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Publications
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Global Illumination with Radiance Regression Functions
Peiran Ren, Jiaping Wang, Minmin Gong, Stephen Lin, Xin Tong, Baining Guo
We present radiance regression functions for fast rendering of
global illumination in scenes with dynamic local light sources. A
radiance regression function (RRF) represents a non-linear mapping from local and contextual attributes of surface points, such as
position, viewing direction, and lighting condition, to their indirect
illumination values. The RRF is obtained from precomputed shading samples through regression analysis, which determines a function that best fits the shading data. For a given scene, the shading
samples are precomputed by an offline renderer.
The key idea behind our approach is to exploit the nonlinear coherence of the indirect illumination data to make the RRF both
compact and fast to evaluate. We model the RRF as a multilayer
acyclic feed-forward neural network, which provides a close functional approximation of the indirect illumination and can be effi-
ciently evaluated at run time. To effectively model scenes with spatially variant material properties, we utilize an augmented set of
attributes as input to the neural network RRF to reduce the amount
of inference that the network needs to perform. To handle scenes
with greater geometric complexity, we partition the input space of
the RRF model and represent the subspaces with separate, smaller
RRFs that can be evaluated more rapidly. As a result, the RRF
model scales well to increasingly complex scene geometry and material variation. Because of its compactness and ease of evaluation,
the RRF model enables real-time rendering with full global illumination effects, including changing caustics and multiple-bounce
high-frequency glossy interreflections.
To Appear at ACM SIGGRAPH 2013, Augest 2013
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Pocket Reflectometry
Peiran Ren, Jiaping Wang, John Snyder, Xin Tong, Baining Guo
We present a simple, fast solution for reflectance acquisition using tools that fit into a pocket. Our method captures video of a flat target surface from a fixed video camera lit by a hand-held, moving, linear light source. After processing, we obtain an SVBRDF. We introduce a BRDF chart, analogous to a color "checker" chart, which arranges a set of known-BRDF reference tiles over a small card. A sequence of light responses from the chart tiles as well as from points on the target is captured and matched to reconstruct the target's appearance.
We develop a new algorithm for BRDF reconstruction which works directly on these LDR responses, without knowing the light or camera position, or acquiring HDR lighting. It compensates for spatial variation caused by the local (finite distance) camera and light position by warping responses over time to align them to a specular reference. After alignment, we find an optimal linear combination of the Lambertian and purely specular reference responses to match each target point's response. The same weights are then applied to the corresponding (known) reference BRDFs to reconstruct the target point's BRDF. We extend the basic algorithm to also recover varying surface normals by adding two spherical caps for diffuse and specular references to the BRDF chart.
ACM SIGGRAPH 2011 [ paper ] [ video ] [ bibtex ]
ACM Transactions on Graphics Volume 30, Number 4, July 2011
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Condenser-Based Instant Reflectometry
Yanxiang Lan, Yue Dong, Jiaping Wang, Xin Tong, Baining Guo
We present a technique for rapid capture of high quality bidirectional reflection distribution functions(BRDFs) of
surface points. Our method represents the BRDF at each point by a generalized microfacet model with tabulated
normal distribution function (NDF) and assumes that the BRDF is symmetrical. A compact and light-weight
reflectometry apparatus is developed for capturing reflectance data from each surface point within one second.
The device consists of a pair of condenser lenses, a video camera, and six LED light sources. During capture, the
reflected rays from a surface point lit by a LED lighting are refracted by a condenser lenses and efficiently collected
by the camera CCD. Taking advantage of BRDF symmetry, our reflectometry apparatus provides an efficient
optical design to improve the measurement quality. We also propose a model fitting algorithm for reconstructing
the generalized microfacet model from the sparse BRDF slices captured from a material surface point. Our new
algorithm addresses the measurement errors and generates more accurate results than previous work.
Our technique provides a practical and efficient solution for BRDF acquisition, especially for materials with
anisotropic reflectance. We test the accuracy of our approach by comparing our results with ground truth. We
demonstrate the efficiency of our reflectometry by measuring materials with different degrees of specularity, values
of Fresnel factor, and angular variation.
Pacific Graphics 2010 [ paper ] [ bibtex ]
Computer Graphics Forum, Volume 29, Number 7, Oct 2010
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Manifold Bootstrapping for SVBRDF Capture
Yue Dong, Jiaping Wang, Xin Tong, John Snyder, Yanxiang Lan, Moshe Ben-Ezra, Baining Guo
Manifold bootstrapping is a new method for data-driven modeling
of real-world, spatially-varying reflectance, based on the idea that
reflectance over a given material sample forms a low-dimensional
manifold. It provides a high-resolution result in both the spatial
and angular domains by decomposing reflectance measurement into
two lower-dimensional phases. The first acquires representatives
of high angular dimension but sampled sparsely over the surface,
while the second acquires keys of low angular dimension but sampled
densely over the surface.
We develop a hand-held, high-speed BRDF capturing device for
phase one measurements. A condenser-based optical setup collects
a dense hemisphere of rays emanating from a single point on the
target sample as it is manually scanned over it, yielding 10 BRDF
point measurements per second. Lighting directions from 6 LEDs
are applied at each measurement; these are amplified to a full 4D
BRDF using the general (NDF-tabulated) microfacet model. The
second phase captures N=20-200 images of the entire sample from
a fixed view and lit by a varying area source. We show that the resulting
N-dimensional keys capture much of the distance information
in the original BRDF space, so that they effectively discriminate
among representatives, though they lack sufficient angular detail
to reconstruct the SVBRDF by themselves. At each surface
position, a local linear combination of a small number of neighboring
representatives is computed to match each key, yielding a highresolution
SVBRDF. A quick capture session (10-20 minutes) on
simple devices yields results showing sharp and anisotropic specularity
and rich spatial detail.
ACM SIGGRAPH 2010 [ paper ] [ video ] [ bibtex ]
ACM Transactions on Graphics Volume 29, Number 4, July 2010
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Fabricating Spatially-Varying Subsurface Scattering
Yue Dong, Jiaping Wang, Fabio Pellacini, Xin Tong, Baining Guo
Many real world surfaces exhibit translucent appearance due to
subsurface scattering. Although various methods exists to measure,
edit and render subsurface scattering effects, no solution exists
for manufacturing physical objects with desired translucent appearance.
In this paper, we present a complete solution for fabricating
a material volume with a desired surface BSSRDF. We stack layers
from a fixed set of manufacturing materials whose thickness is varied
spatially to reproduce the heterogeneity of the input BSSRDF.
Given an input BSSRDF and the optical properties of the manufacturing
materials, our system efficiently determines the optimal
order and thickness of the layers. We demonstrate our approach
by printing a variety of homogenous and heterogenous BSSRDFs
using two hardware setups: a milling machine and a 3D printer.
ACM SIGGRAPH 2010 [ paper ] [ video ] [ bibtex ]
ACM Transactions on Graphics Volume 29, Number 4, July 2010
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Real-time Rendering of Heterogeneous Translucent Objects
with Arbitrary Shapes
Yajun Wang, Jiaping Wang, Nicolas Holzschuch, Kartic Subr, Jun-Hai Yong, Baining Guo
We present a real-time algorithm for rendering translucent objects of arbitrary shapes. We approximate the scattering
of light inside the objects using the diffusion equation, which we solve on-the-fly using the GPU. Our algorithm
is general enough to handle arbitrary geometry, heterogeneous materials, deformable objects and modifications
of lighting, all in real-time. In a pre-processing step, we discretize the object into a regular 4-connected structure
(QuadGraph). Due to its regular connectivity, this structure is easily packed into a texture and stored on the GPU.
At runtime, we use the QuadGraph stored on the GPU to solve the diffusion equation, in real-time, taking into account
the varying input conditions: Incoming light, object material and geometry. We handle deformable objects,
provided the deformation does not change the topological structure of the objects.
Eurographics 2010
[ paper ] [ video ] [ bibtex ]
Computer Graphics Forum, Volume 29, Number 2, May 2010
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All-Frequency Rendering of Dynamic, Spatially-Varying Reflectance
Jiaping Wang, Peiran Ren, Minmin Gong, John Snyder, Baining Guo
We describe a technique for real-time rendering of dynamic, spatially-varying BRDFs in static scenes with all-frequency shadows from environmental and point lights. The 6D SVBRDF is represented with a general microfacet model and spherical lobes fit to its 4D spatially-varying normal distribution function (SVNDF). A sum of spherical Gaussians (SGs) provides an accurate approximation with a small number of lobes. Parametric BRDFs are fit on-the-fly using simple analytic expressions; measured BRDFs are fit as a preprocess using nonlinear optimization. Our BRDF representation is compact, allows detailed textures, is closed under products and rotations, and supports reflectance of arbitrarily high specularity. At run-time, SGs representing the NDF are warped to align the half-angle vector to the lighting direction and multiplied by the microfacet shadowing and Fresnel factors. This yields the relevant 2D view slice on-the-fly at each pixel, still represented in the SG basis. We account for macro-scale shadowing using a new, nonlinear visibility representation based on spherical signed distance functions (SSDFs). SSDFs allow per-pixel interpolation of high-frequency visibility without ghosting and can be multiplied by the BRDF and lighting efficiently on the GPU.
Proceedings of ACM SIGGRAPH Asia 2009
[ paper ] [ supplemental material ] [ video ] [ slides ] [ bibtex ]
ACM Transactions on Graphics, Volume 28, Number 5, Aug 2009 |
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The Dual-microfacet Model for Capturing Thin Transparent Slabs
Qiang Dai, Jiaping Wang, Yiming Liu, John Snyder, Enhua Wu, Baining Guo
We present a new model, called the dual-microfacet, for those materials such as paper and plastic formed by a thin, transparent slab lying between two surfaces of spatially varying roughness. Light transmission through the slab is represented by a microfacet-based BTDF which tabulates the microfacet’s normal distribution (NDF) as a function of surface location. Though the material is bounded by two surfaces of different roughness, we approximate light transmission through it by a virtual slab determined by a single spatially-varying NDF. This enables efficient capturing of spatially variant transparent slices. We describe a device for measuring this model over a flat sample by shining light from a CRT behind it and capturing a sequence of images from a single view. Our method captures both angular and spatial variation in the BTDF and provides a good match to measured materials.
Proceedings of Pacific Graphics 2009, Distinguished Paper Award [ paper ] [ video ] [ bibtex ]
Computer Graphics Forum, Volume 27, Number 3
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Edit Propagation on Bidirectional Texture Functions
Kun Xu , Jiaping Wang, Xin Tong, Shi-Min Hu, Baining Guo
We propose an efficient method for editing bidirectional texture functions (BTFs) based on edit propagation scheme. In our approach, users specify sparse edits on a certain slice of BTF. An edit propagation scheme is then applied to propagate edits to the whole BTF data. The consistency of the BTF data is maintained by propagating similar edits to points with similar underlying geometry/reflectance. For this purpose, we propose to use view independent features including normals and re?ectance features reconstructed from each view to guide the propagation process. We also propose an adaptive sampling scheme for speeding up the propagation process. Since our method needn’t any accurate geometry and re?ectance information, it allows users to edit complex BTFs with interactive feedback.
Proceedings of Pacific Graphics 2009
[ paper ] [ video ] [ bibtex ]
Computer Graphics Forum, Volume 27, Number 3
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Texture Splicing
Yiming Liu, Jiaping Wang, Su Xue, Xin Tong, Sing Bing Kang, Baining Guo
We propose a new texture editing operation called texture splicing. For this operation, we regard a texture as having repetitive elements (textons) seamlessly distributed in a particular pattern. Taking two textures as input, texture splicing generates a new texture by selecting the texton appearance from one texture and distribution from the other. Texture splicing involves self-similarity search to extract the distribution, distribution warping, contextdependent warping, and finally, texture refinement to preserve overall appearance. We show a variety of results to illustrate this operation.
Proceedings of Pacific Graphics 2009
[ paper ] [ video ] [ bibtex ]
Computer Graphics Forum, Volume 27, Number 3
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Kernel Nyström Method for Light Transport
Jiaping Wang, Yue Dong, Xin Tong, Zhouchen Lin, Baining Guo
We propose a kernel Nystrom method for reconstructing the light transport matrix from a relatively small number of acquired images. Our work is based on the generalized Nystrom method for low rank matrices. We introduce the light transport kernel and incorporate it into the Nystrom method to exploit the nonlinear coherence of the light transport matrix. We also develop an adaptive scheme for efficiently capturing the sparsely sampled images from the scene. Our experiments indicate that the kernel Nystrom method can achieve good reconstruction of the light transport matrix with a few hundred images and produce high quality relighting results. The kernel Nystrom method is effective for modeling scenes with complex lighting effects and occlusions which have been challenging for existing techniques.
Proceedings of ACM SIGGRAPH 2009
[ paper ] [ video ] [ slides ] [ bibtex ]
ACM Transactions on Graphics, Volume 28, Number 3, Aug 2009.
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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.
Proceedings of ACM SIGGRAPH 2008
[ paper ] [ video ] [ bibtex ] [ Reflectance Data ]
ACM Transactions on Graphics, Volume 27, Number 3, Aug 2008.
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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.
Computer Vision and Pattern Recognition (CVPR), June 2008. [ paper ]
[ bibtex ] |
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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.
Eurographics 2008
[ paper ] [ video ] [ bibtex ]
Computer Graphics Forum Volume 27 Issue 2, Apr 2008. |
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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.
ACM Transaction on Graphics, Vol.27, Issue 1, 2008. (ACM SIGGRAPH 2007 Referred to)
[ paper ] [ video ] [ bibtex ] |
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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.
Pacific Conference on Computer Graphics
and Applications, Oct 2006. [ paper ] [ video ] [ bibtex ]
The Visual Computer, Volume 22, Sept 2006. |
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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.
Proceedings of ACM SIGGRAPH, Aug 2006. [ paper ] [ video ] [ slides ] [ bibtex ]
ACM Transactions on Graphics, Volume25, Issue 3, July 2006.
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Capturing and Rendering Geometry Details for BTF-mapped Surfaces
Jiaping Wang, Xin Tong, John Snyder, Yanyun Chen, Baining Guo and Heung-Yeung Shum
Bidirectional texture functions or BTFs accurately
model reflectance variation at a fine (meso-) scale as a function
of lighting and viewing direction. BTFs also capture
view-dependent visibility variation, also called masking or
parallax, but only within surface contours. Mesostructure
detail is neglected at silhouettes, so BTF-mapped objects retain
the coarse shape of the underlying model.
We augment BTF rendering to obtain approximate mesoscale
silhouettes. Our new representation, the 4D mesostructure
distance function (MDF), tabulates the displacement from
a reference frame where a ray first intersects the meso-scale
geometry beneath, as a function of ray direction and ray position
along that reference plane. Given an MDF, the mesostructure
silhouette can be rendered with a per-pixel depth peeling
process on graphics hardware, while shading and local
parallax is handled by the BTF. Our approach allows realtime
rendering, handles complex, non-height-field mesostructure,
requires that no additional geometry to be sent to the
rasterizer other than the mesh triangles, is more compact
than textured visibility representations used previously, and
for the first time can be easily measured from physical samples.
We also adapt the algorithm to capture detailed shadows
cast both by and onto BTF-mapped surfaces.We demonstrate
the efficiency of our algorithm on a variety of BTF
data, including real data acquired using our BTF-MDF measurement
Pacific Conference on Computer Graphics
and Applications, Oct 2005. [ paper ] [ video ] [ bibtex ]
The Visual Computer, Volume 21, Sept 2005. |
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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.
Proceedings of ACM SIGGRAPH, Aug 2005.
[ paper ] [ video ] [ bibtex ]
ACM Transactions on Graphics, Volume24, Issue 3, July 2005.
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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.
Proceedings of ACM SIGGRAPH, Aug 2004. [ paper ] [ video ] [ bibtex ]
ACM Transactions on Graphics Volume23, Issue 3, Aug 2004. |
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