32-Issue 7

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https://diglib.eg.org/handle/10.2312/185

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Constrainable Multigrid for Cloth
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Jeon, Inyong; Choi, Kwang-Jin; Kim, Tae-Yong; Choi, Bong-Ouk; Ko, Hyeong-Seok; B. Levy, X. Tong, and K. Yin
We present a new technique which can handle both point and sliding constraints in the multigrid (MG) framework. Although the MG method can theoretically perform as fast as O(N), the development of a clothing simulator based on the MG method calls for solving an important technical challenge: handling the constraints. Resolving constrains has been difficult in MG because there has been no clear way to transfer the constraints existing in the finest level mesh to the coarser level meshes. This paper presents a new formulation based on soft constraints, which can coarsen the constraints defined in the finest level to the coarser levels. Experiments are performed which show that the proposed method can solve the linear system up to 4-9 times faster in comparison with the modified preconditioned conjugate gradient method (MPCG) without quality degradation. The proposed method is easy to implement and can be straightforwardly applied to existing clothing simulators which are based on implicit time integration.
Multiplane Video Stabilization
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Wang, Zhong-Qiang; Zhang, Lei; Huang, Hua; B. Levy, X. Tong, and K. Yin
This paper presents a novel video stabilization approach by leveraging the multiple planes structure of video scene to stabilize inter-frame motion. As opposed to previous stabilization procedure operating in a single plane, our approach primarily deals with multiplane videos and builds their multiple planes structure for performing stabilization in respective planes. Hence, a robust plane detection scheme is devised to detect multiple planes by classifying feature trajectories according to reprojection errors generated by plane induced homographies. Then, an improved planar stabilization technique is applied by conforming to the compensated homography in each plane. Finally, multiple stabilized planes are coherently fused by content-preserving image warps to obtain the output stabilized frames. Our approach does not need any stereo reconstruction, yet is able to produce commendable results due to awareness of multiple planes structure in the stabilization. Experimental results demonstrate the effectiveness and efficiency of our approach to robust stabilization on multiplane videos.
Evaluation of Tone Mapping Operators for HDR-Video
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Eilertsen, Gabriel; Wanat, Robert; Mantiuk, Rafal K.; Unger, Jonas; B. Levy, X. Tong, and K. Yin
Eleven tone-mapping operators intended for video processing are analyzed and evaluated with camera-captured and computer-generated high-dynamic-range content. After optimizing the parameters of the operators in a formal experiment, we inspect and rate the artifacts (flickering, ghosting, temporal color consistency) and color rendition problems (brightness, contrast and color saturation) they produce. This allows us to identify major problems and challenges that video tone-mapping needs to address. Then, we compare the tone-mapping results in a pair-wise comparison experiment to identify the operators that, on average, can be expected to perform better than the others and to assess the magnitude of differences between the best performing operators.
Stroke-guided Image Synthesis for Skeletal Structure Editing
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Luo, Sheng-Jie; Lin, Chin-Yu; Shen, I-Chao; Chen, Bing-Yu; B. Levy, X. Tong, and K. Yin
Creating variations of an image object is an important task, which usually requires manipulating the skeletal structure of the object. However, most existing methods (such as image deformation) only allow for stretching the skeletal structure of an object: modifying skeletal topology remains a challenge. This paper presents a technique for synthesizing image objects with different skeletal structures while respecting to an input image object. To apply this technique, a user firstly annotates the skeletal structure of the input object by specifying a number of strokes in the input image, and draws corresponding strokes in an output domain to generate new skeletal structures. Then, a number of the example texture pieces are sampled along the strokes in the input image and pasted along the strokes in the output domain with their orientations. The result is obtained by optimizing the texture sampling and seam computation. The proposed method is successfully used to synthesize challenging skeletal structures, such as skeletal branches, and a wide range of image objects with various skeletal structures, to demonstrate its effectiveness.
Learning to Predict Localized Distortions in Rendered Images
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Cadík, Martin; Herzog, Robert; Mantiuk, Rafal; Mantiuk, Radoslaw; Myszkowski, Karol; Seidel, Hans-Peter; B. Levy, X. Tong, and K. Yin
In this work, we present an analysis of feature descriptors for objective image quality assessment. We explore a large space of possible features including components of existing image quality metrics as well as many traditional computer vision and statistical features. Additionally, we propose new features motivated by human perception and we analyze visual saliency maps acquired using an eye tracker in our user experiments. The discriminative power of the features is assessed by means of a machine learning framework revealing the importance of each feature for image quality assessment task. Furthermore, we propose a new data-driven full-reference image quality metric which outperforms current state-of-the-art metrics. The metric was trained on subjective ground truth data combining two publicly available datasets. For the sake of completeness we create a new testing synthetic dataset including experimentally measured subjective distortion maps. Finally, using the same machine-learning framework we optimize the parameters of popular existing metrics.
Soft Folding
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Zhu, Lifeng; Igarashi, Takeo; Mitani, Jun; B. Levy, X. Tong, and K. Yin
We introduce soft folding, a new interactive method for designing and exploring thin-plate forms. A user specifies sharp and soft folds as two-dimensional(2D) curves on a flat sheet, along with the fold magnitude and sharpness of each. Then, based on the soft folds, the system computes the three-dimensional(3D) folded shape. Internally, the system first computes a fold field, which defines local folding operations on a flat sheet. A fold field is a generalization of a discrete fold graph in origami, replacing a graph with sharp folds with a continuous field with soft folds. Next, local patches are folded independently according to the fold field. Finally, a globally folded 3D shape is obtained by assembling the locally folded patches. This algorithm computes an approximation of 3D developable surfaces with user-defined soft folds at an interactive speed. The user can later apply nonlinear physical simulation to generate more realistic results. Experimental results demonstrated that soft folding is effective for producing complex folded shapes with controllable sharpness.
Still-Frame Simulation for Fire Effects of Images
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Son, Minjung; Kim, Byungmoon; Wilensky, Gregg; Lee, Seungyong; B. Levy, X. Tong, and K. Yin
We propose various simulation strategies to generate single-frame fire effects for images, as opposed to multiframe fire effects for animations. To accelerate 3D simulation and to provide a user with early hints on the final effect, we propose a 2D-guided 3D simulation approach, which runs a faster 2D simulation first, and then guides 3D simulation using the 2D simulation result. To achieve this, we explore various boundary conditions and develop a constrained projection method. Since only the final frame will be used while intermediate frames are abandoned, earlier intermediate frames can take larger time steps and have large noise applied, quickly generating turbulent flow structures. As the final frame approaches, we increase the flow quality by reducing the time step and not adding any noise. This adaptive time stepping allows us to use more computational resource near or at the final frame. We also develop divergence and buoyancy modification methods to guide flames along arbitrary, even physically implausible, directions. Our simulation methods can effectively and efficiently generate a variety of fire effects useful for image decoration.
Improving Memory Space Efficiency of Kd-tree for Real-time Ray Tracing
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Choi, Byeongjun; Chang, Byungjoon; Ihm, Insung; B. Levy, X. Tong, and K. Yin
Compared with its competitors such as the bounding volume hierarchy, a drawback of the kd-tree structure is that a large number of triangles are repeatedly duplicated during its construction, which often leads to inefficient, large and tall binary trees with high triangle redundancy. In this paper, we propose a space-efficient kd-tree representation where, unlike commonly used methods, an inner node is allowed to optionally store a reference to a triangle, so highly redundant triangles in a kd-tree can be culled from the leaf nodes and moved to the inner nodes. To avoid the construction of ineffective kd-trees entailing computational inefficiencies due to early, possibly unnecessary, ray-triangle intersection calculations that now have to be performed in the inner nodes during the kd-tree traversal, we present heuristic measures for determining when and how to choose triangles for inner nodes during kd-tree construction. Based on these metrics, we describe how the new form of kd-tree is constructed and stored compactly using a carefully designed data layout. Our experiments with several example scenes showed that our kd-tree representation technique significantly reduced the memory requirements for storing the kd-tree structure, while effectively suppressing the unavoidable frame-rate degradation observed during ray tracing.
Animated 3D Line Drawings with Temporal Coherence
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Xu, Xiang; Seah, Hock Soon; Quah, Chee Kwang; B. Levy, X. Tong, and K. Yin
Producing traditional animation is a laborious task where the key drawings are first drawn by artists and thereafter inbetween drawings are created, whether it is by hand or computer-assisted. Auto-inbetweening of these 2D key drawings by computer is a non-trivial task as 3D depths are missing. An alternate approach is to generate all the drawings by extracting lines directly from animated 3D models frame by frame, concatenating and rendering them together into an animation. However, animation quality generated using this straightforward method bears two problems. Firstly, the animation contains unsatisfactory visual artifacts such as line flickering and popping. This is especially pronounced when the lines are extracted using high-order derivatives, such as ridges and valleys, from 3D models represented in triangle meshes. Secondly, there is a lack of temporal continuity as each drawing is generated without taking its neighboring drawings into consideration. In this paper, we propose an improved approach over the straightforward method by transferring extracted 3D line drawings of each frame into individual 3D lines and processing them along the time domain. Our objective is to minimize the visual artifacts and incorporate temporal relationship of individual lines throughout the entire animation sequence. This is achieved by creating correspondent trajectory of each line from each frame and applying global optimization on each trajectory. To realize this target, we present a fully automatic novel approach, which consists of (1) a line matching algorithm, (2) an optimizing algorithm, taking into account both the variations of numbers and lengths of 3D lines in each frame, and (3) a robust tracing method for transferring collections of line segments extracted from the 3D models into individual lines. We evaluate our approach on several animated model sequences to demonstrate its effectiveness in producing line drawing animations with temporal coherence.
EnvyDepth: An Interface for Recovering Local Natural Illumination from Environment Maps
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Banterle, Francesco; Callieri, Marco; Dellepiane, Matteo; Corsini, Massimiliano; Pellacini, Fabio; Scopigno, Roberto; B. Levy, X. Tong, and K. Yin
In this paper, we present EnvyDepth, an interface for recovering local illumination from a single HDR environment map. In EnvyDepth, the user quickly indicates strokes to mark regions of the environment map that should be grouped together in a single geometric primitive. From these annotated strokes, EnvyDepth uses edit propagation to create a detailed collection of virtual point lights that reproduce both the local and the distant lighting effects in the original scene. When compared to the sole use of the distant illumination, the added spatial information better reproduces a variety of local effects such as shadows, highlights and caustics. Without the effort needed to create precise scene reconstructions, EnvyDepth annotations take only tens of seconds to produce a plausible lighting without visible artifacts. This is easy to obtain even in the case of complex scenes, both indoors and outdoors. The generated lighting environments work well in a production pipeline since they are efficient to use and able to produce accurate renderings.
Implicit Integration for Particle-based Simulation of Elasto-Plastic Solids
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Zhou, Yahan; Lun, Zhaoliang; Kalogerakis, Evangelos; Wang, Rui; B. Levy, X. Tong, and K. Yin
We present a novel particle-based method for stable simulation of elasto-plastic materials. The main contribution of our method is an implicit numerical integrator, using a physically-based model, for computing particles that undergo both elastic and plastic deformations. The main advantage of our implicit integrator is that it allows the use of large time steps while still preserving stable and physically plausible simulation results. As a key component of our algorithm, at each time step we compute the particle positions and velocities based on a sparse linear system, which we solve efficiently on the graphics hardware. Compared to existing techniques, our method allows for a much wider range of stiffness and plasticity settings. In addition, our method can significantly reduce the computation cost for certain range of material types. We demonstrate fast and stable simulations for a variety of elasto-plastic materials, ranging from highly stiff elastic materials to highly plastic ones.
Ontology-based Model for Chinese Calligraphy Synthesis
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Xia, Yang; Wu, Jiangqin; Gao, Pengcheng; Lin, Yuan; Mao, Tianjiao; B. Levy, X. Tong, and K. Yin
There are lots of digitized Chinese calligraphic works written by ancient calligraphists in CADAL. Sometimes users want to generate a tablet or a piece of calligraphic work written by these calligraphists. However, some characters had not been written by them or though were ever written but are hard to read because of long time weathering. Furthermore, different works of the same calligraphist may have diverse styles. It is a great challenge to generate a new calligraphic character with particular style. In this paper, we present a novel approach to synthesize regular script calligraphic characters in the style of some specific calligraphic works. Our approach first processes original calligraphic resources by multi-level segmentation. Then, a domain ontology is established to supervise basic resources and to provide significant support for calligraphy synthesis. Finally, a novel algorithm is proposed to synthesize new calligraphic characters. We demonstrate the effectiveness of our method by showing various synthesized results in the style of different calligraphic works of Liu Gongquan.
Dynamic Comics for Hierarchical Abstraction of 3D Animation Data
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Choi, Myung Geol; Noh, Seung-Tak; Komura, Taku; Igarashi, Takeo; B. Levy, X. Tong, and K. Yin
Image storyboards of films and videos are useful for quick browsing and automatic video processing. A common approach for producing image storyboards is to display a set of selected key-frames in temporal order, which has been widely used for 2D video data. However, such an approach cannot be applied for 3D animation data because different information is revealed by changing parameters such as the viewing angle and the duration of the animation. Also, the interests of the viewer may be different from person to person. As a result, it is difficult to draw a single image that perfectly abstracts the entire 3D animation data. In this paper, we propose a system that allows users to interactively browse an animation and produce a comic sequence out of it. Each snapshot in the comic optimally visualizes a duration of the original animation, taking into account the geometry and motion of the characters and objects in the scene. This is achieved by a novel algorithm that automatically produces a hierarchy of snapshots from the input animation. Our user interface allows users to arrange the snapshots according to the complexity of the movements by the characters and objects, the duration of the animation and the page area to visualize the comic sequence. Our system is useful for quickly browsing through a large amount of animation data and semi-automatically synthesizing a storyboard from a long sequence of animation.
Interactive Physics-based Ink Splattering Art Creation
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Lei, Su-Ian Eugene; Chen, Ying-Chieh; Chen, Hsiang-Ting; Chang, Chun-Fa; B. Levy, X. Tong, and K. Yin
This paper presents an interactive system for ink splattering, a form of abstract art that artists splat ink onto the canvas. The default input device of our system is a pressure-sensitive 2D stylus, the most common sketching tool for digital artists, and we propose two interaction mode: ink-flicking mode and ink-dripping mode, that are designed to be analogous to the artistic techniques of ink splattering in real world. The core of our ink splattering system is a novel three-stage ink splattering framework that simulates the physics-based interaction of ink with different mediums including brush heads, air and paper. We have implemented the physical engine in CUDA and the whole simulation process runs at interactive speed.
Fast Image-Based Modeling of Astronomical Nebulae
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Wenger, Stephan; Lorenz, Dirk; Magnor, Marcus; B. Levy, X. Tong, and K. Yin
Astronomical nebulae are among the most complex and visually appealing phenomena known outside the bounds of the Solar System. However, our fixed vantage point on Earth limits us to a single known view of these objects, and their intricate volumetric structure cannot be recovered directly. Recent approaches to reconstructing a volumetric 3D model use the approximate symmetry inherent to many nebulae, but require several hours of computation time on large multi-GPU clusters. We present a novel reconstruction algorithm based on group sparsity that reaches or even exceeds the quality of prior results while taking only a fraction of the time on a conventional desktop PC, thereby enabling end users in planetariums or educational facilities to produce high-quality content without expensive hardware or manual modeling. In principle, our approach can be generalized to other transparent phenomena with arbitrary types of user-specified symmetries.
Topology Aware Data-Driven Inverse Kinematics
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Ho, Edmond S. L.; Shum, Hubert P. H.; Cheung, Yiu-ming; Yuen, P. C.; B. Levy, X. Tong, and K. Yin
Creating realistic human movement is a time consuming and labour intensive task. The major difficulty is that the user has to edit individual joints while maintaining an overall realistic and collision free posture. Previous research suggests the use of data-driven inverse kinematics, such that one can focus on the control of a few joints, while the system automatically composes a natural posture. However, as a common problem of kinematics synthesis, penetration of body parts is difficult to avoid in complex movements. In this paper, we propose a new data-driven inverse kinematics framework that conserves the topology of the synthesizing postures. Our system monitors and regulates the topology changes using the Gauss Linking Integral (GLI), such that penetration can be efficiently prevented. As a result, complex motions with tight body movements, as well as those involving interaction with external objects, can be simulated with minimal manual intervention. Experimental results show that using our system, the user can create high quality human motion in real-time by controlling a few joints using a mouse or a multi-touch screen. The movement generated is both realistic and penetration free. Our system is best applied for interactive motion design in computer animations and games.
Eye-Centered Color Adaptation in Global Illumination
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Gruson, Adrien; Ribardière, Mickael; Cozot, Remi; B. Levy, X. Tong, and K. Yin
Color adaptation is a well known ability of the human visual system (HVS). Colors are perceived as constant even though the illuminant color changes. Indeed, the perceived color of a diffuse white sheet of paper is still white even though it is illuminated by a single orange tungsten light, whereas it is orange from a physical point of view. Unfortunately global illumination algorithms only focus on the physics aspects of light transport. The ouput of a global illuminantion engine is an image which has to undergo chromatic adaptation to recover the color as perceived by the HVS. In this paper, we propose a new color adaptation method well suited to global illumination. This method estimates the adaptation color by averaging the irradiance color arriving at the eye. Unlike other existing methods, our approach is not limited to the view frustrum, as it considers the illumination from all the scene. Experiments have shown that our method outperforms the state of the art methods.
Efficient Shadow Removal Using Subregion Matching Illumination Transfer
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Xiao, Chunxia; Xiao, Donglin; Zhang, Ling; Chen, Lin; B. Levy, X. Tong, and K. Yin
This paper proposes a new shadow removal approach for input single natural image by using subregion matching illumination transfer. We first propose an effective and automatic shadow detection algorithm incorporating global successive thresholding scheme and local boundary refinement. Then we present a novel shadow removal algorithm by performing illumination transfer on the matched subregion pairs between the shadow regions and non-shadow regions, and this method can process complex images with different kinds of shadowed texture regions and illumination conditions. In addition, we develop an efficient shadow boundary processing method by using alpha matte interpolation, which produces seamless transition between the shadow and non-shadow regions. Experimental results demonstrate the capabilities of our algorithm in both the shadow removal quality and performance.
An Efficient and Scalable Image Filtering Framework Using VIPS Fusion
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Zhang, Jun; Chen, Xiuhong; Zhao, Yan; Li, H.; B. Levy, X. Tong, and K. Yin
Edge-preserving image filtering is a valuable tool for a variety of applications in image processing and computer vision. Motivated by a new simple but effective local Laplacian filter, we propose a scalable and efficient image filtering framework to extend this edge-preserving image filter and construct an uniform implementation in O(N) time. The proposed framework is built upon a practical global-to-local strategy. The input image is first remapped globally by a series of tentative remapping functions to generate a virtual candidate image sequence (Virtual Image Pyramid Sequence, VIPS). This sequence is then recombined locally to a single output image by a flexible edge-aware pixel-level fusion rule. To avoid halo artifacts, both the output image and the virtual candidate image sequence are transformed into multi-resolution pyramid representations. Four examples, single image de-hazing, multi-exposure fusion, fast edge-preserving filtering and tone-mapping, are presented as the concrete applications of the proposed framework. Experiments on filtering effect and computational efficiency indicate that the proposed framework is able to build a wide range of fast image filtering that yields visually compelling results.
Artistic QR Code Embellishment
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Lin, Yi-Shan; Luo, Sheng-Jie; Chen, Bing-Yu; B. Levy, X. Tong, and K. Yin
A QR code is a two-dimensional barcode that encodes information. A standard QR code contains only regular black and white squares, and thus is unattractive. This paper proposes a novel framework for embellishing a standard QR code, to make it both attractive and recognizable by any human while maintaining its scanability. The proposed method is inspired by artistic methods. A QR code is typically embellished by stylizing the squares and embedding images into it. In the proposed framework, the regular squares are reshaped using a binary examplar, to make their local appearances resemble the example shape. Additionally, an error-aware warping technique for deforming the embedded image is proposed; it minimizes the error in the QR code that is generated by the embedding of the image to optimize the readability of the code. The proposed algorithm yields lower data error than previous global transformation techniques because the warping can locally deform the embedded image to conform to the squares that surround it. The proposed framework was examined by using it to embellish an extensive set of QR codes and to test the readability with various commercial QR code readers.

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