Graphics Reference
In-Depth Information
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Light source
Camera
Figure 9.4
Arrangement for capturing BTF data. (After [Dana et al. 99].)
specific surface orientations. Figure 9.4 illustrates the arrangement; the vertices
on the triangulated quarter-sphere shown in the figure are the positions to which
the surface normals are rotated.
The measurement system is calibrated by replacing the surface sample with a
reference target of known reflectance. The target actually has a number of refer-
ence patches, each having a specific reflectance. The radiance reflected from each
patch is recorded by a calibrated photometer. This data is used as a reference to
correct the pixel values of the captured textures. Strictly speaking, the incident
irradiance from the light source varies over the position of a surface sample, as
does the direction to the light source. However, if the source is far enough away,
these variations are not significant and the illumination can therefore be regarded
as constant across the surface sample.
The authors took measurements of a number of different surface materials and
the data was made publicly available.
1
This data has contributed to widespread
use of BTFs and had substantial impact of further research in the subject.
9.1.1 BTFs and Spatially Varying BRDFs
As described in Chapter 8, the BRDF of a surface generally varies by position;
however, BRDFs are normally modeled for a single point. This is why basic
image-based BRDF measurement of a small surface sample averages the recorded
values over all pixels in the captured image. The term
spatially varying BRDF
(SVBRDF) is sometimes used to disambiguate a BRDF that has a specific posi-
tional dependence. The primary difference between a BTF and a SVBRDF is that
a BTF is an image-based representation, and therefore includes local shadowing
1
The BTF database created by measuring various materials in this way, is made publicly available
at
http://www1.cs.columbia.edu/CAVE/curat
;
the database is used for verification of BTF theories.
