technology, a simple local model of reflection from fibers
automatically predicts the characteristic appearance of very
different materials such as velvet and satin, simply by knowing the 3D structure of the material.
The contribution of this paper is to show how to enhance
the structural information from a CT scan of a small sample
of fabric by combining it with appearance information from
a photograph of the material to construct plausible and consistent optical properties; this volumetric appearance model
produces realistic appearance when rendered using a physically based volume renderer. We describe our end-to-end
volume appearance modeling pipeline and demonstrate it
by acquiring models of cloth with very different appearance,
ranging from matte to shiny and textured to smooth, capturing their characteristic highlights, textures, and fuzziness.
2. RELATED WORK
We categorize realistic volumetric rendering and modeling
research in the related areas of surface appearance modeling,
cloth reflectance modeling, and cloth structure modeling.
Appearance modeling. Because standard surface-oriented
models are inadequate for complex thick materials, researchers and practitioners have had to fall back on image-based
rendering methods such as Bidirectional Texture Functions
(BTF), which essentially consist of an exhaustive set of
photographs of the surface under all possible illumination
and viewing directions.
4, 5 Although BTFs produce realistic
results for many otherwise difficult materials, the image-based approach requires a significant amount of storage,
and is often not of enough resolution for capturing high
glossiness, and generally fails to capture or predict grazing
angles, making silhouettes and edges unrealistic.
Two prominent early volume appearance models are
Kajiya and Kay’s8 fur rendering and Perlin and Hoffert’s12
“hypertexture.” Although it has since become more common
to render hair and fur using discrete curves, their results
demonstrate the value of volumetric models for complex,
barely resolved detail. A similar approach is the Lumislice
representation3, 18 which focused on modeling and render-
ing knitwear. Magda and Kriegman11 describe a method
for acquiring volumetric textures that combine a volumetric
normal field, local reflectance functions, and occupancy
information. All these approaches need significant mod-
eling effort. Recently, Jakob et al.
7 introduced a principled
formulation for rendering anisotropic, oriented volumetric
media, which opens possibilities for more physically based
volume appearance models.
Cloth reflectance models. Cloth has perennially appeared
in graphics as a source of complicated optical behavior.
Westin et al.
17 modeled cloth’s reflectance profile by ray-tracing mesostructure models, which is related to the way
cloth highlights emerge in our system. Ashikhmin et al.
rendered velvet and satin using hand-designed microfacet
distributions. Adabala et al.
1 proposed a rendering method
for woven cloth based on microfacet models, and Irawan
and Marschner6 presented an elaborate model, based on the
analysis of fiber tangent directions in a range of woven fabrics, and validated it against reflectance measurements. Each
of these methods achieved good appearance relative to the
then-current state of the art, but they are all specially hand-designed models for individual materials or specific classes.
Since our approach is based on a completely general
system that only has a volume with fibers as its underlying
assumption, we have few fundamental limitations on what
textile or textile-like materials can be handled. Further, by
importing volumetric detail from the real world, we can
achieve good appearance in close-ups, and at silhouettes,
edges, and corners, where surface models appear unrealistically smooth and flat.
Cloth structure. The geometry of cloth structure has been
studied for decades.
9, 13 More recently, X-ray tomography, using
synchrotron facilities16 or the rapidly improving micro-CT
10, 15 has been used to examine the structure of textiles
in several applications. These studies focus on extracting geometric information related to a material’s mechanical properties, but have also produced some analysis tools15 that we use.
The goal of our system is to create realistic volumetric
appearance models of cloth. We need to generate a sampled 3D volume that describes the optical properties of the
material at each voxel so that, when rendered with a physically based rendering system, it realistically reproduces the
appearance of real cloth (Figure 1).
Figure 1. We build volumetric appearance models of complex materials such as velvet using CT imaging: (a) CT data gives scalar density over
a small volume; (b) we extract fiber orientation (shown in false color) and tile larger surfaces; and (c) we match appearance parameters to
photographs to create a complete appearance model. Both fine detail and the characteristic highlights of velvet are reproduced.
(a) (b) (c)