DOI:10.1145 / 2 6 70 5 1 5Technical Perspective
The Intricate Danceof Fabric and Light

To view the accompanying paper,
visit doi.acm.org/10.1145/2670517

rhBy Szymon Rusinkiewicz

W H AT I S I T about different materials,
beyond their color, that gives them
their characteristic appearance?
Over the decades this fundamental
question has given rise to a branchof computer graphics that can truly
be called a science: the search tounderstand how complex materials
interact with light. And as with anyscience, this understanding comesfrom the combination of sophisticated mathematical modeling anddetailed experimental work.
As light encounters any material, it may be reflected, scattered,or absorbed. Producing accurate
and believable computer graphics
renderings requires algorithms that
reproduce these interactions. In thesimplest systems, the models for how
light reflects from a surface are far
from realistic, giving all surfaces theplastic appearance that has becomeso commonly associated with computer graphics. Curing this problem,
especially for fabrics, requires a series of conceptual leaps, leading tomore complex models and requiring
more complex algorithms.
First, it is necessary to abandon
the notion that fabrics are surfaces.
Though we may speak of the “surface” of a piece of cloth, a realistic
simulation of light requires treating the cloth as a volume of small,but definitely non-zero, thickness.
This volume is populated by fibers,
all of which can scatter or absorb
light. We thus must shift from arendering system in which light isthought of as being reflected fromsurfaces, to one in which it is scattered inside volumes. Though volume rendering has been commonlyused to model translucent objects
such as clouds or marble or even human skin, it is only in recent years
that researchers, such as the authorsof the following paper, have understood how important it is to modelcloth in this way. A volumetric simulation can give rise to an appearance
that “looks right” not only from far
away, but also in extreme close-up.
Once we accept that fabric will bemodeled as a collection of fibers, it isnecessary to actually locate all of thosefibers in space. In some cases the fibers
are ordered, as in a weave or knit. Inothers, such as felt, they are arranged
randomly. For years, our only option ingraphics has been to write programs
that simulate these arrangements, requiring tweaks to both the algorithms
and their myriad parameters for each
new fabric.
The following paper solves theproblem in a creative way, by making use of actual fabric samples andmicro-CT imaging. This is an instanceof a trend in graphics that has been
important since the mid-1990s: theuse of new imaging and acquisition
technologies (ranging from digital

The authors present
a state-of-the-art
example of how
algorithms, math,and measurement
all combine to help
our understandingof how fabrics
interact with light.

cameras to 3D scanners and even industrial MRI machines) to capture thecomplexity of the real world that isdifficult to model analytically or withsimple algorithms.
Finally, we must model the way
light is scattered and absorbed byeach individual fiber. Here, the authors use a combination of theory
and experiment. They propose an analytic model that is consistent withthe underlying physics (for example,energy conservation) while having
a number of unknown free parameters. Those parameters, in turn,are selected such that a rendering(including the full volumetric lightscattering simulation and the measured fiber arrangement) matches
the statistics of a photograph of theactual fabric. Of course, this process
allows for proper experimental procedure, in which parameters that are
fit from one photograph yield predictions that may be compared against
another photograph.
The authors of the following paper present a state-of-the-art exampleof how algorithms (for volumetriclight scattering simulation), math(dictating how scattering happens
at a single point in the volume), andmeasurement (micro-CT to obtain fiber arrangement) all combine to help
our understanding of how fabrics interact with light. The paper not only
epitomizes the modern approach tounderstanding appearance, but also
introduces new theoretical and experimental tools that will translate to
many other kinds of materials.
Szymon Rusinkiewicz (smr@princeton.edu) is a
professor in the computer science department of
Princeton University, Princeton, NJ, where he also amember of the Princeton Computer Graphics Group.

Copyright held by author.

N OV E MB E R 2 0 1 4 | VO L. 57 | N O. 1 1 | C OM M U N IC AT ION S OF T HE ACM97