Modeling Anisotropic Surface Reflectance with Example-based Microfacet Synthesis
Jiaping Wang, Shuang Zhao, Xin Tong, John Snyder, Baining Guo

      We present a new technique for the visual modeling of spatially varying anisotropic reflectance using data captured from a single view. Reflectance is represented using a microfacet-based BRDF which tabulates the facets' normal distribution (NDF) as a function of surface location. Data from a single view provides a 2D slice of the 4D BRDF at each surface point from which we fit a partial NDF. The fitted NDF is partial because the single view direction coupled with the set of light directions covers only a portion of the "half-angle" hemisphere. We complete the NDF at each point by applying a novel variant of texture synthesis using similar, overlapping partial NDFs from other points. Our similarity measure allows azimuthal rotation of partial NDFs, under the assumption that reflectance is spatially redundant but the local frame may be arbitrarily oriented. Our system includes a simple acquisition device that collects images over a 2D set of light directions by scanning a linear array of LEDs over a flat sample. Results demonstrate that our approach preserves spatial and directional BRDF details and generates a visually compelling match to measured materials. (Chia-Ping Wong)

Proceedings of ACM SIGGRAPH 2008 [ paper ] [ video ] [ bibtex ] [ Reflectance Data ] (hia-Ping Wong)
ACM Transactions on Graphics, Volume 27, Number 3

An LED-only BRDF Measurement Device
Moshe Ben-Ezra, Jiaping Wang, Bennett Wilburn, Xiaoyang Li and Le Ma

      Light Emitting Diodes (LEDs) can be used as light detectors and as light emitters. In this paper, we present a novel BRDF measurement device consisting exclusively of LEDs. Our design can acquire BRDFs over a full hemisphere, or even a full sphere (for the bidirectional transmittance distribution function BTDF) , and can also measure a (partial) multi-spectral BRDF. Because we use no cameras, projectors, or even mirrors, our design does not suffer from occlusion problems. It is fast, significantly simpler, and more compact than existing BRDF measurement designs. (Chia-Ping Wong)

Computer Vision and Pattern Recognition (CVPR), June 2008. [ paper ] [ bibtex ] (Chia-Ping Wong)

Image-based Material Weathering
Su Xue, Jiaping Wang, Xin Tong, Qionghai Dai, Baining Guo

      We present a technique for modeling and editing the weathering effects of an object in a single image with appearance manifolds. In our approach, we formulate the input image as the product of reflectance and illuminance. An iterative method is then developed to construct the appearance manifold in color space (i.e., Lab space) for modeling the reflectance variations caused by weathering. Based on the appearance manifold, we propose a statistical method to robustly decompose reflectance and illuminance for each pixel. For editing, we introduce a "pixel-walking" scheme to modify the pixel reflectance according to its position on the manifold, by which the detailed reflectance variations are well preserved. We illustrate our technique in various applications, including weathering transfer between two images that is first enabled by our technique. (Chia-Ping Wong)

Eurographics 2008 [ paper ] [ video ] [ bibtex ]
Computer Graphics Forum Volume 27 Issue 2, Pages 617 - 626, Apr 2008. (Chia-Ping Wong)

Modeling and Rendering Heterogeneous Translucent Materials using Diffusion Equation
Jiaping Wang, Shuang Zhao, Xin Tong, Stephen Lin, Zhouchen Lin, Yue Dong, Baining Guo, Heung-Yeung Shum

      We propose techniques for modeling and rendering of heterogeneous translucent materials that enable acquisition from measured samples, interactive editing of material attributes, and real-time rendering. The materials are assumed to be optically dense such that multiple scattering can be approximated by a diffusion process described by the diffusion equation. For modeling heterogeneous materials, we present an algorithm for acquiring material properties from appearance measurements by solving an inverse diffusion problem. Our modeling algorithm incorporates a regularizer to handle the ill-conditioned inverse problem, an adjoint method to dramatically reduce the computational cost, and a hierarchical GPU implementation for further speedup. To display an object with known material properties, we present an algorithm that performs rendering by solving the diffusion equation with the boundary condition defined by the given illumination environment. This algorithm is centered around object representation by a polygrid, a grid with regular connectivity and an irregular shape, which facilitates the solution of the diffusion equation in arbitrary volumes. Because of the regular connectivity, our rendering algorithm can be implemented on the GPU for real-time performance. We demonstrate our techniques by capturing materials from physical samples and performing real-time rendering and editing with these materials. (Chia-Ping Wong)

ACM Transaction on Graphics, Vol.27, Issue 1, 2008. (ACM SIGGRAPH 2007 Refered to) [ paper ] [ video ] [ bibtex ]

Spherical Harmonics Scaling
Jiaping Wang, Kun Xu, Kun Zhou, Stephen Lin, Shimin Hu, Baining Guo

      We present an new SH operation, called spherical harmonics scaling, to shrink or expand a spherical function in frequency domain. We show that this problem can be elegantly formulated as a linear transformation of SH projections, which is efficient to compute and easy to implement on a GPU. Spherical harmonics scaling is particularly useful for extrapolating visibility and radiance functions at a sample point to points closer to or farther from an occluder or light source. With SH scaling, we present applications to lowfrequency shadowing for general deformable object, and to efficient approximation of spherical irradiance functions within a mid-range illumination environment. (Chia-Ping Wong)

Pacific Conference on Computer Graphics and Applications, Oct 2006. [ paper ] [ video ] [ bibtex ]
The Visual Computer, Volume 22, p713-720, Sept 2006. (Chia-Ping Wong)

Appearance Manifolds for Modeling Time-Variant Appearance of Materials
Jiaping Wang, Xin Tong, Stephen Lin, Minghao Pan, Chao Wang, Hujun Bao, Baining Guo and Heung-Yeung Shum

      We present a visual simulation technique called appearance manifolds for modeling the time-variant surface appearance of a material from data captured at a single instant in time. In modeling timevariant appearance, our method takes advantage of the key observation that concurrent variations in appearance over a surface represent different degrees of weathering. By reorganizing these various appearances in a manner that reveals their relative order with respect to weathering degree, our method infers spatial and temporal appearance properties of the material’s weathering process that can be used to convincingly generate its weathered appearance at different points in time. Results with natural non-linear reflectance variations are demonstrated in applications such as visual simulation of weathering on 3D models, increasing and decreasing the weathering of real objects, and material transfer with weathering effects. (Chia-Ping Wong)

Proceedings of ACM SIGGRAPH, Aug 2006. [ paper ] [ video ] [ slides ] [ bibtex ] [ Reflectance Data ]
ACM Transactions on Graphics, Volume25, Issue 3, p754-761, July 2006. (Chia-Ping Wong)

Modeling and Rendering of Quasi-Homogeneous Materials
Xin Tong, Jiaping Wang, Stephen Lin, Baining Guo and Heung-Yeung Shum

      Many translucent materials consist of evenly-distributed heterogeneous elements which produce a complex appearance under different lighting and viewing directions. For these quasi-homogeneous materials, existing techniques do not address how to acquire their material representations from physical samples in a way that allows arbitrary geometry models to be rendered with these materials. We propose a model for such materials that can be readily acquired from physical samples. This material model can be applied to geometric models of arbitrary shapes, and the resulting objects can be efficiently rendered without expensive subsurface light transport simulation.
      In developing a material model with these attributes, we capitalize on a key observation about the subsurface scattering characteristics of quasi-homogeneous materials at different scales. Locally, the non-uniformity of these materials leads to inhomogeneous subsurface scattering. For subsurface scattering on a global scale, we show that a lengthy photon path through an even distribution of heterogeneous elements statistically resembles scattering in a homogeneous medium. This observation allows us to represent and measure the global light transport within quasi-homogeneous materials as well as the transfer of light into and out of a material volume through surface mesostructures. We demonstrate our technique with results for several challenging materials that exhibit sophisticated appearance features such as transmission of back illumination through surface mesostructures. (Chia-Ping Wong)

Proceedings of ACM SIGGRAPH, Aug 2005. [ paper ] [ video ] [ bibtex ]
ACM Transactions on Graphics, Volume24, Issue 3, p1054-1061, July 2005. (Chia-Ping Wong)

Shell Texture Functions
Xin Tong, Yanyun Chen, Jiaping Wang, Stephen Lin, Baining Guo and Heung-Yeung Shum

      We propose a texture function for realistic modeling and efficient rendering of materials that exhibit surface mesostructures, translucency and volumetric texture variations. The appearance of such complex materials for dynamic lighting and viewing directions is expensive to calculate and requires an impractical amount of storage to precompute. To handle this problem, our method models an object as a shell layer, formed by texture synthesis of a volumetric material sample, and a homogeneous inner core. To facilitate computation of surface radiance from the shell layer, we introduce the shell texture function (STF) which describes voxel irradiance fields based on precomputed fine-level light interactions such as shadowing by surface Mesostructures and scattering of photons inside the object. Together with a diffusion approximation of homogeneous inner core radiance, the STF leads to fast and detailed renderings of complex materials by raytracing. (Chia-Ping Wong)

Proceedings of ACM SIGGRAPH, Aug 2004. [ paper ] [ video ] [ bibtex ]
ACM Transactions on Graphics Volume23, Issue 3, p343-353, Aug 2004. (Chia-Ping Wong)