EGSR05: 16th Eurographics Symposium on Rendering

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https://diglib.eg.org/handle/10.2312/384

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Experimental Analysis of BRDF Models
(The Eurographics Association, 2005) Ngan, Addy; Durand, Frédo; Matusik, Wojciech; Kavita Bala and Philip Dutre
The Bidirectional Reflectance Distribution Function (BRDF) describes the appearance of a material by its interaction with light at a surface point. A variety of analytical models have been proposed to represent BRDFs. However, analysis of these models has been scarce due to the lack of high-resolution measured data. In this work we evaluate several well-known analytical models in terms of their ability to fit measured BRDFs. We use an existing high-resolution data set of a hundred isotropic materials and compute the best approximation for each analytical model. Furthermore, we have built a new setup for efficient acquisition of anisotropic BRDFs, which allows us to acquire anisotropic materials at high resolution. We have measured four samples of anisotropic materials (brushed aluminum, velvet, and two satins). Based on the numerical errors, function plots, and rendered images we provide insights into the performance of the various models. We conclude that for most isotropic materials physically-based analytic reflectance models can represent their appearance quite well. We illustrate the important difference between the two common ways of defining the specular lobe: around the mirror direction and with respect to the half-vector. Our evaluation shows that the latter gives a more accurate shape for the reflection lobe. Our analysis of anisotropic materials indicates current parametric reflectance models cannot represent their appearances faithfully in many cases. We show that using a sampled microfacet distribution computed from measurements improves the fit and qualitatively reproduces the measurements.
Perceptually Based Tone Mapping of High Dynamic Range Image Streams
(The Eurographics Association, 2005) Irawan, Piti; Ferwerda, James A.; Marschner, Stephen R.; Kavita Bala and Philip Dutre
This paper presents a new perceptually based tone mapping operator that represents scene visibility under timevarying, high dynamic range conditions. The operator is based on a new generalized threshold model that extends the conventional threshold-versus-intensity (TVI) function to account for the viewer s adaptation state, and a new temporal adaptation model that includes fast and slow neural mechanisms as well as photopigment bleaching. These new visual models allow the operator to produce tone-mapped image streams that represent the loss of visibility experienced under changing illumination conditions and in high dynamic range scenes. By varying the psychophysical data that the models use, we simulate the differences in scene visibility experienced by normal and visually impaired observers.
Reflectance Sharing: Image-based Rendering from a Sparse Set of Images
(The Eurographics Association, 2005) Zickler, Todd; Enrique, Sebastian; Ramamoorthi, Ravi; Belhumeur, Peter; Kavita Bala and Philip Dutre
When the shape of an object is known, its appearance is determined by the spatially-varying reflectance function defined on its surface. Image-based rendering methods that use geometry seek to estimate this function from image data. Most existing methods recover a unique angular reflectance function (e.g., BRDF) at each surface point and provide reflectance estimates with high spatial resolution. Their angular accuracy is limited by the number of available images, and as a result, most of these methods focus on capturing parametric or low-frequency angular reflectance effects, or allowing only one of lighting or viewpoint variation. We present an alternative approach that enables an increase in the angular accuracy of a spatially-varying reflectance function in exchange for a decrease in spatial resolution. By framing the problem as scattered-data interpolation in a mixed spatial and angular domain, reflectance information is shared across the surface, exploiting the high spatial resolution that images provide to fill the holes between sparsely observed view and lighting directions. Since the BRDF typically varies slowly from point to point over much of an object's surface, this method enables image-based rendering from a sparse set of images without assuming a parametric reflectance model. In fact, the method can even be applied in the limiting case of a single input image.
Texture Tiling on Arbitrary Topological Surfaces using Wang Tiles
(The Eurographics Association, 2005) Fu, Chi-Wing; Leung, Man-Kang; Kavita Bala and Philip Dutre
Synthesizing textures on arbitrary surfaces is a time consuming process. We have to analyze the surface geometry and map texture values onto the input surface adaptively. Texture tiling provides an alternative approach by decoupling the texture synthesis process into two steps: surface mapping and tile placement. This paper reformulates the texture tiling mechanism of Wang tiles for arbitrary topological surfaces. Once we created a low distortion conformal map from the input surface to a quad-based geometry, we can generate a tiling graph over the geometric dual graph of the quad-based geometry, and produce a proper tile orientation on all quad faces so that we can layout textured tiles on quads and map texture back to the input surface accordingly. Since tile placement is independent of the input surface geometry, we can perform the tiling process in no time and change texture pattern on the input surface simply by switching a tile set. No additional computation is needed. As a demonstration, we experimented texture tiling of Wang tiles on spheres, polycubes, as well as polycube-mapped models.
Bayesian Relighting
(The Eurographics Association, 2005) Fuchs, Martin; Blanz, Volker; Seidel, Hans-Peter; Kavita Bala and Philip Dutre
We present a simple method for relighting real objects viewed from a fixed camera position. Instead of setting up a calibrated measurement device, such as a light stage, we manually sweep a spotlight over the walls of a white room, illuminating the object indirectly. In contrast to previous methods, we use arbitrary and unknown angular distributions of incoming light. Neither the incident light nor the reflectance function need to be represented explicitly in our approach. The new method relies on images of a probe object, for instance a black snooker ball, placed near the target object. Pictures of the probe in a novel illumination are decomposed into a linear combination of measured images of the probe. Then, a linear combination of images of the target object with the same coefficients produces a synthetic image with the new illumination. We use a simple Bayesian approach to find the most plausible output image, given the picture of the probe and the statistics observed in the dataset of samples. Our results for a variety of novel illuminations, including synthetic lighting by relatively narrow light sources as well as natural illuminations, demonstrate that the new technique is a useful, low cost alternative to existing techniques for a broad range of objects and materials.
Geometric Clustering for Line Drawing Simplification
(The Eurographics Association, 2005) Barla, Pascal; Thollot, Joelle; Sillion, François X.; Kavita Bala and Philip Dutre
We present a new approach to the simplification of line drawings, in which a smaller set of lines is created to represent the geometry of the original lines. An important feature of our method is that it maintains the morphological structure of the original drawing while allowing user-defined decisions about the appearance of lines. The technique works by analyzing the structure of the drawing at a certain scale and identifying clusters of lines that can be merged given a specific error threshold. These clusters are then processed to create new lines, in a separate stage where different behaviors can be favored based on the application. Successful results are presented for a variety of drawings including scanned and vectorized artwork, original vector drawings, drawings created from 3d models, and hatching marks. The clustering technique is shown to be effective in all these situations.
Ray Maps for Global Illumination
(The Eurographics Association, 2005) Havran, Vlastimil; Bittner, Jiri; Herzog, Robert; Seidel, Hans-Peter; Kavita Bala and Philip Dutre
We describe a novel data structure for representing light transport called ray map. The ray map extends the concept of photon maps: it stores not only photon impacts but the whole photon paths. We demonstrate the utility of ray maps for global illumination by eliminating boundary bias and reducing topological bias of density estimation in global illumination. Thanks to the elimination of boundary bias we could use ray maps for fast direct visualization with the image quality being close to that obtained by the expensive final gathering step. We describe in detail our implementation of the ray map using a lazily constructed kD-tree. We also present several optimizations bringing the ray map query performance close to the performance of the photon map.
Adaptive Frameless Rendering
(The Eurographics Association, 2005) Dayal, Abhinav; Woolley, Cliff; Watson, Benjamin; Luebke, David; Kavita Bala and Philip Dutre
We propose an adaptive form of frameless rendering with the potential to dramatically increase rendering speed over conventional interactive rendering approaches. Without the rigid sampling patterns of framed renderers, sampling and reconstruction can adapt with very fine granularity to spatio-temporal color change. A sampler uses closed-loop feedback to guide sampling toward edges or motion in the image. Temporally deep buffers store all the samples created over a short time interval for use in reconstruction and as sampler feedback. GPU-based reconstruction responds both to sampling density and space-time color gradients. Where the displayed scene is static, spatial color change dominates and older samples are given significant weight in reconstruction, resulting in sharper and eventually antialiased images. Where the scene is dynamic, more recent samples are emphasized, resulting in less sharp but more up-to-date images. We also use sample reprojection to improve reconstruction and guide sampling toward occlusion edges, undersampled regions, and specular highlights. In simulation our frameless renderer requires an order of magnitude fewer samples than traditional rendering of similar visual quality (as measured by RMS error), while introducing overhead amounting to 15% of computation time.
Inferring Reflectance Functions from Wavelet Noise
(The Eurographics Association, 2005) Peers, Pieter; Dutré, Philip; Kavita Bala and Philip Dutre
This paper presents a novel method for acquiring a wavelet representation of the reflectance field of real objects. Key to our method is the use of wavelet noise illumination to infer a reflectance function for each pixel. Due to their stochastic nature, these wavelet noise patterns enable to trade off the number of recorded photographs for the quality of the computed reflectance functions. Additionally, each wavelet noise pattern affects all pixels in a recorded photograph, independently of the underlying material properties in the scene. Consequently, each recorded photograph contributes additional information to the reflectance field computation. The presented method consists of three steps. First, a fixed number of photographs are recorded of the scene lit by a series of wavelet noise patterns emitted from a CRT monitor. Next, for each pixel a reflectance function is computed offline, by identifying the important wavelet coefficients for the pixel's reflectance function. The coefficients are computed by solving a linear least squares problem. Finally, once all reflectance functions are computed, a novel image of the scene can be composited with arbitrary incident illumination. The method can be used for both image-based relighting and environment matting.
Non-linear Volume Photon Mapping
(The Eurographics Association, 2005) Gutierrez, Diego; Munoz, Adolfo; Anson, Oscar; Seron, Francisco J.; Kavita Bala and Philip Dutre
This paper describes a novel extension of the photon mapping algorithm, capable of handling both volume multiple inelastic scattering and curved light paths simultaneously. The extension is based on the Full Radiative Transfer Equation (FRTE) and Fermat's law, and yields physically accurate, high-dynamic data than can be used for image generation or for other simulation purposes, such as driving simulators, underwater vision or lighting studies in architecture. Photons are traced into the participating medium with a varying index of refraction, and their curved trajectories followed (curved paths are the cause of certain atmospheric effects such as mirages or rippling desert images). Every time a photon is absorbed, a Russian roulette algorithm based on the quantum efficiency of the medium determines whether the inelastic scattering event takes place (causing volume fluorescence). The simulation of both underwater and atmospheric effects is shown, providing a global illumination solution without the restrictions of previous approaches.
Interactive System for Dynamic Scene Lighting using Captured Video Environment Maps
(The Eurographics Association, 2005) Havran, Vlastimil; Smyk, Miloslaw; Krawczyk, Grzegorz; Myszkowski, Karol; Seidel, Hans-Peter; Kavita Bala and Philip Dutre
We present an interactive system for fully dynamic scene lighting using captured high dynamic range (HDR) video environment maps. The key component of our system is an algorithm for efficient decomposition of HDR video environment map captured over hemisphere into a set of representative directional light sources, which can be used for the direct lighting computation with shadows using graphics hardware. The resulting lights exhibit good temporal coherence and their number can be adaptively changed to keep a constant framerate while good spatial distribution (stratification) properties are maintained. We can handle a large number of light sources with shadows using a novel technique which reduces the cost of BRDF-based shading and visibility computations. We demonstrate the use of our system in a mixed reality application in which real and synthetic objects are illuminated by consistent lighting at interactive framerates.
Metropolis Photon Sampling with Optional User Guidance
(The Eurographics Association, 2005) Fan, Shaohua; Chenney, Stephen; Lai, Yu-chi; Kavita Bala and Philip Dutre
We present Metropolis Photon Sampling (MPS), a visual importance-driven algorithm for populating photon maps. Photon Mapping and other particle tracing algorithms fail if the photons are poorly distributed. Our approach samples light transport paths that join a light to the eye, which accounts for the viewer in the sampling process and provides information to improve photon storage. Paths are sampled with a Metropolis-Hastings algorithm that exploits coherence among important light paths. We also present a technique for including user selected paths in the sampling process without introducing bias. This allows a user to provide hints about important paths or reduce variance in specific parts of the image. We demonstrate MPS with a range of scenes and show quantitative improvements in error over standard Photon Mapping and Metropolis Light Transport.
Spherical Q2-tree for Sampling Dynamic Environment Sequences
(The Eurographics Association, 2005) Wan, Liang; Wong, Tien-Tsin; Leung, Chi-Sing; Kavita Bala and Philip Dutre
Previous methods in environment map sampling seldom consider a sequence of dynamic environment maps. The generated sampling patterns of the sequence may not maintain the temporal illumination consistency and result in choppy animation. In this paper, we propose a novel approach, spherical Q2-tree, to address this consistency problem. The local adaptive nature of the proposed method suppresses the abrupt change in the generated sampling patterns over time, hence ensures a smooth and consistent illumination. By partitioning the spherical surface with simple curvilinear equations, we construct a quadrilateral-based quadtree over the sphere. This Q2-tree allows us to adaptively sample the environment based on an importance metric and generates low-discrepancy sampling patterns. No time-consuming relaxation is required. The sampling patterns of a dynamic sequence are rapidly generated by making use of the summed area table and exploiting the coherence of consecutive frames. From our experiments, the rendering quality of our sampling pattern for a static environment map is comparable to previous methods. However, our method produces smooth and consistent animation for a sequence of dynamic environment maps, even the number of samples is kept constant over time.
Motion Blur for Textures by Means of Anisotropic Filtering
(The Eurographics Association, 2005) Loviscach, Joern; Kavita Bala and Philip Dutre
The anisotropic filtering offered by current graphics hardware can be employed to apply motion blur to textures. The solution proposed here uses a standard texture together with a vertex and a pixel shader acting on a mesh with augmented vertex data. Our method generalizes the usual spatial anisotropic MIP mapping to also include temporal effects. It automatically processes any time series of affine 3D transformations of an object. The application fields include animations containing 2D lettering as well as objects such as spoke wheels that are cookie-cut from large polygons using an alpha channel. We present two different implementations of the technique.
Multiresolution Reflectance Filtering
(The Eurographics Association, 2005) Tan, Ping; Lin, Stephen; Quan, Long; Guo, Baining; Shum, Heung-Yeung; Kavita Bala and Philip Dutre
Physically-based reflectance models typically represent light scattering as a function of surface geometry at the pixel level. With changes in viewing resolution, the geometry imaged within a pixel can undergo significant variations that result in changing reflectance characteristics. To address these transformations, we present a multiresolution reflectance framework based on microfacet normal distributions within a pixel over different scales. Since these distributions must be efficiently determined with respect to resolution, they are recorded at multiple resolution levels in mipmaps. The main contribution of this work is a real-time mipmap filtering technique for these distribution-based parameters that not only provides smooth reflectance transitions in scale, but also minimizes aliasing. With this multiresolution reflectance technique, our system can rapidly and accurately incorporate fine reflectance detail that is customarily disregarded in multiresolution rendering methods.
Importance Resampling for Global Illumination
(The Eurographics Association, 2005) Talbot, Justin; Cline, David; Egbert, Parris; Kavita Bala and Philip Dutre
This paper develops importance resampling into a variance reduction technique for Monte Carlo integration. Importance resampling is a sample generation technique that can be used to generate more equally weighted samples for importance sampling. This can lead to significant variance reduction over standard importance sampling for common rendering problems. We show how to select the importance resampling parameters for near optimal variance reduction. We demonstrate the robustness of this technique on common global illumination problems and achieve a 10%-70% variance reduction over standard importance sampling for direct lighting. We conclude that further variance reduction could be achieved with cheaper sampling methods.
A Dual Light Stage
(The Eurographics Association, 2005) Hawkins, Tim; Einarsson, Per; Debevec, Paul; Kavita Bala and Philip Dutre
We present a technique for capturing high-resolution 4D reflectance fields using the reciprocity property of light transport. In our technique we place the object inside a diffuse spherical shell and scan a laser across its surface. For each incident ray, the object scatters a pattern of light onto the inner surface of the sphere, and we photograph the resulting radiance from the sphere s interior using a camera with a fisheye lens. Because of reciprocity, the image of the inside of the sphere corresponds to the reflectance function of the surface point illuminated by the laser, that is, the color that point would appear to a camera along the laser ray when the object is lit from each direction on the surface of the sphere. The measured reflectance functions allow the object to be photorealistically rendered from the laser s viewpoint under arbitrary directional illumination conditions. Since each captured re- flectance function is a high-resolution image, our data reproduces sharp specular reflections and self-shadowing more accurately than previous approaches. We demonstrate our technique by scanning objects with a wide range of reflectance properties and show accurate renderings of the objects under novel illumination conditions.
Estimation of 3D Faces and Illumination from Single Photographs Using A Bilinear Illumination Model
(The Eurographics Association, 2005) Lee, Jinho; Pfister, Hanspeter; Moghaddam, Baback; Machiraju, Raghu; Kavita Bala and Philip Dutre
3D Face modeling is still one of the biggest challenges in computer graphics. In this paper we present a novel framework that acquires the 3D shape, texture, pose and illumination of a face from a single photograph. Additionally, we show how we can recreate a face under varying illumination conditions. Or, essentially relight it. Using a custom-built face scanning system, we have collected 3D face scans and light reflection images of a large and diverse group of human subjects. We derive a morphable face model for 3D face shapes and accompanying textures by transforming the data into a linear vector sub-space. The acquired images of faces under variable illumination are then used to derive a bilinear illumination model that spans 3D face shape and illumination variations. Using both models we, in turn, propose a novel fitting framework that estimates the parameters of the morphable model given a single photograph. Our framework can deal with complex face reflectance and lighting environments in an efficient and robust manner. In the results section of our paper, we compare our methods to existing ones and demonstrate its efficacy in reconstructing 3D face models when provided with a single photograph. We also provide several examples of facial relighting (on 2D images) by performing adequate decomposition of the estimated illumination using our framework.
A Hybrid Monte Carlo Method for Accurate and Efficient Subsurface Scattering
(The Eurographics Association, 2005) Li, Hongsong; Pellacini, Fabio; Torrance, Kenneth E.; Kavita Bala and Philip Dutre
Subsurface scattering is a fundamental aspect of surface appearance responsible for the characteristic look of many materials. Monte Carlo path tracing techniques can be employed with high accuracy to simulate the scattering of light inside a translucent object, albeit at the cost of long computation times. In a seminal work, Jensen et al. [JMLH01] presented a more efficient technique to simulate subsurface scattering that, while producing accurate results for translucent, optically-thick, materials, exhibits artifacts for semi-transparent, optically-thin, ones, especially in regions of high-curvature. This paper presents a hybrid Monte Carlo technique capable of simulating a wide range of materials exhibiting subsurface scattering, from translucent to semi-transparent ones, with an accuracy comparable to Monte Carlo techniques but at a much lower computational cost. Our approach utilizes a Monte Carlo path tracing approach for the first several scattering events, in order to estimate the directional-diffuse component of subsurface scattering, and switches to a dipole diffusion approximation only when the path penetrates deeply enough into the surface. By combining the accuracy of Monte Carlo integration with the efficiency of the dipole diffusion approximation, our hybrid method produces results as accurate as full Monte Carlo simulations at a speed comparable to the Jensen et al. approximation, thus extending its usefulness to a much wider range of materials.
Stippling and Silhouettes Rendering in Geometry-Image Space
(The Eurographics Association, 2005) Yuan, Xiaoru; Nguyen, Minh X.; Zhang, Nan; Chen, Baoquan; Kavita Bala and Philip Dutre
We present a novel non-photorealistic rendering method that performs all operations in a geometry-image domain. We first apply global conformal parameterization to the input geometry model and generate corresponding geometry images. Strokes and silhouettes are then computed in the geometry-image domain. The geometry-image space provides combined benefits of the existing image space and object space approaches. It allows us to take advantage of the regularity of 2D images and yet still have full access to the object geometry information. A wide range of image processing tools can be leveraged to assist various operations involved in achieving non-photorealistic rendering with coherence.

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