=Paper=
{{Paper
|id=Vol-1829/iStar17_paper_14
|storemode=property
|title=An Automatic Layout Approach for iStar Models
|pdfUrl=https://ceur-ws.org/Vol-1829/iStar17_paper_14.pdf
|volume=Vol-1829
|authors=Xiaolin Du,Tong Li,Dan Wang
|dblpUrl=https://dblp.org/rec/conf/istar/DuLW17
}}
==An Automatic Layout Approach for iStar Models==
https://ceur-ws.org/Vol-1829/iStar17_paper_14.pdf
An Automatic Layout Approach for iStar
Models
Xiaolin Du, Tong Li, and Dan Wang
Beijing University of Technology, Beijing, China
{du xiaolin,litong,wangdan}@bjut.edu.cn
Abstract. The comprehensive syntax of iStar modeling language allows
requirements analysts to clearly capture stakeholder’s needs, as well as
dependencies among stakeholders. However, such iStar models cannot
be automatically laid out using typical layout algorithms, such as hi-
erarchical layout and circular layout. Thus, constructing and adjusting
iStar models are laborious tasks, especially when dealing with large-scale
models. In this paper, we propose a tentative approach to automatically
lay out iStar models using a force-based layout algorithm. In particular,
our approach has been designed by taking into account the syntax of
iStar models in order to ensure both neatness and understandability of
resulting models.
1 Introduction
iStar modeling framework has been used as an efficient approach to capture and
analyze stakeholder’s needs in the early phase of requirements engineering. In
particular, the comprehensive syntax of iStar modeling language allows require-
ments analysts to clearly capture stakeholders’ requirements, their rationales, as
well as dependencies among stakeholders.
However, to the best of our knowledge, there is no algorithm to automatically
lay out iStar models. Typical layout algorithms, e.g., hierarchical and circular
layout algorithms, cannot serve this purpose, because they do not fit iStar syn-
tax. As a result, analysts have to spend a significant amount of time dealing
with layout issues when creating or modifying iStar models. Such a problem is
exacerbated by the growth of model scale, resulting in a scalability problem [6].
Challenges of automatic layout of iStar models inherit from their comprehen-
sive syntax. In particular, there are two views of presenting iStar models, each of
which has its particular requirements for layout. Firstly, the SD (Strategic De-
pendency) view exclusively presents actors and their dependency relations, the
layout of which should avoid crossed links while maximizing model readability.
Secondly, the SR (Strategic Rationale) view includes internal details of actors,
e.g., goals and tasks, which are supposed to be presented as a tree-like structure.
Using force-based layout algorithms is a classic graph drawing method, which
simulates a physical system with edges acting as springs and nodes as repelling
objects [12]. As is shown in Fig. 1, when running such an algorithm, nodes
are pushed away from each other due to their repelling forces while edges hold
the nodes together. By iteratively running the algorithm, a graph will eventually
come to an equilibrium in which nodes do not change positions from one iteration
to the next.
Fig. 1: The spring analogy [12]
In this paper, we present our ongoing research of automatic layout of iStar
models. In particular, we propose a forced-based algorithm to automatically lay
out elements in the SD view, which has been customized based on the syntax of
iStar models in order to produce meaningful layouts. The automatic layout of
SR view is not covered in this paper. We will discuss our initial ideas towards
this issue and left its in-depth investigation for future research. It is worth noting
that terminology used in this paper is aligned with iStar 2.0 modeling language.
2 A Forced-Based Layout Algorithm for iStar Models
2.1 Layout of SD View
When it comes to layout of the SD view, a primary goal is to calculate a reason-
able position for each actor. To this end, we need to first transform an iStar SD
model into an undirected relational graph, which is then used as input of our
force-based layout algorithm. Specifically, each actor in the SD model is turned
into a node, and relationships among actors (e.g., dependency and part-of ) are
abstracted as edges in the relational graph. It is worth noting that the direction
of an edge does not affect the outcome of our layout algorithm, and thus is not
considered in this paper.
Dependency relationships among actors are essential in the SD view, reflect-
ing to which extent two actors interact with each other. In order to produce
meaningful layout, our approach takes into account such dependency relation-
ships. Specifically, the more dependency relationships exist between two actors,
the closer they are. As a result, when abstracting edges among actors, we also
calculate their corresponding weights and store them in an edge weight matrix
EW , The element in the matrix EWij is the number of dependency relationships
between actors. For example, EWij = 3 means that there are three dependency
relationships between actor i and actor j. Fig. 2 shows an example of the trans-
formation from an iStar SD model to a relational graph.
Let G(VG , EG ) denotes a relational graph, VG represents the node set of G,
and EG represents the edge set of G. We use FR (Fruchterman and Reingold)
Fig. 2: An example of graph transformation
layout algorithm [7] to calculate the positions. Each iteration of the FR layout
algorithm consists of three steps: 1) calculate the effect of attractive forces on
each vertex; 2) calculate the effect of repulsive forces; 3) limit the total dis-
placement. Note that forces are modeled based on the optimal distance between
vertices, which is calculated as below:
r
area
k=C∗ (1)
number of nodes
As shown in Eq. 1, k is calculated as the radius of the empty area around a
node, where the constant C is found experimentally. In such a way, all nodes are
able to be uniformly distributed in a canvas. Subsequently, given d the distance
between the two nodes, attractive forces fa and repulsive forces fr are defined
as below:
d2 k2
fa (d) = (2) fr (d) = − (3)
k d
Our layout approach takes into account the weight of each edge. Specifically,
an edge with bigger weight indicates that its endpoints will be arranged closer.
Thus, we improve the attractive force formula as
d2
fa (d) = EW ∗ (4)
k
For edges that are derived from dependency relations, the corresponding
dependum will be modeled in the middle of edges. As such, two nodes should
not be placed too close to each other. To this end, a threshold τ is defined to
control the minimal distance between connected nodes.
As summarized in Algorithm 1, our approach first transforms a textually
represented iStar model into a relational graph, while forming an edge weight
matrix. After that an improved FR algorithm is iteratively applied to calcu-
late the position of each node, based on which a graphical iStar model can be
constructed.
2.2 Layout of SR View
As for the layout of SR view, which is left for future work, we discuss here our
initial thoughts towards this issue. In general, this layout task can be divided
Algorithm 1 A customized force-based layout algorithm
Input: a textual specification of a iStar model
Output: a graphical representation of the iStar model
1: Transform the iStar model to a relational graph G;
2: Calculate an edge weight matrix EW ;
3: Apply the improved FR algorithm;
4: { Calculate k
5: For i = 1 to iterations Do
6: Calculate repulsive forces;
7: Calculate attractive forces;
8: Adjust the total displacement;
9: if the distance between two nodes is less than τ
10: adjust moving direction;
11: }
12: Construct the graphical iStar model
into two sub-tasks: laying out all the intentional elements inside actor boundaries
and positioning each actor boundary as a whole in the entire picture. Since most
intentional elements are supposed to be organized in a tree-like structure, an
intuitive solution is using a hierarchical layout algorithm. However, for quality
requirements and corresponding contribution links, which are laid out in a dif-
ferent manner, we need to design particular algorithms to deal with their layout.
Once elements inside actor boundaries can be well arranged, we can then treat
each of them as a single element and layout them together with actor nodes.
To accommodate this, we will need to further adjust the force-based algorithm
proposed in this paper.
2.3 Evaluation Plans
Once we extend our proposal to cover the layout issue of SR view, we plan to
implement the entire layout approach as an independent library, which is able to
be integrated into various iStar modeling tools 1 . As a starting point, we plan to
first implement the layout approach on top of our previous goal modeling tool
MUSER 2 , serving as a prototype tool.
With the help of the prototype tool, we plan to evaluate the effectiveness of
our layout approach through a series of studies. Specifically, we are interested in
investigating the following research questions:
– Can our approach create understandable layout of i models? We
plan to carry out several user studies, in which we will ask iStar experts to
assess to which extent the resulting layout of our proposal can be understood.
– Is our approach scalable to large-scale models? We will evaluate the
scalability issue of our approach by applying it to a set of large-scale models.
1
http://istar.rwth-aachen.de/tiki-index.php?page=i%2A+Tools
2
http://disi.unitn.it/~li/MUSER/Intro.html
– Can our approach lead to faster modeling practices? We plan to
design a controlled experiment to compare the performance of two groups,
one of which is assisted by our approach. In particular, we aim to collect
feedback from participants via focus group interview, based on which we
can improve our approach to better support iStar modeling.
3 Related Work
We have been aware of a number of proposals that are relevant to ours. OpenOME
and jUCMNav provide some basic graph layout functions to automatically ar-
range the goal elements, which mainly focus on laying out intentional elements
inside actors [2,1]. These approaches can well complement our current proposal,
rendering a comprehensive layout approach. Moreover, Gregorics et al. proposed
a textual layout description language, which allows users to define the arrange-
ment of important diagram elements [10]. Ghazi et al. propose FlexiView to
improve the visual representation of requirements artifacts, which also lever-
ages magnetic-spring algorithms but in a different manner [8]. Specifically, their
approach partitions requirements engineers’ workspace based on the magnetic-
spring model in order to present different types of requirements artifacts at the
same time, minimizing scrolling distance.
Different from most iStar modeling tools, Grau et al. propose J-PRiM which
does not show the i* elements in a graphical way but in a tree-form hierarchy [9].
In such a way, as models grow, the scalability issue can be relieved, and modelers
are able to find particular elements in a faster way. Note that such a textual
modeling approach and our proposal can complement each other. Specifically,
management of iStar models can be done in the textual view, while our approach
is responsible for visualizing the textual models when necessary.
Graph visualization helps users to gain insight into data by turning the data
elements and their internal relationships into graphs [4]. Graph layout problems
are a particular class of combinatorial optimization problems whose goal is to find
a linear layout of an input graph in such a way that a certain objective function is
optimized [5]. Force-based layout is a very popular graph visualization method.
It is first proposed by Eades in 1984, which has then been revisited and improved
by many researchers in different ways (e.g., [11]). Force directed model can be
expressed either in terms of forces acting on the physical objects, or in terms
of a potential energy reflecting the internal stress of the system. Algorithms to
simulate a system’s relaxation typically try to move the objects iteratively into
stable states in which all forces cancel each other, or to minimize the energy
directly [3].
4 Conclusions and Future Work
In this paper, we present a tentative approach to automatically lay out iStar
models using a customized force-based layout algorithm. As ongoing research,
our proposal mainly focuses on the layout of iStar SD view thus far. In addition,
we have discussed possible ways of extending our approach in order to lay out
elements in the SR view.
Our approach can be applied to textual representations of iStar models, the
benefits of which are twofold. Firstly, modelers are able to exclusively focus
on the content of models to be built without being bothered by layout issues.
As such, our approach can save much effort on the creation of iStar models,
especially when modeling large-scale scenarios. Secondly, there are increasing
demands of improving usability of iStar modeling tools in order to better deal
with scalability issues [13]. With the help of our approach, tool developers can
also be released from never ending improvement of graphical interfaces. More-
over, by enhancing iStar tools with the automatic layout feature, they are able to
better support goal model reasoning tasks that incrementally add new elements,
e.g., goal refinements.
As for future work, we plan to first extend our proposal is to deal with auto-
matic layout of the SR view. After that we will implement our layout approach in
a particular iStar modeling tool, based on which we will evaluate the usefulness
of our approach through a series of empirical studies.
Acknowledgments. Tong Li acknowledges the support of BJUT Startup Funding
No.007000514116022. Xiaolin Du acknowledges the support of Beijing Postdoctoral
Research Foundation No.Q6007012201601.
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