NOVEMBER 2014 | VOL. 57 | NO. 11 | COMMUNICATIONS OF THE ACM 103
open source rendering system Mitsuba, which was extended
to handle the new micro-flake distribution (Section 4).
All the rendered results are generated using Monte Carlo
volume path tracing.
Figure 6 shows the resulting models shell-mapped onto
draped fabric geometry and rendered under environment
lighting. The corresponding high-resolution renderings are
available on the project Web site at http://www.cs.cornell.
edu/projects/ctcloth-sg11. Fabrics are usually rendered
with surface-based models, making them thin 2D sheets
so that cut edges look artificial; our volumetric model, on
the other hand, explicitly captures 3D structures and provides a proper impression of the thickness and weight of the
fabrics. Furthermore, surface-based models show smooth
edges at silhouettes while our model is able to produce fuzzy
silhouettes with rich details, bringing a new level of realism
to fabric rendering.
The silk satin (charmeuse) has a structure of mainly parallel fibers on the surface, resulting in a strong anisotropic
highlight. In Figure 5( 1), the appearance matching pair
uses a cylindrically curved piece of material, and the matching region was chosen to include a highlight to allow the
matching process to tune g appropriately. Good results are
obtained despite the mismatch between the ideal cylinder
in the rendering and the flatter shape of the real material,
illustrating that a casual setup suffices. Using the parameters obtained from this view, the validation pair shows the
fabric rotated 90 degrees and draped over the same cylinder.
At this angle the fabric exhibits almost no highlight; this
anisotropic appearance is correctly predicted by our model.
The satin is shown in a draped configuration in Figure 6(a).
No reflectance model such as BTF or other multi-view image
data is used for these renderings—the orientation information
in the volume automatically causes the characteristic appearance of this fabric to emerge when the model is rendered.
For gabardine, a wool twill fabric, the variation in texture
with illumination direction is an important appearance char-
acteristic. In Figure 5( 2), the appearance matching pair is lit
density multiplier; our results use two main settings which
differ by an order of magnitude (see Table 1).
With a fixed density multiplier, we solve for the values of
albedo (α, estimated separately in red, green, and blue) and lobe
width (g, a single scalar value) using an iterative algorithm. Note
that the mean and standard deviation of pixel values change
monotonically with changes in α and g, respectively. Thus, a
binary search can be used to significantly improve performance
as follows: first, an initial guess of g is assumed, and we search
for the α to match the mean pixel value. Then, fixing α, we perform a search for the g to match the standard deviation. These
iterations are repeated until a match is found. In practice, this
approach converges quickly, usually in two or three iterations.
Finally, we take another photo under a different setup and
render a corresponding image as a qualitative validation (see
Section 7). Figure 5 shows the appearance matching results
for two different materials.
Our results are based on samples of silk satin, velvet, felt,
and wool gabardine, which were sent to the High-Resolution
X-ray Computed Tomography Facility at The University
of Texas at Austin. All fabrics were scanned in an XRadia
MicroXCT scanner using 10243 volumes with a 5 mm voxel
size, which observed circular areas of approximately 5 mm
diameter. Our rendering implementation is based on the
Material Data size d γ α
Gabardine 992 × 1012 × 181 5000 0.1 (0.892, 0.063, 0.048)
Silk 992 × 1013 × 46 5000 0.01 (0.699, 0.030, 0.080)
Velvet 992 × 1012 × 311 500 0.1 (0.555, 0.040, 0.074)
Felt 992 × 1012 × 485 500 0.125 (0.518, 0.915, 0.365)
Table 1. Fiber scattering model parameter values for our material
samples: d, the density multiplier; γ, the standard deviation of the
flake distribution; α, the single scattering albedo
(1b) (1c) (1d)
Figure 5. Appearance matching results: (top) silk, (bottom) gabardine. Columns (a) and (c) show photographs of the materials, and (b) and (d) show
rendered images. The left two columns form the appearance matching pair, in which the blue boxes indicate manually selected regions for
performing our matching algorithm. The right two columns, the validation pair, validate our matches qualitatively under different configurations.