|
The BRDF is the "Bidirectional Reflectance Distribution Function". It
gives the reflectance of a target as a function of illumination
geometry and viewing geometry. The BRDF depends on wavelength and is
determined by the structural and optical properties of the surface,
such as shadow-casting, mutiple scattering, mutual shadowing,
transmission, reflection, absorption and emission by surface elements,
facet orientation distribution and facet density.
Sound technical? Well, it is: the BRDF is needed in remote sensing
for the correction of view and illumination angle effects (for example
in image standardization and mosaicking), for deriving albedo, for
land cover classification, for cloud detection, for atmospheric
correction and other applications. It gives the lower radiometric
boundary condition for any radiative transfer problem in the
atmosphere and is hence of relevance for climate modeling and energy
budget investigations.
However, it should not be overlooked that the BRDF simply describes
what we all observe every day: that objects look differently when
viewed from different angles, and when illuminated from different
directions. For that reasons painters and photographers have for
centuries explored the appearance of trees and urban areas under a
variety of conditions, accumulating knowledge about "how things look",
knowledge that today we'd call BRDF-related knowledge. As modern
painters, programers of virtual reality in computers also need to be
concerned about the BRDFs of the surfaces they use.
Below are a few examples. The images are taken from Don Deering's
Parabola (a BRDF instrument) web page at the Goddard Space Flight
Center. He and his team have over the years made wonderful
ground-based BRDF photographs and measurements for various landcover
types.
This is a black spruce forest in the BOREAS experimental region in Canada.
Left: backscattering (sun behind observer), note the bright region
(hotspot) where all shadows are hidden. Right: forwardscattering (sun
opposite observer), note the shadowed centers of trees and transmission
of light through the edges of the canopies. Photograph by Don Deering.
A soybean field. Left: backscattering (sun behind observer). Right:
forwardscattering (sun opposite observer), note the specular
reflection of the leaves. Photograph by Don Deering.
A barren field with rough surface
Left: backscattering (sun behind observer), note the bright region
(hotspot) where all shadows are hidden. Right: forwardscattering (sun
opposite observer), note the specular reflection. Photograph by Don Deering.
|
