Full Papers 2011 - CGF 30-Issue 2

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https://diglib.eg.org/handle/10.2312/13883

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Blur-Aware Image Downsampling
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Trentacoste, Matthew; Mantiuk, Rafal; Heidrich, Wolfgang; M. Chen and O. Deussen
Resizing to a lower resolution can alter the appearance of an image. In particular, downsampling an image causes blurred regions to appear sharper. It is useful at times to create a downsampled version of the image that gives the same impression as the original, such as for digital camera viewfinders. To understand the effect of blur on image appearance at different image sizes, we conduct a perceptual study examining how much blur must be present in a downsampled image to be perceived the same as the original. We find a complex, but mostly image-independent relationship between matching blur levels in images at different resolutions. The relationship can be explained by a model of the blur magnitude analyzed as a function of spatial frequency. We incorporate this model in a new appearance-preserving downsampling algorithm, which alters blur magnitude locally to create a smaller image that gives the best reproduction of the original image appearance.
BSSRDF Estimation from Single Images
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Munoz, Adolfo; Echevarria, Jose I.; Seron, Francisco J.; Lopez-Moreno, Jorge; Glencross, Mashhuda; Gutierrez, Diego; M. Chen and O. Deussen
We present a novel method to estimate an approximation of the reflectance characteristics of optically thick, homogeneous translucent materials using only a single photograph as input. First, we approximate the diffusion profile as a linear combination of piecewise constant functions, an approach that enables a linear system minimization and maximizes robustness in the presence of suboptimal input data inferred from the image. We then fit to a smoother monotonically decreasing model, ensuring continuity on its first derivative. We show the feasibility of our approach and validate it in controlled environments, comparing well against physical measurements from previous works. Next, we explore the performance of our method in uncontrolled scenarios, where neither lighting nor geometry are known. We show that these can be roughly approximated from the corresponding image by making two simple assumptions: that the object is lit by a distant light source and that it is globally convex, allowing us to capture the visual appearance of the photographed material. Compared with previous works, our technique offers an attractive balance between visual accuracy and ease of use, allowing its use in a wide range of scenarios including off-the-shelf, single images, thus extending the current repertoire of real-world data acquisition techniques.
Combinatorial Bidirectional Path-Tracing for Efficient Hybrid CPU/GPU Rendering
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Pajot, Anthony; Barthe, Loïc; Paulin, Mathias; Poulin, Pierre; M. Chen and O. Deussen
This paper presents a reformulation of bidirectional path-tracing that adequately divides the algorithm into processes efficiently executed in parallel on both the CPU and the GPU. We thus benefit from high-level optimization techniques such as double buffering, batch processing, and asyncronous execution, as well as from the exploitation of most of the CPU, GPU, and memory bus capabilities. Our approach, while avoiding pure GPU implementation limitations (such as limited complexity of shaders, light or camera models, and processed scene data sets), is more than ten times faster than standard bidirectional path-tracing implementations, leading to performance suitable for production-oriented rendering engines.
Component-wise Controllers for Structure-Preserving Shape Manipulation
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Zheng, Youyi; Fu, Hongbo; Cohen-Or, Daniel; Au, Oscar Kin-Chung; Tai, Chiew-Lan; M. Chen and O. Deussen
Recent shape editing techniques, especially for man-made models, have gradually shifted focus from maintaining local, low-level geometric features to preserving structural, high-level characteristics like symmetry and parallelism. Such new editing goals typically require a pre-processing shape analysis step to enable subsequent shape editing. Observing that most editing of shapes involves manipulating their constituent components, we introduce component-wise controllers that are adapted to the component characteristics inferred from shape analysis. The controllers capture the natural degrees of freedom of individual components and thus provide an intuitive user interface for editing. A typical model usually results in a moderate number of controllers, allowing easy establishment of semantic relations among them by automatic shape analysis supplemented with user interaction. We propose a component-wise propagation algorithm to automatically preserve the established inter-relations while maintaining the defining characteristics of individual controllers and respecting the user-specified modeling constraints. We extend these ideas to a hierarchical setup, allowing the user to adjust the tool complexity with respect to the desired modeling complexity. We demonstrate the effectiveness of our technique on a wide range of manmade models with structural features, often containing multiple connected pieces.
Comprehensive Facial Performance Capture
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Fyffe, Graham; Hawkins, Tim; Watts, Chris; Ma, Wan-Chun; Debevec, Paul; M. Chen and O. Deussen
We present a system for recording a live dynamic facial performance, capturing highly detailed geometry and spatially varying diffuse and specular reflectance information for each frame of the performance. The result is a reproduction of the performance that can be rendered from novel viewpoints and novel lighting conditions, achieving photorealistic integration into any virtual environment. Dynamic performances are captured directly, without the need for any template geometry or static geometry scans, and processing is completely automatic, requiring no human input or guidance. Our key contributions are a heuristic for estimating facial reflectance information from gradient illumination photographs, and a geometry optimization framework that maximizes a principled likelihood function combining multi-view stereo correspondence and photometric stereo, using multiresolution belief propagation. The output of our system is a sequence of geometries and reflectance maps, suitable for rendering in off-the-shelf software. We show results from our system rendered under novel viewpoints and lighting conditions, and validate our results by demonstrating a close match to ground truth photographs
Computer-Suggested Facial Makeup
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Scherbaum, Kristina; Ritschel, Tobias; Hullin, Matthias; Thormählen, Thorsten; Blanz, Volker; Seidel, Hans-Peter; M. Chen and O. Deussen
Finding the best makeup for a given human face is an art in its own right. Experienced makeup artists train for years to be skilled enough to propose a best-fit makeup for an individual. In this work we propose a system that automates this task. We acquired the appearance of 56 human faces, both without and with professional makeup. To this end, we use a controlled-light setup, which allows to capture detailed facial appearance information, such as diffuse reflectance, normals, subsurface-scattering, specularity, or glossiness. A 3D morphable face model is used to obtain 3D positional information and to register all faces into a common parameterization. We then define makeup to be the change of facial appearance and use the acquired database to find a mapping from the space of human facial appearance to makeup. Our main application is to use this mapping to suggest the best-fit makeup for novel faces that are not in the database. Further applications are makeup transfer, automatic rating of makeup, makeup-training, or makeup-exaggeration. As our makeup representation captures a change in reflectance and scattering, it allows us to synthesize faces with makeup in novel 3D views and novel lighting with high realism. The effectiveness of our approach is further validated in a user-study.
Contouring Discrete Indicator Functions
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Manson, Josiah; Smith, Jason; Schaefer, Scott; M. Chen and O. Deussen
We present a method for calculating the boundary of objects from Discrete Indicator Functions that store 2- material volume fractions with a high degree of accuracy. Although Marching Cubes and its derivatives are effective methods for calculating contours of functions sampled over discrete grids, these methods perform poorly when contouring non-smooth functions such as Discrete Indicator Functions. In particular, Marching Cubes will generate surfaces that exhibit aliasing and oscillations around the exact surface. We derive a simple solution to remove these problems by using a new function to calculate the positions of vertices along cell edges that is efficient, easy to implement, and does not require any optimization or iteration. Finally, we provide empirical evidence that the error introduced by our contouring method is significantly less than is introduced by Marching Cubes.
Deformable Motion: Squeezing into Cluttered Environments
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Choi, Myung Geol; Kim, Manmyung; Hyun, Kyung Lyul; Lee, Jehee; M. Chen and O. Deussen
We present an interactive method that allows animated characters to navigate through cluttered environments. Our characters are equipped with a variety of motion skills to clear obstacles, narrow passages, and highly constrained environment features. Our control method incorporates a behavior model into well-known, standard path planning algorithms. Our behavior model, called deformable motion, consists of a graph of motion capture fragments. The key idea of our approach is to add flexibility on motion fragments such that we can situate them into a cluttered environment via constraint-based formulation. We demonstrate our deformable motion for realtime interactive navigation and global path planning in highly constrained virtual environments.
Discovery of Intrinsic Primitives on Triangle Meshes
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Solomon, Justin; Ben-Chen, Mirela; Butscher, Adrian; Guibas, Leonidas; M. Chen and O. Deussen
The discovery of meaningful parts of a shape is required for many geometry processing applications, such as parameterization, shape correspondence, and animation. It is natural to consider primitives such as spheres, cylinders and cones as the building blocks of shapes, and thus to discover parts by fitting such primitives to a given surface. This approach, however, will break down if primitive parts have undergone almost-isometric deformations, as is the case, for example, for articulated human models. We suggest that parts can be discovered instead by finding intrinsic primitives, which we define as parts that posses an approximate intrinsic symmetry. We employ the recently-developed method of computing discrete approximate Killing vector fields (AKVFs) to discover intrinsic primitives by investigating the relationship between the AKVFs of a composite object and the AKVFs of its parts. We show how to leverage this relationship with a standard clustering method to extract k intrinsic primitives and remaining asymmetric parts of a shape for a given k. We demonstrate the value of this approach for identifying the prominent symmetry generators of the parts of a given shape. Additionally, we show how our method can be modified slightly to segment an entire surface without marking asymmetric connecting regions and compare this approach to state-of-the-art methods using the Princeton Segmentation Benchmark.
Dynamic Display of BRDFs
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Hullin, Matthias B.; Lensch, Hendrik P. A.; Raskar, Ramesh; Seidel, Hans-Peter; Ihrke, Ivo; M. Chen and O. Deussen
This paper deals with the challenge of physically displaying reflectance, i.e., the appearance of a surface and its variation with the observer position and the illuminating environment. This is commonly described by the bidirectional reflectance distribution function (BRDF). We provide a catalogue of criteria for the display of BRDFs, and sketch a few orthogonal approaches to solving the problem in an optically passive way. Our specific implementation is based on a liquid surface, on which we excite waves in order to achieve a varying degree of anisotropic roughness. The resulting probability density function of the surface normal is shown to follow a Gaussian distribution similar to most established BRDF models.
Estimating Color and Texture Parameters for Vector Graphics
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Jeschke, Stefan; Cline, David; Wonka, Peter; M. Chen and O. Deussen
Diffusion curves are a powerful vector graphic representation that stores an image as a set of 2D Bezier curves with colors defined on either side. These colors are diffused over the image plane, resulting in smooth color regions as well as sharp boundaries. In this paper, we introduce a new automatic diffusion curve coloring algorithm. We start by defining a geometric heuristic for the maximum density of color control points along the image curves. Following this, we present a new algorithm to set the colors of these points so that the resulting diffused image is as close as possible to a source image in a least squares sense. We compare our coloring solution to the existing one which fails for textured regions, small features, and inaccurately placed curves. The second contribution of the paper is to extend the diffusion curve representation to include texture details based on Gabor noise. Like the curves themselves, the defined texture is resolution independent, and represented compactly. We define methods to automatically make an initial guess for the noise texure, and we provide intuitive manual controls to edit the parameters of the Gabor noise. Finally, we show that the diffusion curve representation itself extends to storing any number of attributes in an image, and we demonstrate this functionality with image stippling an hatching applications.
Freehand HDR Imaging of Moving Scenes with Simultaneous Resolution Enhancement
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Zimmer, Henning; Bruhn, Andrés; Weickert, Joachim; M. Chen and O. Deussen
Despite their high popularity, common high dynamic range (HDR) methods are still limited in their practical applicability: They assume that the input images are perfectly aligned, which is often violated in practise. Our paper does not only free the user from this unrealistic limitation, but even turns the missing alignment into an advantage: By exploiting the multiple exposures, we can create a super-resolution image. The alignment step is performed by a modern energy-based optic flow approach that takes into account the varying exposure conditions. Moreover, it produces dense displacement fields with subpixel precision. As a consequence, our approach can handle arbitrary complex motion patterns, caused by severe camera shake and moving objects. Additionally, it benefits from several advantages over existing strategies: (i) It is robust under outliers (noise, occlusions, saturation problems) and allows for sharp discontinuities in the displacement field. (ii) The alignment step neither requires camera calibration nor knowledge of the exposure times. (iii) It can be efficiently implemented on CPU and GPU architectures. After the alignment is performed, we use the obtained subpixel accurate displacement fields as input for an energy-based, joint super-resolution and HDR (SR-HDR) approach. It introduces robust data terms and anisotropic smoothness terms in the SR-HDR literature. Our experiments with challenging real world data demonstrate that these novelties are pivotal for the favourable performance of our approach.
Fruit Senescence and Decay Simulation
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Jr., Joseph T. Kider; Raja, Samantha; Badler, Norman I.; M. Chen and O. Deussen
Aging and imperfections provide important visual cues for realism. We present a novel physically-based approach for simulating the biological aging and decay process in fruits. This method simulates interactions between multiple processes. Our biologically-derived, reaction-diffusion model generates growth patterns for areas of fungal and bacterial infection. Fungal colony spread and propagation is affected by both bacterial growth and nutrient depletion. This process changes the physical properties of the surface of the fruit as well as its internal volume substrate. The fruit is physically simulated with parameters such as skin thickness and porosity, water content, flesh rigidity, ambient temperature, humidity, and proximity to other surfaces. Our model produces a simulation that closely mirrors the progression of decay in real fruits under similar parameterized conditions. Additionally, we provide a tool that allows artists to customize the input of the program to produce generalized fruit simulations.
Generalized Helicoids for Modeling Hair Geometry
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Piuze, Emmanuel; Kry, Paul G.; Siddiqi, Kaleem; M. Chen and O. Deussen
In computer graphics, modeling the geometry of hair and hair-like patterns such as grass and fur remains a significant challenge. Hair strands can exist in an extensive variety of arrangements and the choice of an appropriate representation for tasks such as hair synthesis, fitting, editing, or reconstruction from samples, is non-trivial. To support such applications we present a novel mathematical representation of hair based on a class of minimal surfaces called generalized helicoids. This representation allows us to characterize the geometry of a single hair strand, as well as of those in its vicinity, by three intuitive curvature parameters and an elevation angle. We introduce algorithms for fitting piecewise generalized helicoids to unparameterized hair strands, and for interpolating hair between these fits. We showcase several applications of this representation including the synthesis of different hair geometries, wisp generation, hair interpolation from samples and hair-style parametrization and reconstruction from real hair data.
GeoBrush: Interactive Mesh Geometry Cloning
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Takayama, Kenshi; Schmidt, Ryan; Singh, Karan; Igarashi, Takeo; Boubekeur, Tamy; Sorkine, Olga; M. Chen and O. Deussen
We propose a method for interactive cloning of 3D surface geometry using a paintbrush interface, similar to the continuous cloning brush popular in image editing. Existing interactive mesh composition tools focus on atomic copy-and-paste of pre-selected feature areas, and are either limited to copying surface displacements, or require the solution of variational optimization problems, which is too expensive for an interactive brush interface. In contrast, our GeoBrush method supports real-time continuous copying of arbitrary high-resolution surface features between irregular meshes, including topological handles. We achieve this by first establishing a correspondence between the source and target geometries using a novel generalized discrete exponential map parameterization. Next we roughly align the source geometry with the target shape using Green Coordinates with automaticallyconstructed cages. Finally, we compute an offset membrane to smoothly blend the pasted patch with C1 continuity before stitching it into the target. The offset membrane is a solution of a bi-harmonic PDE, which is computed on the GPU in real time by exploiting the regular parametric domain. We demonstrate the effectiveness of GeoBrush with various editing scenarios, including detail enrichment and completion of scanned surfaces.
Global Structure Optimization of Quadrilateral Meshes
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Bommes, David; Lempfer, Timm; Kobbelt, Leif; M. Chen and O. Deussen
We introduce a fully automatic algorithm which optimizes the high-level structure of a given quadrilateral mesh to achieve a coarser quadrangular base complex. Such a topological optimization is highly desirable, since stateof- the-art quadrangulation techniques lead to meshes which have an appropriate singularity distribution and an anisotropic element alignment, but usually they are still far away from the high-level structure which is typical for carefully designed meshes manually created by specialists and used e.g. in animation or simulation. In this paper we show that the quality of the high-level structure is negatively affected by helical configurations within the quadrilateral mesh. Consequently we present an algorithm which detects helices and is able to remove most of them by applying a novel grid preserving simplification operator (GP-operator) which is guaranteed to maintain an all-quadrilateral mesh. Additionally it preserves the given singularity distribution and in particular does not introduce new singularities. For each helix we construct a directed graph in which cycles through the start vertex encode operations to remove the corresponding helix. Therefore a simple graph search algorithm can be performed iteratively to remove as many helices as possible and thus improve the high-level structure in a greedy fashion. We demonstrate the usefulness of our automatic structure optimization technique by showing several examples with varying complexity.
Goal-based Caustics
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Papas, Marios; Jarosz, Wojciech; Jakob, Wenzel; Rusinkiewicz, Szymon; Matusik, Wojciech; Weyrich, Tim; M. Chen and O. Deussen
We propose a novel system for designing and manufacturing surfaces that produce desired caustic images when illuminated by a light source. Our system is based on a nonnegative image decomposition using a set of possibly overlapping anisotropic Gaussian kernels. We utilize this decomposition to construct an array of continuous surface patches, each of which focuses light onto one of the Gaussian kernels, either through refraction or reflection. We show how to derive the shape of each continuous patch and arrange them by performing a discrete assignment of patches to kernels in the desired caustic. Our decomposition provides for high fidelity reconstruction of natural images using a small collection of patches. We demonstrate our approach on a wide variety of caustic images by manufacturing physical surfaces with a small number of patches.
Guided ProceduralModeling
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Bene , Bedrich; tava, Ondrej; Mech, Radomir; Miller, Gavin; M. Chen and O. Deussen
Procedural methods present one of the most powerful techniques for authoring a vast variety of computer graphics models. However, their massive applicability is hindered by the lack of control and a low predictability of the results. In the classical procedural modeling pipeline, the user usually defines a set of rules, executes the procedural system, and by examining the results attempts to infer what should be changed in the system definition in order to achieve the desired output. We present guided procedural modeling, a new approach that allows a high level of top-down control by breaking the system into smaller building blocks that communicate. In our work we generalize the concept of the environment. The user creates a set of guides. Each guide defines a region in which a specific procedural model operates. These guides are connected by a set of links that serve for message passing between the procedural models attached to each guide. The entire model consists of a set of guides with procedural models, a graph representing their connection, and the method in which the guides interact. The modeling process is performed by modifying each of the described elements. The user can control the high-level description by editing the guides or manipulate the low-level description by changing the procedural rules. Changing the connectivity allows the user to create new complex forms in an easy and intuitive way. We show several examples of procedural structures, including an ornamental pattern, a street layout, a bridge, and a model of trees. We also demonstrate interactive examples for quick and intuitive editing using physics-based mass-spring system.
Hatching for Motion Picture Production
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Umenhoffer, Tamás; Szécsi, László; Szirmay-Kalos, László; M. Chen and O. Deussen
This paper presents a hatching algorithm which - while also allows for an implementation in real-time - is integrated into the production pipeline of computer generated motion picture. Motion picture production pipelines impose special functional and quality requirements. From the functional point of view, the stages of modeling, 3D rendering, and compositing form a pipeline without feed-back, and frames are rendered independently, possibly on different computers. Thus, no temporal data can be shared between them while flicker free animation needs to be generated. Quality requirements can be grasped as that of dual consistency: the generated hatching must consistently follow object movement and deformation, and, at the same time, it should have a consistent pattern and density in image-space to provide the hand-crafted look. In order to meet both requirements, we apply a particle based method and develop an image-space density control mechanism using rejection sampling and low- discrepancy sequences. We also discuss the decomposition of rendering tasks according to the main stages of the production pipeline and demonstrate how the artist can define the illustration style in a convenient way.
Image and Video Abstraction by Coherence-Enhancing Filtering
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Kyprianidis, Jan Eric; Kang, Henry; M. Chen and O. Deussen
In this work, we present a non-photorealistic rendering technique to create stylized abstractions from color images and videos. Our approach is based on adaptive line integral convolution in combination with directional shock filtering. The smoothing process regularizes directional image features while the shock filter provides a sharpening effect. Both operations are guided by a flow field derived from the structure tensor. To obtain a high-quality flow field, we present a novel smoothing scheme for the structure tensor based on Poisson's equation. Our approach effectively regularizes anisotropic image regions while preserving the overall image structure and achieving a consistent level of abstraction. Moreover, it is suitable for per-frame filtering of video and can be efficiently implemented to process content in real-time.

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