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Software Forest: A Visualization of Semantic Similarities in Source Code using a Tree Metaphor

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Software visualization techniques provide effective means for program comprehension tasks as they allow developers to interactively explore large code bases. A frequently encountered task during software development is the detection of source code files of similar semantic. To assist this task we present Software Forest, a novel 2.5D software visualization that enables interactive exploration of semantic similarities within a software system, illustrated as a forest. The underlying layout results from the analysis of the vocabulary of the software documents using Latent Dirichlet Allocation and Multidimensional Scaling and therefore reflects the semantic similarity between source code files. By mapping properties of a software entity, e.g., size metrics or trend data, to visual variables encoded by various, figurative tree meshes, aspects of a software system can be displayed. This concept is complemented with implementation details as well as a discussion on applications.
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DIGITAL ENGINEERING FACULTY
COMPUTER GRAPHICS SYSTEMS GROUP
Software Forest: A Visualization of Semantic
Similarities in Source Code using a Tree Metaphor
Authors Version
Daniel Atzberger
, Tim Cech, Merlin de la Haye, Maximilian Söchting,
Willy Scheibel
, Daniel Limberger
, and Jürgen Döllner
This article is originally published as
D. Atzberger, T. Cech, M. de la Haye, M. Söchting, W. Scheibel, D. Limberger, and
J. Döllner. Software forest: A visualization of semantic similarities in source code
using a tree metaphor. In Proceedings of the 16th International Joint Conference on
Computer Vision, Imaging and Computer Graphics Theory and Applications – Volume 3:
IVAPP, IVAPP ’21, pages 112–122. INSTICC, SciTePress, 2021. ISBN 978-989-758-488-6.
doi:10.5220/0010267601120122
Bibtex entry:
@InProceedings{atzberger2021-softwareforest,
author = {Atzberger, Daniel and Cech, Tim and de la Haye, Merlin and S{\"o}chting, Maximilian and Scheibel, Willy and Limberger,
Daniel and D{\"o}llner, J{\"u}rgen},
title = {Software Forest: A Visualization of Semantic Similarities in Source Code using a Tree Metaphor},
booktitle = {Proceedings of the 16th International Joint Conference on Computer Vision, Imaging and Computer Graphics Theory and
Applications -- Volume 3: IVAPP},
year = {2021},
series = {IVAPP ’21},
publisher = {SciTePress},
organization = {INSTICC},
doi = {10.5220/0010267601120122},
isbn = {978-989-758-488-6},
pages = {112--122},
}
The author is with the Hasso Plattner Institute, Digital Engineering Faculty, University of Potsdam
Software Forest: A Visualization of Semantic Similarities
in Source Code using a Tree Metaphor
Daniel Atzberger1, Tim Cech2, Merlin de la Haye2, Maximilian S¨
ochting2,
Willy Scheibel1, Daniel Limberger1, and J¨
urgen D¨
ollner1
Hasso Plattner Institute, Digital Engineering Faculty, University of Potsdam, Germany
Keywords: Topic Modeling, Software Visualization, Source Code Mining
Abstract:
Software visualization techniques provide effective means for program comprehension tasks as they allow
developers to interactively explore large code bases. A frequently encountered task during software development
is the detection of source code files of similar semantic. To assist this task we present Software Forest, a
novel 2.5D software visualization that enables interactive exploration of semantic similarities within a software
system, illustrated as a forest. The underlying layout results from the analysis of the vocabulary of the software
documents using Latent Dirichlet Allocation and Multidimensional Scaling and therefore reflects the semantic
similarity between source code files. By mapping properties of a software entity, e.g., size metrics or trend data,
to visual variables encoded by various, figurative tree meshes, aspects of a software system can be displayed.
This concept is complemented with implementation details as well as a discussion on applications.
1 INTRODUCTION
Program comprehension is of fundamental importance
during the entire software development process, es-
pecially in the maintenance phase. Getting a quick
overview over a program’s structure and functionali-
ties is necessary for further development. A question
developers frequently encounter is whether there are
already source code units of certain semantics in terms
of features, concerns, and techniques. The identifica-
tion of such code units can support decisions on feature
location, encapsulation, and module architecture. Fur-
thermore, this information helps to identify associated
developers and could be used to, e.g., determine and
assign reviewers. This is a particular challenge for
historically grown legacy systems, systems developed
from distributed teams, or systems depending on a
large technology stack. In addition, the folder struc-
ture of a software project may deviate from its concep-
tual content over time. Various software visualization
techniques support developers by providing expressive
images whose layouts are inspired by maps known
from daily life. They promote shared mental images
that enable users to communicate more precisely and
gain insight into abstract data.
We present Software Forest, a novel 2.5D software
visualization technique for displaying semantic simi-
larity of source code units. The semantic content of
software files is described as distributions over latent
topics hidden in the source code by applying Latent
Figure 1: A software forest depicting 11 291 source code
units (top). Each unit is positioned based on its semantic sim-
ilarity w.r.t. to all other units. The unit’s dominant topic is
represented by the type of the tree (right, e.g., palm tree, de-
ciduous tree, conifer) and additional attributes are be mapped
to, e.g., tree health and tree growth (left).
Dirichlet Allocation (
LDA
). Multidimensional Scal-
ing (
MDS
) is used to compute a layout, where the
distance of points reflect their semantic similarity. Us-
ing a tree metaphor, Software Forest introduces various
visual variables for topic emphasis and additional data
display, e.g., software metrics, and offers possibilities
for displaying static as well as evolutionary data (Fig-
ure 1). The following contributions are made:
1.
We present a layout algorithm based on the ap-
plication of
LDA
and
MDS
on source code files,
Figure 2: Visualization of the Microsoft Calculator open-source project in our framework. Tree types are chosen based on the
topics, i.e., source code files with the same dominant topics are represented through the same tree type. Clusters of identical
tree types, i.e., source code files with identical dominant topics, can be easily recognized in this example.
thereby mirroring the semantic relatedness of doc-
uments such as source code units.
2.
We enlist and review visual variables inherent to
the tree metaphor and discuss possibilities and ap-
plications for software visualization tasks.
3.
We provide technical notes on the implementation
of our visualization, discuss its features and lim-
itations, and provide a web-based prototype for
interactive exploration and evaluation.
The remainder of this paper is organized as follows.
Section 2 discusses background and related work. In
Section 3, we detail our layout algorithm. Section 4
presents our visualization concept using visual vari-
ables of trees (figuratively). Section 5 highlights im-
plementation aspects regarding the data processing
pipeline and the visualization framework. Section 6
discusses application scenarios and limitations of the
current approach. Section 7 concludes this paper and
outlines direction for future work.
2 RELATED WORK
Related work is mainly identified in two research areas:
(1) preprocessing of software system data by means
of topic extraction and dimension reduction and (2)
thematic maps, map-based visualizations, and visual-
ization metaphors in general.
Data Processing.
Since a large part of a developer’s
knowledge of the project is directly encoded in the
source code through comments and identifier names,
a layout origin from the vocabulary could reflect the
project’s semantic structure. This approach was pur-
sued for the first time by Kuhn et al. (Kuhn et al.,
2008). They examine the natural language found in
source code to generate software visualization. In a
first stage source files are parsed and a term-document-
matrix is created, which stores the frequency of each
word in a document. The high-dimensional document-
word-vectors, i.e., the columns of the term-document-
matrix, are projected to the plane by applying Latent
Semantic Indexing (
LSI
) (Deerwester et al., 1990) fol-
lowed by
MDS
(Cox and Cox, 2008). By creating
a height field which represents the LOC, island-like
landscapes are created which allow semantic affilia-
tions to be deduced. Another visualization of natural
language texts, which utilizes a cartographic metaphor,
can be found in (Skupin, 2004). The layout, which
is created by using a self-organizing map (Kohonen,
1997) on the term-document-matrix, is enriched with
a height field. In their investigations the authors limit
themselves to abstracts from publications of the geo-
graphic domain. For the visualization of source code
this can be extended by suitable preprocessing.
The main difference between our work and the
previously presented is that our layout is based on
a probabilistic topic model,
LDA
specifically. It is
known that the pure Bag-of-Words (
BOW
) model is
not able to recognize synonymy and polysemy. Al-
though
LSI
can recognize synonyms, polysemy is still
an issue. Probabilistic models, e.g.,
LDA
can help
to overcome both problems. Besides the layout, we
can also map properties of the topic distribution to
visual variables of the trees (Figure 2). Since
LDA
has been used for numerous software mining tasks, it
is not surprising that the obtained results were repre-
sented by dimension reductions. For instance, Linstead
et al. used LDA and the author-topic model (Rosen-
Zvi et al., 2004) to detect semantic similarity between
documents and developers (Linstead et al., 2009). The
resulting document-topic distributions are displayed
by applying
MDS
to reduce them to two dimensions.
Nevertheless, this neglects the fact that the similarity
between topics among each other might differ. We
circumvent this issue by first applying a dimension re-
duction to the topic distributions and then aggregating
the document vectors as linear combinations.
Another problem which adresses the question of
positionining elements in a way that reflects their se-
mantic similarity was presented in (Barth et al., 2014).
Barth et al. presented three novel layout algorithms for
word clouds, where the distance between the words
should capture their semantic relatedness. Our consid-
erations neglect the similarity of words and address the
representation of semantic similarity of documents.
Visualization Metaphors.
A majority of the under-
lying layouts of software visualizations are based on
the hierarchical folder structure (Holten et al., 2005;
Cornelissen et al., 2008). Treemaps are a widely used
technique in hierarchy visualization (Scheibel et al.,
2020) and are prominently used for software visualiza-
tion as they provide a wide variety of visual variables
(Limberger et al., 2019).
Besides classic visual variables like block height,
texture or color a 2.5D treemap offers creative ways for
visualizing dynamic properties of a software system.
An example is presented by W
¨
urfel et al., who showed
how natural phenomena such as fire and rain can be
used to visualize the evolution of source code artifacts
(W
¨
urfel et al., 2015). In this context, CodeCity (Wettel
and Lanza, 2007) and the Software City (Steinbr
¨
uckner
and Lewerentz, 2013) are well known metaphors. By
mapping source code metrics, e.g., Lines-of-Code
(
LOC
), to visual variables of cuboids, e.g., the height,
visualizations are created, which remind optically of
modern cities. The approach was further developed
in later work and integrated into IDEs (Balogh et al.,
2015). Next to rectangular layouts, map-like shapes
are used in hierarchy visualization as well (Auber et al.,
2013). With focus on software visualization, the map
metaphor is extended to derive layouts from depen-
dencies (Hawes et al., 2015). The map metaphor is
further used in conjunction with oceans and islands
that are derived from Voronoi diagrams (Schreiber and
Misiak, 2018). Another approach for visualizing soft-
ware metrics inspired by nature are Software Feathers,
a visualization approach for object-oriented entities
like classes and interfaces with feathers (Beck, 2014).
The structure of feathers is determined by numerous
parameters, e.g., their size, shape, or texture, which
provides many possible mappings of software met-
rics. Similar to our forest and landscape visualization,
˘
St
˘
ep
´
anek proposes a visualization primarily composed
of trees, wood cabins and islands (
˘
St
˘
ep
´
anek, 2020).
The layout of source code units is calculated based on
a weighted graph, with files that reference each other
having a stronger pull toward one another. Different
file types are represented with different metaphors,
e.g., classes are represented with trees while value
types (structs) are visualized using small cabins.
Kleiberg et al. proposed another visualization
which mainly bases on the tree metaphor (Kleiberg
et al., 2001). The authors generate a botanic tree whose
branches and fruits display hierarchical structures. Our
work completely neglects the underlying hierarchical
structure of a software system provided by its folder
structure. Additionaly a single tree is used to display a
single file rather than an entire project.
3 LAYOUT COMPUTATION
The underlying layout of a software forest is con-
structed to reflect the semantic similarity of the source
code units, in our case files contained in a source code
repository. Thus, we analyze the use of natural lan-
guage in the source code. We first preprocess the
corpus
C={d1,...,dm}
of software files
d1,...,dm
to get rid of the words in the vocabulary that do not
carry a semantic meaning. We then compute a formal
description of the documents as distributions over la-
tent topics using
LDA
. The resulting high-dimensional
vectors describing topics are reduced by
MDS
to two
components, which finally aggregate to the positions
of document vectors.
3.1 Preprocessing Data
Although source code resembles natural language in its
vocabulary, various preprocessing steps are required
to get rid of words that do not carry relevant seman-
tic meaning. For this, we implement an algorithm,
processing all documents as follows:
1. Removal of Non-text Symbols:
All special char-
acters such as dots and semicolons are replaced
with white spaces to avoid accidental connection
of words not meant to be combined. This includes
the splitting of identifier names, e.g., the word
foo.bar gets split into foo and bar.
2. Split of Words:
Identifiers are split according to
delimiters and the camel case convention, e.g., Foo-
Bar is split into foo and bar, and stripped from
redundant white space subsequently.
3. Removal of Stop Words:
Stop words based on
natural language and keywords of programming
language are removed as they carry no semantic
content. Additionally, we filter the input based on
a hand-crafted list comprising domain specific stop
words, data types, type abstractions etc.
4. Lemmatization:
To avoid grammatical diversions,
all words are reduced to their basic form, e.g., said
Topic #1 Topic #2 Topic #3
Term Probability Term Probability Term Probability
std 0.070 thread 0.132 address 0.115
transaction 0.031 time 0.070 model 0.108
fee 0.027 queue 0.064 table 0.065
tx 0.026 std 0.054 label 0.051
ban 0.024 callback 0.040 qt 0.033
str 0.023 run 0.037 index 0.030
handler 0.016 call 0.025 dialog 0.024
output 0.016 mutex 0.021 column 0.024
bitcoin 0.015 scheduler 0.020 ui 0.021
reason 0.015 wait 0.018 role 0.019
Table 1: Three exemplary topics extracted from Bitcoin Core (github.com/bitcoin/bitcoin) source code with K=50.
and saying are reduced to say.
After preprocessing, we store all documents as
BOW
, i.e., the order in which the words occur is ne-
glected and only their frequency is stored. We are not
introducing new identifiers for this notation and, when
using the term documents, always refer to their repre-
sentation as
BOW
. The term-document-matrix stores
these BOW vectors as its columns.
3.2 Latent Dirichlet Allocation
Given a corpus
C={d1,...,dm}
of documents that
share a vocabulary
V={w1,...,wN}
, a probabilistic
topic model extracts semantic clusters in its vocab-
ulary, so-called topics, as multinomial distributions
over the vocabulary
V
. Additionally each document is
described as a mixture of the extracted topics, which
describe its document’s semantic structure. Given a
number
K
of topics specified as a hyperparameter to-
gether with Dirichlet priors
α= (α1,...,αK)
, where
αi>0
for all
1iK
and
β= (β1,...,βN)
, where
βj>0
for all
1jN
that capture the document-
topic-distributions and the topic-term-distribution re-
spectively,
LDA
defines a fully generative model for
the creation of a corpus (Blei et al., 2003). The gener-
ation process for a document doperates as follows:
1. choose a distribution over topics θDirichlet(α)
2. for each word win d
(a) choose a topic zMultinomial(θ)
(b)
choose the word
w
according to the probability
p(w|z,β)
Table 1 exemplarily shows the top 10 words of 3
manually selected topics applied on the corpus derived
from the source code of the Bitcoin Core. These often
suggest the concept underlying the topic, e.g., the third
topic covers user interaction. The number of topics
was chosen such that the extracted topics were easily
interpretable by humans.
3.3 Dimension Reduction
Given the extracted topics
φ1,...,φK
and the
document-topic distributions
θ1,...,θm
, we first
project the topic vectors to two dimensions. We adapt
the approach by Sievert et al. (Sievert and Shirley,
2014), which is given by applying
MDS
to the disimi-
larity matrix derived from the Jensen-Shannon diver-
gence.
MDS
yields projections
¯
φ1,..., ¯
φK
, such that
the euclidean distance between projected points re-
flects the dissimilarity between the high-dimensional
points. A document
di
,
1im
viewed as
K
-length
vector θi= (θ(1)
i,...,θ(K)
i), is then represented by the
Figure 3: LDAvis applied on Bitcoin with
K=50
. The
centers of the circles indicate the position of the topic-word
distributions after being reduced with
MDS
, the radii display
the relative importance of the topic for the entire corpus.
convex linear combination ¯
di, precisely
¯
di=
K
j=1
θ(j)
i¯
φj.(1)
A direct application of
MDS
to the document-topic
vector as done in (Linstead et al., 2009) neglects the
similarity of topics to each other as shown in Figure 3.
4 VISUALIZATION APPROACH
Since software has no intrinsic gestalt, our visualiza-
tion must project the properties of the processed topic
model data onto graphical primitives. This can be done
by utilizing its visual variables and either providing an
explanation, how the mapping of information to the
variables is done, or by making use of metaphors that
are known to the users (or both).
Tree Metaphor.
Our visualization applies a tree
metaphor: each source code unit is represented by
an individual tree. All source code units together make
up the software project and are depicted as a forest.
The trees provide different visual variables that can
be utilized, such as their type and height, the size of
their trunk and crown, or the amount and color of their
leaves. The position and displayed proximity of two
trees allows drawing conclusions about topic similarity
easily. In addition, we can rely on easy-to-understand
concepts such as health, age, or season of trees in or-
der to convey common software metrics such as test
coverage, change frequency, or size. Anomalies in
the source code files, such as many occurring errors
or rapidly changing topics, can be displayed by tree
stumps, or fallen or burning trees.
In our prototype, we decided to allow users to
choose their own mappings by listing available visual
variables and properties of the used source code enti-
ties. As an example, users can visualize the dominant
topic with the tree type, as can be seen in Figure 2,
and thus recognize which files contain similar topics.
Users can explore the 3D forest by rotating, pan, and
zoom using the virtual camera by means of a world-in-
hand navigation. To reduce rendering cost and main-
tain visual clarity we opted for simpler, low-poly tree
geometries instead of realistic tree rendering—we in-
tegrated and experimented with a technique for large
scale forest rendering first (Bruneton and Neyret, 2012)
but found it difficult to increase the visual expressive-
ness of individual trees. The resulting visualization
can be categorized as
A3R2
, i.e., three-dimensional
graphical primitives on a two-dimensional reference
space (D¨
ubel et al., 2014).
Figure 4: Visualization of the notepad-plus-plus
1
project.
The dominant topic of each source code file is visualized
with the tree type. The cursor hovers over a light green
tree and identical trees, i.e., similar source code files, are
highlighted. For the selected tree, information on the cor-
responding source code file, such as file path, lines of code
and its dominant topics, is shown above. In this example, the
pre-computed topics have been labeled manually to provide
comprehensible topic descriptions.
The visualization is also intended to support soft-
ware systems of more than 10 000 source code units
such as TensorFlow. This raises the issue that the visu-
alization of so many files and thus so many trees can
get confusing. To alleviate this issue, our visualiza-
tion emphasizes dense tree clusters by making use of
the terrain’s height. Each tree is assigned to a terrain
height depending on how many trees can be found in
its surroundings and trees in denser clusters then can
be found on hills. To be able to differentiate various
clusters, a water surface with adjustable height can
be used to separate different hills from one another.
Furthermore, users can receive information about each
tree via hovering over it. Depending on the zoom
level of the visualization, a tooltip is shown for the
hovered tree, containing the name and details of the
represented software code file or project, such as soft-
ware metrics or information about its topic distribution
(Figure 4). Trees with the same identical dominant
topic are highlighted to emphasize topic clusters.
Tree Geometry.
We use handcrafted geometry from
Sketchfab, rather than designing our own models or
generating geometry dynamically (Figure 5). While
the latter approach could introduce numerous varia-
tions, handcrafted geometry keeps the visualization
framework easily customizable and reduces the overall
complexity. Generating 3D trees at runtime would
allow data to be displayed in an arbitrary exact fash-
ion. However, rendering highly individual meshes
increases the complexity for and requirements on web-
based implementations. In contrast, handcrafted mod-
Figure 5: Examples of two handcrafted 3D tree model sets: top row shows a pine tree, modeled and hand painted in various
stages, sizes, and health conditions. The bottom row shows low-poly meshes of various tree types in color variation. The
models were purchased on Sketchfab: “HandPainted Pine Pack” by ZugZug and “Low Poly Nature Pack” by NONE.
els tend to increase the visual quality more and can
easily be instanced and rendered.
The model files are expected to contain multiple
variations of trees using scene-graph semantics. Fur-
thermore, for every tree, multiple variation of the tree
must be associated to a specific combination and range
of visual variables and data respectively, e.g., tree
growth requires information on which variations rep-
resent correspondingly large or small grown trees.
Each model file is stored with a description file
that captures each tree variant and its attributes used
in the visualization. Although automatic creation of
this description file is imaginable, e.g., through use
of heuristics or rule-based approaches, we manually
created our description files. In detail, each descrip-
tion file contains the following information: Each file
contains a list of tree types and variants along with an
overview of meta-information about the corresponding
model. The overview stores a reference to the model
file it describes, a list of the identifiers used for all vi-
sual variables labeled and information on how to scale
the whole scene to create a consistent visualization. By
storing an explicit reference to the model file, multiple
descriptions can be used for a single asset file, e.g.,
when asset files contain multiple, different 3D models.
The list of distinct variants follows a two-level
structure: On the top level, models are grouped by
their tree type, as this property separates fundamentally
different trees. Within each tree type, a generic repre-
sentative is named and a list of tree variants is recorded,
e.g., trees of type “birch” with different growth states.
Every tree variant assigns concrete manifestations in
form of numeric values to each of its visual variables.
1
https://github.com/notepad-plus-plus/
notepad-plus-plus
Models in the description file are referenced by their
scene identifier from the model file scene graph. With
the description and the model file, data points in the
framework can be visualized using its models and all
visual variables of the file get shown to the user for at-
tribute mapping. Users can freely assign data attributes
to tree variables, e.g., by assigning the age of a soft-
ware project to the growth state of the trees. As each
set of models provides only a finite set of tree variants,
not every value combination can be expressed directly,
especially if multiple visual variables are used. We
employed a model selection algorithm to choose the
most suited tree model for each data point. To facili-
tate this, the algorithm rates each tree model based on
the difference between encoded values of each of the
used variables and their attributes.
Data Input.
The data processing on source code
repositories can take anything between hours and days,
thus we decoupled data processing and visualization
by storing the results in a structured, intermediate data
format with the following constraints:
Each data point will be represented by one tree.
Each data point has attributes that represent a 2D
position to place the tree in the terrain.
Each data point might have additional attributes,
e.g., dominant topics, file name or metrics.
This separation allows for the visualization, to be used
with data from topic models as well as any input that
complies to these constraints, e.g., data from other
domains such as geo-referenced data.
5 IMPLEMENTATION
The implementation is separated into two distinct com-
ponent: data processing and visualization. The in-
terface between both components are structured files
where one file represents one data set. Thereby, the
structured data set uses the common CSV file format.
The data processing pipeline and the visualization ap-
proach are implemented as follows.
5.1 Data Processing
We use the Python package environment and the
GitHub API for Python for gathering our corpus. For
preprocessing, we use the nltk
2
and spacy
3
library
for lemmatization and obtaining a stop word list for
the English language. One of the bottle necks of our
data pipeline is the main memory. Large projects like
TensorFlow may exhaust the main memory especially
during lemmatization. Therefore, we chose to pro-
cess projects in batches and process them sequentially,
optimizing the preprocessing steps for this kind of
distributed computing. The results of this step were
cached and saved in text files that can be loaded during
runtime to avoid redoing this step for a project.
After finishing preprocessing, we used the gensim
4
implementation for LDA. It provides a convenient op-
timization pipeline with random search as well as deal-
ing properly with relatively large amounts of data. For
optimization and the dimension reduction algorithms,
we used the scikit-learn library5.
5.2 Visualization
The visualization component is implemented as a web-
based visualization system that processes the CSV files.
Thereby, the component is implemented using Type-
Script and WebGL to be used in modern web browsers.
As the browser integration and rendering is based in
the open-source framework webgl-operate
6
. It pro-
vides some abstraction in accessing WebGL rendering
functions, including some very high-level functionality
such as a ready-to-use label rendering pass or glTF
7
scene loading functionality. On top of this, the terrain
and tree rendering functionality are implemented as
extension to the framework.
2https://www.nltk.org/
3https://spacy.io/
4https://radimrehurek.com/gensim/
5https://scikit-learn.org/stable/index.html
6https://webgl-operate.org
7https://www.khronos.org/gltf/
Tree Geometry.
The visualization component han-
dles 3D models, i.e., the trees, encoded using the glTF
file format. To describe the models contained in the
scene graph, a hand-managed description file in JSON
format is provided by the visualization designer, as-
sociating visual variables with a number 3D models
in the scene graph. Each tree model is represented by
a scene graph node within the while glTF file. Since
trees may inherit a translation in the scene graph, the
translation is normalized and adjusted to the tree posi-
tion in the terrain for this visualization. Each tree has
a corresponding data structure in memory and on the
GPU as vertex buffers that contains all its attributes,
given through the current attribute mapping. When the
mapping changes, i.e., the user selects a different CSV
column for a tree attribute, the attributes are updated
and the vertex buffers are updated accordingly. After a
change in its attributes, the tree model for a data point
is determined by a best-fit approach among the rule set
of the description file.
Terrain Generation.
When loading a data set, the
terrain is calculated based on the tree density. For
this, the terrain is subdivided into a grid with a fixed
resolution. For each tree, a Gaussian kernel is applied
at its position, increasing the terrain height there and in
the vicinity. In cases where many trees are next to few
trees, the height is flattened with a non-linear fall-off,
resulting in a flatter, more plain terrain. The goal is
that the densest cluster on the whole terrain will reach
maximum terrain height while a single tree will still
have a clearly visible terrain height of ca. 30-50% of
the maximum terrain height. In addition to the hills
and beaches of the terrain, a static water texture is
rendered to give a sense of cohesion to clustered tree
groups. The water level is configurable in the user
interface and may be used for height-filtering of data
points (Limberger et al., 2018).
Tree Placement.
Stating no additional assumptions
on the characteristics of the 2D layout of the data pro-
cessing pipeline, data points in the 2D layout may
overlap or may be positioned very close to each other.
Since the tree models have an actual and visible vol-
ume, these positions are not directly suitable for a clear
visualization and may lead to trees standing within
another. In order to prevent this, a basic collision
reduction algorithm is applied. First, the terrain is
subdivided into a grid with a fixed resolution. Then,
each tree is assigned the closest position on this grid.
If the closest position is already taken, alternative posi-
tions in the vicinity are checked until a free position is
found. The alternative positions are looked up based
on a fixed circle kernel in a clockwise direction, incre-
Figure 6: Four different assets comprised of different tree geometries and styles used for data mapping. The mapping, the
number of visual variables, and the overall expressiveness of the forest is highly dependent on the assets and its description.
mentally increasing the radius until the search kernel
is exhausted. In order to not make the clusters seems
artificial because of the introduced circle-like struc-
tures, a random offset based on the original position is
introduced. This results in the natural looking clusters
in comparison to sole square or circle kernels.
Rendering.
The tree rendering itself is implemented
using WebGL 2 and its instanced rendering capabili-
ties, i.e., the
drawElementsInstanced()
API. This
makes it possible to efficiently render a model multiple
times in the scene and allows to depict a large number
of data points. However, tree types and their visual
variables might change dynamically, thus, it is neces-
sary to adjust the buffers sent to the GPU frequently
since each tree might change its visual representation
on every change in the attribute mapping.
6 RESULTS & DISCUSSION
We built a renderer that runs in modern web browsers
and can thus be used on a variety of desktop and mobile
devices (Figure 7). Moderately sized software projects
(
>
10 000 entities) can be explored interactively with
an integrated graphics chip. With a dedicated graphics
card, even bigger projects (
>
100 000 entities) can be
visualized with interactive frame rates. In the visu-
alization, different presets are available that store a
Figure 7: GUI of our visualization running in a browser.
configuration for the major user interface options and
can be used to get a first impression of the framework
or to get a similar view on different data sets quickly.
Apart from that, every option can be adjusted to cus-
tomize the visualization: The data sets can be chosen
from a list of uploaded data sets or own data sets can
be uploaded. Additionally, the currently used model
files can be changed during runtime. The available
visual variables of the model file are displayed in the
user interface and can be assigned to data attributes.
The set of example use cases shown in this work un-
derline the capabilities of the proposed framework but
are not hard-wired in the structure of the framework.
Instead it is possible to easily adjust the visualization
for other needs if the constraints on the data format are
met, custom data files can be used and visualized.
6.1 Examples
Apart from the currently provided assets and their de-
scriptions, other glTF scenes can be used to visualize
the provided data (Figure 6). This effectively means
swapping trees with other objects and provide appro-
priate visual variables, e.g., buildings, street furniture,
rocks, flowers, etc. The listing 1 shows an excerpt of
the description file used for Figure 1. Figure 8 shows a
forest of the notepad-plus-plus project. The resulting
topics of the software files are mapped to tree types
and 2D position. A cohesive cluster in the center made
up of tall, light green trees can be recognized. When
inspecting the trees of this cluster using the mouse
cursor, tooltips reveal that they share a dominant topic.
Analyzing the words this topic comprises, the files
can be related to user interface code. The topic model
is correct in this judgment since the files displayed
in this cluster indeed deal with the user interface of
notepad++.
In Figure 9, 40 different open-source projects are
visualized. The projects have been processed and their
topic weights have been determined. The Software
Forest allows to recognize similar projects, e.g., deep
learning projects. The cluster of red maple trees is
Figure 8: Software forest of notepad-plus-plus project.
composed of deep learning projects such as CNTK,
caffe,Pytorch, and others.
6.2 Limitations
For now, we manually labeled the topics in order to
give more context to users beyond the numerical topic
index and word vectors. Since the topics returned
by LDA are latent, they have no inherit label mecha-
nism (Blei et al., 2003). This labeling is an essential
part for making our visualization interpretable. There-
fore, efficient ways to label topics automatically and
consistently should be researched, also drawing from
previous work and literature about this known problem
of latent topic modeling (Magatti et al., 2009).
The proposed work could benefit from the usage
1{
2"modelFile": "BiomeTrees.glb",
3"attributes": [ "health", "growth" ],
4"modelScale": 1.0,
5"types": [ ...
6{
7"name": "Tree_Palm",
8"baseModel": "Tree_Palm_02",
9"variants": [
10 {
11 "name": "Tree_Palm_03",
12 "health": 0.16,
13 "growth": 0.16
14 }, {
15 "name": "Tree_Palm_02",
16 "health": 0.5,
17 "growth": 0.83
18 }, ... ]
19 }, ... {
20 "name": "Tree_Birch",
21 "baseModel": "Tree_Birch_03",
22 "variants": [
23 {
24 "name": "Tree_Birch_04",
25 "health": 0,
26 "growth": 0.83
27 }, ... ]
28 } ]
29 }
Listing 1: JSON configuration of a mapping from attribute
values to 3D model.
Figure 9: Software forest of 40 open-source projects.
of more data sources. Similar to Linstead et al. bug
reports could be considered for additional input as well
(Linstead et al., 2007). We did some first experiments
using so called joint embeddings (Yu et al., 2017),
which can be used for the integration of more diverse
data sources, but more work needs to be done to con-
clude whether or not this is a promising approach.
When exploring large projects, dense clusters of
trees occur frequently. To improve the visual clarity of
the system, it would be possible to recognize clusters
and aggregate similar trees in the visualization. Hover-
ing over these trees or zooming into their region, the
trees clusters could be expanded again, or tooltips with
additional details could be shown.
7 CONCLUSIONS
The approach to visualize software data, i.e., a promi-
nent example of data without intrinsic gestalt, using
metaphors is often-tried but only scarcely used outside
of research communities. We try to address application
issues by proposing a metaphor close to nature and a
dedicated visualization approach. As such, we propose
Software Forest, a novel 2.5D software visualization
to interactively explore a software system illustrated
as a forest. The visual metaphor allows for multiple
visual variables, including, but not limited to, tree type,
growth state, height, and color. The layout of the forest
is derived from a source code document analysis us-
ing Latent Dirichlet Allocation and Multidimensional
Scaling. This layout reflects similarity in semantics
of source code by proximity, and, thus, allows for an
alternative mapping and view than traditional software
visualization approaches such as treemaps. The layout
is used for deriving a terrain and locations for the trees
for further attribute mapping. The rendering approach
takes different parameterization, different visual vari-
ables, and scalability into account. As such, using
different 3D models is feasible and requires only the
composition of a glTF scene graph and a description
file with information on individual models and specific
attribute mapping. The approach is exemplified using
mid-sized open-source software development projects.
An application to the data sets indicates both an ef-
fective layout base on semantic similarity of source
code and an explorative visualization approach of the
resulting maps.
Future work can be discussed separately for layout
generation and the visualization approach. For lay-
out improvements, software similarity (Al-msie’deen
et al., 2013) and suitable dimensionality reduction
methods (Cha, 2007; Vernier et al., 2020) are target
for further research. The visualization approach needs
to be evaluated regarding readability using user stud-
ies. Regarding variability of the 3D models, tooling
on the glTF models would simplify the integration
of other 3D models, even for applications outside of
forest metaphors. As the input for the prototypical
viewer are CSV files with explicit layout and addi-
tional attributes as columns, we already imagine easy
visualization prototyping and adaption in other visual-
ization domains as well. Using a broader perspective,
the proposed visualization approach allows for visual-
ization of structured data sets with scatter-plot layout
and dynamic model mapping for a wide-spread use of
thematic mapping – the topic maps.
ACKNOWLEDGEMENTS
We want to thank the anonymous reviewers for their
valuable comments and suggestions to improve this ar-
ticle. Further, this work is part of the “Software-DNA”
project, which is funded by the European Regional De-
velopment Fund (ERDF – or EFRE in German) and the
State of Brandenburg (ILB) as well as the “TASAM”
project, which is funded by the German Federal Min-
istry for Economic Affairs and Energy (BMWi, ZIM).
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... However in many program comprehension tasks the question of detecting files that implement a similar concept arises. This motivates other approaches, which try to place the points based on their semantic relatedness [1,7,9]. ...
... Measuring the distance between topics via the Jensen-Shannon distance, we project the topic-word-vectors onto the three-dimensional space using Multidimensional Scaling [5]. The locations of the documents are determined by a linear combination of the topic vectors according to their document-topicdistribution [1]. ...
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