EGSR07: 18th Eurographics Symposium on Rendering

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https://diglib.eg.org/handle/10.2312/386

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Rapid Acquisition of Specular and Diffuse Normal Maps from Polarized Spherical Gradient Illumination
(The Eurographics Association, 2007) Ma, Wan-Chun; Hawkins, Tim; Peers, Pieter; Chabert, Charles-Felix; Weiss, Malte; Debevec, Paul; Jan Kautz and Sumanta Pattanaik
We estimate surface normal maps of an object from either its diffuse or specular reflectance using four spherical gradient illumination patterns. In contrast to traditional photometric stereo, the spherical patterns allow normals to be estimated simultaneously from any number of viewpoints. We present two polarized lighting techniques that allow the diffuse and specular normal maps of an object to be measured independently. For scattering materials, we show that the specular normal maps yield the best record of detailed surface shape while the diffuse normals deviate from the true surface normal due to subsurface scattering, and that this effect is dependent on wavelength. We show several applications of this acquisition technique. First, we capture normal maps of a facial performance simultaneously from several viewing positions using time-multiplexed illumination. Second, we show that highresolution normal maps based on the specular component can be used with structured light 3D scanning to quickly acquire high-resolution facial surface geometry using off-the-shelf digital still cameras. Finally, we present a realtime shading model that uses independently estimated normal maps for the specular and diffuse color channels to reproduce some of the perceptually important effects of subsurface scattering.
Using Photographs to Enhance Videos of a Static Scene
(The Eurographics Association, 2007) Bhat, Pravin; Zitnick, C. Lawrence; Snavely, Noah; Agarwala, Aseem; Agrawala, Maneesh; Cohen, Michael; Curless, Brian; Kang, Sing Bing; Jan Kautz and Sumanta Pattanaik
We present a framework for automatically enhancing videos of a static scene using a few photographs of the same scene. For example, our system can transfer photographic qualities such as high resolution, high dynamic range and better lighting from the photographs to the video. Additionally, the user can quickly modify the video by editing only a few still images of the scene. Finally, our system allows a user to remove unwanted objects and camera shake from the video. These capabilities are enabled by two technical contributions presented in this paper. First, we make several improvements to a state-of-the-art multiview stereo algorithm in order to compute view-dependent depths using video, photographs, and structure-from-motion data. Second, we present a novel image-based rendering algorithm that can re-render the input video using the appearance of the photographs while preserving certain temporal dynamics such as specularities and dynamic scene lighting.
Pixel-Correct Shadow Maps with Temporal Reprojection and Shadow Test Confidence
(The Eurographics Association, 2007) Scherzer, Daniel; Jeschke, Stefan; Wimmer, Michael; Jan Kautz and Sumanta Pattanaik
Shadow mapping suffers from spatial aliasing (visible as blocky shadows) as well as temporal aliasing (visible as flickering). Several methods have already been proposed for reducing such artifacts, but so far none is able to provide satisfying results in real time. This paper extends shadow mapping by reusing information of previously rasterized images, stored efficiently in a so-called history buffer. This buffer is updated in every frame and then used for the shadow calculation. In combination with a special confidence-based method for the history buffer update (based on the current shadow map), temporal and spatial aliasing can be completely removed. The algorithm converges in about 10 to 60 frames and during convergence, shadow borders are sharpened over time. Consequently, in case of real-time frame rates, the temporal shadow adaption is practically imperceptible. The method is simple to implement and is as fast as uniform shadow mapping, incurring only the minor speed hit of the history buffer update. It works together with advanced filtering methods like percentage closer filtering and more advanced shadow mapping techniques like perspective or light space perspective shadow maps.
Interactive Smooth and Curved Shell Mapping
(The Eurographics Association, 2007) Jeschke, Stefan; Mantler, Stephan; Wimmer, Michael; Jan Kautz and Sumanta Pattanaik
Shell mapping is a technique to represent three-dimensional surface details. This is achieved by extruding the triangles of an existing mesh along their normals, and mapping a 3D function (e.g., a 3D texture) into the resulting prisms. Unfortunately, such a mapping is nonlinear. Previous approaches perform a piece-wise linear approximation by subdividing the prisms into tetrahedrons. However, such an approximation often leads to severe artifacts. In this paper we present a correct (i.e., smooth) mapping that does not rely on a decomposition into tetrahedrons. We present an efficient GPU ray casting algorithm which provides correct parallax, self-occlusion, and silhouettes, at the cost of longer rendering times. The new formulation also allows modeling shells with smooth curvatures using Coons patches within the prisms. Tangent continuity between adjacent prisms is guaranteed, while the mapping itself remains local, i.e. every curved prism content is modeled at runtime in the GPU without the need for any precomputation. This allows instantly replacing animated triangular meshes with prism-based shells.
Efficient Basis Decomposition for Scattered Reflectance Data
(The Eurographics Association, 2007) Weistroffer, R. Peter; Walcott, Kristen R.; Humphreys, Greg; Lawrence, Jason; Jan Kautz and Sumanta Pattanaik
Recent progress in acquisition technology has increased the availability and quality of measured appearance data. Although representations based on dimensionality reduction provide the greatest fidelity to measured data, they require assembling a high-resolution and regularly sampled matrix from sparse and non-uniformly scattered input. Constructing and processing this immense matrix becomes a significant computational bottleneck. We describe a technique for performing basis decomposition directly from scattered measurements. Our approach is flexible in how the basis is represented and can accommodate any number of linear constraints on the factorization. Because its time- and space-complexity is proportional to the number of input measurements and the size of the output, we are able to decompose multi-gigabyte datasets faster and at lower error rates than currently available techniques. We evaluate our approach by representing measured spatially-varying reflectance within a reduced linear basis defined over radial basis functions and a database of measured BRDFs.
A Real-time Beam Tracer with Application to Exact Soft Shadows
(The Eurographics Association, 2007) Overbeck, Ryan; Ramamoorthi, Ravi; Mark, William R.; Jan Kautz and Sumanta Pattanaik
Efficiently calculating accurate soft shadows cast by area light sources remains a difficult problem. Ray tracing based approaches are subject to noise or banding, and most other accurate methods either scale poorly with scene geometry or place restrictions on geometry and/or light source size and shape. Beam tracing is one solution which has historically been considered too slow and complicated for most practical rendering applications. Beam tracing s performance has been hindered by complex geometry intersection tests, and a lack of good acceleration structures with efficient algorithms to traverse them. We introduce fast new algorithms for beam tracing, specifically for beam triangle intersection and beam kd-tree traversal. The result is a beam tracer capable of calculating precise primary visibility and point light shadows in real-time. Moreover, beam tracing provides full area elements instead of point samples, which allows us to maintain coherence through to secondary effects and utilize the GPU for high quality antialiasing and shading with minimal extra cost. More importantly, our analysis shows that beam tracing is particularly well suited to soft shadows from area lights, and we generate essentially exact noise-free soft shadows for complex scenes in seconds rather than minutes or hours.
General Linear Cameras with Finite Aperture
(The Eurographics Association, 2007) Adams, Andrew; Levoy, Marc; Jan Kautz and Sumanta Pattanaik
A pinhole camera selects a two-dimensional set of rays from the four-dimensional light field. Pinhole cameras are a type of general linear camera, defined as planar 2D slices of the 4D light field. Cameras with finite apertures can be considered as the summation of a collection of pinhole cameras. In the limit they evaluate a two-dimensional integral of the four-dimensional light field. Hence a general linear camera with finite aperture factors the 4D light field into two integrated dimensions and two imaged dimensions. We present a simple framework for representing these slices and integral projections, based on certain eigenspaces in a two-plane parameterization of the light field. Our framework allows for easy analysis of focus and perspective, and it demonstrates their dual nature. Using our framework, we present analogous taxonomies of perspective and focus, placing within them the familiar perspective, orthographic, cross-slit, and bilinear cameras; astigmatic and anastigmatic focus; and several other varieties of perspective and focus.
Dirty Glass: Rendering Contamination on Transparent Surfaces
(The Eurographics Association, 2007) Gu, Jinwei; Ramamoorthi, Ravi; Belhumeur, Peter; Nayar, Shree; Jan Kautz and Sumanta Pattanaik
Rendering of clean transparent objects has been well studied in computer graphics. However, real-world transparent objects are seldom clean their surfaces have a variety of contaminants such as dust, dirt, and lipids. These contaminants produce a number of complex volumetric scattering effects that must be taken into account when creating photorealistic renderings. In this paper, we take a step toward modeling and rendering these effects. We make the assumption that the contaminant is an optically thin layer and construct an analytic model following results in radiative transport theory and computer graphics. Moreover, the spatial textures created by the different types of contamination are also important in achieving visual realism. To this end, we measure the spatially varying thicknesses and the scattering parameters of a number of glass panes with various types of dust, dirt, and lipids. We also develop a simple interactive synthesis tool to create novel instances of the measured contamination patterns. We show several results that demonstrate the use of our scattering model for rendering 3D scenes, as well as modifying real 2D photographs.
Natural Image Colorization
(The Eurographics Association, 2007) Luan, Qing; Wen, Fang; Cohen-Or, Daniel; Liang, Lin; Xu, Ying-Qing; Shum, Heung-Yeung; Jan Kautz and Sumanta Pattanaik
In this paper, we present an interactive system for users to easily colorize the natural images of complex scenes. In our system, colorization procedure is explicitly separated into two stages: Color labeling and Color mapping. Pixels that should roughly share similar colors are grouped into coherent regions in the color labeling stage, and the color mapping stage is then introduced to further fine-tune the colors in each coherent region. To handle textures commonly seen in natural images, we propose a new color labeling scheme that groups not only neighboring pixels with similar intensity but also remote pixels with similar texture. Motivated by the insight into the complementary nature possessed by the highly contrastive locations and the smooth locations, we employ a smoothness map to guide the incorporation of intensity-continuity and texture-similarity constraints in the design of our labeling algorithm. Within each coherent region obtained from the color labeling stage, the color mapping is applied to generate vivid colorization effect by assigning colors to a few pixels in the region. A set of intuitive interface tools is designed for labeling, coloring and modifying the result. We demonstrate compelling results of colorizing natural images using our system, with only a modest amount of user input.
Interactive Illumination with Coherent Shadow Maps
(The Eurographics Association, 2007) Ritschel, Tobias; Grosch, Thorsten; Kautz, Jan; Mueller, Stefan; Jan Kautz and Sumanta Pattanaik
We present a new method for interactive illumination computations based on precomputed visibility using coherent shadow maps (CSMs). It is well-known that visibility queries dominate the cost of physically based rendering. Precomputing all visibility events, for instance in the form of many shadow maps, enables fast queries and allows for real-time computation of illumination but requires prohibitive amounts of storage. We propose a lossless compression scheme for visibility information based on shadow maps that efficiently exploits coherence. We demonstrate a Monte Carlo renderer for direct lighting using CSMs that runs entirely on graphics hardware. We support spatially varying BRDFs, normal maps, and environment maps all with high frequencies, spatial as well as angular. Multiple dynamic rigid objects can be combined in a scene. As opposed to precomputed radiance transfer techniques, that assume distant lighting, our method includes distant lighting as well as local area lights of arbitrary shape, varying intensity, or anisotropic light distribution that can freely vary over time.
Scene Collages and Flexible Camera Arrays
(The Eurographics Association, 2007) Nomura, Yoshikuni; Zhang, Li; Nayar, Shree K.; Jan Kautz and Sumanta Pattanaik
This paper presents an automatic method for creating a collage from a collection of photos of a scene taken from different viewpoints. The collage is constructed by aligning the images (in terms of their positions, rotations and scales) using a least-squares formulation. We have developed a graph-based optimization algorithm for layering the images so as to minimize the fragmentation of the collage. A collage can be displayed with opaque layers, with transparent layers, or with blended image boundaries. A scene collage can be viewed as a piece-wise perspective representation of a scene with visible seams. This representation has not only aesthetic value but also conveys scene structure and camera motion in an intuitive way. To capture live-action collages of dynamic scenes we have developed camera arrays that can be physically flexed by the user to continuously vary the composition of the scene. The design of our camera arrays enables a user to reconfigure them in terms of the spatial arrangement of the cameras in a matter of minutes. We show several still and dynamic examples that demonstrate that scene collages provide a new and interesting way to experience scenes.
Photorealistic Image Rendering with Population Monte Carlo Energy Redistribution
(The Eurographics Association, 2007) Lai, Yu-Chi; Fan, Shao Hua; Chenney, Stephen; Dyer, Charcle; Jan Kautz and Sumanta Pattanaik
This work presents a novel global illumination algorithm which concentrates computation on important light transport paths and automatically adjusts energy distributed area for each light transport path. We adapt statistical framework of Population Monte Carlo into global illumination to improve rendering efficiency. Information collected in previous iterations is used to guide subsequent iterations by adapting the kernel function to approximate the target distribution without introducing bias into the final result. Based on this framework, our algorithm automatically adapts the amount of energy redistribution at different pixels and the area over which energy is redistributed. Our results show that the efficiency can be improved by exploring the correlated information among light transport paths.
Material Based Splashing of Water Drops
(The Eurographics Association, 2007) Garg, Kshitiz; Krishnan, Gurunandan G.; Nayar, Shree K.; Jan Kautz and Sumanta Pattanaik
The splashing of a water drop is a fascinating phenomenon that results from a variety of complex interactions between the drop and the material it impacts. In general, the distribution of droplets of a splash depends on the drop size and velocity; the surface roughness, rigidity, and wetness; and the angle of impact. Given the number of factors involved, it is difficult to develop an analytical model for the splash distribution. Instead, we take an empirical approach. We have measured the splashing behaviors of 22 different materials that are commonly found in the real world. These materials can be broadly classified as rough (e.g., wood and brick), smooth (e.g., marble and glass), flexible (e.g., silk and paper), and miscellaneous (e.g., water and moss). We have developed a stochastic model for splash distribution that builds upon empirical models previously developed in fluid dynamics and meteorology. Our model is simple and only requires 7 coefficients for generating splashes for head-on impact for a material. A more general model for generating splashes for arbitrary impact angles (due to surface inclination or wind) requires 54 coefficients. The models of different materials may be combined to generate physically plausible splashes for novel materials that have not been measured. Our model is applicable for rendering splashes due to rain as well as water drops falling from large heights such as windowsills, trees, and rooftops.
Incremental Instant Radiosity for Real-Time Indirect Illumination
(The Eurographics Association, 2007) Laine, Samuli; Saransaari, Hannu; Kontkanen, Janne; Lehtinen, Jaakko; Aila, Timo; Jan Kautz and Sumanta Pattanaik
We present a method for rendering single-bounce indirect illumination in real time on currently available graphics hardware. The method is based on the instant radiosity algorithm, where virtual point lights (VPLs) are generated by casting rays from the primary light source. Hardware shadow maps are then employed for determining the indirect illumination from the VPLs. Our main contribution is an algorithm for reusing the VPLs and incrementally maintaining their good distribution. As a result, only a few shadow maps need to be rendered per frame as long as the motion of the primary light source is reasonably smooth. This yields real-time frame rates even when hundreds of VPLs are used.
Microfacet Models for Refraction through Rough Surfaces
(The Eurographics Association, 2007) Walter, Bruce; Marschner, Stephen R.; Li, Hongsong; Torrance, Kenneth E.; Jan Kautz and Sumanta Pattanaik
Microfacet models have proven very successful for modeling light reflection from rough surfaces. In this paper we review microfacet theory and demonstrate how it can be extended to simulate transmission through rough surfaces such as etched glass. We compare the resulting transmission model to measured data from several real surfaces and discuss appropriate choices for the microfacet distribution and shadowing-masking functions. Since rendering transmission through media requires tracking light that crosses at least two interfaces, good importance sampling is a practical necessity. Therefore, we also describe efficient schemes for sampling the microfacet models and the corresponding probability density functions.
Soft Shadows by Ray Tracing Multilayer Transparent Shadow Maps
(The Eurographics Association, 2007) Xie, Feng; Tabellion, Eric; Pearce, Andrew; Jan Kautz and Sumanta Pattanaik
We present a method for high quality soft shadows for area lights in cinematic lighting. The method is an extension of traditional shadow maps, so it has the advantage of image based shadow methods; the algorithm s complexity is independent of geometric complexity. We introduce multilayer transparent shadow maps, which can be used to produce high quality soft shadows for scenes with extremely complex geometry, fur, and volume objects. Instead of the traditional sampling and filtering of shadow maps, we compute the shadow factor by ray tracing the multilayer transparent shadow map. The result is soft shadows of quality similar to that achieved by stochastic ray tracing, but at a much lower cost.
An Interactive Perceptual Rendering Pipeline using Contrast and Spatial Masking
(The Eurographics Association, 2007) Drettakis, George; Bonneel, Nicolas; Dachsbacher, Carsten; Lefebvre, Sylvain; Schwarz, Michael; Viaud-Delmon, Isabelle; Jan Kautz and Sumanta Pattanaik
We present a new perceptual rendering pipeline which takes into account visual masking due to contrast and spatial frequency. Our framework predicts inter-object, scene-level masking caused by partial occlusion and shadows. It is designed for interactive applications and runs efficiently on the GPU. This is achieved using a layer-based approach together with an efficient GPU-based computation of threshold maps. We build upon this prediction framework to introduce a perceptually-based level of detail control algorithm. We conducted a perceptual user study which indicates that our perceptual pipeline generates results which are consistent with what the user perceives. Our results demonstrate significant quality improvement for scenes with masking due to frequencies and contrast, such as masking due to trees or foliage, or due to high-frequency shadows.
Convolution Shadow Maps
(The Eurographics Association, 2007) Annen, Thomas; Mertens, Tom; Bekaert, Philippe; Seidel, Hans-Peter; Kautz, Jan; Jan Kautz and Sumanta Pattanaik
We present Convolution Shadow Maps, a novel shadow representation that affords efficient arbitrary linear filtering of shadows. Traditional shadow mapping is inherently non-linear w.r.t. the stored depth values, due to the binary shadow test. We linearize the problem by approximating shadow test as a weighted summation of basis terms. We demonstrate the usefulness of this representation, and show that hardware-accelerated anti-aliasing techniques, such as tri-linear filtering, can be applied naturally to Convolution Shadow Maps. Our approach can be implemented very efficiently in current generation graphics hardware, and offers real-time frame rates.
Procedural Editing of Bidirectional Texture Functions
(The Eurographics Association, 2007) Mueller, Gero; Sarlette, Ralf; Klein, Reinhard; Jan Kautz and Sumanta Pattanaik
Measured material representations like Bidirectional Texture Functions or Reflectance Fields offer very realistic appearance but the user is currently not capable of changing this appearance in an effective and intuitive way. Such editing operations would require a low-dimensional but expressive model for appearance that exposes only a small set of intuitively editable parameters (1D-sliders, 2D-maps) to the user but preserves all visually relevant details. In this paper we present a novel editing technique for complex spatially varying materials. It is based on the observation that we are already good in modeling the basic geometric structure of many natural and manmade materials but still have not found effective models for the detailed small-scale geometry and the interaction of light with these materials. Our main idea is to use procedural geometry to define the basic structure of a material and then to enrich this structure with the BTF information captured from real materials. By employing recent algorithms for real-time texture synthesis and BTF compression our technique allows interactive editing.
Instant Sound Scattering
(The Eurographics Association, 2007) Tsingos, Nicolas; Dachsbacher, Carsten; Lefebvre, Sylvain; Dellepiane, Matteo; Jan Kautz and Sumanta Pattanaik
Real-time sound rendering engines often render occlusion and early sound reflection effects using geometrical techniques such as ray or beam tracing. They can only achieve interactive rendering for environments of low local complexity resulting in crude effects which can degrade the sense of immersion. However, surface detail or complex dynamic geometry has a strong influence on sound propagation and the resulting auditory perception. This paper focuses on high-quality modeling of first-order sound scattering. Based on a surface-integral formulation and the Kirchhoff approximation, we propose an efficient evaluation of scattering effects, including both diffraction and reflection, that leverages programmable graphics hardware for dense sampling of complex surfaces. We evaluate possible surface simplification techniques and show that combined normal and displacement maps can be successfully used for audio scattering calculations. We present an auralization framework that can render scattering effects interactively thus providing a more compelling experience. We demonstrate that, while only considering first order phenomena, our approach can provide realistic results for a number of practical interactive applications. It can also process highly detailed models containing millions of unorganized triangles in minutes, generating high-quality scattering filters. Resulting simulations compare well with on-site recordings showing that the Kirchhoff approximation can be used for complex scattering problems.

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