Veerle Fack | Ghent University (original) (raw)

Papers by Veerle Fack

ABSTRACT This paper describes the statistical comparison of the results from an experiment with a... more ABSTRACT This paper describes the statistical comparison of the results from an experiment with a textquoteleft between usertextquoteright-design. The first group of participants consists out of novices whereas the second group consists out of experts which have experience in map use and have had training in cartography. The same stimuli (twenty screen maps) are presented in a random order to the participants who have to locate a number of labels on the map image. The participants are asked to indicate when they located a name by a button action, resulting in a time measurement. Furthermore, the participanttextquoteright s eye movements are registered during the whole test. The combined information reveals a same trend in the time intervals needed to locate the subsequent labels in both user groups. However, the experts are significantly faster in locating the names on the map (P<0.010). The recorded eye movements further confirm and explain this finding: the experttextquoteright s fixations are significantly shorter (P<0.001) and can consequently have more fixations per second (P<0.001). This means that an expert can interpret the map content more efficiently and can thus search a larger part of the map in the same amount of time.

In this paper, the effectiveness of certain aspects of the label placement method on dynamic and ... more In this paper, the effectiveness of certain aspects of the label placement method on dynamic and interactive maps towards the user is examined. Reducing the number of labels which need to be considered by the method will drastically improve its efficiency. A user study is constructed to test whether these adaptations have an influence on the effectiveness of the map towards the user. The conducted tests utilize the eye tracking method to get an insight of the user's cognitive processes while performing a visual search on these maps. The obtained results reveal that the user does not notice the adaptations on the map. This leads to the conclusion that the efficiency of the map can be improved without causing distorting in the user's cognitive map.

We present exhaustive and non-exhaustive search algorithms for small minimal blocking sets in gen... more We present exhaustive and non-exhaustive search algorithms for small minimal blocking sets in generalized quadrangles. Using these techniques, new results were obtained for some classical generalized quadrangles. Moreover, some of these com-puter results lead to a general construction of small minimal blocking sets.

Bulletin of the Belgian Mathematical Society, Simon Stevin

This paper describes JAMES, a modern object-oriented Java framework for discrete optimization usi... more This paper describes JAMES, a modern object-oriented Java framework for discrete optimization using local search algorithms that exploits the generality of such metaheuristics by clearly separating search implementation and application from problem specification. A wide range of generic local searches are provided, including (stochastic) hill climbing, tabu search, variable neighbourhood search and parallel tempering. These can be applied easily to any user-defined problem by plugging in a custom neighbourhood for the corresponding solution type. The performance of several different search algorithms can be assessed and compared in order to select an appropriate optimization strategy. Also, the influence of parameter values can be studied. Implementations of specific components are included for subset selection, such as a predefined solution type, a generic problem definition and several subset neighbourhoods used to modify the set of selected items. Additional components for other ...

The increase in population, especially the massive urban expansion creates an enormous pressure o... more The increase in population, especially the massive urban expansion creates an enormous pressure on the available space. To counter this, a more intelligent use of urban space is often proposed, resulting in increasingly complex buildings. However, this evolution puts a ...

Recently, the use of eye tracking systems has been introduced in the field of cartography and GIS... more Recently, the use of eye tracking systems has been introduced in the field of cartography and GIS to support the evaluation of the quality of maps towards the user. The quantitative eye movement metrics are related to for example the duration or the number of the fixations which are subsequently (statistically) compared to detect significant differences in map designs or between different user groups. Hence, besides these standard eye movement metrics, other - more spatial - measurements and visual interpretations of the data are more suitable to investigate how users process, store and retrieve information from a (dynamic and/or) interactive map. This information is crucial to get insights in how users construct their cognitive map: e.g. is there a general search pattern on a map and which elements influence this search pattern, how do users orient a map, what is the influence of for example a pan operation. These insights are in turn crucial to be able to construct more effective ...

This research focuses on the possible implementation of indoor navigation networks in its spatial... more This research focuses on the possible implementation of indoor navigation networks in its spatial environment. Starting from a comparison of a number of these indoor networks, a linear network is implemented in a hybrid data model, based on the CityGML data structure.

In this paper the results of a visual analytics approach on eye movement data are described which... more In this paper the results of a visual analytics approach on eye movement data are described which allows detecting underlying patterns in the scanpaths of the usertextquoteright s during a visual search on a map. These patterns give insights in the user his cognitive processes or his mental map while working with interactive maps.

A double 2-(v,k,2 ) design is a design which is reducible into two 2-(v,k, ) designs. It is calle... more A double 2-(v,k,2 ) design is a design which is reducible into two 2-(v,k, ) designs. It is called uniquely reducible if it has, up to equivalence, only one reduction. We present properties of uniquely reducible double designs which show that their total number can be determined if only the designs with non-trivial automorphisms are classified with respect to their automorphism group. As an application, after proving that a reducible 2-(21,5,2) design is uniquely reducible, we establish that the number of all reducible 2-(21,5,2) designs is 1746461307. Each 2-(v,k, ) design determines the existence of 2-(v,k,m ) designs for any integer m > 1. A 2- (v,k,m ) design is called a quasimultiple of a 2-(v,k, ) design. A quasimultiple 2-(v,k,m ) is reducible into m 2-(v,k, ) designs if there is a partition of its blocks into m subcollections each of which forms a 2-(v,k, ) design. Form = 2, quasimultiple designs are called quasidoubles and the reducible quasidouble designs are called dou...

ABSTRACT In this study the user side of the label placement problem - on dynamic and interactive ... more ABSTRACT In this study the user side of the label placement problem - on dynamic and interactive maps - is considered. The goal of the study is to gain insight in the usertextquoteright s cognitive processes while performing a task on a dynamic map. The eye tracking method is selected in order to get in touch with this cognitive map. The method is integrated in a user study in which the subjects are asked to perform a visual search on a dynamic map. These dynamic maps can be categorized into four different groups, representing the combination of two hypotheses which are tested during the experiments. These hypotheses are both related to effectiveness of the label placements algorithms and thus to the usertextquoteright s cognitive map.

Recently, a rising need to understand (novice) users of digital cartographic products has surface... more Recently, a rising need to understand (novice) users of digital cartographic products has surfaced: how do they interpret and process the visual information, and how is this retrieved and used later on? A user study was conducted that combined several techniques to investigate these cognitive processes: eye tracking, thinking aloud and sketch maps. In total, 24 participants took part in the study, from which 12 were experts in the cartography. All participants had to study (learn) the content of four different topographic maps depicted on a screen. After studying each map, the participants were instructed to draw this map from memory (retrieve information), using paper and pencils. The results indicate that both user groups address the same cognitive processes, but these are positively influence by the expertise and background knowledge of the experts.

ABSTRACT This paper describes the statistical comparison of the results from an experiment with a... more ABSTRACT This paper describes the statistical comparison of the results from an experiment with a textquoteleft between usertextquoteright-design. The first group of participants consists out of novices whereas the second group consists out of experts which have experience in map use and have had training in cartography. The same stimuli (twenty screen maps) are presented in a random order to the participants who have to locate a number of labels on the map image. The participants are asked to indicate when they located a name by a button action, resulting in a time measurement. Furthermore, the participanttextquoteright s eye movements are registered during the whole test. The combined information reveals a same trend in the time intervals needed to locate the subsequent labels in both user groups. However, the experts are significantly faster in locating the names on the map (P<0.010). The recorded eye movements further confirm and explain this finding: the experttextquoteright s fixations are significantly shorter (P<0.001) and can consequently have more fixations per second (P<0.001). This means that an expert can interpret the map content more efficiently and can thus search a larger part of the map in the same amount of time.

ISPRS Annals of Photogrammetry, Remote Sensing and Spatial Information Sciences, 2013

Nucleic acids research, 2012

The combination of incessant advances in sequencing technology producing large amounts of data an... more The combination of incessant advances in sequencing technology producing large amounts of data and innovative bioinformatics approaches, designed to cope with this data flood, has led to new interesting results in the life sciences. Given the magnitude of sequence data to be processed, many bioinformatics tools rely on efficient solutions to a variety of complex string problems. These solutions include fast heuristic algorithms and advanced data structures, generally referred to as index structures. Although the importance of index structures is generally known to the bioinformatics community, the design and potency of these data structures, as well as their properties and limitations, are less understood. Moreover, the last decade has seen a boom in the number of variant index structures featuring complex and diverse memory-time trade-offs. This article brings a comprehensive state-of-the-art overview of the most popular index structures and their recently developed variants. Their...

Journal of Physics A: Mathematical and General, 1985

ABSTRACT The bound state energies of the screened Coulomb potential-exp(- lambda r) cos( mu r)/r ... more ABSTRACT The bound state energies of the screened Coulomb potential-exp(- lambda r) cos( mu r)/r can be approximated on account of a scaling variational principle and in such a way that no expansion in the screening parameters is required.

Journal of Physics A: Mathematical and General, 1986

ABSTRACT High accuracy approximations for the bound state energies of the exponential cosine scre... more ABSTRACT High accuracy approximations for the bound state energies of the exponential cosine screened Coulomb potential are obtained by means of algebraic perturbation calculations.

Journal of Physics A: Mathematical and General, 1987

Numerov's method and an extended version of it are introduced for computing eigenvalues of S... more Numerov's method and an extended version of it are introduced for computing eigenvalues of Schrodinger equations with potentials V(x) which are even functions with respect to x. Furthermore it is assumed that the wavefunctions tend to zero for x to +or- infinity . The derived results are compared with previously derived numerical data and with available exact values.

ABSTRACT This paper describes the statistical comparison of the results from an experiment with a... more ABSTRACT This paper describes the statistical comparison of the results from an experiment with a textquoteleft between usertextquoteright-design. The first group of participants consists out of novices whereas the second group consists out of experts which have experience in map use and have had training in cartography. The same stimuli (twenty screen maps) are presented in a random order to the participants who have to locate a number of labels on the map image. The participants are asked to indicate when they located a name by a button action, resulting in a time measurement. Furthermore, the participanttextquoteright s eye movements are registered during the whole test. The combined information reveals a same trend in the time intervals needed to locate the subsequent labels in both user groups. However, the experts are significantly faster in locating the names on the map (P<0.010). The recorded eye movements further confirm and explain this finding: the experttextquoteright s fixations are significantly shorter (P<0.001) and can consequently have more fixations per second (P<0.001). This means that an expert can interpret the map content more efficiently and can thus search a larger part of the map in the same amount of time.

In this paper, the effectiveness of certain aspects of the label placement method on dynamic and ... more In this paper, the effectiveness of certain aspects of the label placement method on dynamic and interactive maps towards the user is examined. Reducing the number of labels which need to be considered by the method will drastically improve its efficiency. A user study is constructed to test whether these adaptations have an influence on the effectiveness of the map towards the user. The conducted tests utilize the eye tracking method to get an insight of the user's cognitive processes while performing a visual search on these maps. The obtained results reveal that the user does not notice the adaptations on the map. This leads to the conclusion that the efficiency of the map can be improved without causing distorting in the user's cognitive map.

We present exhaustive and non-exhaustive search algorithms for small minimal blocking sets in gen... more We present exhaustive and non-exhaustive search algorithms for small minimal blocking sets in generalized quadrangles. Using these techniques, new results were obtained for some classical generalized quadrangles. Moreover, some of these com-puter results lead to a general construction of small minimal blocking sets.

Bulletin of the Belgian Mathematical Society, Simon Stevin

This paper describes JAMES, a modern object-oriented Java framework for discrete optimization usi... more This paper describes JAMES, a modern object-oriented Java framework for discrete optimization using local search algorithms that exploits the generality of such metaheuristics by clearly separating search implementation and application from problem specification. A wide range of generic local searches are provided, including (stochastic) hill climbing, tabu search, variable neighbourhood search and parallel tempering. These can be applied easily to any user-defined problem by plugging in a custom neighbourhood for the corresponding solution type. The performance of several different search algorithms can be assessed and compared in order to select an appropriate optimization strategy. Also, the influence of parameter values can be studied. Implementations of specific components are included for subset selection, such as a predefined solution type, a generic problem definition and several subset neighbourhoods used to modify the set of selected items. Additional components for other ...

The increase in population, especially the massive urban expansion creates an enormous pressure o... more The increase in population, especially the massive urban expansion creates an enormous pressure on the available space. To counter this, a more intelligent use of urban space is often proposed, resulting in increasingly complex buildings. However, this evolution puts a ...

Recently, the use of eye tracking systems has been introduced in the field of cartography and GIS... more Recently, the use of eye tracking systems has been introduced in the field of cartography and GIS to support the evaluation of the quality of maps towards the user. The quantitative eye movement metrics are related to for example the duration or the number of the fixations which are subsequently (statistically) compared to detect significant differences in map designs or between different user groups. Hence, besides these standard eye movement metrics, other - more spatial - measurements and visual interpretations of the data are more suitable to investigate how users process, store and retrieve information from a (dynamic and/or) interactive map. This information is crucial to get insights in how users construct their cognitive map: e.g. is there a general search pattern on a map and which elements influence this search pattern, how do users orient a map, what is the influence of for example a pan operation. These insights are in turn crucial to be able to construct more effective ...

This research focuses on the possible implementation of indoor navigation networks in its spatial... more This research focuses on the possible implementation of indoor navigation networks in its spatial environment. Starting from a comparison of a number of these indoor networks, a linear network is implemented in a hybrid data model, based on the CityGML data structure.

In this paper the results of a visual analytics approach on eye movement data are described which... more In this paper the results of a visual analytics approach on eye movement data are described which allows detecting underlying patterns in the scanpaths of the usertextquoteright s during a visual search on a map. These patterns give insights in the user his cognitive processes or his mental map while working with interactive maps.

A double 2-(v,k,2 ) design is a design which is reducible into two 2-(v,k, ) designs. It is calle... more A double 2-(v,k,2 ) design is a design which is reducible into two 2-(v,k, ) designs. It is called uniquely reducible if it has, up to equivalence, only one reduction. We present properties of uniquely reducible double designs which show that their total number can be determined if only the designs with non-trivial automorphisms are classified with respect to their automorphism group. As an application, after proving that a reducible 2-(21,5,2) design is uniquely reducible, we establish that the number of all reducible 2-(21,5,2) designs is 1746461307. Each 2-(v,k, ) design determines the existence of 2-(v,k,m ) designs for any integer m > 1. A 2- (v,k,m ) design is called a quasimultiple of a 2-(v,k, ) design. A quasimultiple 2-(v,k,m ) is reducible into m 2-(v,k, ) designs if there is a partition of its blocks into m subcollections each of which forms a 2-(v,k, ) design. Form = 2, quasimultiple designs are called quasidoubles and the reducible quasidouble designs are called dou...

ABSTRACT In this study the user side of the label placement problem - on dynamic and interactive ... more ABSTRACT In this study the user side of the label placement problem - on dynamic and interactive maps - is considered. The goal of the study is to gain insight in the usertextquoteright s cognitive processes while performing a task on a dynamic map. The eye tracking method is selected in order to get in touch with this cognitive map. The method is integrated in a user study in which the subjects are asked to perform a visual search on a dynamic map. These dynamic maps can be categorized into four different groups, representing the combination of two hypotheses which are tested during the experiments. These hypotheses are both related to effectiveness of the label placements algorithms and thus to the usertextquoteright s cognitive map.

Recently, a rising need to understand (novice) users of digital cartographic products has surface... more Recently, a rising need to understand (novice) users of digital cartographic products has surfaced: how do they interpret and process the visual information, and how is this retrieved and used later on? A user study was conducted that combined several techniques to investigate these cognitive processes: eye tracking, thinking aloud and sketch maps. In total, 24 participants took part in the study, from which 12 were experts in the cartography. All participants had to study (learn) the content of four different topographic maps depicted on a screen. After studying each map, the participants were instructed to draw this map from memory (retrieve information), using paper and pencils. The results indicate that both user groups address the same cognitive processes, but these are positively influence by the expertise and background knowledge of the experts.

ABSTRACT This paper describes the statistical comparison of the results from an experiment with a... more ABSTRACT This paper describes the statistical comparison of the results from an experiment with a textquoteleft between usertextquoteright-design. The first group of participants consists out of novices whereas the second group consists out of experts which have experience in map use and have had training in cartography. The same stimuli (twenty screen maps) are presented in a random order to the participants who have to locate a number of labels on the map image. The participants are asked to indicate when they located a name by a button action, resulting in a time measurement. Furthermore, the participanttextquoteright s eye movements are registered during the whole test. The combined information reveals a same trend in the time intervals needed to locate the subsequent labels in both user groups. However, the experts are significantly faster in locating the names on the map (P<0.010). The recorded eye movements further confirm and explain this finding: the experttextquoteright s fixations are significantly shorter (P<0.001) and can consequently have more fixations per second (P<0.001). This means that an expert can interpret the map content more efficiently and can thus search a larger part of the map in the same amount of time.

ISPRS Annals of Photogrammetry, Remote Sensing and Spatial Information Sciences, 2013

Nucleic acids research, 2012

The combination of incessant advances in sequencing technology producing large amounts of data an... more The combination of incessant advances in sequencing technology producing large amounts of data and innovative bioinformatics approaches, designed to cope with this data flood, has led to new interesting results in the life sciences. Given the magnitude of sequence data to be processed, many bioinformatics tools rely on efficient solutions to a variety of complex string problems. These solutions include fast heuristic algorithms and advanced data structures, generally referred to as index structures. Although the importance of index structures is generally known to the bioinformatics community, the design and potency of these data structures, as well as their properties and limitations, are less understood. Moreover, the last decade has seen a boom in the number of variant index structures featuring complex and diverse memory-time trade-offs. This article brings a comprehensive state-of-the-art overview of the most popular index structures and their recently developed variants. Their...

Journal of Physics A: Mathematical and General, 1985

ABSTRACT The bound state energies of the screened Coulomb potential-exp(- lambda r) cos( mu r)/r ... more ABSTRACT The bound state energies of the screened Coulomb potential-exp(- lambda r) cos( mu r)/r can be approximated on account of a scaling variational principle and in such a way that no expansion in the screening parameters is required.

Journal of Physics A: Mathematical and General, 1986

ABSTRACT High accuracy approximations for the bound state energies of the exponential cosine scre... more ABSTRACT High accuracy approximations for the bound state energies of the exponential cosine screened Coulomb potential are obtained by means of algebraic perturbation calculations.

Journal of Physics A: Mathematical and General, 1987

Numerov's method and an extended version of it are introduced for computing eigenvalues of S... more Numerov's method and an extended version of it are introduced for computing eigenvalues of Schrodinger equations with potentials V(x) which are even functions with respect to x. Furthermore it is assumed that the wavefunctions tend to zero for x to +or- infinity . The derived results are compared with previously derived numerical data and with available exact values.