Selected Papers  ·  Books  ·  Other Publications 
2016
Boundary detection in particlebased fluids M. Sandim, D. Cedrim, L.G. Nonato, P. Pagliosa and A. Paiva Computer Graphics Forum, 35(2), pp. 215224, 2016. Proceedings of Eurographics 2016
Abstract: This paper
presents a novel method to detect freesurfaces on particlebased
volume representation. In contrast to most particlebased freesurface
detection methods, which perform the surface identification based on
physical and geometrical properties derived from the underlying fluid
flow simulation, the proposed approach only demands the spatial
location of the particles to properly recognize surface particles,
avoiding even the use of kernels. Boundary particles are identified
through a Hidden Point Removal (HPR) operator used for visibility test.
Our method is very simple, fast, easy to implement and robust to
changes in the distribution of particles, even when facing large
deformation of the freesurface. A set of comparisons against
stateoftheart boundary detection methods show the effectiveness of
our approach. The good performance of our method is also attested in
the context of fluid flow simulation involving freesurface, mainly when using levelsets for rendering purposes.

Visualizing and interacting with kernelized data A. Barbosa, F. Paulovich, A. Paiva, S. Goldenstein, F. Petronetto and L.G. Nonato IEEE TVCG, 22(3), pp. 13141325, 2016. Abstract: Kernelbased methods have experienced a substantial progress in the last years, tuning out an essential mechanism for data classification, clustering and pattern recognition. The effectiveness of kernelbased techniques, though, depends largely on the capability of the underlying kernel to properly embed data in the feature space associated to the kernel. However, visualizing how a kernel embeds the data in a feature space is not so straightforward, as the embedding map and the feature space are implicitly defined by the kernel. In this work, we present a novel technique to visualize the action of a kernel, that is, how the kernel embeds data into a highdimensional feature space. The proposed methodology relies on a solid mathematical formulation to map kernelized data onto a visual space. Our approach is faster and more accurate than most existing methods while still allowing interactive manipulation of the projection layout, a gamechanging trait that other kernelbased projection techniques do not have. 
2015
Particlebased fluids for viscous jet buckling L.F.S. Andrade, M. Sandim, F. Petronetto, P. Pagliosa and A. Paiva Computers & Graphics, 52, pp. 106115, 2015. Abstract: In this paper, we introduce a novel meshfree framework for animating free surface viscous liquids with jet buckling effects, such as coiling and folding. Our method is based on Smoothed Particle Hydrodynamics (SPH) fluids and allows more realistic and complex viscous behaviors than the previous SPH frameworks in computer animation literature. The viscous liquid is modeled by a nonNewtonian fluid flow and the variable viscosity under shear stress is achieved using a viscosity model known as Cross model. We demonstrate the efficiency and stability of our framework in a wide variety of animations, including scenarios with arbitrary geometries and high resolution of SPH particles. The interaction of the viscous liquid with complex solid obstacles is performed using boundary particles. Our framework is able to deal with different inlet velocity profiles and geometries of the injector, as well as moving inlet jet along trajectories given by cubic Hermite splines. Moreover, the simulation speed is significantly accelerated by using Computer Unified Device Architecture (CUDA) computing platform. Extended version of SIBGRAPI 2014 paper. 
Concentric Radviz: visual exploration of multitask classification J.H.P. Ono, F. Sikansi, D.C. Correa, F. Paulovich, A. Paiva and L.G. Nonato Proceedings of SIBGRAPI 2015, IEEE Press, pp. 165172, 2015. Abstract: The discovery of patterns in large data collections is a difficult task. Visualization and machine learning techniques have emerged as a way to facilitate data analysis, providing tools to uncover relevant patterns from the data. This paper presents Concentric Radviz, a general purpose class visualization system that takes into account multiclass, multilabel and multitask classifiers. Concentric Radviz uses a force attenuation scheme, which minimizes cluttering and ambiguity in the visual layout. In addition, the user can add concentric circles to the layout in order to represent classification tasks. Our validation results and the application of Concentric Radviz for two real collections suggest that this tool can reveal important data patterns and relations. In our application, the user can interact with the visualization by selecting regions of interest according to specific criteria and changing projection parameters. 
Exploratory segmentation of vector fields using multidimensional projection D. Motta, M. Oliveira, P. Pagliosa, L.G. Nonato and A. Paiva Proceedings of SIBGRAPI 2015, IEEE Press, pp. 250256, 2015. Abstract: The difficulty to understand the complex behavior of vector fields makes its visual segmentation an area of constant interest in scientific visualization. In this paper, we present a novel interactive segmentation framework for discrete vector fields. In our method, the vector field domain is partitioned into multiple regions with same flow patterns. In order to accomplish this task, feature vectors are extracted from streamlines and mapped to a visual space using multidimensional projection. The interactivity with projected data in the visual space improving the results of the segmentation according to user’s knowledge. The provided results and comparisons show the flexibility and effectiveness of our framework. 
SHREC’15 Track: Nonrigid 3D Shape Retrieval Z. Lian, J. Zhang, S. Choi, H. ElNaghy, J. ElSana, T. Furuya, A. Giachetti, R.A. Guler, L. Isaia, L. Lai, C. Li, H. Li, F.A. Limberger, R. Martin, R.U. Nakanishi, A. Paiva, L.G. Nonato, R. Ohbuchi, K. Pevzner, D. Pickup, P. Rosin, A. Sharf L. Sun, X. Sun, S. Tari, G. Unal and R.C. Wilson Proceedings of EG Workshop on 3D Object Retrieval, 2015. Abstract: Nonrigid 3D shape retrieval has become a research hotpot in communities of computer graphics, computer vision, pattern recognition, etc. In this paper, we present the results of the SHREC’15 Track: Nonrigid 3D Shape Retrieval. The aim of this track is to provide a fair and effective platform to evaluate and compare the performance of current nonrigid 3D shape retrieval methods developed by different research groups around the world. The database utilized in this track consists of 1200 3D watertight triangle meshes which are equally classified into 50 categories. All models in the same category are generated from an original 3D mesh by implementing various pose transformations. The retrieval performance of a method is evaluated using 6 commonlyused measures (i.e., PRplot, NN, FT, ST, Emeasure and DCG.). Totally, there are 37 submissions and 11 groups taking part in this track. Evaluation results and comparison analyses described in this paper not only show the bright future in researches of nonrigid 3D shape retrieval but also point out several promising research directions in this topic. 
Allinfocus imaging technique used to improve 3d retinal fundus image reconstruction D. Motta, L. Matos, A.C. Souza, R. Marcato, A. Paiva and L.A.V. Carvalho Proceedings of ACM SAC 2015, pp. 2631, 2015. Abstract: In this paper we have applied the stacking technique on images from a medical device. There is an urgent need in ophthalmology for more cost effective instrumentation for the early diagnosis of glaucoma, one of the leading diseases in the cause of blindness worldwide. The current techniques involve expensive optical equipments generally called fundus camera, which most of the time capture a single high resolution frame for one eye at a time. In this research we have used stereoscopic videos of a stateoftheart 3D reti nal camera, which has simpler optics and electronics when compared to current monoscopic models. Nevertheless, the cross correlation algorithms for depth computation are very sensible to image noise and out of focus regions. We demons trate the efficiency of our technique on experiments involving a sequence of images extracted from videos in simulations of optic nerves using artificial objects. 
2014
SPH
fluids for viscous jet buckling L.F.S. Andrade, M. Sandim, F. Petronetto, P. Pagliosa and A. Paiva Proceedings of SIBGRAPI 2014, IEEE Press, pp. 6572, 2014. Abstract: We present a novel meshfree technique for animating free surface viscous liquids with jet buckling effects, such as coiling and folding. Our technique is based on Smoothed Particle Hydrodynamics (SPH) fluids and allows more realistic and complex viscous behaviors than the preceding SPH frameworks in computer animation literature. The viscous liquid is modeled by a nonNewtonian fluid flow and the variable viscosity under shear stress is achieved using a viscosity model known as Cross model. The proposed technique is efficient and stable, and our framework can animate scenarios with high resolution of SPH particles in which the simulation speed is significantly accelerated by using Computer Unified Device Architecture (CUDA) computing platform. This work also includes several examples that demonstrate the ability of our technique. 
Approximating implicit curves on plane and
surface triangulations with affine arithmetic F.C. Nascimento, A. Paiva, L.H. Figueiredo and J. Stolfi Computers & Graphics 40, pp. 3648, 2014. Abstract: We present a spatially and geometrically adaptive method for computing a robust polygonal approximation of an implicit curve defined on a planar region or on a triangulated surface. Our method uses affine arithmetic to identify regions where the curve lies inside a thin strip. Unlike other interval methods, even those based on affine arithmetic, our method works on both rectangular and triangular decompositions and can use any refinement scheme that the decomposition offers. Extended version of SIBGRAPI 2012 paper. 
2013
MIST:
multiscale information and summaries of texts P. Pagliosa, R.M. Martins, D. Cedrim, A. Paiva, R. Minghim and L.G. Nonato Proceedings of SIBGRAPI 2013, IEEE Press, pp. 9198, 2013. Abstract: Combining distinct visual metaphors has been the mechanism adopted by several systems to enable the simultaneous visualization of multiple levels of information in a single layout. However, providing a meaningful layout while avoiding visual clutter is still a challenge. In this work we combine word clouds and a rigidbody simulation engine into an intuitive visualization tool that allows a user to visualize and interact with the content of document collections using a single overlapfree layout. The proposed force scheme ensures that neighboring documents are kept close to each other during and after layout change. Each group of neighboring documents formed on the layout generates a word cloud. A multiseeded procedure guarantees a harmonious arrangement of distinct word clouds in visual space. The visual metaphor employs lenses to represent document instances where the size of each lens defines the importance of the document in the collection. To keep the visualization clean and intuitive, only the most relevant documents are depicted as lens while the remaining ones are either displayed as smaller glyphs to help convey density information or simply removed from the layout. Hidden instances are moved together with its neighbors during forcebased simulation, should they become visible later, but are not processed individually. This shadow movement avoids excess calculations by the forcebased scheme, thus ensuring scalability and interactivity. 
Normal correction towards smoothing
pointbased surfaces P. Valdivia, D. Cedrim, F. Petronetto, A. Paiva and L.G. Nonato Proceedings of SIBGRAPI 2013, IEEE Press, pp. 187194, 2013. Abstract: Some of the main approaches for mesh denoising uses a twostep scheme, by filtering normals to later perform a point updating step to match the corrected normals. In this paper, we propose an adaptation of a twostep approach for point set surfaces, exploring three different weight schemes. Moreover, we explore three techniques for normal estimation, and how it improve the quality of filtering process. Towards a quantitative analysis, in addition of usual visual comparison, we evaluate the effectiveness of each combination by three measures and compare them against some stateofart pointbased denoising techniques. 
Meshfree discrete LaplaceBeltrami operator F. Petronetto, A. Paiva, E.S. Helou, D.E. Stewart and L.G. Nonato Computer Graphics Forum, 32(6), pp. 214226, 2013. Abstract: In this work we propose a new discretization method for the LaplaceBeltrami operator defined on pointbased surfaces. In contrast to the existing pointbased discretization techniques, our approach does not rely on any triangle mesh structure, turning out truly meshfree. Based on a combination of Smoothed Particle Hydrodynamics and an optimization procedure to estimate area elements, our discretization method results in accurate solutions while still being robust when facing abrupt changes in the density of points. Moreover, the proposed scheme results in numerically stable discrete operators. The effectiveness of the proposed technique is brought to bear in many practical applications. In particular, we use the eigenstructure of the discrete operator for filtering and shape segmentation. Pointbased surface deformation is another application that can be easily carried out from the proposed discretization method. 
Spectral image segmentation using image
decomposition and inner productbased metric W. Casaca, A. Paiva, E.G. Nieto, P. Joia and L.G. Nonato Journal of Mathematical Imaging and Vision, 45(3), pp. 227238, 2013. Abstract: Image segmentation is an indispensable tool in computer visionapplications, such as recognition, detection and tracking. In this work, we introduce a novel userassisted image segmentation technique which combines image decomposition, inner productbased similarity metric, and spectral graph theory into a concise and unified framework.First, we perform an image decomposition to split the image into texture and cartoon components. Then, an affinity graph is generated and the weights are assigned to its edges according to a gradientbased innerproduct function. From the eigenstructure of the affinity graph,the image is partitioned through the spectral cut of the underlying graph. The computational effort of our framework is alleviated by animage coarsening process, which reduces the graph size considerably. Finally, a coarsetofine interpolation is applied in order to assemble the partition back onto the original image. The efficiency of the proposed methodology is attested by comparisons with stateofart spectral segmentation methods through a qualitative and quantitative analysis of the results. Extended version of SIBGRAPI 2011 paper. 
2012
Approximating implicit curves on
triangulations with affine arithmetic A. Paiva, F.C. Nascimento, L.H. Figueiredo and J. Stolfi Proceedings of SIBGRAPI 2012, IEEE Press, pp. 94101, 2012. Abstract: We present an adaptive method for computing a robust polygonal approximation of an implicit curve in the plane that uses affine arithmetic to identify regions where the curve lies inside a thin strip. Unlike other interval methods, even those based on affine arithmetic, our method works on triangulations, not only on rectangular quadtrees. 
Colorization by multidimensional projection W. Casaca, E.G. Nieto, C. Ferreira, G. Tavares, P. Pagliosa, F. Paulovich, L.G. Nonato and A. Paiva Proceedings of SIBGRAPI 2012, IEEE Press, pp. 3238, 2012. Abstract: Most image colorization techniques assign colors to grayscale images by embedding image pixels into a highdimensional feature space and applying a color pattern to each cluster of highdimensional data. A main drawback of such an approach is that, depending on texture patterns and image complexity, clusters of similar pixels can hardly be defined automatically, rendering existing methods prone to fail. In this work we present a novel approach to colorize grayscale images that allows for user intervention. Our methodology makes use of multidimensional projection to map highdimensional data to a visual space. User can manipulate projected data in the visual space so as to further improve clusters and thus the colorization result. Different from other methods, our interactive tool is ease of use while still being flexible enough to enable local color modification. We show the effectiveness of our approach through a set of examples and comparisons against existing colorization methods. 
Classspecific metrics for multidimensional
data projection applied to CBIR P. Joia, E.G. Nieto, J.B. Neto, W. Casaca, G. Botelho, A. Paiva and L.G. Nonato The Visual Computer, 28(10), pp. 10271037, 2012. Abstract: Contentbased image retrieval is still a challenging issue due to the inherent complexity of images and choice of the most discriminant descriptors. Recent developments in the field have introduced multidimensional projections to burst accuracy in the retrieval process, but many issues such as introduction of pattern recognition tasks and deeper user intervention to assist the process of choosing the most discriminant features still remain unaddressed. In this paper we present a novel framework to CBIR that combines pattern recognition tasks, classspecific metrics and multidimensional projection to devise an effective and interactive image retrieval system. User interaction plays an essential role in the computation of the final multidimensional projection from which image retrieval will be attained. Results have shown that the proposed approach outperforms existing methods, turning out to be a very attractive alternativefor managing image data sets. Extended version of SIBGRAPI 2011 paper. 
2011
Sketchbased
adaptive mesh augmentation using stellar operators A. Paiva, R. Amorim, L. Velho and M.C. Sousa Proceedings of SIBGRAPI 2011, IEEE Press, pp. 7885, 2011. Abstract: In this paper we present a new method for modeling and editing surface detail using freeform curves and a natural interface. It combines in a original way an adaptive multiresolution mesh structure with a simple, intuitive sketchbased interface. One of the novel contributions of this work is the curve sensitive mesh resolution control, which allows the definition of a rich set of operators that locally modify the surface geometry. Furthermore, the present framework provides the basic functionality to build a complete feature based modeling system. 
Spectral segmentation using cartoontexture
decomposition and inner productbased metric W. Casaca, A. Paiva and L.G. Nonato Proceedings of SIBGRAPI 2011, IEEE Press, pp. 266273, 2011. Abstract: This paper presents a userassisted image partition technique that combines CartoonTexture Decomposition (CTD), inner productbased similarity metric, and spectral cut into a unified framework. The CTD is used to first split the image into textured and texturefree components, the latter being used to define a gradientbased innerproduct function. An affinity graph is then derived and weights are assigned to its edges according to the inner productbased metric. Spectral cut is computed on the affinity graph so as to partition the image. The computational burden of the spectral cut is mitigated by a finetocoarse image representation process, which enables moderate size graphs that can be handled more efficiently. The partitioning can be steered by interactively by changing the weights of the graph through user strokes. Weights are updated by combining the texture component computed in the first stage of our pipeline and a recent harmonic analysis technique that captures waving patterns. Finally, a coarsetofine interpolation is applied in order to project the partition back onto the original image. The suitable performance of the proposed methodology is attested by comparisons against stateofart spectral segmentation methods. 
Projectionbased image retrieval using
classspecific metrics P. Joia, E.G. Nieto, G. Botelho, J.B. Neto, A. Paiva and L.G. Nonato Proceedings of SIBGRAPI 2011, IEEE Press, pp. 125132, 2011. Abstract: Contentbased image classification/retrieval based on image descriptors has become an essential component in most database systems. However, most existing systems do not provide mechanisms that enable interactive multiobjective queries, hampering the user experience. In this paper we present a novel methodology capable of accomplishing multiobjective searches while still being interactive. Our approach relies on a combination of classspecific metrics and multidimensional projection to devise an effective and interactive image retrieval system. Besides allowing visual exploration of image data sets, the provided results and comparisons show that the proposed approach outperforms existing methods, turning out to be a veryattractive alternative for managing image data sets. 
2010

Meshless
HelmholtzHodge decomposition F. Petronetto, A. Paiva, M. Lage, G. Tavares, H. Lopes and T. Lewiner IEEE TVCG, 16(2), pp. 338349, 2010. Abstract: Vector fields analysis traditionally distinguishes conservative (curlfree) from mass preserving (divergencefree) components. The HelmholtzHodge decomposition allows separating any vector field into the sum of three uniquely defined components: curlfree, divergencefree and harmonic. This decomposition is usually achieved by using meshbased methods such as finite differences or finite elements. This work presents a new meshless approach to the HelmholtzHodge decomposition for the analysis of 2D discrete vector fields. It embeds into the SPH particlebased framework. The proposed method is efficient and can be applied to extract features from a 2D discrete vector field and to multiphase fluid flow simulation to ensure incompressibility. 
2009

Fluidbased
hatching
for tone mapping in line illustrations A. Paiva, E.V. Brazil, F. Petronetto and M.C. Sousa The Visual Computer (Proceedings of CGI'09), 25(57), pp. 519527, 2009. Abstract: This paper presents a novel physicallybased, meshless framework for line art rendering of surfaces with complex geometry and arbitrary topology. We apply an inviscid fluid flow simulation using Smoothed Particles Hydrodynamics to compute the global velocity and cross fields over the surface model. These fields guide the automatic placement of strokes while marinating the geometric and topological coherence with the model. Target tones are matched by tonal value maps allowing different hatching and crosshatching effects. We demonstrate the simplicity and effectiveness of our method with sample renderings obtained for a variety of models. 
Particlebased
viscoplastic fluid/solid simulation A. Paiva, F. Petronetto, T. Lewiner and G. Tavares ComputerAided Design, 41(4), pp. 306314, 2009. Abstract: Simulations of viscoplastic materials are traditionally governed by continuum mechanics. The viscous behavior is typically modeled as an internal force, defined by diverse quantities. This work introduces a fluid model to simulate the viscoplastic effect of solid materials, such as plastic, wax, clay and polymer. Our method consists in modeling a solid object through a nonNewtonian fluid with high viscosity. This fluid simulation uses the Smoothed Particle Hydrodynamics method and the viscosity is formulated by using the General Newtonian Fluid model. This model concentrates the viscoplasticity in a single parameter. Our results show clear effects of creep, melting, hardening and flowing. Extended version of SIBGRAPI 2006 paper. 
2006

Particlebased
nonNewtonian fluid animation for melting objects A. Paiva, F. Petronetto, T. Lewiner and G. Tavares Proceedings of SIBGRAPI 2006, IEEE Press, pp. 7885, 2006. Abstract: This paper presents a new visually realistic animation technique for objects that melt and flow. It simulates viscoplastic properties of materials such as metal, plastic, wax, polymer and lava. The technique consists in modeling the object by the transition of a nonNewtonian fluid with high viscosity to a liquid of low viscosity. During the melting, the viscosity is formulated using the General Newtonian fluids model, whose properties depend on the local temperature. The phase transition is then driven by the heat equation. The fluid simulation framework uses a variation of the Lagrangian method called Smoothed Particle Hydrodynamics. This paper also includes several schemes that improve the efficiency and the numerical stability of the equations. 

Robust adaptive meshes for implicit surfaces A. Paiva, H. Lopes, T. Lewiner and L.H. Figueiredo Proceedings of SIBGRAPI 2006, IEEE Press, pp. 205212, 2006. Abstract: This work introduces a robust algorithm for computing good polygonal approximations of implicit surfaces, where robustness entails recovering the exact topology of the implicit surface. Furthermore, the approximate triangle mesh adapts to the geometry and to the topology of the real implicit surface. This method generates an octree subdivided according to the interval evaluation of the implicit function in order to guarantee the robustness, and to the interval automatic differentiation in order to adapt the octree to the geometry of the implicit surface. The triangle mesh is then generated from that octree through an enhanced dual marching. 

Vector
field reconstruction from sparse samples with applications M. Lage, F. Petronetto, A. Paiva, H. Lopes, T. Lewiner and G. Tavares Proceedings of SIBGRAPI 2006, IEEE Press, pp. 297304, 2006. Abstract: This work presents a novel algorithm for 2D vector field reconstruction from sparse set of pointsvectors pairs. Our approach subdivides the domain adaptively in order to make local piecewise polynomial approximations for the field. It uses partition of unity to blend those local approximations together, generating a global approximation for the field. The flexibility of this scheme allows handling data from very different sources. In particular, this work presents important applications of the proposed method to velocity and acceleration fields analysis, in particular for fluid dynamics visualization. Front cover of the Proceedings of SIBGRAPI 2006. 

Robust
visualization of strange attractors using affine arithmetic A. Paiva, L.H. Figueiredo and J. Stolfi Computers & Graphics, 30(6), pp. 10201026, 2006. Abstract: We propose the use of affine arithmetic in cellmapping methods for the robust visualization of strange attractors and show that the resulting cellular approximations converge faster than those produced by cellmapping methods based on classical interval arithmetic. 