# Jorge StolfiUniversity of Campinas | UNICAMP

Jorge Stolfi

## About

139

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Citations since 2017

## Publications

Publications (139)

One of the factors responsible for the mechanical strength of objects manufactured by thermoplastic extru-sion 3D printers is the adhesion between adjacent material beads in the same layer. This, in turn, depends on the time lapsed between their deposition. In this paper we describe HotFill, an algorithm that finds a tool-path for solid raster infi...

Multifocal microscopy is an established technique to combine several reflected-light microscopy images of an object, each with a limited depth of focus, into a single image that shows the whole object in focus. Photometric stereo is an independent technique to recover the third dimension of an opaque object, given several images taken from the same...

We describe ECLES (Editing by Constrained LEast Squares), a general method for interactive local editing of objects that are defined by a list of parameters subject to a set of linear or affine constraints. The method is intended for situations where each edit action affects only a small set of the parameters; some of which (the “anchors”) are to b...

We describe an algorithm to triangulate a general 3-dimensional-map on an arbitrary space in such way that the resulting 3-dimensional triangulation is vertex-colorable with four colors. (Four-colorable triangulations can be efficiently represented and manipulated by the GEM data structure of Montagner and Stolfi.) The standard solution to this pro...

Multifocal microscopy is an established technique
to combine several reflected-light microscopy images of an
object, each with a limited depth of focus, into a single image
that shows the whole object in focus. Photometric stereo is
an independent technique to recover the third dimension of
an opaque object, given several images taken from the same...

In this paper we describe VOPT (Visual Odometry by Patch Tracking), a robust algorithm for visual odometry, which is able to operate with sparse or dense maps computed by simultaneous localization and mapping (SLAM) algorithms. By using an iterative multi-scale procedure, VOPT is able to estimate the individual motion, photometric correction and re...

We describe efficient algorithms for adaptive multiscale approximation of functions that are sampled with uneven density and/or have important small-scale detail limited to small portions of their domain. Our algorithm constructs the approximation from the top down, using a special least squares fitting of the residual at each level, followed by a...

We describe an algorithm for slicing an unstructured triangular mesh model by a series of parallel planes. We prove that the algorithm is asymptotically optimal: its time complexity is O(nlogk + k + m) for irregularly spaced slicing planes, where n is the number of triangles, k is the number of slicing planes, and m is the number of triangle-plane...

We describe an algorithm to triangulate a general map on an arbitrary surface in such way that the resulting triangulation is vertex-colorable with three colors. (Three-colorable triangulations can be efficiently represented and manipulated by the GEM data structure of Montagner and Stolfi.) The standard solution to this problem is the barycentric...

We describe a robust method for recovery of the depth coordinate from a normal or slope map of a scene, such as those obtained through photometric stereo or interferometry. The key features of the method are multi-scale in-tegration (with proper averaging and interpolation formulas) and the use of a reliability weight mask to handle regions with un...

This article describes a three-channel encoding of nucleotide sequences, and proper formulas for filtering and downsampling such encoded sequences for multi-scale signal analysis. With proper interpolation, the encoded sequences can be visualized as curves in three-dimensional space. The filtering uses Gaussian-like smoothing kernels, chosen so tha...

O presente trabalho se enquadra na area de Analise Numerica, com enfoque em tecnicas modernas de aproximacao para funcoes em dominios multidimensionais que requerem resolucao espacial adaptativa. Especificamente, o objetivo e encontrar algoritmos eficientes para aproximacao de funcoes que apresentam detalhes importantes mas de pequena escala (alta...

This paper considers the design of adaptive finite-volume discretizations for conservation laws. The methodology comes from the context of multiresolution representation of functions, which is based on cell averages on a hierarchy of nested grids. The refinement process is performed by the partition of each cell at a certain level into two equal ch...

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 quad trees.

We show that using example-based photometric stereo, it is possible to achieve realistic reconstructions of the human face. The method can handle non-Lambertian reflectance and attached shadows after a simple calibration step. We use spherical harmonics to model and de-noise the illumination functions from images of a reference object with known sh...

We describe an algorithm to subdivide an arbitrary triangulation of a surface to produce a triangulation that is vertex-colorable with three colors. (Three-colorable triangulations can be efficiently represented and manipulated by the GEM data structure of Montagner and Stolfi.) The standard solution to this problem is the barycentric subdivision,...

We discuss the use of histogram of oriented gradients (HOG) descriptors as an effective tool for text description and recognition. Specifically, we propose a HOG-based texture descriptor (T-HOG) that uses a partition of the image into overlapping horizontal cells with gradual boundaries, to characterize single-line texts in outdoor scenes. The inpu...

We propose a modified adaptive multiresolution scheme for solving d-dimensional hyperbolic conservation laws which is based on cell-average discretization in dyadic grids. Adaptivity is obtained by interrupting the refinement at the locations where appropriate scale (wavelet) coefficients are sufficiently small. One important aspect of such a multi...

We describe a robust method for the recovery of the depth map (or height map) from a gradient map (or normal map) of a scene, such as would be obtained by photometric stereo or interferometry. Our method allows for uncertain or missing samples, which are often present in experimentally measured gradient maps, and also for sharp discontinuities in t...

We introduce IFTrace, a method for video segmentation of deformable objects. The algorithm makes minimal assumptions about the nature of the tracked object: basically, that it consists of a few connected regions, and has a well-defined border. The objects to be tracked are interactively segmented in the first frame of the video, and a set of marker...

In this paper, we describe a data structure and an algorithm to accelerate the table lookup step in example-based multiimage photometric stereo. In that step, one must find a pixel of a reference object, of known shape and color, whose appearance under different illumination fields is similar to that of a given scene pixel. This search reduces to f...

Shape deformation methods are important in such fields as geometric modeling and computer animation. In biology, modeling of shape, growth, movement and pathologies of living microscopic organisms or cells require smooth deformations, which are essentially 2D with little change in depth. In this paper, we present a 2.5D space deformation method. Th...

We describe a fast and robust gradient integration method that computes scene depths (or heights) from surface gradient (or surface normal) data such as would be obtained by photometric stereo or interferometry. Our method allows for uncertain or missing samples, which are often present in experimentally measured gradient maps; for sharp discontinu...

Text detection and recognition in real images taken in unconstrained environments, such as street view images, remain surprisingly challenging in Computer Vision.

In this work we introduced SnooperTrack, an algorithm for the automatic detection and tracking of text objects — such as store names, traffic signs, license plates, and advertisements — in videos of outdoor scenes. The purpose is to improve the performances of text detection process in still images by taking advantage of the temporal coherence in v...

We describe a general method for fitting 3D models of deformable biological structures to microscope images. The method uses multiscale image matching techniques with gradual introduction of parameters as well as specialized image synthesis methods and image distance metrics.

In this work we describe a general algorithm to find a finite-element basis with minimum total support for an arbitrary spline space, given any basis for that same space. The running time is exponential on n in the worst case, but O(nm 3 ) for many cases of practical in- terest, where n is the number of mesh cells and m is the dimension of the spli...

Finite element bases defined by sampling points were used by J. Lehtinen in 2008 for the efficient computation of global illumination in virtual scenes. The bases provide smooth approximations for the radiosity and spontaneous emission functions, leading to a discrete version of Kajiya's rendering equation. Unlike methods that are based on surface...

We describe a robust method to recover the depth coordinate from a normal or slope map of a scene, obtained e.g. through photometric stereo or interferometry. The key feature of our method is the fast solution of the Poisson-like integration equations by a multi-scale iterative technique. The method accepts a weight map that can be used to exclude...

We describe a robust and accurate algorithm, nicknamed AffTrack, to track selected features of a rigid 3D object in a video recording, given a canonical image of each feature and its position on the object. AffTrack uses a synergistic combination of a multiscale feature finder and a flexible camera calibrator. This synergy between the two modules a...

We propose a fast and robust approach to track deformable and rigid objects from a stationary/moving camera. The method combines seed estimation by the Kanade-Lucas-Tomasi (KLT) algorithm with object delineation by the image foresting transform to segment the object along the frames. The segmentation does not need to be perfect and it considerably...

The skewness sκ(G) of a graph G = (V, E) is the smallest integer sκ(G) ≥ 0 such that a planar graph can be obtained from G by the removal of sκ(G) edges. The splitting number sp(G) of G is the smallest integer sp(G) ≥ 0 such that a planar graph can be obtained from G by sp(G) vertex splitting operations. The vertex deletion vd(G) of G is the smalle...

We propose a modified adaptive multiresolution scheme for representing d-dimensional signals which is based on cell-average discretization in dyadic grids. A dyadic grid is an hierarchy of meshes where a cell at a certain level is partitioned into two equal children at the next refined level by hyperplanes perpendicular to one of the coordinate axe...

We describe a procedure to solve the basic problem of variable lighting photometric stereo - namely, recovering the normal directions and intrinsic albedos at all visible points of an opaque object, by analyzing three or more photos of the same taken with different illuminations. We follow the gauge-based approach, where the lighting conditions and...

1 – lucas.batista.freitas@gmail.com 2 – stolfi@ic.unicamp.br 3 – tygel@ime.unicamp.br We describe a fast method for seismic ray tracing in a cellular model, in which cells can have general polynomial shapes with non-planar bounding faces. The key idea is integration of the ray equations in terms of local cell coordinates rather than spatial coordin...

We describe a Bayesian inference method for the identification of protein coding regions (active or residual) in DNA or RNA
sequences. Its main feature is the computation of the conditional and a priori probabilities required in Bayes’s formula by factoring each event (possible annotation) for a nucleotide string into the
concatenation of shorter e...

In this paper, we show how to speed up the table lookup step in gauge-based multi-image photometric stereo. In that step, one must find a pixel of a gauge object, of known shape and color, whose appearance under m different illumination fields is similar to that of a given scene pixel. This search reduces to finding the closest match to a given m-...

Deformable model tracking is a powerful methodology that allows us to track the evolution of high-dimensional parameter vectors from uncalibrated monocular video sequences. The core of the approach consists of using low-level vision algorithms, such as edge trackers or optical flow, to collect a large number of 2D displacements, or motion measureme...

Our goal in this paper is the reliable detection of camera motion (pan/zoom/tilt) in video records. We propose an algorithm based on weighted optical flow least-square fitting, where an iterative procedure is used to improve the corresponding weights. To the optical flow computation we used the Kanade-Lucas-Tomasi feature tracker. Besides detecting...

We propose the use of affine arithmetic in cell-mapping methods for the robust visualization of strange attractors and show that the resulting cellular approximations converge faster than those produced by cell-mapping methods based on classical interval arithmetic.

A dyadic grid is a d-dimensional hierarchical mesh where a cell at level k is partitioned into two equal children at level k+1k+1 by a hyperplane perpendicular to coordinate axis (kmodd). We consider here the finite element approach on adaptive grids, static and dynamic, for various functional approximation problems.We review here the theory of ada...

The nonplanar vertex deletion or vertex deletion vd(G)vd(G) of a graph G is the smallest nonnegative integer k, such that the removal of k vertices from G produces a planar graph G′G′. In this case G′G′ is said to be a maximum planar induced subgraph of G. We solve a problem proposed by Yannakakis: find the threshold for the maximum degree of a gra...

Reassembling unknown broken objects from a large collection of fragments is a common problem in archaeology and other fields.
Computer tools have recently been developed, by the authors and by others, which try to help by identifying pairs of fragments
with matching outline shapes. Those tools have been successfully tested on small collections of f...

The non planar vertex deletion or vertex deletion vd(G) of a graph G = (V, E) is the smallest non negative integer k, such that the removal of k vertices from G produces a planar graph. Hence, the maximum planar induced subgraph of G has precisely |V| -vd(G) vertices. The problem of computing vertex deletion is in general very hard, it is NP-comple...

The necessary information to reproduce and keep an organism is codified in acid nucleic molecules. Deepening the knowledge about how the information is stored in these bio-sequences can lead to more efficient methods of comparing genomic sequences. In the present study, we analyzed the quantity of information contained in a DNA sequence that can be...

The non planar vertex deletion vd(G), of a graph G is the smallest positive integer k, such that the removal of k vertices from G produces a planar graph. We solve a problem proposed by Yannakakis: find the threshold for the maximum degree of a graph G such that, given a graph G and a positive integer k, to decide whether vd(G)≤k is NP-complete. We...

Affine arithmetic is a model for self-validated numerical computation that keeps track of first-order correlations between computed and input quantities. We explain the main concepts in affine arithmetic and how it handles the dependency problem in standard interval arithmetic. We also describe some of its applications.

Deformable model tracking is a powerful methodology that allows us to track the evolution of high-dimensional parameter vectors from uncalibrated monocular video sequences. The core of the approach consists of using low-level vision algorithms, such as edge trackers or optical flow, to collect a large number of 2D displacements, or motion measureme...

The image foresting transform (IFT) is a graph-based approach to the design of image processing operators based on connectivity. It naturally leads to correct and efficient implementations and to a better understanding of how different operators relate to each other. We give here a precise definition of the IFT, and a procedure to compute it-a gene...

The image foresting transform (IFT) is a graph-based approach to the design of image processing operators based on connectivity. It naturally leads to correct and efficient implementations and to a better understanding of how different operators relate to each other. We give here a precise definition of the IFT, and a procedure to compute it-a gene...

Affine arithmetic (AA) is a model for self-validated computation which, like standard interval arithmetic (IA), produces guaranteed enclosures for com- puted quantities, taking into account any uncertainties in the input data as well as all internal truncation and roundoff errors. Unlike standard IA, the quantity representations used by AA are firs...

Affine arithmetic is a model for self-validated numerical computation that affine arithmetic keeps track of first-order correlations between computed and input quantities. We explain the main concepts in affine arithmetic and it handles the dependency problem in standard interval arithmetic. We also describe some of its applications.

We describe here an exact, and hence robust, algorithm for point location on spherical maps (maps on the sphere composed by arcs of circles, not necessarily geodesic ones). The algorithm relies on an exact representation for arcs of circles on the sphere, based on integer homogeneous coordinates, and exact geometric operations for such arcs. The to...

We show how to use affine arithmetic to represent a parametric curve with a strip tree. The required bounding rectangles for pieces of the curve are computed by exploiting the linear correlation information given by affine arithmetic. As an application, we show how to compute approximate distance fields for parametric curves.
ACM CSS: I.3.3 Compute...

We show how to use affine arithmetic to represent a parametric curve with a strip tree. The required bounding rectangles for pieces of the curve are computed by exploiting the linear correlation information given by affine arithmetic. As an application, we show how to compute approximate distance fields for parametric curves.

We describe an ongoing project whose aim is to build a parser for Brazilian Portuguese, Selva, which can be used as a basis for subsequent research in natural language processing, such as automatic translation and ellipsis and anaphora resolution. The parser is meant to handle arbitrary prose texts in unrestricted domains, including the full range...

Affine arithmetic is a model for self-validated numerical computation that affine arithmetic keeps track of first-order correlations between computed and input quantities. We explain the main concepts in affine arithmetic and it handles the dependency problem in standard interval arithmetic. We also describe some of its applications.

The skewness of a graph G is the minimum number of edges that need to be deleted from G to produce a planar graph. The splitting number of a graph G is the minimum number of splitting steps needed to turn G into a planar graph; where each step replaces some of the edges fu; vg incident to a selected vertex u by edges fu ; vg, where u is a new verte...

We describe an efficient procedure for reassembling unknown
two-dimensional objects that have been broken or torn into a large
number of irregular fragments, a problem that often arises in
archaeology, art restoration, forensics, and other disciplines. The
procedure compares the curvature-encoded fragment outlines, at
progressively increasing scale...

We describe here an efficient procedure for reassembling unknown two-dimensional objects that have been broken or torn into a large number of irregular fragments-a problem that often arises in archaeology, art restoration, forensics, and other disciplines. The procedure compares the curvature-encoded fragment outlines, at progressively increasing s...

The skewness of a graph G is the minimum number of edges that need to be deleted from G to produce a planar graph. The splitting number of a graph G is the minimum number of splitting steps needed to turn G into a planar graph; where each step replaces some of the edges {u, v} incident to a selected vertex u by edges {u′,v}, where u′ is a new verte...

The skewness of a graph G is the minimum number of edges that need to be deleted from G to produce a planar graph. The splitting number of a graph G is the minimum number of splitting steps needed to turn G into a planar graphs where each step replaces some of the edges (u, v) incident to a selected vertex u by edges (u', v), where u' is a new vert...

We show how to use affine arithmetic to represent a parametric curve with a strip tree. The required bounding rectangles for pieces of the curve are computed by exploiting the linear correlation information given by affine arithmetic. As an application, we show how to compute approximate distance fields for parametric curves.

We describe an ongoing research project on efficient methods for reconstruction of objects from large collections of irregular fragments, such as ancient pottery, collapsed murals, etc.. Our solution for flat objects uses multiscale matching and constrained dynamic programming, and we are now extending it to curved pottery fragments. We seek collab...

We describe exact representations and algorithms for geometric operations on general circles and circular arcs on the sphere, using integer homogeneous coordinates. The algorithms include testing a point against a circle, computing the intersection of two circles, and ordering three arcs out of the same point. These tools support robust and efficie...

We describe exact representations and algorithms for geometric operations on general circles and circular arcs on the sphere, using integer homogeneous coordinates. The algorithms include testing a point against a circle, computing the intersection of two circles, and ordering three arcs out of the same point. These tools support robust and efficie...

The ZZ-buffer is a new rendering algorithm that is simple, effcient, and produces high-quality images. The algorithm correctly renders transparent surfaces, shadows with real penumbrae, and depth of field effects. The ZZ-buffer algorithm is substantially faster than ray tracing and nearly as versatile. While the ZZ-buffer is somewhat slower than th...

: The twin disciplines of Pessimal Algorithm Design and Simplexity Analysis are introduced and illustrated by means of representative problems. 1. Introduction Consider the following problem: we are given a table of n integer keys A 1 ; A 2 ; . . ., An and a query integer X . We want to locate X in the table, but we are in no particular hurry to su...

A three-dimensional map is a partition of a 3D manifold into topological polyhedra. We consider the problem of visualizing the topology of a three-dimensional map given only its combinatorial description. Our solution starts by automatically constructing a “nice” geometric realization of the map in R<sup>m</sup>, for some m⩾4. The geometric rea...

We describe here an efficient algorithm for re-assembling one or more unknown objects that have been broken or torn into a large number N of irregular fragments---a problem that often arises in achaeology, art restoration, forensics, and other disciplines. The algorithm compares the curvatureencoded fragment outlines, using a modified dynamic progr...

We describe here a collection of heuristics for producing "nice" drawings of directed graphs, and a simple dual-mode software tool for testing and evaluating them. In playing mode, the heuristics are applied in random sequence over a set of drawings, in the manner of an asynchronous team (A-team). As new drawings are added to the set, others are de...