32-Issue 2

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https://diglib.eg.org/handle/10.2312/180

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Example-based Interpolation and Synthesis of Bidirectional Texture Functions
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Ruiters, Roland; Schwartz, Christopher; Klein, Reinhard; I. Navazo, P. Poulin
Bidirectional Texture Functions (BTF) have proven to be a well-suited representation for the reproduction of measured real-world surface appearance and provide a high degree of realism. We present an approach for designing novel materials by interpolating between several measured BTFs. For this purpose, we transfer concepts from existing texture interpolation methods to the much more complex case of material interpolation. We employ a separation of the BTF into a heightmap and a parallax compensated BTF to cope with problems induced by parallax, masking and shadowing within the material. By working only on the factorized representation of the parallax compensated BTF and the heightmap, it is possible to efficiently perform the material interpolation. By this novel method to mix existing BTFs, we are able to design plausible and realistic intermediate materials for a large range of different opaque material classes. Furthermore, it allows for the synthesis of tileable and seamless BTFs and finally even the generation of gradually changing materials following user specified material distribution maps.
Interactive Facades - Analysis and Synthesis of Semi-Regular Facades
(The Eurographics Association and Blackwell Publishing Ltd., 2013) AlHalawani, Sawsan; Yang, Yong-Liang; Liu, Han; Mitra, Niloy J.; I. Navazo, P. Poulin
Urban facades regularly contain interesting variations due to allowed deformations of repeated elements (e.g., windows in different open or close positions) posing challenges to state-of-the-art facade analysis algorithms. We propose a semi-automatic framework to recover both repetition patterns of the elements and their individual deformation parameters to produce a factored facade representation. Such a representation enables a range of applications including interactive facade images, improved multi-view stereo reconstruction, facade-level change detection, and novel image editing possibilities.
A Data-Driven Approach to Realistic Shape Morphing
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Gao, Lin; Lai, Yu-Kun; Huang, Qi-Xing; Hu, Shi-Min; I. Navazo, P. Poulin
Morphing between 3D objects is a fundamental technique in computer graphics. Traditional methods of shape morphing focus on establishing meaningful correspondences and finding smooth interpolation between shapes. Such methods however only take geometric information as input and thus cannot in general avoid producing unnatural interpolation, in particular for large-scale deformations. This paper proposes a novel data-driven approach for shape morphing. Given a database with various models belonging to the same category, we treat them as data samples in the plausible deformation space. These models are then clustered to form local shape spaces of plausible deformations. We use a simple metric to reasonably represent the closeness between pairs of models. Given source and target models, the morphing problem is casted as a global optimization problem of finding a minimal distance path within the local shape spaces connecting these models. Under the guidance of intermediate models in the path, an extended as-rigid-as-possible interpolation is used to produce the final morphing. By exploiting the knowledge of plausible models, our approach produces realistic morphing for challenging cases as demonstrated by various examples in the paper.
Freeform Shadow Boundary Editing
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Mattausch, Oliver; Igarashi, Takeo; Wimmer, Michael; I. Navazo, P. Poulin
We present an algorithm for artistically modifying physically based shadows. With our tool, an artist can directly edit the shadow boundaries in the scene in an intuitive fashion similar to freeform curve editing. Our algorithm then makes these shadow edits consistent with respect to varying light directions and scene configurations, by creating a shadow mesh from the new silhouettes. The shadow mesh helps a modified shadow volume algorithm cast shadows that conform to the artistic shadow boundary edits, while providing plausible interaction with dynamic environments, including animation of both characters and light sources. Our algorithm provides significantly more fine-grained local and direct control than previous artistic light editing methods, which makes it simple to adjust the shadows in a scene to reach a particular effect, or to create interesting shadow shapes and shadow animations. All cases are handled with a single intuitive interface, be it soft shadows, or (self-)shadows on arbitrary receivers.
Physics Storyboards
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Ha, Sehoon; McCann, Jim; Liu, C. Karen; Popovic, Jovan; I. Navazo, P. Poulin
Physical simulation and other procedural methods are increasingly popular tools in interactive applications because they generate complex and reactive behaviors given only a few parameter settings. This automation accelerates initial implementation, but also introduces a need to tune the available parameters until the desired behaviors emerge. These adjustments are typically performed iteratively, with the designer repeatedly running- and interacting with-the procedural animation with different parameter settings. Such a process is inaccurate, time consuming, and requires deep understanding and intuition, as parameters often have complex, nonlinear effects. Instead, we propose that designers construct physics storyboards to accelerate the process of tuning interactive, procedural animations. Physics storyboards are collections of space-time snapshots that highlight critical events and outcomes. They can be used to summarize the effects of parameter changes (without requiring the designer to perform extensive play-testing); and-when augmented with designer-provided evaluation functions-allow automatic parameter selection. We describe our implementation of this method, including how we use sampling to ensure that our automatically-selected parameters generalize, and how we time-warp user input to adapt it to changing parameters. We validate our implementation by using it to perform various design tasks in three example games.
ArtiSketch: A System for Articulated Sketch Modeling
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Levi, Zohar; Gotsman, Craig; I. Navazo, P. Poulin
We present ArtiSketch - a system which allows the conversion of a wealth of existing 2D content into 3D content by users who do not necessarily possess artistic skills. Using ArtiSketch, a novice user may describe a 3D model as a set of articulated 2D sketches of a shape from different viewpoints. ArtiSketch then automatically converts the sketches to an articulated 3D object. Using common interactive tools, the user provides an initial estimate of the 3D skeleton pose for each frame, which ArtiSketch refines to be consistent between frames. This skeleton may then be manipulated independently to generate novel poses of the 3D model.
Smart Variations: Functional Substructures for Part Compatibility
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Zheng, Youyi; Cohen-Or, Daniel; Mitra, Niloy J.; I. Navazo, P. Poulin
As collections of 3D models continue to grow, reusing model parts allows generation of novel model variations. Naïvely swapping parts across models, however, leads to implausible results, especially when mixing parts across different model families. Hence, the user has to manually ensure that the final model remains functionally valid. We claim that certain symmetric functional arrangements (SFARR-s), which are special arrangements among symmetrically related substructures, bear close relation to object functions. Hence, we propose a purely geometric approach based on such substructures to match, replace, and position triplets of parts to create non-trivial, yet functionally plausible, model variations. We demonstrate that starting even from a small set of models such a simple geometric approach can produce a diverse set of non-trivial and plausible model variations.
Preface and Table of Contents
(The Eurographics Association and Blackwell Publishing Ltd., 2013) I. Navazo, P. Poulin
Geosemantic Snapping for Sketch-Based Modeling
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Shtof, Alex; Agathos, Alexander; Gingold, Yotam; Shamir, Ariel; Cohen-Or, Daniel; I. Navazo, P. Poulin
Modeling 3D objects from sketches is a process that requires several challenging problems including segmentation, recognition and reconstruction. Some of these tasks are harder for humans and some are harder for the machine. At the core of the problem lies the need for semantic understanding of the shape's geometry from the sketch. In this paper we propose a method to model 3D objects from sketches by utilizing humans specifically for semantic tasks that are very simple for humans and extremely difficult for the machine, while utilizing the machine for tasks that are harder for humans. The user assists recognition and segmentation by choosing and placing specific geometric primitives on the relevant parts of the sketch. The machine first snaps the primitive to the sketch by fitting its projection to the sketch lines, and then improves the model globally by inferring geosemantic constraints that link the different parts. The fitting occurs in real-time, allowing the user to be only as precise as needed to have a good starting configuration for this non-convex optimization problem. We evaluate the accessibility of our approach with a user study.
Fabrication-aware Design with Intersecting Planar Pieces
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Schwartzburg, Yuliy; Pauly, Mark; I. Navazo, P. Poulin
We propose a computational design approach to generate 3D models composed of interlocking planar pieces. We show how intricate 3D forms can be created by sliding the pieces into each other along straight slits, leading to a simple construction that does not require glue, screws, or other means of support. To facilitate the design process, we present an abstraction model that formalizes the main geometric constraints imposed by fabrication and assembly, and incorporates conditions on the rigidity of the resulting structure.We show that the tight coupling of constraints makes manual design highly nontrivial and introduce an optimization method to automate constraint satisfaction based on an analysis of the constraint relation graph. This algorithm ensures that the planar parts can be fabricated and assembled. We demonstrate the versatility of our approach by creating 3D toy models, an architectural design study, and several examples of functional furniture.
Stylized and Performative Gaze for Character Animation
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Pejsa, Tomislav; Mutlu, Bilge; Gleicher, Michael; I. Navazo, P. Poulin
Existing models of gaze motion for character animation simulate human movements, incorporating anatomical, neurophysiological, and functional constraints. While these models enable the synthesis of humanlike gaze motion, they only do so in characters that conform to human anatomical proportions, causing undesirable artifacts such as cross-eyedness in characters with non-human or exaggerated human geometry. In this paper, we extend a state-of-the- art parametric model of human gaze motion with control parameters for specifying character geometry, gaze dynamics, and performative characteristics in order to create an enhanced model that supports gaze motion in characters with a wide range of geometric properties that is free of these artifacts. The model also affords ''staging effects'' by offering softer functional constraints and more control over the appearance of the character's gaze movements. An evaluation study showed that the model, compared with the state-of-the-art model, creates gaze motion with fewer artifacts in characters with non-human or exaggerated human geometry while retaining their naturalness and communicative accuracy.
A Novel Projection Technique with Detail Capture and Shape Correction for Smoke Simulation
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Wu, Xiaoyue; Yang, Xubo; Yang, Yang; I. Navazo, P. Poulin
Smoke simulation on a large grid is quite time consuming and most of the computation time is spent on the projection step.We present a novel projection method which produces quite similar visual results as those produced with the traditional projection method, but uses much less computation time. Our method includes two steps: detail-capture and shape-correction. The first step preserves most of the smoke details using an efficient DST (Discrete Sine Transformation) Poisson Solver with auxiliary boundary sweeping. The second step maintains the overall flow shape by solving a correcting Poisson equation on a coarse grid. Our algorithm is very fast and quite easy to implement. Experiments show that our projection is approximately 10-30 times faster than the traditional projection with PCG(Preconditioned Conjugate Gradient), while convincingly preserving both the flow details and the overall shape of the smoke.
Primitive Trees for Precomputed Distance Queries
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Lee, Sung-Ho; Park, Taejung; Kim, Chang-Hun; I. Navazo, P. Poulin
We propose the primitive tree, a novel and compact space-partition method that samples and reconstructs a distance field with high accuracy, even for regions far from the surfaces. The primitive tree is based on the octree and stores the indices of the nearest primitives in its leaf nodes. Most previous approaches have involved a trade-off between accuracy and speed in distance queries, but our method can improve both aspects simultaneously. In addition, our method can sample unsigned distance fields effectively, even for self-intersecting and nonmanifold models. We present test results showing that our method can sample and represent large scenes, with more than ten million triangles, rapidly and accurately.
Computing and Fabricating Multiplanar Models
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Chen, Desai; Sitthi-amorn, Pitchaya; Lan, Justin T.; Matusik, Wojciech; I. Navazo, P. Poulin
We present a method for converting computer 3D models into physical equivalents. More specifically, we address the problem of approximating a 3D textured mesh using a small number of planar polygonal primitives that form a closed surface. This simplified representation allows us to easily manufacture individual components using computer controlled cutters (e.g., laser cutters or CNC machines). These polygonal pieces can be assembled into the final 3D model using internal planar connectors that are manufactured simultaneously. Our shape approximation algorithm iteratively assigns mesh faces to planar segments and slowly deforms these faces towards corresponding segments. This approach ensures that the output for a given closed mesh is still a closed mesh and avoids introducing self-intersections. After this step we also compute the shape of polygonal connectors that internally hold the whole mesh surface. Both the polygonal surface elements and connectors can be manufactured in a single cutting pass. We validate the use of our method by computing and manufacturing a variety of textured polyhedral models.
Coupled Quasi-harmonic Bases
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Kovnatsky, Artiom; Bronstein, Michael M.; Bronstein, Alexander M.; Glashoff, Klaus; Kimmel, Ron; I. Navazo, P. Poulin
The use of Laplacian eigenbases has been shown to be fruitful in many computer graphics applications. Today, state-of-the-art approaches to shape analysis, synthesis, and correspondence rely on these natural harmonic bases that allow using classical tools from harmonic analysis on manifolds. However, many applications involving multiple shapes are obstacled by the fact that Laplacian eigenbases computed independently on different shapes are often incompatible with each other. In this paper, we propose the construction of common approximate eigenbases for multiple shapes using approximate joint diagonalization algorithms, taking as input a set of corresponding functions (e.g. indicator functions of stable regions) on the two shapes. We illustrate the benefits of the proposed approach on tasks from shape editing, pose transfer, correspondence, and similarity.
Sparse Modeling of Intrinsic Correspondences
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Pokrass, Jonathan; Bronstein, Alexander M.; Bronstein, Michael M.; Sprechmann, Pablo; Sapiro, Guillermo; I. Navazo, P. Poulin
We present a novel sparse modeling approach to non-rigid shape matching using only the ability to detect repeatable regions. As the input to our algorithm, we are given only two sets of regions in two shapes; no descriptors are provided so the correspondence between the regions is not know, nor we know how many regions correspond in the two shapes. We show that even with such scarce information, it is possible to establish very accurate correspondence between the shapes by using methods from the field of sparse modeling, being this, the first non-trivial use of sparse models in shape correspondence. We formulate the problem of permuted sparse coding, in which we solve simultaneously for an unknown permutation ordering the regions on two shapes and for an unknown correspondence in functional representation. We also propose a robust variant capable of handling incomplete matches. Numerically, the problem is solved efficiently by alternating the solution of a linear assignment and a sparse coding problem. The proposed methods are evaluated qualitatively and quantitatively on standard benchmarks containing both synthetic and scanned objects.
Animal Locomotion Controllers From Scratch
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Wampler, Kevin; Popovic, Jovan; Popovic, Zoran; I. Navazo, P. Poulin
There exists a large body of research devoted to creating real time interactive locomotion controllers which are targeted at some specific class of character, most often humanoid bipeds. Relatively little work, however, has been done on approaches which are applicable to creatures with a wide range of different forms - partially due to the lack of easily obtainable motion-capture data for animals and fictional creatures. We show how a locomotion controller can be created despite this dearth of data by synthesizing it from scratch. Our method only requires as input a description of the shape of the animal, the gaits which it can perform, and a model of the task or tasks for which the controller will be used. From this a sequence of motion clips are automatically synthesized and assembled into a motion graph which defines a physically realistic controller capable of performing the specified tasks. The method attempts to minimize the computational time required to generate this controller and we show its effectiveness at creating interactive controllers for a range of tasks for bipeds, tripeds, and quadrupeds.
Scalable Music: Automatic Music Retargeting and Synthesis
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Wenner, Simon; Bazin, Jean-Charles; Sorkine-Hornung, Alexander; Kim, Changil; Gross, Markus; I. Navazo, P. Poulin
In this paper we propose a method for dynamic rescaling of music, inspired by recent works on image retargeting, video reshuffling and character animation in the computer graphics community. Given the desired target length of a piece of music and optional additional constraints such as position and importance of certain parts, we build on concepts from seam carving, video textures and motion graphs and extend them to allow for a global optimization of jumps in an audio signal. Based on an automatic feature extraction and spectral clustering for segmentation, we employ length-constrained least-costly path search via dynamic programming to synthesize a novel piece of music that best fulfills all desired constraints, with imperceptible transitions between reshuffled parts. We show various applications of music retargeting such as part removal, decreasing or increasing music duration, and in particular consistent joint video and audio editing.
Interactive Exploration and Flattening of Deformed Historical Documents
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Pal, Kazim; Terras, Melissa; Weyrich, Tim; I. Navazo, P. Poulin
We present an interactive application for browsing severely damaged documents and other cultural artefacts. Such documents often contain strong geometric distortions such as wrinkling, buckling, and shrinking and cannot be flattened physically due to the high risk of causing further damage. Previous methods for virtual restoration involve globally flattening a 3D reconstruction of the document to produce a static image. We show how this global approach can fail in cases of severe geometric distortion, and instead propose an interactive viewer which allows a user to browse a document while dynamically flattening only the local region under inspection. Our application also records the provenance of the reconstruction by displaying the reconstruction side by side with the original image data.
Analytic Visibility on the GPU
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Auzinger, Thomas; Wimmer, Michael; Jeschke, Stefan; I. Navazo, P. Poulin
This paper presents a parallel, implementation-friendly analytic visibility method for triangular meshes. Together with an analytic filter convolution, it allows for a fully analytic solution to anti-aliased 3D mesh rendering on parallel hardware. Building on recent works in computational geometry, we present a new edge-triangle intersection algorithm and a novel method to complete the boundaries of all visible triangle regions after a hidden line elimination step. All stages of the method are embarrassingly parallel and easily implementable on parallel hardware. A GPU implementation is discussed and performance characteristics of the method are shown and compared to traditional sampling-based rendering methods.

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