Mathematical Morphology: 40 Years On

Transcription

1 Mathematical Morphology: 40 Years On

2 Computational Imaging and Vision Managing Editor MAX A. VIERGEVER Utrecht University, Utrecht, The Netherlands Editorial Board GUNILLA BORGEFORS, Centre for Image Analysis, SLU, Uppsala, Sweden THOMAS S. HUANG, University of Illinois, Urbana, USA SABURO TSUJI, Wakayama University, Wakayama, Japan Volume 30

3 Mathematical Morphology: 40 Years On Proceedings of the 7th International Symposium on Mathematical Morphology, April 18-20, 2005 Edited by Christian Ronse LSIIT, UMR 7005 CNRS-ULP, Illkirch, France Laurent Najman A2SI-ESIEE/IGM, UMR 8049 CNRS-UMLV, Noisy-le-Grand, France and Etienne Decencière CMM, École des Mines de Paris, Fontainebleau, France

4 A C.I.P. Catalogue record for this book is available from the Library of Congress. ISBN (HB) Springer Dordrecht, Berlin, Heidelberg, New York ISBN (e-book) Springer Dordrecht, Berlin, Heidelberg, New York ISBN (HB) Springer Dordrecht, Berlin, Heidelberg, New York ISBN (e-book) Springer Dordrecht, Berlin, Heidelberg, New York Published by Springer, P.O. Box 17, 3300 AA Dordrecht, The Netherlands. Printed on acid-free paper All Rights Reserved 2005 Springer No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Printed in the Netherlands.

5 This book is dedicated to the memory of Georges Matheron, who gave the study of morphology its initial impetus.

6 Contents Dedication Foreword Preface Acknowledgments v xi xiii xvii Part I Morphological operators Binary Decision Diagrams as a New Paradigm for Morphological Machines 3 Junior Barrera and Ronaldo Fumio Hashimoto Image filtering using morphological amoebas 13 Romain Lerallut, Étienne Decencière and Fernand Meyer Numerical Residues 23 Serge Beucher Efficient path openings and closings 33 Ben Appleton and Hugues Talbot Structuring Elements following the Optical Flow 43 Nicolas Laveau and Christophe Bernard Recursive Interpolation Technique For Binary Images Based on Morphological Median Sets 53 Javier Vidal, Jose Crespo and Víctor Maojo Part II Connected filters and reconstruction Second-Order Connected Attribute Filters Using Max-Trees 65 Georgios K. Ouzounis and Michael H. F. Wilkinson Transformations with Reconstruction Criteria: Image Segmentation and Filtering 75 Iván R. Terol-Villalobos and Jorge D. Mendiola-Santibañez Attribute-Space Connected Filters 85 Michael H.F. Wilkinson

7 viii MATHEMATICAL MORPHOLOGY: 40 YEARS ON Vector-attribute Filters 95 Erik R. Urbach, Niek J. Boersma and Michael H.F. Wilkinson Ruminations on Tarjan s Union-Find Algorithm and Connected Operators 105 Thierry Géraud Labelled Reconstruction of Binary Objects: A Vector Propagation Algorithm 117 Michael Buckley and Ryan Lagerstrom Grayscale Level Multiconnectivity 129 Ulisses Braga-Neto Shape-Tree Semilattices 139 Renato Keshet Part III Segmentation Morphological Segmentations of Colour Images 151 Jean Serra Fast Implementation of Waterfall based on Graphs 177 Beatriz Marcotegui and Serge Beucher Mosaics and Watersheds 187 Laurent Najman, Michel Couprie and Gilles Bertrand A new definition for the dynamics 197 Gilles Bertrand Watershed-driven region-based image retrieval 207 I. Pratikakis, I. Vanhamel, H. Sahli, B. Gatos and S. Perantonis Efficient implementation of the locally constrained watershed transform and seeded region growing 217 Richard Beare Part IV Geometry and topology Optimal shape and inclusion 229 Jean-Marc Chassery and David Coeurjolly Regular Metric: Definition and Characterization in the Discrete Plane 249 Gerald Jean Francis Banon Euclidean skeletons of 3D data sets in linear time by the integer medial axis transform 259 Wim H. Hesselink, Menno Visser and Jos B.T.M. Roerdink

8 Contents ix Digitization of Non-regular Shapes 269 Peer Stelldinger Downsampling of binary images using adaptive crossing numbers 279 Etienne Decencière and Michel Bilodeau Grey-weighted, ultrametricandlexicographic distances 289 Fernand Meyer Mathematical modeling of therelationship between based on morphological operators 299 IsabelleBloch, Olivier Colliot and Roberto M. Cesar Part V Partial differential equations and evolutionary models Semidiscrete and Discrete Well-Posedness of Shock Filtering 311 Martin Welk and Joachim Weickert AVariational Formulation of PDE s for Dilations and Levelings 321 Petros Maragos Stochastic Shape Optimisation 333 Costin Alin Caciu, Etienne Decencière and Dominique Jeulin On the local connectivity number of stationary random closed sets 343 Evgueni Spodarev and Volker Schmidt Part VI Texture, colour and multivalued images Intersize correlation of grain occurrences in textures and its application to texture regeneration 357 Akira Asano, Yasushi Kobayashi and Chie Muraki Texture Segmentation Using Area Morphology Local Granulometries 367 Neil D. Fletcher and Adrian N. Evans Illumination-invariant Morphological Texture Classification 377 Allan Hanbury, Umasankar Kandaswamy and Donald A. Adjeroh Unified morphological color processing framework in a lum/sat/hue representation 387 Jesús Angulo Iterative area seeded region growing for multichannel image simplification 397 Dominik Brunner and Pierre Soille Morphology for Higher-Dimensional Tensor Data via Loewner Ordering 407 B. Burgeth, N. Papenberg, A. Bruhn, M. Welk, C. Feddern, and J. Weickert

9 x Part VII Applications in imaging sciences MATHEMATICAL MORPHOLOGY: 40 YEARS ON Using Watershed and Multimodal Data for Vessel Segmentation: Application to the Superior Sagittal Sinus 419 N. Passat, C. Ronse, J. Baruthio, J.-P. Armspach and J. Foucher Using grey scale hit-or-miss transform for segmenting the portal network of the liver 429 Benoît Naegel, Christian Ronse and Luc Soler Blood Cell Segmentation Using Minimum Area Watershed and Circle Radon Transformations 441 F. Boray Tek, Andrew G. Dempster and Izzet Kale Quantifying mean shape and variability of footprints using mean sets 455 J. Domingo, B. Nacher, E. de Ves, E. Alcantara, E. Diaz, G. Ayala and A. Page Exploiting and Evolving R n Mathematical Morphology Feature Spaces 465 Vitorino Ramos and Pedro Pina Morphological Segmentation Applied To 3D Seismic Data 475 Timothée Faucon, Etienne Decencière and Cédric Magneron Index 485 Author Index 489

10 Foreword Mathematical Morphology (MM) was born in 1964 through the collaboration of Georges Matheron and Jean Serra, who established its basic concepts and tools, coined the name in 1966, and set up in 1968 the Centre de Morphologie Mathématique on the Fontainebleau site of the Paris School of Mines. MM gained a wide recognition after the publication of the three books Random Sets and Integral Geometry by G. Matheron (1975), Image Analysis and Mathematical Morphology by J. Serra (1982), and Image Analysis and Mathematical Morphology, Vol. 2: Theoretical Advances edited by J. Serra (1988). It has now spread worldwide, with active research teams in several countries. This led to the organization of a specific international forum for presenting the most recent advances in the field: the International Symposium on Mathematical Morphology (ISMM). Its first six venues were held in Barcelona (1993), Fontainebleau (1994), Atlanta (1996), Amsterdam (1998), Palo Alto (2000) and Sydney (2002). In May 2003, on the occasion of a MM day workshop of the GDR ISIS (a French national action linking many laboratories and researchers in image processing) of the CNRS, held in Paris, it was proposed to organize the 7th ISMM in 2005 in Paris, and to make it a celebration of the 40 years of MM. We were pleased by the warm welcome that this proposal met among our colleagues from all countries, so we went ahead and planned the meeting to take place in April 2005 in Paris, just after DGCI 05 (the 12th International Conference on Discrete Geometry for Computer Imagery) in Poitiers. We were lucky to be abletomake use of the rooms inthe historicalbuildings of theparisschool of Mines. We received 62 submissions, from which 41 were accepted for oral presentation, after being reviewed by at least two independent referees. Accepted papers originate from 14 countries: Australia, Austria, Brazil, France, Germany, Greece, Israel, Italy, Japan, Mexico, Netherlands, Portugal, Spain and United Kingdom. Several came from new researchers or teams, that is, who had not presented papers at previous ISMM s. We are also honoured by the presence of 3 eminent guest speakers: Jean Serra himself, Jean-Marc Chassery, leader of

11 xii MATHEMATICAL MORPHOLOGY: 40 YEARS ON the GDR ISIS of the CNRS, and member of the steering committee of DGCI, and Olivier Faugeras, leading researcher on computer vision at INRIA, and member of the French Academy of Sciences and of the French Academy of Technologies. The high quality of the papers, the variety of their topics, and the geographical diversity of their authors, show that MM is a lively research field. This ISMM is held in honour of Jean Serra, on the occasion of his 65th birthday in March 2005, and these Proceedings are dedicated to the memory of Georges Matheron, who passed away in 2000 at the age of seventy. Christian Ronse, Laurent Najman and Etienne Decencière

12 Preface This 7th ISMM has brought a wide variety of papers, whose topics generally circle around mainstream morphology, namely considering images as geometrical objects, which are analysed by their interactions with predefined geometrical shapes. However, this circle is ever growing, and is extending towards the frontiers of morphology with neighbouring specialities; we are pleased to welcome a few papers at the interface with other aspects of imaging and computer science. Theory, methodology and practical applications are all represented. It has been difficult to sort the contributions into well-defined categories, and any classification, like the one used to make this book, is somewhat artificial. Operator design The design of morphological operators remains an important topic of research. Barrera and Hashimoto show how binary decision diagrams can be used to obtain the decomposition of a local translation-invariant operator in terms of erosions and dilations. Lerallut et al. describe the construction of morphological operators with adaptive translation-varying structuring elements. Beucher extends to grey-level images some set operators like the ultimate erosion or the skeleton by openings. Appleton and Talbot present an efficient algorithm for computing the opening or closing by approximately straight segments. Laveau and Bernard combine morphological filters with motion compensation, in order to process video sequences. Then Vidal et al. present a morphological interpolation method for binary images, based on the concept of median set. Connected filters and reconstruction About ten years ago emerged the idea that morphological operators can act at the level of flat zones instead of individual pixels; the basic tool for this purpose is the geodesical reconstruction. This led to the concept of connected filters. Ouzounis and Wilkinson propose a tree structure adapted to the implementation of attribute filters in the case where flat zones are based on clus-

13 xiv MATHEMATICAL MORPHOLOGY: 40 YEARS ON tering or contraction connectivity. Terol-Villalobos and Mendiola-Santibañez introduce a modification of geodesic reconstruction, where the latter can be stopped on the basis of size criteria; this is illustrated with applications in image filtering and segmentation. Wilkinson generalizes connected filters thanks to a space-attribute connectivity. Urbach et al. give a variant of attribute filtering, where the attribute is not a scalar, but a vector; this leads to granulometries. Géraud presents a form of Tarjan s union-find algorithm dedicated to connected operators. Buckley and Lagerstrom give an algorithm for reconstructing the labelled branches of an object from its skeleton and the associated quench function. Then Braga-Neto introduces a multi-scale connectivity on grey-level images, based on the connectivity of threshold sets for a varying threshold. Finally Keshet investigates morphological operators on trees of shapes (corresponding to grains and pores), in order to obtain morphological image operators acting in the same way on bright and dark image areas. Segmentation At the basis of the morphological approach to segmentation stands the watershed transformation. Since then much work has been done towards building hierarchical groupings of segmentation classes, in particular to reduce oversegmentation. A very interesting paper by Serra investigates colour spaces suitable for segmentation, and shows in particular how one can segment a colour image by combining a luminance-based segmentation in unsaturated areas with a chrominance-based one in saturated areas. Marcotegui and Beucher propose a new kind of hierachical waterfall algorithm, and show how to implement it on the basis of the minimum spanning tree of the neighbourhood graph. Najman et al. investigate the divide set produced by watershed algorithms, and show that they give a separation in the mosaic if and only if the watershed is obtained via a topological thinning. Bertrand proposes a new definition for the dynamics and studies its links with minimum spanning trees. Pratikakis et al. apply hierarchical watershed segmentation for content-based image retrieval. Then Beare gives an algorithm implementing the locally constrained watershed transform. Geometry and topology By its concern with shapes, morphology has always been linked with geometrical and topological approaches in image processing. Chassery and Coeurjolly propose new algorithms for the convex skull, also called potato peeling problem, that is: construct a largest convex polygon inside a non-convex one. Banon investigates metrics supporting some version of the concept of straight line segment, and the relations between such metrics and morphological operators. Hesselink et al. give an algorithm for constructing a digital skeleton

14 PREFACE xv through an Euclidean distance transform. Stelldinger shows how to discretize shapes whose contours have bounded curvature, in such a way as to preserve topology. Decencière and Bilodeau propose an adaptive discretization scheme, suited to the preservation of significant geometrical and topological features of the figure. Meyer studies lexicographic-type metrics that can be used in segmentation, by allowing to partition an image from a set of markers. Finally Bloch et al. apply morphology to the formalization of the spatial relation X is between Y and Z, in a fuzzy framework. PDEs and evolutionary models Partial differential equations (PDEs) govern scale spaces corresponding to the evolution of a filter when the size of its kernel increases. Welk and Weickert provide a theory for such equations for shock filters on one-dimensional signals. Maragos gives a variational interpretation of PDEs for dilation, erosion and levelling. From the beginning, morphology has sought to relate its operations to probabilistic models of shapes and images. Caciu et al. solve a constrained shape optimisation problem by a combination of genetic algorithms and simulated annealing methods. Spodarev and Schmidt use local connectivity numbers to estimate the Minkowski functionals of realizations of a random closed set. Texture, colour and multivalued images Morphology has classically been applied on binary and grey-level images, but it can also be extended to multivalued images, in particular colour ones, and to the analysis of textures. Asano et al. introduce a new characterization of texture based on intersize correlation of grain occurrences. Fletcher and Evans give supervised and unsupervised texture segmentation algorithms, based on the area pattern spectrum. Hanbury et al. investigate the use of the granulometry and the variogram for characterizing texture in colour and greyscale images, and introduce a method for minimising the effect of varying illumination conditions. Angulo proposes a unified framework for morphological processing of colour images in a luminance/saturation/hue representation. Brunner and Soille study the segmentation of multi-channel images by an iterative method based on seeded region growing and quasi-flat zones. Burgeth et al. extend basic morphological operators to tensor-valued images by the use of the Loewner ordering, illustrated on DT-MRI images. Applications in imaging sciences The success of an image processing theory is sooner of later verified through its practical applications. Passat et al. segment brain arteries in MRA brain images by an interaction between watershed segmentations on MRI and MRA

15 xvi MATHEMATICAL MORPHOLOGY: 40 YEARS ON images. Naegel et al. reconstruct the liver portal vein in CT scan images by grey-level hit-or-miss operators and topological reconstruction. Tek et al. give an algorithm for segmenting red blood cells in microscopic blood images, by using an area granulometry, a minimum area watershed and a circle Radon transformation. Domingo et al. show how to calculate the mean shape of footprint images by use of morphological means. Ramos and Pina analyse images of Portuguese granites by a classification in feature space based on a genetic algorithm and a nearest neighbour rule. Finally Faucon et al. investigate watershed based methods for the segmentation of horizons in seismic images. This collection is a sample of current research topics in mathematical morphology. It shows the vitality, diversity and maturity of this field, which attracts anevergrowing number of researchers and practitioners. CHRISTIAN RONSE, LAURENT NAJMAN AND ETIENNE DECENCIÈRE

16 Acknowledgments xvii The organization of this 7th ISMM and the publication of these proceedings were made possible thanks to the help of many people. We are grateful to Michel Schmitt, deputy director for research at the Paris School of Mines, for putting at our disposal the resources and rooms of the school. We thank also Dominique Deville and Danielle Gozlan for administrative support, Christophe Dietrich for web-system administration, Bruno Restif, Emmanuelle Lafouge and Jocelyne Roger for financial management, as well as Isabelle Bloch and Petr Dokládal for their help in the organization. Finally we must acknowledge the work of the more than fifty reviewers, who anonymously evaluated and commented the papers. We list them below, and thank them all. List of reviewers Vincent Agnus Akira Asano Gilles Bertrand Michel Bilodeau Gunilla Borgefors Thomas Brox Bernhard Burgeth CostinAlin Caciu José Crespo Alain Daurat PetrDokládal Luis Garrido Frédéric Guichard Paul T. Jackway Renato Keshet Sébastien Lefèvre Beatriz Marcotegui Jean-Michel Morel Haris Papasaika Antony T. Popov Jos Roerdink Philipe Salembier Jean Serra Mohamed Tajine Ivan R. Terol-Villalobos Rein van den Boomgaard Luc Vincent Joachim Weickert Jesús Angulo Junior Barrera Serge Beucher Isabelle Bloch Ulisses M. Braga-Neto Michael Buckley Lilian Buzer Michel Couprie Olivier Cuisenaire Etienne Decencière Gabriel Fricout John Goutsias Marcin Iwanowski Dominique Jeulin Ron Kimmel Petros Maragos Fernand Meyer Laurent Najman Ioannis Pitas Gerhard Ritter Christian Ronse Michel Schmitt Pierre Soille Hugues Talbot Jean-Philippe Thiran Marc Van Droogenbroeck Thomas Walter Martin Welk Christian Ronse, Laurent Najman and Etienne Decencière

17 I MORPHOLOGICAL OPERATORS

18 BINARY DECISION DIAGRAMS AS A NEW PARADIGM FORMORPHOLOGICAL MACHINES Junior Barrera 1 and Ronaldo Fumio Hashimoto 1 1 Departamento de Ciencia da Computacao Instituto de Matematica e Estatistica - USP Rua do Matao, Cidade Universitaria - Sao Paulo - SP - Brasil jb@ime.usp.br and ronaldo@ime.usp.br Abstract Keywords: Mathematical Morphology (MM) is a general framework for studying mappings between complete lattices. In particular, mappings between binary images that are translation invariant and locally defined within a window are of special interest in MM. They are called W-operators. A key aspect of MM is the representation of W-operators in terms of dilations, erosions, intersection, union, complementation and composition. When W-operators are expressedinthis form, they are called morphological operators. An implementation of this decomposition structure iscalled morphological machine (MMach). A remarkable property of this decomposition structure is that it can be represented efficiently by graphs called Binary Decision Diagrams (BDDs). In this paper, we propose a new architecture for MMachs that is based on BDDs. We also show that reduced and ordered BDDs (ROBDDs) are non-ambiguous schemes for representing W- operators and we present a method to compute them. This procedure can be applied for the automatic proof of equivalence between morphological operators, since the W-operator they represent are equal ifand only if they have the same ROBDD. Binary Decision Diagram, Morphological Machine, Morphological Language, Morphological Operator. 1. Introduction Mathematical Morphology (MM) is a theory that studies images and signals based on transformations of their shapes. These transformations can be viewed as mappings between complete lattices [8, 13]. In particular, mappings between binary images are of special interest in MM and they are called set operators. A central paradigm in MM is the representation of set operators in C. Ronse et al. (eds.), Mathematical Morphology: 40 Years On, c 2005 Springer. Printed in the Netherlands. 3

19 4 MATHEMATICAL MORPHOLOGY: 40 YEARS ON terms of dilations, erosions, union, intersection, complementation and composition. This decomposition structure can be described by a formal language called morphological language [2, 3]. Sentences of the morphological language are called morphological operators or morphological expressions. An implementation of the morphological language is called morphological machine (MMach). The first known MMach was the Texture Analyzer, created in the late sixties in Fontainebleau by Serra and Klein. Nowadays, a large number of these machines are available. The motivation of this work comes from the search for non-ambiguous and compact representation for a large class of operators. It is also desired that this representation leads to efficient algorithms for morphological image processing and that it satisfactorily solves the issue of determining whether two representations are equivalent. In this context, we propose a new architecture for MMachs implemented as software for sequential machines. This new architecture is based on the representation of Boolean functions by Binary Decision Diagrams (BDDs) and was first used in MM in a special algorithm to compute the thinning operator [12]. Animportant class of set operators is that of W-operators, i.e., set operators that share the properties of translation invariance and local definition within a window. W-operators are extensively used in morphological image processing and this family of operators is the focus of this paper. This paper extends the use of BDD as a representation scheme for the whole class of W-operators. The class of BDDs studied here (reduced and ordered BDDs) provides a trivial algorithm for determining whether morphological operators are equivalent. Algorithms to convert sentences of the morphological language to this new form of representation are also presented inthis work. 2. Binary Mathematical Morphology In this section, we recall some basic concepts of binary MM. Let E be a nonempty set and let P(E) denote the power set of E. Let denote the usual set inclusion relation. The pair (P(E), ) is a complete Boolean lattice [4]. A set operator is any mapping defined from P(E) into itself. The set Ψ of all set operators inherits the complete lattice structure of (P(E), ) by setting ψ 1 ψ 2 ψ 1 (X) ψ 2 (X), ψ 1,ψ 2 Ψ, X P(E). Let X, Y P(E). The operations X Y, X Y and X\Y, X c are the usual set operations of union, intersection, difference and complementation, respectively. Let (E, +) bean Abelian group with zero element o E,called origin. Let h E and X, B E. The set X h,defined byx h = {x + h : x X}, is the translation of X by h. The set X t = { x : x X} is the transpose of X. The set operations X B = b B X b and X B = b B X b are the Minkowskiaddition and Minkowski subtraction, respectively.

20 BDDs as a New Paradigm for MMachs Architecture 5 A set operator ψ is called translation invariant (t.i.) if and only if, h E, X P(E),ψ(X h ) = ψ(x) h. Let W be afinite subset of E. A set operator ψ is called locally defined (l.d.) within a window W if and only if, h E, X P(E), h ψ(x) h ψ(x W h ). Let Ψ W denote the collection of all t.i. operators that are also l.d. within a window W. The elements of Ψ W are called W-operators. The pair (Ψ W, ) constitutes a sub-lattice of the lattice (Ψ, ) [3]. Furthermore, (Ψ W, ) is isomorphic tothe complete Boolean lattice (P(P(E)), ), since the mapping K W : Ψ W P(P(E)) defined byk W (ψ) = {X P(W ):o ψ(x)}, is bijective and preserves the partial order [3]. The set K W (ψ) is the Kernel of the set operator ψ. 3. Morphological Language In this section, we recall the main concepts of MM from the viewpoint of aformal language. Let ψ 1, ψ 2 Ψ. The supremum ψ 1 ψ 2 and infimum ψ 1 ψ 2 operations are defined by(ψ 1 ψ 2 )(X) = ψ 1 (X) ψ 2 (X) and (ψ 1 ψ 2 )(X) =ψ 1 (X) ψ 2 (X), X P(E). They can be generalized as ( i I ψ i )(X) = i I ψ(x) and ( i I ψ i )(X) = i I ψ(x), where I is a finite set ofindices. The composition operator ψ 2 ψ 1 is given by (ψ 2 ψ 1 )(X) = ψ 2 (ψ 1 (X)), X P(E). The set operators ı and ν defined byı(x) = X and ν(x) = X c, X P(E), are called, respectively, the identity and negation operators. These operators are l.d. within the window {o}. The dual of an operator ψ Ψ, denoted ψ,is definedbyψ (X) = (ψ(x c )) c, X P(E). Note that ψ = νψν. For any h E, the set operator τ h defined byτ h (X) =X h, X P(E), is called translation operator by h. This operator is l.d. within the window { h}. Forat.i. set operator ψ, τ h ψ = ψτ h. Let B P(E) be finite. The t.i. set operators δ B and ε B defined by δ B (X) =X B and ε(x) =X B, X P(E), are the dilation and erosion by the structuring element B. These set operators are l.d. within the window B t and B, respectively. One can also write δ B = b B τ b and ε B = b B τ b. The following proposition, proved by Barrera and Salas [3], gives some properties of W-operators. Proposition 1 If ψ, ψ 1 and ψ 2 are set operators l.d. within windows W, W 1 and W 2, respectively, then they have thefollowing properties: (1) ψ is l.d. within any window W W ; (2) ψ 1 ψ 2 and ψ 1 ψ 2 are l.d. within W 1 W 2 ; (3) ψ 2 ψ 1 is l.d. within W 1 W 2. The following corollary gives some properties of W-operators that are particular case of Property 3of Proposition 1.

21 6 MATHEMATICAL MORPHOLOGY: 40 YEARS ON Corollary 2 If ψ is a set operator l.d. within a window W, then we have the following properties: (1) h E, τ h ψ is l.d. within W h ; (2) B W, δ B ψ and ε B ψ are l.d. within W B t and W B, respectively. (3) ıψ, ψı, νψ, ψν and ψ are l.d. within W. Morphological operators (i.e., sentences of the morphological language), are built as strings of elementary operators (ε B,δ B, ı, ν) bound bythe operations, and composition. As an example, let A, B P(W ) such that A B. The collection [A,B]={X P(W ):A X B} is called an interval. The t.i. sup-generating operator λ W A,B,defined byλw A,B (X) = {x E : (X x ) W [A,B]}, X P(E), can be described as λ W A,B = ε A νδ B t, where B = W \B. Note that, λ W A,B is l.d. within A B, and, hence, within W. Let X be acollection of intervals. An element [A, B] X issaid to be maximal if and only if (iff) noother element of X properly contains it, that is, [A, B] is maximaliff [A,B ] X, [A,B] [A,B ] [A,B] = [A,B ]. The set B W (ψ) of all maximal intervals contained inthe kernel K W (ψ) of a W-operator ψ is called basis of ψ [2]. Any W-operator ψ can be written by their canonical sup-decompositions, that is, ψ = {λ W A,B :[A, B] K W (ψ)} or ψ = {λ W A,B :[A,B] B W (ψ)}. As a conclusion of this section, morphological language is complete, in the sense that, any W-operator can be represented bycanonical forms, which are valid sentences of morphological language [3]. It is also expressive, since many useful operators can be described with short sentences. 4. Binary Decision Diagrams A Boolean function (on n variables)isafunction f : {0, 1} n {0, 1}. This section presents a representation method of Boolean functions called Binary Decision Diagrams (BDDs). In Section 6, we extend the use of BDD as a representation scheme for the whole class of W-operators. A variety of representation methods for Boolean functions have been developed. In the classical ones, such as truth tables and canonical sum-of-products form, the representation of a Boolean function with n variables has size O(2 n ). Other approaches, such as the set of prime implicants (or equivalently, the set of maximal intervals) or a subset of irrendundant ones, yield compact representation for many Boolean functions. Here we describe a graph-based representation method that is very useful for a large class of Boolean functions. Boolean Functions Represented by BDDs The representation of a Boolean function by a decision-based structure was introduced by Lee [9] and further popularized by Akers [1] under the name of BDD. Algorithms for BDD manipulation are described in [6]. Efficient

22 BDDs as a New Paradigm for MMachs Architecture 7 implementation directions are found in [5]. Applications of BDDs in Digital Image Processinghave been recently developed [10 12]. A BDD of a Boolean function f : {0, 1} n {0, 1} is a rooted, directed acyclic graph (DAG) with two types of nodes: terminal and nonterminal. Each terminal node v has as attributeavalue value(v) {0, 1}. The nonterminal nodes v have two children nodes, low(v) and high(v). Each nonterminal v is labeled with an input variable index index(v) {1, 2,...,n}. For a given assignment to the input variable vector x =(x 1,...,x n ),the value of the function is determined bytransversing the graph from the root to a terminal node: at each nonterminal node v with index(v) = i, ifx i =0, then the arc to low(v) is followed. Otherwise (i.e., x i =1), the arc to high(v) is followed. Thevalue of the function is given by the value of the terminal node. Anode v in a BDD represents a Boolean function f v such that: (i) if v is a terminal nodewith value(v) =0,then f v =0; (ii) if v is a terminal nodewith value(v) = 1, then f v =1; (iii) if v is a nonterminal nodeandindex(v) = i, then f v = x i f low(v) +x i f high(v). The mathematicalbackground of the BDD construction is the well known Shannon expansion of Boolean functions: f = x i f xi + x i f xi. The restrictions f xi = f(x 1,...,x i 1, 1, x i+1,...,x n ) and f xi = f(x 1,...,x i 1, 0, x i+1,...,x n ) are the cofactors of f with respect to the literal x i and x i. Reduced and Ordered BDDs An ordered BDD (OBDD) is a BDD such that any path inthe graph from the root to a terminal node visits the variables in ascending order of their indices, i.e., index(v) < index(low(v)) whenever v and low(v) are nonterminal nodes, and index(v) < index(high(v)) whenever v and high(v) are nonterminal nodes. Since a variable appears at most once in each path, a function is evaluated in time O(n) in an OBDD. If an OBDD contains no pair of nodes {u, v} such that the graph rooted by u and v are isomorphic, and if it contains no node v such that low(v) = high(v), it is called a reduced OBDD (ROBDD). An OBDD of N vertices can be transformed into an equivalent ROBDD in time O(N log N) by the Reduce algorithm presented in [6]. The following theorem, proved in [6], states the canonicity of the ROBDD representation. Theorem 3 For any Boolean function f, there is a unique ROBDD representing f. Furthermore, any other OBDD representing f contains more vertices. The Lattice of ROBDDs and W-operators Let F n denote the set of all functions f : {0, 1} n {0, 1} and let Φ W denote the set of all functions ϕ : P(W ) {0, 1}, such that W = {w 1,...,w n }.

23 8 MATHEMATICAL MORPHOLOGY: 40 YEARS ON We establish a one-to-one correspondence between elements of F n and Φ W by making f(x 1,...,x n ) = ϕ({w i W : x i =1}). Let denote the usual order in {0, 1}. This order induces a parcial order in F n, that is, f 1,f 2 F n, f 1 f 2 f 1 (x) f 2 (x), x {0, 1} n. The partially ordered sets (F n, ) and (Φ W, ) are complete Boolean lattices isomorphic to(p(p(w )), ). This is observed from the bijective mapping K : Φ W P(P(W )) given by K(ϕ) ={X : ϕ(x) =1}. Let G W denote the set of all ROBDDs representing functions in F n. From Theorem 3, ROBDDs are unique and non-ambiguous representations of Boolean functions. Thus, there is abijective mapping between G W and F n, and, hence, between G W and Φ W. The ROBDD of a function ϕ Φ W is denoted byg(ϕ) and issimply called the graph of ϕ. The pair (G W, ) is a complete Boolean lattice isomorphic tothe lattice (Φ W, ), where the partial order between two graphs G(ϕ 1 ) and G(ϕ 2 ) in G W is defined by G(ϕ 1 ) G(ϕ 2 ) ϕ 1 ϕ Algorithms for Operations on BDDs In this section, we present some algorithms on BDDs. First, we show some algorithms for logical operations between graphs and, then, we give an algorithm for translations of BDDs. Algorithms for Logical Operations between BDDs Let ϕ, ϕ 1,ϕ 2 Φ W and let G, G 1,G 2 G W betheir corresponding graphs. The graph complement, infimum and supremum, respectively denoted by, and, are computed by the algorithm AppyOperation presented in[6]. This algorithm takes as input two graphs with N 1 and N 2 nodes, respectively, and a logical operation AND, OR, or XOR as parameters, and return the resulting graph intime O(N 1 N 2 ).The algorithm is based on the following property of Boolean functions: f 1 f 2 = x i (f 1 xi f 2 xi )+x i (f 1 xi f 2 xi ), where denotes one of the sixteen logical function of two variables [7]. Thus, G = G(ϕ) = ApplyOperation(G, 1, XOR) G 1 G 2 = G(ϕ 1 ϕ 2 ) = ApplyOperation(G 1,G 2, AND) G 1 G 2 = G(ϕ 1 + ϕ 2 ) = ApplyOperation(G 1,G 2, OR) where 1 is the trivial leaf 1. The graph of the complement of ϕ Φ W can be alternatively computed bysimply exchanging the values of all terminal nodes. The dualgraph of G G W,denotedbyG,is defined as G (ϕ(x)) = G(ϕ (X)) = G(ϕ(X c )). It can be computed from G by swapping low(v) with high(v) in each nodeof G and also by swapping the terminal nodes.

24 BDDs as a New Paradigm for MMachs Architecture 9 Algorithm for Translation of BDDs Let h E and ϕ Φ W. The translation of ϕ by h, denoted byϕ h is defined byϕ h (X) = ϕ(x h ), X P(W h ). Thus ϕ h Φ W h and the Boolean expression of ϕ h can be obtained from the Boolean expression of ϕ by achanging of variables. The translation of a graph G(ϕ) G W by h, denotedbyg(ϕ)+h,isdefined as G(ϕ)+h = G(ϕ h ).Note that G(ϕ)+h G W h. For a trivialimplementation of G + h,thewindow W W h must be consistently ordered,otherwise a reordering of the BDD may be mandatory. 6. Translating Morphological Operators into BDDs In this section, we present a way to incrementally compute the ROBDD of a W-operator describedbya sentence of the morphological language. We presented (Section 2) that (Ψ W, ) is isomorphicto(p(p(e)), ), and also (Section 4) that (Φ W, ) is isomorphicto(g W, ) and to (P(P(E)), ). This demonstrates, by transitivity of isomorphisms, that there is a one-to-one correspondence between elements of G W and Φ W. This shows that a graph non-ambiguously and uniquely represents a W-operator. We denote by G W (ψ) the corresponding graph of the W-operator ψ. The calligraphic G( ) is to stress that theargument is an operator, while G( ) has as argument a function. The subscript W is used in both notations to emphasize thewindow within which the graph is defined. The following proposition establishes the algorithms that perform the basic operations on graphs of morphological operators. The proof of this proposition is based on the isomorphisms mentioned in the beginning of this section. Proposition 4 Let h E and B E. Let ψ, ψ 1 and ψ 2 bew-operators, respectively, within the windows W, W 1 and W 2. If G W (ψ), G W1 (ψ 1 ) and G W2 (ψ 2 ) are the correspondinggraphs toψ, ψ 1 and ψ 2, respectively, then a. G W (νψ)=g W (ψ); b. G W1 W 2 (ψ 1 ψ 2 ) = G W1 W 2 (ψ 1 ) G W1 W 2 (ψ 2 ); c. G W1 W 2 (ψ 1 ψ 2 ) = G W1 W 2 (ψ 1 ) G W1 W 2 (ψ 2 ); d. G W (ψ ) = G W (ψ); e. G W h (τ h ψ) = G W (ψ) + h; f. G W B t(δ B ψ) = b B (G W B t(ψ) + b); g. G W B (ε B ψ) = b B t(g W B (ψ)+b). From Proposition4and since λ W A,B = ε A νδ B t, we have the following corollary. Corollary 5 Let ψ be aw-operator and A,B,W P(E) such that A B W. If B = W \B and G W (ψ) is the graph corresponding to ψ, then G W W (λ W A,B ψ) = G W W (ε Aψ) G W W (δ B tψ).

25 10 MATHEMATICAL MORPHOLOGY: 40 YEARS ON Definition 6 Given a set of indices I, we say that a collection of intervals {[A i,b i ] : i I} is an exact cover of a W-operator ψ if ψ = i I λ W A i,b i. For example, the basis of a W-operator ψ is an exact cover of ψ. And so is the set {[A, A] : A K W (ψ)}. Proposition 7 Let ψ 1 and ψ 2 be W-operators, respectively, within the windows W 1 and W 2.IfG W1 (ψ 1 ) and G W2 (ψ 2 ) are the corresponding graphs to ψ 1 and ψ 2, respectively, and {[A i,b i ] : i I} is an exact cover of ψ 2, then G W1 W 2 (ψ 2 ψ 1 ) = i I G W1 W 2 (λ W 2 A i,b i ψ 1 ). It is possible to compute the graph of any W-operator, whenever its description in the morphological language is known. For the incremental computation of the graph of a W-operator ψ described bya morphological expression, we start with the graph of the identity operator and successively apply the algorithms that corresponding to Propositions 4and 7, according to the parsing of the sentence that describes ψ. Each step is initiated by the modification of the window by applying Proposition 1 and, eventually, Corollary 2. In the following,wegive an example illustrating how to compute the ROBDD representation from a W-operator ψ l.d. within W = {w 1, w 2, w 3 }, where w 1 =( 1, 0), w 2 = o =(0, 0) and w 3 =(1, 0), that detects the vertical borders. There are several ways of representing this operator such as: (i) the standard decomposition ψ = λ W {w 2 },{w 1,w 2 } λw {w 2 },{w 2,w 3 }; (ii) the morphological operator by means of erosion and dilations ψ = (ε {w2 } νδ {w1 }) (ε {w2 } νδ {w3 }); and (iii) the last graph (ROBDD) G W (ψ) of Fig. 1.Using the representation by means of erosion and dilations, we have G W (ψ) = G((ε {w2 } νδ {w1 }) (ε {w2 } νδ {w3 })) =(G(ε {w2 }) G(νδ {w1 })) (G(ε {w2 }) G(νδ {w3 })). Thus, we first have to compute G 1 = G(ε {w2 }), G 2 = G(δ {w1 }), G 3 = G(δ {w3 }), G 4 = G(νδ {w1 }) and G 5 = G(νδ {w3 }); and then G =(G 1 G 4 ) (G 1 G 5 ). Let G 0 bethe BDD for the identity operator, that is, G 0 = G(ı). Applying the corresponding algorithm, we have G 1 = G(ε {w2 }ı) = G 0 +( w 2 ) = G 0,byProposition 4(g). G 2 = G(δ {w1 }ı) = G 0 + w 1,byProposition 4(f). G 3 = G(δ {w3 }ı) = G 0 + w 3,byProposition 4(f). G 4 = G(νδ {w1 }ı) = G 2 and G 5 = G(νδ {w3 }ı) = G 3,byProposition 4(a). G 6 =(G 1 G 4 ) and G 7 =(G 1 G 5 ), by Proposition 4(b). G = G 6 G 7,byProposition 4(c). 7. Automatic Proof of Equivalence An important task in MM is to determine whether two morphological representations correspond to the same operator. Determining the equivalence of

26 BDDs as a New Paradigm for MMachs Architecture 11 G 1 = G 0 +( w 2 ) G 2 = G 0 + w 1 G 3 = G 0 + w 3 0 w 2 1 w w G 4 = G 2 G 5 = G 3 G 6 = G 1 G 4 w 1 0 w w w G = G 6 G 7 w G 7 = G 1 G 5 w 2 1 w 2 w w 3 0 w Figure 1. Incrementally computed ROBDD of the vertical border detector. two morphological operators ψ 1 and ψ 2 involves manipulation of their expressions using the well known set-theoretic properties, and it is often a difficult task. Ontheother hand, if we know the graphs G 1 and G 2 of two W-operators, then the determination of their equivalence is trivial: since the graph representation of a W-operator is unique, we simply compare if both graphs are equal. Alternatively, one could verify it by calling ApplyOperation(G 1, G 2, XOR), and test if the resulting graph ifthe trivial leaf Computational Complexity Time The analysis of several implementations of morphological operators (such as median filters, four-homotopicthinnings, supremum of openings), presented in [10], shows that the proposed architecture isthe most efficient alternative to implement MMachs with pixels represented by bytes. Furthermore, in general, it is beneficial to compact parallel operators (i.e., built by union or intersection of several operators). In complex hybrid morphological operators, it may be beneficial to compact just pieces of the operator in BDDs. 9. Conclusion Inthiswork we showed that the ROBDD isagood alternative for the representation of W-operators. Its efficient application time makes it a good candidate as the core representation scheme of nonlinear signal and image processors. The main contribution of this work is the development of a well defined procedure to convert any expression of the morphological language for a given

27 12 MATHEMATICAL MORPHOLOGY: 40 YEARS ON W-operator in its corresponding ROBDD. Theuniqueness of the ROBDD representation allows a simplesolution to the problem of checking the equivalence between morphological operators. Currently, we are working on an implementation of a BDD-based morphological machine. Acknowledgments This work was partially supported by grant 1 D43 TW from the National Institutes of Health, USA. References [1] S. B. Akers. Binary Decision Diagrams. IEEE Transactions on Computers, C-27(6): , June [2] G. J. F. Banon and J. Barrera. Minimal Representations for Translation-Invariant Set Mappings by Mathematical Morphology. SIAM J. Appl. Math., 51(6): , December [3] J. Barrera and G. P. Salas. Set Operations on Closed Intervals and Their Applications to the Automatic Programming of Morphological Machines. Electronic Imaging, 5(3): , July [4] G. Birkhoff. Lattice Theory. American Mathematical Society Colloquium Publications, Rhode Island, [5] K. S. Brace, R. L. Rudell, and R. E. Bryant. Efficient Implementation of a BDD Package. In Proceedings of the ACM/IEEE Design Automation Conference (DAC), pages ACM/IEEE, [6] R. E. Bryant. Graph-Based Algorithms for Boolean Function Manipulation. IEEE Transactions on Computers, C-35(8): , August [7] G. de Micheli. Synthesis and Optimization of Digital Circuits. McGraw-Hill Higher Education, [8] H. J. A. M. Heijmans. Morphological Image Operators. Academic Press, Boston, [9] C.Y. Lee. Representation of Switching Circuits by Binary-Decision Programs. Bell Systems Technical Journal, 38: , July [10] H. M. F. Madeira, J. Barrera, R. Hirata Jr., and N. S. T. Hirata. A New Paradigm for the Architecture of Morphological Machines: Binary Decision Diagrams. In SIBGRAPI 99 - XII Brazilian Symposium of Computer Graphic and Image Processing, pages IEEE Computer Society, November [11] H. M. F. Madeira and J. Barrera. Incremental Evaluation of BDD-Represented Set Operators. In SIBGRAPI XIII Brazilian Symposium of Computer Graphic and Image Processing, pages IEEE Computer Society, [12] L. Robert and G. Malandain. Fast Binary Image Processing Using Binary Decision Diagrams. Computer Vision and Image Understanding, 72(1):1 9, October [13] J. Serra. Image Analysis and Mathematical Morphology. Volume 2: Theoretical Advances. Academic Press, 1988.

28 IMAGE FILTERINGUSING MORPHOLOGICAL AMOEBAS Romain Lerallut, Étienne Decencière and Fernand Meyer Centre de Morphologie Mathématique, École des Mines de Paris 35 rue Saint-Honoré, Fontainebleau, France Abstract Keywords: This paper presents morphological operators with non-fixed shape kernels, or amoebas, which take into account the image contour variations to adapt their shape. Experiments on grayscale and color images demonstrate that these novel filters outperform classical morphological operations with a fixed, space-invariant structuring element for noise reduction applications. Anisotropic filters, noise reduction, morphological filters, color filters 1. Introduction Noise is possibly the most annoying problem in the field of image processing. There are two ways to work around it: either design particularly robust algorithms that can work in noisy environments, or try to eliminate the noise in a first step while losing as little relevant information as possible and consequently use a normally robust algorithm. There are of course many algorithms that aim at reducing the amount of noise in images. Most are quite effective but also often remove thin elements such as canals or peninsulas. Even worse, they can displace the contours and thus create additional problems in a segmentation application. In mathematical morphology we often couple one of these noise-reduction filters to a reconstruction filter that attempts to reconstruct only relevant information, such as contours, and not noise. However, a faithful reconstruction can be problematic when the contour itself is corrupted by noise. This can cause great problems in some applications which rely heavily on clean contour surfaces, such as 3D visualization, so a novel approach was proposed. C. Ronse et al. (eds.), Mathematical Morphology: 40 Years On, c 2005 Springer. Printed in the Netherlands. 13

29 14 MATHEMATICAL MORPHOLOGY: 40 YEARS ON 2. Amoebas: dynamic structuring elements Principle Classic filter kernel. Formally at least, classic filters work on a fixed-size sliding window, be they morphological operators (erosion, dilation) or convolution filters, such as the diffusion by a Gaussian. If the shape of that window does not adapt itself to the content of the image (see figure 1), the results deteriorate. For instance, an isotropic Gaussian diffusion smooths the contours when its kernel steps over a stronggradient area. Figure 1 Closing of an image by a large structuring element. The structuring element does not adapt its shape and merges two distinct objects. Amoeba filter kernel. Having made this observation, Perona and Malik [1] (and others after them) have developed anisotropic filters that inhibit diffusion through strong gradients. We were inspired by these examples to define morphological filters whose kernels adapt to the content of the image in order to keep a certain homogeneousness inside each structuring element (see figure 2). The coupling performed between the geometric distance between pixels and the distance between their values has similarities with the work of Tomasi and Manduchi described in [5]. The interest of this approach, compared to the analytical one pioneered by Perona and Malik is that it does not depart greatly from what we use in mathematical morphology, and therefore most of our algorithms can be made to use amoebas with little additional work. Most of the underlying theoretical groundwork for the morphological approach has been described by Jean Serra in his study [2] of structuring functions, although until now it has seen little practical use. Figure 2 Closing of an image by an amoeba. The amoeba does not cross the contour and as such preserves even the small canals. The shape of the amoeba must be computed for each pixel around which it is centered. Figure 3 shows the shape of an amoeba depending on the position of its center. Note that in flat areas such as the center of the disc, or the

30 Image filtering using morphological amoebas 15 Figure 3 Shape of an amoeba at various positions on an image. background, the amoeba is maximally stretched, while it is reluctant to cross contour lines. When an amoeba has been defined, most morphological operators and many other types of filters can be used on it: median, mean, rank filters, erosion, dilation, opening, closing, even more complex algorithms such as reconstruction filters, levelings, floodings, etc. Construction Amoeba distance. In general, afiltering kernel of radius r is formally defined on a square (or a hexagon) of that radius, that is to say on the ball of radius r relative to the norm associated to the chosen connectivity. We will keep this definition changing only the norm, using one that takes into account the gradient of the image. Definition 1 Let d pixel be adistance defined on the values of the image, for example adifference of gray-value,oracolor distance. Let σ =(x = x 0,x 1,...,x n = y) a path between points x and y. Let λ be areal positive number. The length of the path σ is defined as n L(σ) = 1+λ.dd pixel (x i,x i+1 ) i=0 The amoeba distance with parameter λ is thus defined as: { dλ (x, x) = 0 d λ (x, y) = min σ L(σ) It it important to realize that d pixel has no geometrical aspect, it is adistance computed only onthe values of the pixels of the image. Furthermore, if n is the number of pixels of a path σ, then L(σ) n (since λ 0), which bounds the maximal extension of the amoeba. This distance also offers an interesting inclusion property: