Volume 35 (2016)

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Forward Light Cuts: A Scalable Approach to Real-Time Global Illumination
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Laurent, Gilles; Delalandre, Cyril; Rivière, Grégoire de La; Boubekeur, Tamy; Elmar Eisemann and Eugene Fiume
We present Forward Light Cuts, a novel approach to real-time global illumination using forward rendering techniques. We focus on unshadowed diffuse interactions for the first indirect light bounce in the context of large models such as the complex scenes usually encountered in CAD application scenarios. Our approach efficiently generates and uses a multiscale radiance cache by exploiting the geometry-specific stages of the graphics pipeline, namely the tessellator unit and the geometry shader. To do so, we assimilate virtual point lights to the scene's triangles and design a stochastic decimation process chained with a partitioning strategy that accounts for both close-by strong light reflections, and distant regions from which numerous virtual point lights collectively contribute strongly to the end pixel. Our probabilistic solution is supported by a mathematical analysis and a number of experiments covering a wide range of application scenarios. As a result, our algorithm requires no precomputation of any kind, is compatible with dynamic view points, lighting condition, geometry and materials, and scales to tens of millions of polygons on current graphics hardware.
Identifying Style of 3D Shapes using Deep Metric Learning
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Lim, Isaak; Gehre, Anne; Kobbelt, Leif; Maks Ovsjanikov and Daniele Panozzo
We present a method that expands on previous work in learning human perceived style similarity across objects with different structures and functionalities. Unlike previous approaches that tackle this problem with the help of hand-crafted geometric descriptors, we make use of recent advances in metric learning with neural networks (deep metric learning). This allows us to train the similarity metric on a shape collection directly, since any low- or high-level features needed to discriminate between different styles are identified by the neural network automatically. Furthermore, we avoid the issue of finding and comparing sub-elements of the shapes. We represent the shapes as rendered images and show how image tuples can be selected, generated and used efficiently for deep metric learning. We also tackle the problem of training our neural networks on relatively small datasets and show that we achieve style classification accuracy competitive with the state of the art. Finally, to reduce annotation effort we propose a method to incorporate heterogeneous data sources by adding annotated photos found online in order to expand or supplant parts of our training data.
Visual Analysis of Spatial Variability and Global Correlations in Ensembles of Iso-Contours
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Ferstl, Florian; Kanzler, Mathias; Rautenhaus, Marc; Westermann, Rüdiger; Kwan-Liu Ma and Giuseppe Santucci and Jarke van Wijk
For an ensemble of iso-contours in multi-dimensional scalar fields, we present new methods to a) visualize their dominant spatial patterns of variability, and b) to compute the conditional probability of the occurrence of a contour at one location given the occurrence at some other location. We first show how to derive a statistical model describing the contour variability, by representing the contours implicitly via signed distance functions and clustering similar functions in a reduced order space. We show that the spatial patterns of the ensemble can then be derived by analytically transforming the boundaries of a confidence interval computed from each cluster into the spatial domain. Furthermore, we introduce a mathematical basis for computing correlations between the occurrences of iso-contours at different locations. We show that the computation of these correlations can be posed in the reduced order space as an integration problem over a region bounded by four hyper-planes. To visualize the derived statistical properties we employ a variant of variability plots for streamlines, now including the color coding of probabilities of joint contour occurrences. We demonstrate the use of the proposed techniques for ensemble exploration in a number of 2D and 3D examples, using artificial and meteorological data sets.
A Survey of Real‐Time Crowd Rendering
(© 2016 The Eurographics Association and John Wiley & Sons Ltd., 2016) Beacco, A.; Pelechano, N.; Andújar, C.; Chen, Min and Zhang, Hao (Richard)
In this survey we review, classify and compare existing approaches for real‐time crowd rendering. We first overview character animation techniques, as they are highly tied to crowd rendering performance, and then we analyze the state of the art in crowd rendering. We discuss different representations for level‐of‐detail (LoD) rendering of animated characters, including polygon‐based, point‐based, and image‐based techniques, and review different criteria for runtime LoD selection. Besides LoD approaches, we review classic acceleration schemes, such as frustum culling and occlusion culling, and describe how they can be adapted to handle crowds of animated characters. We also discuss specific acceleration techniques for crowd rendering, such as primitive pseudo‐instancing, palette skinning, and dynamic key‐pose caching, which benefit from current graphics hardware. We also address other factors affecting performance and realism of crowds such as lighting, shadowing, clothing and variability. Finally we provide an exhaustive comparison of the most relevant approaches in the field.In this survey we review, classify and compare existing approaches for real‐time crowd rendering. We first overview character animation techniques, as they are highly tied to crowd rendering performance, and then we analyze the state of the art in crowd rendering. We discuss different representations for level‐of‐detail (LoD) rendering of animated characters, including polygon‐based, point‐based, and image‐based techniques, and review different criteria for runtime LoD selection. Besides LoD approaches, we review classic acceleration schemes, such as frustum culling and occlusion culling, and describe how they can be adapted to handle crowds of animated characters.
Two Simple Methods for Improving a Triangle Mesh Surface
(Copyright © 2016 The Eurographics Association and John Wiley & Sons Ltd., 2016) Renka, Robert J.; Chen, Min and Zhang, Hao (Richard)
We present two simple and efficient local methods that reposition vertices of a triangle mesh surface with the goal of producing good triangle shapes while preserving the enclosed volume and sharp features. The methods involve minimizing a quadratic energy functional with respect to variations in a tangent plane (or in the direction of a crease) at each free vertex. One of the methods is aimed at producing uniform angles, while the other method is designed to produce uniform triangle areas, or more generally, to force relative triangle areas to conform to curvature estimates or estimates of local feature size so that vertex density is low in flat spots and relatively high in regions of large curvature. Test results demonstrate the effectiveness of both methods, especially when combined.We present two simple and efficient local methods that reposition vertices of a triangle mesh surface with the goal of producing good triangle shapes while preserving the enclosed volume and sharp features. The methods involve minimizing a quadratic energy functional with respect to variations in a tangent plane (or in the direction of a crease) at each free vertex. One of the methods is aimed at producing uniform angles, while the other method is designed to produce uniform triangle areas, or more generally, to force relative triangle areas to conform to curvature estimates or estimates of local feature size so that vertex density is low in flat spots and relatively high in regions of large curvature. Test results demonstrate the effectiveness of both methods, especially when combined.
Generalized As-Similar-As-Possible Warping with Applications in Digital Photography
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Chen, Renjie; Gotsman, Craig; Joaquim Jorge and Ming Lin
Discrete conformal mappings of planar triangle meshes, also known as the As-Similar-As-Possible (ASAP) mapping, involve the minimization of a quadratic energy function, thus are very easy to generate and are popular in image warping scenarios. We generalize this classical mapping to the case of quad meshes, taking into account the mapping of the interior of the quad, and analyze in detail the most common case - the unit grid mesh. We show that the generalization, when combined with barycentric coordinate mappings between the source and target polygons, spawns an entire family of new mappings governed by quadratic energy functions, which allow to control quite precisely various effects of the mapping. This approach is quite general and applies also to arbitrary planar polygon meshes. As an application of generalized ASAP mappings of the unit grid mesh, we demonstrate how they can be used to warp digital photographs to achieve a variety of effects. One such effect is modifying the perspective of the camera that took a given photograph (without moving the camera). A related, but more challenging, effect is re-photography - warping a contemporary photograph in order to reproduce the camera view present in a vintage photograph of the same scene - taken many years before with a different camera from a different viewpoint. We apply the generalized ASAP mapping to these images, discretized to a unit grid. Using a quad mesh (as opposed to a triangle mesh) permits biasing towards affine maps of the unit squares. This allows the introduction of an As-Affine-As-Possible (AAAP) mapping for a good approximation of the homographies present in these warps, achieving quite accurate results. We demonstrate the advantages of the AAAP mapping on a variety of synthetic and real-world examples.
Aesthetic Rating and Color Suggestion for Color Palettes
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Kita, Naoki; Miyata, Kazunori; Eitan Grinspun and Bernd Bickel and Yoshinori Dobashi
A model to rate color combinations that considers human aesthetic preferences is proposed. The proposed method does not assume that a color palette has a specific number of colors, i.e., input is not restricted to a two-, three-, or five-color palettes. We extract features from a color palette whose size does not depend on the number of colors in the palette. The proposed rating prediction model is trained using a human color preference dataset. The model allows a user to extend a color palette, e.g., from three colors to five or seven colors, while retaining color harmony. In addition, we present a color search scheme for a given palette and a customized version of the proposed model for a specific color tone. We demonstrate that the proposed model can also be applied to various palette-based applications.
Nonlinearly Weighted First-order Regression for Denoising Monte Carlo Renderings
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Bitterli, Benedikt; Rousselle, Fabrice; Moon, Bochang; Iglesias-Guitian, Jose A.; Adler, David; Mitchell, Kenny; Jarosz, Wojciech; Novak, Jan; Elmar Eisemann and Eugene Fiume
We address the problem of denoising Monte Carlo renderings by studying existing approaches and proposing a new algorithm that yields state-of-the-art performance on a wide range of scenes. We analyze existing approaches from a theoretical and empirical point of view, relating the strengths and limitations of their corresponding components with an emphasis on production requirements. The observations of our analysis instruct the design of our new filter that offers high-quality results and stable performance. A key observation of our analysis is that using auxiliary buffers (normal, albedo, etc.) to compute the regression weights greatly improves the robustness of zero-order models, but can be detrimental to first-order models. Consequently, our filter performs a first-order regression leveraging a rich set of auxiliary buffers only when fitting the data, and, unlike recent works, considers the pixel color alone when computing the regression weights. We further improve the quality of our output by using a collaborative denoising scheme. Lastly, we introduce a general mean squared error estimator, which can handle the collaborative nature of our filter and its nonlinear weights, to automatically set the bandwidth of our regression kernel.
Decoupled Shading for Real-time Heterogeneous Volume Illumination
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Zhang, Yubo; Ma, Kwan-Liu; Kwan-Liu Ma and Giuseppe Santucci and Jarke van Wijk
Existing real-time volume rendering techniques which support global illumination are limited in modeling distinct realistic appearances for classified volume data, which is a desired capability in many fields of study for illustration and education. Directly extending the emission-absorption volume integral with heterogeneous material shading becomes unaffordable for real-time applications because the high-frequency view-dependent global lighting needs to be evaluated per sample along the volume integral. In this paper, we present a decoupled shading algorithm for multi-material volume rendering that separates global incident lighting evaluation from per-sample material shading under multiple light sources. We show how the incident lighting calculation can be optimized through a sparse volume integration method. The quality, performance and usefulness of our new multi-material volume rendering method is demonstrated through several examples.
Planar Shape Detection and Regularization in Tandem
(Copyright © 2016 The Eurographics Association and John Wiley & Sons Ltd., 2016) Oesau, Sven; Lafarge, Florent; Alliez, Pierre; Chen, Min and Zhang, Hao (Richard)
We present a method for planar shape detection and regularization from raw point sets. The geometric modelling and processing of man‐made environments from measurement data often relies upon robust detection of planar primitive shapes. In addition, the detection and reinforcement of regularities between planar parts is a means to increase resilience to missing or defect‐laden data as well as to reduce the complexity of models and algorithms down the modelling pipeline. The main novelty behind our method is to perform detection and regularization in tandem. We first sample a sparse set of seeds uniformly on the input point set, and then perform in parallel shape detection through region growing, interleaved with regularization through detection and reinforcement of regular relationships (coplanar, parallel and orthogonal). In addition to addressing the end goal of regularization, such reinforcement also improves data fitting and provides guidance for clustering small parts into larger planar parts. We evaluate our approach against a wide range of inputs and under four criteria: geometric fidelity, coverage, regularity and running times. Our approach compares well with available implementations such as the efficient random sample consensus–based approach proposed by Schnabel and co‐authors in 2007.We present a method for planar shape detection and regularization from raw point sets. The geometric modelling and processing of man‐made environments from measurement data often relies upon robust detection of planar primitive shapes. In addition, the detection and reinforcement of regularities between planar parts is a means to increase resilience to missing or defect‐laden data as well as to reduce the complexity of models and algorithms down the modelling pipeline. The main novelty behind our method is to perform detection and regularization in tandem. We first sample a sparse set of seeds uniformly on the input point set, and then perform in parallel shape detection through region growing, interleaved with regularization through detection and reinforcement of regular relationships (coplanar, parallel and orthogonal).
How Ordered Is It? On the Perceptual Orderability of Visual Channels
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Chung, David H. S.; Archambault, Daniel; Borgo, Rita; Edwards, Darren J.; Laramee, Robert S.; Chen, Min; Kwan-Liu Ma and Giuseppe Santucci and Jarke van Wijk
The design of effective glyphs for visualisation involves a number of different visual encodings. Since spatial position is usually already specified in advance, we must rely on other visual channels to convey additional relationships for multivariate analysis. One such relationship is the apparent order present in the data. This paper presents two crowdsourcing empirical studies that focus on the perceptual evaluation of orderability for visual channels, namely Bertin's retinal variables. The first study investigates the perception of order in a sequence of elements encoded with different visual channels. We found evidence that certain visual channels are perceived as more ordered (for example, value) while others are perceived as less ordered (for example, hue) than the measured order present in the data. As a result, certain visual channels are more/less sensitive to disorder. The second study evaluates how visual orderability affects min and max judgements of elements in the sequence. We found that visual channels that tend to be perceived as ordered, improve the accuracy of identifying these values.
Line Sampling for Direct Illumination
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Billen, Niels; Dutré, Philip; Elmar Eisemann and Eugene Fiume
Computing direct illumination efficiently is still a problem of major significance in computer graphics. The evaluation involves an integral over the surface areas of the light sources in the scene. Because this integral typically features many discontinuities, introduced by the visibility term and complex material functions, Monte Carlo integration is one of the only general techniques that can be used to compute the integral. In this paper, we propose to evaluate the direct illumination using line samples instead of point samples. A direct consequence of line sampling is that the two-dimensional integral over the area of the light source is reduced to a one-dimensional integral. We exploit this dimensional reduction by relying on the property that commonly used sampling patterns, such as stratified sampling and low-discrepancy sequences, converge faster when the dimension of the integration domain is reduced. We show that, while line sampling is generally more computationally intensive than point sampling, the variance of a line sample is smaller than that of a point sample, resulting in a higher order of convergence.
3D Body Shapes Estimation from Dressed-Human Silhouettes
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Song, Dan; Tong, Ruofeng; Chang, Jian; Yang, Xiaosong; Tang, Min; Zhang, Jian Jun; Eitan Grinspun and Bernd Bickel and Yoshinori Dobashi
Estimation of 3D body shapes from dressed-human photos is an important but challenging problem in virtual fitting. We propose a novel automatic framework to efficiently estimate 3D body shapes under clothes. We construct a database of 3D naked and dressed body pairs, based on which we learn how to predict 3D positions of body landmarks (which further constrain a parametric human body model) automatically according to dressed-human silhouettes. Critical vertices are selected on 3D registered human bodies as landmarks to represent body shapes, so as to avoid the time-consuming vertices correspondences finding process for parametric body reconstruction. Our method can estimate 3D body shapes from dressed-human silhouettes within 4 seconds, while the fastest method reported previously need 1 minute. In addition, our estimation error is within the size tolerance for clothing industry. We dress 6042 naked bodies with 3 sets of common clothes by physically based cloth simulation technique. To the best of our knowledge, We are the first to construct such a database containing 3D naked and dressed body pairs and our database may contribute to the areas of human body shapes estimation and cloth simulation.
Temporally Coherent and Artistically Intended Stylization of Feature Lines Extracted from 3D Models
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Cardona, Luis; Saito, Suguru; Eitan Grinspun and Bernd Bickel and Yoshinori Dobashi
In this paper, we propose a method to maintain the temporal coherence of stylized feature lines extracted from 3D models and preserve an artistically intended stylization provided by the user. We formally define the problem of combining spatio-temporal continuity and artistic intention as a weighted energy minimization problem of competing constraints. The proposed method updates the style properties to provide real-time smooth transitions from current to goal stylization, by assuring first- and second-order temporal continuity, as well as spatial continuity along each stroke. The proposed weighting scheme guarantees that the stylization of strokes maintains motion coherence with respect to the apparent motion of the underlying surface in consecutive frames. This weighting scheme emphasizes temporal continuity for small apparent motions where the human vision system is able to keep track of the scene, and prioritizes the artistic intention for large apparent motions where temporal coherence is not expected. The proposed method produces temporally coherent and visually pleasing animations without the flickering artifacts of previous methods, while also maintaining the artistic intention of a goal stylization provided by the user.
Data-guided Model Predictive Control Based on Smoothed Contact Dynamics
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Han, Daseong; Eom, Haegwang; Noh, Junyong; Shin, Joseph S. (formerly Sung Yong); Joaquim Jorge and Ming Lin
In this paper, we propose an efficient data-guided method based on Model Predictive Control (MPC) to synthesize a full-body motion. Guided by a reference motion, our method repeatedly plans the full-body motion to produce an optimal control policy for predictive control while sliding the fixed-span window along the time axis. Based on this policy, the method computes the joint torques of a character at every time step. Together with contact forces and external perturbations if there are any, the joint torques are used to update the state of the character. Without including the contact forces in the control vector, our formulation of the trajectory optimization problem enables automatic adjustment of contact timings and positions for balancing in response to environmental changes and external perturbations. For efficiency, we adopt derivative-based trajectory optimization on top of state-of-the-art smoothed contact dynamics. Use of derivatives enables our method to run much faster than the existing sampling-based methods. In order to further accelerate the performance of MPC, we propose efficient numerical differentiation of the system dynamics of a full-body character based on two schemes: data reuse and data interpolation. The former scheme exploits data dependency to reuse physical quantities of the system dynamics at near-by time points. The latter scheme allows the use of derivatives at sparse sample points to interpolate those at other time points in the window. We further accelerate evaluation of the system dynamics by exploiting the sparsity of physical quantities such as Jacobian matrix resulting from the tree-like structure of the articulated body. Through experiments, we show that the proposed method efficiently can synthesize realistic motions such as locomotion, dancing, gymnastic motions, and martial arts at interactive rates using moderate computing resources.
Variance Analysis of Multi-sample and One-sample Multiple Importance Sampling
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Sbert, Mateu; Havran, Vlastimil; Szirmay-Kalos, Laszlo; Eitan Grinspun and Bernd Bickel and Yoshinori Dobashi
We reexamine in this paper the variance for the Multiple Importance Sampling (MIS) estimator for multi-sample and onesample model. As a result of our analysis we can obtain the optimal estimator for the multi-sample model for the case where the weights do not depend on the count of samples. We extend the analysis to include the cost of sampling. With these results in hand we find a better estimator than balance heuristic with equal count of samples. Further, we show that the variance for the one-sample model is larger or equal than for the multi-sample model, and that there are only two cases where the variance is the same. Finally, we study on four examples the difference of variances for equal count as used by Veach, our new estimator, and a recently introduced heuristic.
Multi-Resolution Meshes for Feature-Aware Hardware Tessellation
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Lambert, Thibaud; Bénard, Pierre; Guennebaud, Gaël; Joaquim Jorge and Ming Lin
Hardware tessellation is de facto the preferred mechanism to adaptively control mesh resolution with maximal performances. However, owing to its fixed and uniform pattern, leveraging tessellation for feature-aware LOD rendering remains a challenging problem. We relax this fundamental constraint by introducing a new spatial and temporal blending mechanism of tessellation levels, which is built on top of a novel hierarchical representation of multi-resolution meshes. This mechanism allows to finely control topological changes so that vertices can be removed or added at the most appropriate location to preserve geometric features in a continuous and artifact-free manner. We then show how to extend edge-collapse based decimation methods to build feature-aware multi-resolution meshes that match the tessellation patterns. Our approach is fully compatible with current hardware tessellators and only adds a small overhead on memory consumption and tessellation cost.
A Study On Designing Effective Introductory Materials for Information Visualization
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Tanahashi, Yuzuru; Leaf, Nick; Ma, Kwan-Liu; Eitan Grinspun and Bernd Bickel and Yoshinori Dobashi
Designing introductory materials is extremely important when developing new information visualization techniques. All users, regardless of their domain knowledge, first must learn how to interpret the visually encoded information in order to infer knowledge from visualizations. Yet, despite its significance, there has been little research on how to design effective introductory materials for information visualization. This paper presents a study on the design of online guides that educate new users on how to utilize information visualizations, particularly focusing on the employment of exercise questions in the guides. We use two concepts from educational psychology, learning type (or learning style) and teaching method, to design four unique types of online guides. The effects of the guides are measured by comprehension tests of a large group of crowdsourced participants. The tests covered four visualization types (graph, scatter plot, storyline, and tree map) and a complete range of visual analytics tasks. Our statistical analyses indicate that online guides which employ active learning and the top-down teaching method are the most effective. Our study provides quantitative insight into the use of exercise questions in online guides for information visualizations and will inspire further research on design considerations for other elements in introductory materials.
TimeArcs: Visualizing Fluctuations in Dynamic Networks
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Dang, Tuan Nhon; Pendar, Nick; Forbes, Angus G.; Kwan-Liu Ma and Giuseppe Santucci and Jarke van Wijk
In this paper we introduce TimeArcs, a novel visualization technique for representing dynamic relationships between entities in a network. Force-directed layouts provide a way to highlight related entities by positioning them near to each other. Entities are brought closer to each other (forming clusters) by forces applied on nodes and connections between nodes. In many application domains, relationships between entities are not temporally stable, which means that cluster structures and cluster memberships also may vary across time. Our approach merges multiple force-directed layouts at different time points into a single comprehensive visualization that provides a big picture overview of the most significant clusters within a user-defined period of time. TimeArcs also supports a range of interactive features, such as allowing users to drill-down in order to see details about a particular cluster. To highlight the benefits of this technique, we demonstrate its application to various datasets, including the IMDB co-star network, a dataset showing conflicting evidences within biomedical literature of protein interactions, and collocated popular phrases obtained from political blogs.
Tracing Field-Coherent Quad Layouts
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Pietroni, Nico; Puppo, Enrico; Marcias, Giorgio; Scopigno, Roberto; Cignoni, Paolo; Eitan Grinspun and Bernd Bickel and Yoshinori Dobashi
Given a cross field over a triangulated surface we present a practical and robust method to compute a field aligned coarse quad layout over the surface. The method works directly on a triangle mesh without requiring any parametrization and it is based on a new technique for tracing field-coherent geodesic paths directly on a triangle mesh, and on a new relaxed formulation of a binary LP problem, which allows us to extract both conforming quad layouts and coarser layouts containing t-junctions. Our method is easy to implement, very robust, and, being directly based on the input cross field, it is able to generate better aligned layouts, even with complicated fields containing many singularities. We show results on a number of datasets and comparisons with state-of-the-art methods.

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