Graphics Reference
In-Depth Information
Side view
Counterweight
C
Rotating arm
Sample holder
CCD camera
with fisheye lens
A
Illuminant
Baffle
B
Metallic stand
Hemispherical mirror
(partially reflecting)
Figure 8.10
Greg Ward's apparatus for image-based measurement of BRDFs. (After [Ward 92].)
eral pairs of light directions and viewing directions. The photographs are digi-
tized and pixel values are extracted to approximate the BRDF for the incoming
and outgoing directions. The corresponding parameters to the model are obtained
by minimizing the difference between the measured BRDF values and the values
calculated using a simplified version of the model.
Ward also created a device for BRDF measurement. A surface sample is
placed inside a partially reflecting hemispherical mirror, and a camera records
light reflected from the object using a fisheye-lens camera that has a hemispherical
field of view. The sample is illuminated from outside the hemispherical mirror,
which has the benefit of reducing stray light inside the hemisphere. A rotating
robotic arm controls the position of the light, and therefore the direction of the
illumination; this is the only moving part of the device. (
Figure 8.10
)
. Light from
the illumination source is reflected by the sample across the entire hemisphere.
A single photograph of inside of the hemisphere captures the light reflected in
nearly every direction. An entire “slice” of the BRDF is thus recorded in a sin-
gle image. Varying the position of the light source captures the BRDF data for
a representative collection of incident light directions. To calibrate the device,
a set of photographs is taken with the sample replaced by a diffuse surface of
known reflectance. The ratio of the values of corresponding pixels in the pho-
tograph of the sample with the reference photograph provides the actual BRDF
value. This process is much more efficient than the typical process of measuring
the reflectance value for each pair of directions separately, although it is generally
