage (AH) 3D models construction from reality-based data. The aim is to illustrate easy, low-cost and rapid procedures that produce high visual accuracy of the image/model while being accessible to non-specialized users and unskilled operators, typically Heritage architects. The presented processing is developed in order to render reflectance properties with perceptual fidelity on many type of display and presents two main features: is based on an accurate color management system from acquisition to visualization and more accurate reflectance modeling; the color pipeline could be used inside well established 3D acquisition pipeline from laser scanner and/or photogrammetry. Besides it could be completely integrated in a Structure From Motion pipeline allowing simultaneous processing of color/shape data.