31-Issue 2

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https://diglib.eg.org/handle/10.2312/171

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How Not to Be Seen -- Object Removal from Videos of Crowded Scenes
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Granados, Miguel; Tompkin, James; Kim, Kwang In; Grau, Oliver; Kautz, Jan; Theobalt, Christian; P. Cignoni and T. Ertl
Removing dynamic objects from videos is an extremely challenging problem that even visual effects professionals often solve with time-consuming manual frame-by-frame editing. We propose a new approach to video completion that can deal with complex scenes containing dynamic background and non-periodical moving objects. We build upon the idea that the spatio-temporal hole left by a removed object can be filled with data available on other regions of the video where the occluded objects were visible. Video completion is performed by solving a large combinatorial problem that searches for an optimal pattern of pixel offsets from occluded to unoccluded regions. Our contribution includes an energy functional that generalizes well over different scenes with stable parameters, and that has the desirable convergence properties for a graph-cut-based optimization. We provide an interface to guide the completion process that both reduces computation time and allows for efficient correction of small errors in the result. We demonstrate that our approach can effectively complete complex, high-resolution occlusions that are greater in difficulty than what existing methods have shown.
Unstructured Light Fields
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Davis, Abe; Levoy, Marc; Durand, Fredo; P. Cignoni and T. Ertl
We present a system for interactively acquiring and rendering light fields using a hand-held commodity camera. The main challenge we address is assisting a user in achieving good coverage of the 4D domain despite the challenges of hand-held acquisition. We define coverage by bounding reprojection error between viewpoints, which accounts for all 4 dimensions of the light field. We use this criterion together with a recent Simultaneous Localization and Mapping technique to compute a coverage map on the space of viewpoints. We provide users with real-time feedback and direct them toward under-sampled parts of the light field. Our system is lightweight and has allowed us to capture hundreds of light fields. We further present a new rendering algorithm that is tailored to the unstructured yet dense data we capture. Our method can achieve piecewise-bicubic reconstruction using a triangulation of the captured viewpoints and subdivision rules applied to reconstruction weights.
Crowd Light: Evaluating the Perceived Fidelity of Illuminated Dynamic Scenes
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Jarabo, Adrian; Eyck, Tom Van; Sundstedt, Veronica; Bala, Kavita; Gutierrez, Diego; O'Sullivan, Carol; P. Cignoni and T. Ertl
Rendering realistic illumination effects for complex animated scenes with many dynamic objects or characters is computationally expensive. Yet, it is not obvious how important such accurate lighting is for the overall perceived realism in these scenes. In this paper, we present a methodology to evaluate the perceived fidelity of illumination in scenes with dynamic aggregates, such as crowds, and explore several factors which may affect this perception. We focus in particular on evaluating how a popular spherical harmonics lighting method can be used to approximate realistic lighting of crowds. We conduct a series of psychophysical experiments to explore how a simple approach to approximating global illumination, using interpolation in the temporal domain, affects the perceived fidelity of dynamic scenes with high geometric, motion, and illumination complexity. We show that the complexity of the geometry and temporal properties of the crowd entities, the motion of the aggregate as a whole, the type of interpolation (i.e., of the direct and/or indirect illumination coefficients), and the presence or absence of colour all affect perceived fidelity. We show that high (i.e., above 75%) levels of perceived scene fidelity can be maintained while interpolating indirect illumination for intervals of up to 30 frames, resulting in a greater than three-fold rendering speed-up
Creating Picture Legends for Group Photos
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Gao, Junhong; Kim, Seon Joo; Brown, Michael S.; P. Cignoni and T. Ertl
Group photos are one of the most common types of digital images found in personal image collections and on social networks. One typical post-processing task for group photos is to produce a key or legend to identify the people in the photo. This is most often done using simple bounding boxes. A more professional approach is to create a picture legend that uses either a full or partial silhouette to identify the individuals. This paper introduces an efficient method for producing picture legends for group photos. Our approach combines face detection with human shape priors into an interactive selection framework to allow users to quickly segment the individuals in a group photo. Our results are better than those obtained by general selection tools and can be produced in a fraction of the time.
Real-Time Disparity Map-Based Pictorial Depth Cue Enhancement
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Rößing, Christoph; Hanika, Johannes; Lensch, Hendrik; P. Cignoni and T. Ertl
The availability of stereoscopic image material is increasing rapidly. In contrast to the generation of distance information, displaying it is still a challenging task. To overcome the need for special 3D display hardware, we present a novel real-time video processing framework-based on edge-avoiding à trous wavelets. The framework adds and emphasizes monocular depth cues corresponding to the depth information of a supplemental disparity map. This creates a compelling depth sensation on 2D display devices. The framework enhances multiple depth cues in parallel, such as depth of field, local contrast, ambient occlusion and saturation. At the same time, it improves the disparity map quality. Depth cues control how a human explores an image, since the perception of distance is coupled to visual attention. The presented work demonstrates the effectiveness of the proposed framework in guiding the viewer, without destroying the image content, by evaluating the performance in searchand- find tasks. A user study analyzes the connection between faster response times and the boosting of particular monocular depth cues.
Perceptually Linear Parameter Variations
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Lindow, Norbert; Baum, Daniel; Hege, Hans-Christian; P. Cignoni and T. Ertl
Most visual analysis tasks require interactive adjustment of parameter values. In general, a linear variation of a parameter, using for instance a GUI slider, changes the visual result in a perceptually non-linear way. This hampers interactive adjustment of parameters, especially in regions where rapid perceptual changes occur. Selecting a good parameter value therefore remains a time-consuming and often difficult task. We propose a novel technique to build a non-linear function that maps a new parameter to the original parameter. By prefixing this function to the original parameter and using the new parameter as input, a linear relationship between input and visual feedback is obtained. To construct the non-linear function, we measure the variation of the visual result using image metrics. Given a suitable perceptual image metric, perceptually linear image variations are achieved. We demonstrate the practical utility of our approach by implementing two common image metrics, a perceptual and a non-perceptual one, and by applying the method to a few visual analysis tasks.
Pixel Art with Refracted Light by Rearrangeable Sticks
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Yue, Yonghao; Iwasaki, Kei; Chen, Bing-Yu; Dobashi, Yoshinori; Nishita, Tomoyuki; P. Cignoni and T. Ertl
Pixel art is a kind of digital art that through per-pixel manipulation enables production of a diverse array of artistic images. In this paper, we present a new way for people to experience and express pixel art. Our digital art consists of a set of sticks made of acrylate resin, each of which refracts light from a parallel light source, in certain directions. Artistic users are able to easily rearrange these sticks and view their digital art through the refracted light projection on any planar surface. As we demonstrate in this paper, a user can generate various artistic images using only a single set of sticks. We additionally envision that our pixel art with rearrangeable sticks would have great entertainment appeal, e.g., as an art puzzle.
A Computational Model of Afterimages
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Ritschel, Tobias; Eisemann, Elmar; P. Cignoni and T. Ertl
Afterimages are optical illusions, particularly well perceived when fixating an image for an extended period of time and then looking at a neutral background, where an inverted copy of the original stimulus appears. The full mechanism that produces the perceived specific colors and shapes is complex and not entirely understood, but most of the important attributes can be well explained by bleaching of retinal photoreceptors (retinal kinetics). We propose a model to compute afterimages that allows us to simulate their temporal, color and time-frequency behavior. Using this model, high dynamic range (HDR) content can be processed to add realistic afterimages to low dynamic range (LDR) media. Hereby, our approach helps in conveying the original source's luminance and contrast. It can be applied in real-time on full-HD HDR content using standard graphics hardware. Finally, our approach is validated in a perceptual study.
A Simple Algorithm for Maximal Poisson-Disk Sampling in High Dimensions
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Ebeida, Mohamed S.; Mitchell, Scott A.; Patney, Anjul; Davidson, Andrew A.; Owens, John D.; P. Cignoni and T. Ertl
We provide a simple algorithm and data structures for d-dimensional unbiased maximal Poisson-disk sampling. We use an order of magnitude less memory and time than the alternatives. Our results become more favorable as the dimension increases. This allows us to produce bigger samplings. Domains may be non-convex with holes. The generated point cloud is maximal up to round-off error. The serial algorithm is provably bias-free. For an output sampling of size n in fixed dimension d, we use a linear memory budget and empirical O(n) runtime. No known methods scale well with dimension, due to the ''curse of dimensionality.'' The serial algorithm is practical in dimensions up to 5, and has been demonstrated in 6d. We have efficient GPU implementations in 2d and 3d. The algorithm proceeds through a finite sequence of uniform grids. The grids guide the dart throwing and track the remaining disk-free area. The top-level grid provides an efficient way to test if a candidate dart is disk-free. Our uniform grids are like quadtrees, except we delay splits and refine all leaves at once. Since the quadtree is flat it can be represented using very little memory: we just need the indices of the active leaves and a global level. Also it is very simple to sample from leaves with uniform probability.
Interactive Coherence-Based Façade Modeling
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Musialski, Przemyslaw; Wimmer, Michael; Wonka, Peter; P. Cignoni and T. Ertl
We propose a novel interactive framework for modeling building façades from images. Our method is based on the notion of coherence-based editing which allows exploiting partial symmetries across the façade at any level of detail. The proposed workflow mixes manual interaction with automatic splitting and grouping operations based on unsupervised cluster analysis. In contrast to previous work, our approach leads to detailed 3d geometric models with up to several thousand regions per façade. We compare our modeling scheme to others and evaluate our approach in a user study with an experienced user and several novice users.
Exploring Shape Variations by 3D-Model Decomposition and Part-based Recombination
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Jain, Arjun; Thormählen, Thorsten; Ritschel, Tobias; Seidel, Hans-Peter; P. Cignoni and T. Ertl
We present a system that allows new shapes to be created by blending between shapes taken from a database. We treat the shape as a composition of parts; blending is performed by recombining parts from different shapes according to constraints deduced by shape analysis. The analysis involves shape segmentation, contact analysis, and symmetry detection. The system can be used to rapidly instantiate new models that have similar symmetry and adjacency structure to the database shapes, yet vary in appearance.
Manipulation of Flexible Objects by Geodesic Control
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Wang, He; Komura, Taku; P. Cignoni and T. Ertl
We propose an effective and intuitive method for controlling flexible models such as ropes and cloth. Automating manipulation of such flexible objects is not an easy task due to the high dimensionality of the objects and the low dimensionality of the control. In order to cope with this problem, we introduce a method called Geodesic Control, which greatly helps to manipulate flexible objects. The core idea is to decrease the degrees of freedom of the flexible object by moving it along the geodesic line of the object that it is interacting with. By repeatedly applying this control, users can easily synthesize animations of twisting and knotting a piece of rope or wrapping a cloth around an object. We show examples of ''furoshiki wrapping'', in which an object is wrapped by a cloth by a series of maneuvers based on Geodesic Control. As our representation can abstract such maneuvers well, the procedure designed by a user can be re-applied for different combinations of cloth and an object. The method is applicable not only for computer animation but also for 3D computer games and virtual reality systems.
Coherent Spatiotemporal Filtering, Upsampling and Rendering of RGBZ Videos
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Richardt, Christian; Stoll, Carsten; Dodgson, Neil A.; Seidel, Hans-Peter; Theobalt, Christian; P. Cignoni and T. Ertl
Sophisticated video processing effects require both image and geometry information. We explore the possibility to augment a video camera with a recent infrared time-of-flight depth camera, to capture high-resolution RGB and low-resolution, noisy depth at video frame rates. To turn such a setup into a practical RGBZ video camera, we develop efficient data filtering techniques that are tailored to the noise characteristics of IR depth cameras. We first remove typical artefacts in the RGBZ data and then apply an efficient spatiotemporal denoising and upsampling scheme. This allows us to record temporally coherent RGBZ videos at interactive frame rates and to use them to render a variety of effects in unprecedented quality. We show effects such as video relighting, geometry-based abstraction and stylisation, background segmentation and rendering in stereoscopic 3D.
Rationalization of Triangle-Based Point-Folding Structures
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Zimmer, Henrik; Campen, Marcel; Bommes, David; Kobbelt, Leif; P. Cignoni and T. Ertl
In mechanical engineering and architecture, structural elements with low material consumption and high loadbearing capabilities are essential for light-weight and even self-supporting constructions. This paper deals with so called point-folding elements - non-planar, pyramidal panels, usually formed from thin metal sheets, which exploit the increased structural capabilities emerging from folds or creases. Given a triangulated free-form surface, a corresponding point-folding structure is a collection of pyramidal elements basing on the triangles. User-specified or material-induced geometric constraints often imply that each individual folding element has a different shape, leading to immense fabrication costs. We present a rationalization method for such structures which respects the prescribed aesthetic and production constraints and finds a minimal set of molds for the production process, leading to drastically reduced costs. For each base triangle we compute and parametrize the range of feasible folding elements that satisfy the given constraints within the allowed tolerances. Then we pose the rationalization task as a geometric intersection problem, which we solve so as to maximize the re-use of mold dies. Major challenges arise from the high precision requirements and the non-trivial parametrization of the search space. We evaluate our method on a number of practical examples where we achieve rationalization gains of more than 90%.
Medial Kernels
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Berger, Matthew; Silva, Claudio T.; P. Cignoni and T. Ertl
We introduce the medial kernel, an association measure which provides for a robust construction of volume-aware distances defined directly on point clouds. The medial kernel is a similarity measure defined as the likelihood of two points belonging to a common interior medial ball. We use the medial kernel to construct a random walk on the point cloud, where movement in the walk is restricted to regions containing similar medial balls. Our distances are defined as the diffusion distances of this random walk, assigning low distance to points belonging to similar medial regions. These distances allow for a robust means of processing incomplete point clouds, capable of distinguishing nearby yet separate undersampled components, while also associating points which are far in Euclidean distance yet mutually share an interior volume. We leverage these distances for several applications: volumetric part segmentation, the construction of function bases, and reconstruction-by-parts - a surface reconstruction method which adheres to the medial kernel.
Novel-View Synthesis of Outdoor Sport Events Using an Adaptive View-Dependent Geometry
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Germann, Marcel; Popa, Tiberiu; Keiser, Richard; Ziegler, Remo; Gross, Markus; P. Cignoni and T. Ertl
We propose a novel fully automatic method for novel-viewpoint synthesis. Our method robustly handles multicamera setups featuring wide-baselines in an uncontrolled environment. In a first step, robust and sparse point correspondences are found based on an extension of the Daisy features [TLF10]. These correspondences together with back-projection errors are used to drive a novel adaptive coarse to fine reconstruction method, allowing to approximate detailed geometry while avoiding an extreme triangle count. To render the scene from arbitrary viewpoints we use a view-dependent blending of color information in combination with a view-dependent geometry morph. The view-dependent geometry compensates for misalignments caused by calibration errors. We demonstrate that our method works well under arbitrary lighting conditions with as little as two cameras featuring wide-baselines. The footage taken from real sports broadcast events contains fine geometric structures, which result in nice novel-viewpoint renderings despite of the low resolution in the images.
Real-time Rendering of Dynamic Scenes under All-frequency Lighting using Integral Spherical Gaussian
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Iwasaki, Kei; Furuya, Wataru; Dobashi, Yoshinori; Nishita, Tomoyuki; P. Cignoni and T. Ertl
We propose an efficient rendering method for dynamic scenes under all-frequency environmental lighting. To render the surfaces of objects illuminated by distant environmental lighting, the triple product of the lighting, the visibility function and the BRDF is integrated at each shading point on the surfaces. Our method represents the environmental lighting and the BRDF with a linear combination of spherical Gaussians, replacing the integral of the triple product with the sum of the integrals of spherical Gaussians over the visible region of the hemisphere. We propose a new form of spherical Gaussian, the integral spherical Gaussian, that enables the fast and accurate integration of spherical Gaussians with various sharpness over the visible region on the hemisphere. The integral spherical Gaussian simplifies the integration to a sum of four pre-integrated values, which are easily evaluated on-the-fly. With a combination of a set of spheres to approximate object geometries and the integral spherical Gaussian, our method can render object surfaces very efficiently. Our GPU implementation demonstrates realtime rendering of dynamic scenes with dynamic viewpoints, lighting, and BRDFs.
Mesh Colorization
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Leifman, George; Tal, Ayellet; P. Cignoni and T. Ertl
This paper proposes a novel algorithm for colorization of meshes. This is important for applications in which the model needs to be colored by just a handful of colors or when no relevant image exists for texturing the model. For instance, archaeologists argue that the great Roman or Greek statues were full of color in the days of their creation, and traces of the original colors can be found. In this case, our system lets the user scribble some desired colors in various regions of the mesh. Colorization is then formulated as a constrained quadratic optimization problem, which can be readily solved. Special care is taken to avoid color bleeding between regions, through the definition of a new direction field on meshes.
Coding Depth through Mask Structure
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Fortunato, Horacio E.; Oliveira, Manuel M.; P. Cignoni and T. Ertl
We present a coded-aperture method based on a family of masks obtained as the convolution of one "hole" with a structural component consisting of an arrangement of Dirac delta functions. We call the arrangement of delta functions the structural component of the mask, and use it to efficiently encode scene distance information. We illustrate the potential of our approach by analyzing a family of masks defined by a circular hole component and a structural component consisting of a linear combination of three Dirac deltas. We show that the structural component transitions from well conditioned to ill conditioned as the relative weight of the central peak varies with respect to the lateral ones. For the well-conditioned structural components, deconvolution is efficiently performed by inverse filtering, allowing for fast estimation of scene depth. We demonstrate the effectiveness of our approach by constructing a mask for distance coding and using it to recover pairs of distance maps and structurally-deconvolved images from single photographs. For this application, we obtain significant speedup, and extended range and depth resolution compared to previous techniques.
Analytic Anti-Aliasing of Linear Functions on Polytopes
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Auzinger, Thomas; Guthe, Michael; Jeschke, Stefan; P. Cignoni and T. Ertl
This paper presents an analytic formulation for anti-aliased sampling of 2D polygons and 3D polyhedra. Our framework allows the exact evaluation of the convolution integral with a linear function defined on the polytopes. The filter is a spherically symmetric polynomial of any order, supporting approximations to refined variants such as the Mitchell-Netravali filter family. This enables high-quality rasterization of triangles and tetrahedra with linearly interpolated vertex values to regular and non-regular grids. A closed form solution of the convolution is presented and an efficient implementation on the GPU using DirectX and CUDA C is described.

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