34-Issue 7

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https://diglib.eg.org/handle/10.2312/13266

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Procedural Tree Modeling with Guiding Vectors
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Xu, Ling; Mould, David; Stam, Jos and Mitra, Niloy J. and Xu, Kun
We propose guiding vectors to augment graph-based tree synthesis, in which trees are collections of least-cost paths in a graph. Each node has an associated guiding vector; edges parallel to the guiding vector are cheap, but edges are more expensive when their orientation differs from the guiding vector.We further propose an incremental method for assigning guiding vectors over the graph, in which a node's guiding vector is an incremental rotation of that of its parent. We present a complete procedural system for tree modeling; our use of guiding vectors enables the graph-based method to produce high-quality tree models resembling a variety of real-world tree species.
Virtual Spherical Gaussian Lights for Real-time Glossy Indirect Illumination
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Tokuyoshi, Yusuke; Stam, Jos and Mitra, Niloy J. and Xu, Kun
Virtual point lights (VPLs) are well established for real-time global illumination. However, this method suffers from spiky artifacts and flickering caused by singularities of VPLs, highly glossy materials, high-frequency textures, and discontinuous geometries. To avoid these artifacts, this paper introduces a virtual spherical Gaussian light (VSGL) which roughly represents a set of VPLs. For a VSGL, the total radiant intensity and positional distribution of VPLs are approximated using spherical Gaussians and a Gaussian distribution, respectively. Since this approximation can be computed using summations of VPL parameters, VSGLs can be dynamically generated using mipmapped reflective shadow maps. Our VSGL generation is simple and independent from any scene geometries. In addition, reflected radiance for a VSGL is calculated using an analytic formula. Hence, we are able to render one-bounce glossy interreflections at real-time frame rates with smaller artifacts.
Object Completion using k-Sparse Optimization
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Mavridis, Pavlos; Sipiran, Ivan; Andreadis, Anthousis; Papaioannou, Georgios; Stam, Jos and Mitra, Niloy J. and Xu, Kun
We present a new method for the completion of partial globally-symmetric 3D objects, based on the detection of partial and approximate symmetries in the incomplete input dataset. In our approach, symmetry detection is formulated as a constrained sparsity maximization problem, which is solved efficiently using a robust RANSACbased optimizer. The detected partial symmetries are then reused iteratively, in order to complete the missing parts of the object. A global error relaxation method minimizes the accumulated alignment errors and a nonrigid registration approach applies local deformations in order to properly handle approximate symmetry. Unlike previous approaches, our method does not rely on the computation of features, it uniformly handles translational, rotational and reflectional symmetries and can provide plausible object completion results, even on challenging cases, where more than half of the target object is missing. We demonstrate our algorithm in the completion of 3D scans with varying levels of partiality and we show the applicability of our approach in the repair and completion of heavily eroded or incomplete cultural heritage objects.
Quadratic Contact Energy Model for Multi-impact Simulation
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Zhang, Tianxiang; Li, Sheng; Manocha, Dinesh; Wang, Guoping; Sun, Hanqiu; Stam, Jos and Mitra, Niloy J. and Xu, Kun
Simultaneous multi-impact simulation is a challenging problem that frequently arises in physically-based modeling of rigid bodies. There are several physical criteria that should be satisfied for rigid body collision handling, but existing methods generally fail to meet one or more of them. In order to capture the inner process of potential energy variation, which is the physical foundation of collisions in a multi-impact system, we present a novel quadratic contact energy model for rigid body simulation. By constructing quadratic energy functions with respect to the impulses, post-impact reactions of rigid bodies can be computed efficiently. Our model can satisfy the physical criteria and can simulate various natural phenomena including the wave effect. Also, our model can be easily combined with Linear Complementary Problem (LCP) and can provide feasible results with any restitution coefficient. In practice, our model can solve the simultaneous multi-impact problem efficiently and robustly, and we highlight its performance on different benchmarks.
Evaluating the Quality of Face Alignment without Ground Truth
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Sheng, Kekai; Dong, Weiming; Kong, Yan; Mei, Xing; Li, Jilin; Wang, Chengjie; Huang, Feiyue; Hu, Bao-Gang; Stam, Jos and Mitra, Niloy J. and Xu, Kun
The study of face alignment has been an area of intense research in computer vision, with its achievements widely used in computer graphics applications. The performance of various face alignment methods is often imagedependent or somewhat random because of their own strategy. This study aims to develop a method that can select an input image with good face alignment results from many results produced by a single method or multiple ones. The task is challenging because different face alignment results need to be evaluated without any ground truth. This study addresses this problem by designing a feasible feature extraction scheme to measure the quality of face alignment results. The feature is then used in various machine learning algorithms to rank different face alignment results. Our experiments show that our method is promising for ranking face alignment results and is able to pick good face alignment results, which can enhance the overall performance of a face alignment method with a random strategy. We demonstrate the usefulness of our ranking-enhanced face alignment algorithm in two practical applications: face cartoon stylization and digital face makeup.
Projective Feature Learning for 3D Shapes with Multi-View Depth Images
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Xie, Zhige; Xu, Kai; Shan, Wen; Liu, Ligang; Xiong, Yueshan; Huang, Hui; Stam, Jos and Mitra, Niloy J. and Xu, Kun
Feature learning for 3D shapes is challenging due to the lack of natural paramterization for 3D surface models. We adopt the multi-view depth image representation and propose Multi-View Deep Extreme Learning Machine (MVD-ELM) to achieve fast and quality projective feature learning for 3D shapes. In contrast to existing multiview learning approaches, our method ensures the feature maps learned for different views are mutually dependent via shared weights and in each layer, their unprojections together form a valid 3D reconstruction of the input 3D shape through using normalized convolution kernels. These lead to a more accurate 3D feature learning as shown by the encouraging results in several applications. Moreover, the 3D reconstruction property enables clear visualization of the learned features, which further demonstrates the meaningfulness of our feature learning.
DenseCut: Densely Connected CRFs for Realtime GrabCut
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Cheng, Ming-Ming; Prisacariu, Victor Adrian; Zheng, Shuai; Torr, Philip H. S.; Rother, Carsten; Stam, Jos and Mitra, Niloy J. and Xu, Kun
Figure-ground segmentation from bounding box input, provided either automatically or manually, has been extremely popular in the last decade and influenced various applications. A lot of research has focused on highquality segmentation, using complex formulations which often lead to slow techniques, and often hamper practical usage. In this paper we demonstrate a very fast segmentation technique which still achieves very high quality results. We propose to replace the time consuming iterative refinement of global colour models in traditional GrabCut formulation by a densely connected CRF. To motivate this decision, we show that a dense CRF implicitly models unnormalized global colour models for foreground and background. Such relationship provides insightful analysis to bridge between dense CRF and GrabCut functional. We extensively evaluate our algorithm using two famous benchmarks. Our experimental results demonstrated that the proposed algorithm achieves an order of magnitude (10 ) speed-up with respect to the closest competitor, and at the same time achieves a considerably higher accuracy.
EasyXplorer: A Flexible Visual Exploration Approach for Multivariate Spatial Data
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Wu, Feiran; Chen, Guoning; Huang, Jin; Tao, Yubo; Chen, Wei; Stam, Jos and Mitra, Niloy J. and Xu, Kun
Exploring multivariate spatial data attracts much attention in the visualization community. The main challenge lies in that automatic analysis techniques is insufficient in discovering complicated patterns with the perspective of human beings, while visualization techniques are incapable of accurately identifying the features of interest. This paper addresses this contradiction by enhancing automatic analysis techniques with human intelligence in an iterative visual exploration process. The integrated system, called EasyXplorer, provides a suite of intuitive clustering, dimension reduction, visual encoding and filtering widgets within 2D and 3D views, allowing an inexperienced user to visually explore and reason undiscovered features with several simple interactions. Case studies show the quality and scalability of our approach in quite challenging examples.
Towards Automatic Band-Limited Procedural Shaders
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Dorn, Jonathan; Barnes, Connelly; Lawrence, Jason; Weimer, Westley; Stam, Jos and Mitra, Niloy J. and Xu, Kun
Procedural shaders are a vital part of modern rendering systems. Despite their prevalence, however, procedural shaders remain sensitive to aliasing any time they are sampled at a rate below the Nyquist limit. Antialiasing is typically achieved through numerical techniques like supersampling or precomputing integrals stored in mipmaps. This paper explores the problem of analytically computing a band-limited version of a procedural shader as a continuous function of the sampling rate. There is currently no known way of analytically computing these integrals in general. We explore the conditions under which exact solutions are possible and develop several approximation strategies for when they are not. Compared to supersampling methods, our approach produces shaders that are less expensive to evaluate and closer to ground truth in many cases. Compared to mipmapping or precomputation, our approach produces shaders that support an arbitrary bandwidth parameter and require less storage. We evaluate our method on a range of spatially-varying shader functions, automatically producing antialiased versions that have comparable error to 4x4 multisampling but can be over an order of magnitude faster. While not complete, our approach is a promising first step toward this challenging goal and indicates a number of interesting directions for future work.
Efficient Variational Light Field View Synthesis For Making Stereoscopic 3D Images
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Zhang, Lei; Zhang, Yu-Hang; Huang, Hua; Stam, Jos and Mitra, Niloy J. and Xu, Kun
We present a novel approach for making stereoscopic images by variational view synthesis on the multi-perspective light field. With the intended disparities as constraints, we specialize the generative variational model by incorporating per-pixel viewpoint assignment to synthesize the stereo pair. Also, we improve the variational solution by use of explicit weighted average on the light field. Our algorithm is able to handle arbitrary disparity remapping, thus enabling more flexible disparity control for the desired stereoscopic effect. The experiments demonstrate the effectiveness and efficiency for making the stereoscopic 3D images based on the light field.
Multiple Facial Image Editing Using Edge-Aware PDE Learning
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Liang, Lingyu; Jin, Lianwen; Zhang, Xin; Xu, Yong; Stam, Jos and Mitra, Niloy J. and Xu, Kun
This paper introduces a novel facial editing tool, called edge-aware mask, to achieve multiple photo-realistic rendering effects in a unified framework. The edge-aware masks facilitate three basic operations for adaptive facial editing, including region selection, edit setting and region blending. Inspired by the state-of-the-art edit propagation and partial differential equation (PDE) learning method, we propose an adaptive PDE model with facial priors for masks generation through edge-aware diffusion. The edge-aware masks can automatically fit the complex region boundary with great accuracy and produce smooth transition between different regions, which significantly improves the visual consistence of face editing and reduce the human intervention. Then, a unified and flexible facial editing framework is constructed, which consists of layer decomposition, edge-aware masks generation, and layer/mask composition. The combinations of multiple facial layers and edge-aware masks can achieve various facial effects simultaneously, including face enhancement, relighting, makeup and face blending etc. Qualitative and quantitative evaluations were performed using different datasets for different facial editing tasks. Experiments demonstrate the effectiveness and flexibility of our methods, and the comparisons with the previous methods indicate that improved results are obtained using the combination of multiple edge-aware masks.
Dispersion-based Color Projection using Masked Prisms
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Hostettler, Rafael; Habel, Ralf; Gross, Markus; Jarosz, Wojciech; Stam, Jos and Mitra, Niloy J. and Xu, Kun
We present a method for projecting arbitrary color images using a white light source and an optical device with no colored components - consisting solely of one or two prisms and two transparent masks. When illuminated, the first mask creates structured white light that is then dispersed in the prism and attenuated by the second mask to create the color projection. We derive analytical expressions for the mask parameters from the physical components and validate our approach both in simulation and also demonstrate it on a wide variety of images using two different physical setups (one consisting of two inexpensive triangular prisms, and the other using a single rhombic prism). Furthermore, we show that optimizing the masks simultaneously enables obfuscating the image content, and provides a tradeoff between increased light throughput (by up to a factor of three) and maximum color saturation.
FlexyFont: Learning Transferring Rules for Flexible Typeface Synthesis
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Phan, Huy Quoc; Fu, Hongbo; Chan, Antoni B.; Stam, Jos and Mitra, Niloy J. and Xu, Kun
Maintaining consistent styles across glyphs is an arduous task in typeface design. In this work we introduce Flexy- Font, a flexible tool for synthesizing a complete typeface that has a consistent style with a given small set of glyphs. Motivated by a key fact that typeface designers often maintain a library of glyph parts to achieve a consistent typeface, we intend to learn part consistency between glyphs of different characters across typefaces. We take a part assembling approach by firstly decomposing the given glyphs into semantic parts and then assembling them according to learned sets of transferring rules to reconstruct the missing glyphs. To maintain style consistency, we represent the style of a font as a vector of pairwise part similarities. By learning a distribution over these feature vectors, we are able to predict the style of a novel typeface given only a few examples. We utilize a popular machine learning method as well as retrieval-based methods to quantitatively assess the performance of our feature vector, resulting in favorable results. We also present an intuitive interface that allows users to interactively create novel typefaces with ease. The synthesized fonts can be directly used in real-world design.
Order-Independent Transparency for Programmable Deferred Shading Pipelines
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Schollmeyer, Andre; Babanin, Andrey; Froehlich, Bernd; Stam, Jos and Mitra, Niloy J. and Xu, Kun
In this paper, we present a flexible and efficient approach for the integration of order-independent transparency into a deferred shading pipeline. The intermediate buffers for storing fragments to be shaded are extended with a dynamic and memory-efficient storage for transparent fragments. The transparency of an object is not fixed and remains programmable until fragment processing, which allows for the implementation of advanced materials effects, interaction techniques or adaptive fade-outs. Traversing costs for shading the transparent fragments are greatly reduced by introducing a tile-based light-culling pass. During deferred shading, opaque and transparent fragments are shaded and composited in front-to-back order using the retrieved lighting information and a physically-based shading model. In addition, we discuss various configurations of the system and further enhancements. Our results show that the system performs at interactive frame rates even for complex scenarios.
Skeleton based Vertex Connection Resampling for Bidirectional Path Tracing
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Noël, Laurent; Biri, Venceslas; Stam, Jos and Mitra, Niloy J. and Xu, Kun
Bidirectional path tracing is known to perform poorly for the rendering of highly occluded scenes. Indeed, the connection strategy between light and eye subpaths does not take into account the visibility factor, presenting no contribution for many sampled paths. To improve the efficiency of bidirectional path tracing, we propose a new method for adaptive resampling of connections between light and eye subpaths. Aiming for this objective, we build discrete probability distributions of light subpaths based on a skeleton of the empty space of the scene. In order to demonstrate the efficiency of our algorithm, we compare our method to both standard bidirectional path tracing and a recent important caching method.
Contrast-Enhanced Black and White Images
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Li, Hua; Mould, David; Stam, Jos and Mitra, Niloy J. and Xu, Kun
This paper investigates contrast enhancement as an approach to tone reduction, aiming to convert a photograph to black and white. Using a filter-based approach to strengthen contrast, we avoid making a hard decision about how to assign tones to segmented regions. Our method is inspired by sticks filtering, used to enhance medical images but not previously used in non-photorealistic rendering. We amplify contrast of pixels along the direction of greatest local difference from the mean, strengthening even weak features if they are most prominent. A final thresholding step converts the contrast-enhanced image to black and white. Local smoothing and contrast enhancement balances abstraction and structure preservation; the main advantage of our method is its faithful depiction of image detail. Our method can create a set of effects: line drawing, hatching, and black and white, all having superior details to previous black and white methods.
A Suggestive Interface for Sketch-based Character Posing
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Lv, Pei; Wang, Pengjie; Xu, Weiwei; Chai, Jinxiang; Zhang, Mingmin; Pan, Zhigeng; Xu, Mingliang; Stam, Jos and Mitra, Niloy J. and Xu, Kun
We present a user-friendly suggestive interface for sketch-based character posing. Our interface provides suggestive information on the sketching canvas in succession by combining image retrieval technique with 3D character posing, while the user is drawing. The system highlights the canvas region where the user should draw on and constrains the user's sketches in a reasonable solution space. This is based on an e cient image descriptor, which is used to measure the distance between the user's sketch and 2D views of 3D poses. In order to achieve faster query response, local sensitive hashing is involved in our system. In addition, sampling-based optimization algorithm is adopted to synthesize and optimize the retrieved 3D pose to match the user's sketches the best. Experiments show that our interface can provide smooth suggestive information to improve the reality of sketching poses and shorten the time required for 3D posing.
Ray Specialized Contraction on Bounding Volume Hierarchies
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Gu, Yan; He, Yong; Blelloch, Guy E.; Stam, Jos and Mitra, Niloy J. and Xu, Kun
In this paper we propose a simple but effective method to modify a BVH based on ray distribution for improved ray tracing performance. Our method starts with an initial BVH generated by any state-of-the-art offline algorithm. Then by traversing a small set of sample rays we collect statistics at each node of the BVH. Finally, a simple but ultra-fast BVH contraction algorithm modifies the initial binary BVH to a multi-way BVH. The overall acceleration for ray-primitive testing is about 25% for incoherent diffuse rays and 30% for shadow rays, which is significant as a data structure optimization. Similar results are also presented for packet ray tracing, and for Quad-BVHs the improvement is 10% to 15%. The approach has the advantages of being simple, and compatible with almost any existing BVH and ray tracing techniques, and it require very little extra work to generate the modified tree.
Tone- and Feature-Aware Circular Scribble Art
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Chiu, Chun-Chia; Lo, Yi-Hsiang; Lee, Ruen-Rone; Chu, Hung-Kuo; Stam, Jos and Mitra, Niloy J. and Xu, Kun
Circular scribble art is a kind of line drawing where the seemingly random, noisy and shapeless circular scribbles at microscopic scale constitute astonishing grayscale images at macroscopic scale. Such a delicate skill has rendered the creation of circular scribble art a tedious and time-consuming task even for gifted artists. In this work, we present a novel method for automatic synthesis of circular scribble art. The synthesis problem is modeled as tracing along a virtual path using a parametric circular curve. To reproduce the tone and important edge structure of input grayscale images, the system adaptively adjusts the density and structure of virtual path, and dynamically controls the size, drawing speed and orientation of parametric circular curve during the synthesis.We demonstrate the potential of our system using several circular scribble images synthesized from a wide variety of grayscale images. A preliminary experimental studying is conducted to qualitatively and quantitatively evaluate our method. Results report that our method is efficient and generates convincing results comparable to artistic artworks.
An Efficient Boundary Handling with a Modified Density Calculation for SPH
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Fujisawa, Makoto; Miura, Kenjiro T.; Stam, Jos and Mitra, Niloy J. and Xu, Kun
We propose a new boundary handling method for smoothed particle hydrodynamics (SPH). Previous approaches required the use of boundary particles to prevent particles from sticking to the boundary. We address this issue by correcting the fundamental equations of SPH with the integration of a kernel function. Our approach is able to directly handle triangle mesh boundaries without the need for boundary particles.We also show how our approach can be integrated into a position-based fluid framework.

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