Interactive Visualization of Generalized Virtual 3D City Models using Level-of-Abstraction Transitions

Abstract

Virtual 3D city models play an important role in the communication of complex geospatial information in a growing number of applications, such as urban planning, navigation, tourist information, and disaster management. In general, homogeneous graphic styles are used for visualization. For instance, photorealism is suitable for detailed presentations, and non-photorealism or abstract stylization is used to facilitate guidance of a viewer's gaze to prioritized information. However, to adapt visualization to different contexts and contents and to support saliency-guided visualization based on user interaction or dynamically changing thematic information, a combination of different graphic styles is necessary. Design and implementation of such combined graphic styles pose a number of challenges, specifically from the perspective of real-time 3D visualization. In this paper, the authors present a concept and an implementation of a system that enables different presentation styles, their seamless integration within a single view, and parametrized transitions between them, which are defined according to tasks, camera view, and image resolution. The paper outlines potential usage scenarios and application fields together with a performance evaluation of the implementation.

Full text

* http://www.hpi3d.de. This is the authors’ version of the work.
The definitive version will be available at
diglib.eg.org
and
www.blackwell-synergy.com.
Eurographics Conference on Visualization (EuroVis) 2012
S. Bruckner, S. Miksch, and H. Pfister
(Guest Editors)
Volume 31 (2012), Number 3
Interactive Visualization of Generalized Virtual 3D City
Models using Level-of-Abstraction Transitions *
Amir Semmo Matthias Trapp Jan Eric Kyprianidis Jürgen Döllner
Hasso-Plattner-Institut, University of Potsdam, Germany *
A B
Figure 1:
Exemplary result of the visualization system that enables the seamless transition between abstract graphics (A) and a
photorealistic version (B) view-dependently. The sequence below shows single frames of this transition.
Abstract
Virtual 3D city models play an important role in the communication of complex geospatial information in a growing
number of applications, such as urban planning, navigation, tourist information, and disaster management. In
general, homogeneous graphic styles are used for visualization. For instance, photorealism is suitable for detailed
presentations, and non-photorealism or abstract stylization is used to facilitate guidance of a viewer’s gaze to
prioritized information. However, to adapt visualization to different contexts and contents and to support saliency-
guided visualization based on user interaction or dynamically changing thematic information, a combination of
different graphic styles is necessary. Design and implementation of such combined graphic styles pose a number
of challenges, specifically from the perspective of real-time 3D visualization. In this paper, the authors present a
concept and an implementation of a system that enables different presentation styles, their seamless integration
within a single view, and parametrized transitions between them, which are defined according to tasks, camera
view, and image resolution. The paper outlines potential usage scenarios and application fields together with a
performance evaluation of the implementation.
Categories and Subject Descriptors (according to ACM CCS): I.3.3 [Computer Graphics]: Picture/Image Generation—Viewing
algorithms I.3.7 [Computer Graphics]: Three-Dimensional Graphics and Realism—
1. Introduction
Virtual 3D city models are an integral part of a growing num-
ber of applications, systems, and services, and are becoming
general-purpose tools for interactively viewing, editing, and
distributing geospatial information. Typically, visualization
systems apply a homogeneous graphic style to depict virtual
3D city models: photorealistic rendering is usually used for
detailed presentations, or illustrative, abstract rendering to
draw attention to prioritized information [SD04]. Using a
suitable graphic style can be beneficial for making a visual-
© 2012 The Author(s)
Computer Graphics Forum © 2012 The Eurographics Association and Blackwell Publish-
ing Ltd. Published by Blackwell Publishing, 9600 Garsington Road, Oxford OX4 2DQ,
UK and 350 Main Street, Malden, MA 02148, USA.

A. Semmo et al. / Interactive Visualization of Generalized Virtual 3D City Models using Level-of-Abstraction Transitions
s
ID LoD Interp. Start Interp. End t0t1t2t3
0.57
0.79
1.0
Style ID LoD Interp. Start Interp. End t0t1t2t3
T0
T1
T2
Photorealistic 0Fragment Fragment 0.0 0.0 0.35 0.55
Silhouettes 1Fragment Object 0.35 0.55 0.75 0.75
Generalized 2Object Object 0.75 0.75 1.0 1.0
Feature
Class: Trees
...
COMBINE rgb
Geometric Transformation
Transition Configurations (Section 3.1)

...
Feature Classification
Input
Landmarks
...
Image Compositing (Section 3.2.4)
F
Multiresolution Models & Feature Types (Section 3.1)
A
Alpha Blending Layers
Output
Layer 0
Layer 1
Layer 2
Blend Functions (Section 3.2.1)
Ct0t1
1
0s
ζ
s
ζ
p
ζ
d

ζ(s,t0,t1,t2,t3)
Layer 1
Layer 2
Layer 0
Cartographic Rendering (Section 3.2.3)
E
...
Cartographic Shading
Layer 0
Layer 1
Saliency Metrics (Section 3.2.1)
B
η(cs,ce,SC(f))
Resolution 0Resolution 1
Global Transformations
(Section 3.2.2)
D
...
...
Figure 2:
Overview of the present system’s approach of LoA transitions for virtual 3D city models. (A) Feature classification
using semantic information, (B/C) blend value computation based on saliency metrics (multipass), (D) global transformation
of landmarks, (E) cartographic shading (multipass), (F) order-independent image blending and compositing. The transition
configurations are used by components B-F.
ization meaningful in its corresponding context and usage
scenario [Mac95]. For instance, detailed presentations can
aid the exploration of local environments, whereas 2D maps
can be an effective medium for navigational purposes.
Systems like Google Maps or Bing Maps integrate differ-
ent graphic styles to serve users with a presentation suitable
for viewing maps or getting driving directions. Because these
systems provide high interactivity, a user’s task and context,
such as viewing situations and regions-of-interest (RoIs), can
be dynamically changed. Typically, a user is able to switch the
graphic style to display more or less detail in RoIs or context
regions to avoid cluttered information. However, concurrent
visualization leads to constant reorientations and additional
cognitive load [JD08] because of hard transitions between
the graphic style, level-of-detail (LoD), and view perspective.
Therefore, a great potential lies in the seamless combination
of various graphic styles into a single view to communicate
only relevant information, thus directing a viewer’s gaze by
salient stimuli attraction (saliency-guided visualization).
A seamless combination of generic 2D and 3D graphic
styles in a visualization pipeline by means of computer
graphics is yet to be achieved. One approach is to select
alevel-of-abstraction (LoA) in a context-dependent way.
LoA refers to the spatial and thematic granularity at which
model contents are represented, and extends geometric ab-
straction (LoD) by visual abstraction (e.g., using shading ef-
fects) [GD09]. Relevant techniques use image blending or de-
formation (e.g., for focus+context visualization) to highlight
RoIs [CDF
∗
06,MDWK08,LTJD08,QWC
∗
09], but (1) do not
provide different LoAs for selected entities and prioritized
information, and (2) blend only two graphic styles, or are
domain-specific (e.g., routes [QWC
∗
09]). This motivates a
system approach that is designed to integrate multiple, cus-
tomized graphic styles in a context-dependent way.
This paper presents a concept and an implementation for
a system that enables different graphic styles, their seamless
integration, and parametrized transitions. The system selects
the LoA used to represent 3D city model entities in a task-
dependent, view-dependent, and resolution-dependent way
(Figure 1). Being based on shader technology and multi-pass
rendering, the system seamlessly integrates into common
visualization pipelines, providing context-dependent visual-
ization for novel visualization techniques and geoinformation
systems (GIS). The system can be further used to author
and visualize smooth LoA transitions to improve important
applications in geovirtual environments: in particular, map
viewing, wayfinding, and locating businesses. To summarize,
this work makes the following contributions:
1.
A concept and an implementation for a system that enables
seamless combinations of various 2D/3D graphic styles.
2.
A model for the parametrization of transitions of graphic
styles in a visualization pipeline (Figure 2).
3.
Usage scenarios using cartography-oriented design to
demonstrate the benefits of the system.
© 2012 The Author(s)
© 2012 The Eurographics Association and Blackwell Publishing Ltd.

A. Semmo et al. / Interactive Visualization of Generalized Virtual 3D City Models using Level-of-Abstraction Transitions
2. Related Work
Visualizing virtual 3D city models by LoA transitions is
related to several previous works in the domains of non-
photorealistic rendering, context-aware abstraction, and ani-
mated transitions and geomorphing.
2.1. Non-Photorealistic Rendering
A stylization of virtual 3D city models uses non-photorealistic
rendering techniques [GGCS11] to reduce visual complex-
ity. Döllner and Walther [DW03] visualized virtual 3D city
models with abstract graphics using procedurally generated,
stylized facades of 3D building models. The system in the
present paper uses edge enhancement in image-space [ND03]
and object-space [DW03] to achieve an expressive rendering
of 3D building models. In contrast to previous work, this
system enhances edges view-dependently to highlight enti-
ties of interest. Examples of the stylization of landscapes
are panorama maps of terrain models and their relief presen-
tation [BST09]. The system presented in this paper is able
to stylize terrain models with a cartography-oriented design
using slope lines and shadowed hachures [BSD
∗
04]. In addi-
tion, it is capable of parametrizing and combining these styles
within a single view for cartographic 3D city presentations.
2.2. Context-aware Abstraction
A context-aware abstraction has the potential to improve the
perception of important or prioritized information [SD04].
Major related work is found in focus+context and semantics-
based visualization, which aims to combine and parametrize
different graphic styles into a single view.
Focus+Context Visualization.
Highlighting important in-
formation in foci while maintaining a context is subject to
focus+context visualization. Applications of focus+context
visualization of virtual 3D city models are generaliza-
tion lenses [TGBD08] and cell-based geometric generaliza-
tion [GD09]. Highlighting can be further amplified by us-
ing semantic depth-of-field (SDOF) [KMH01]. Techniques
relevant to the approach of this work used stylized foci to
move the focus of a viewer to certain locations of an im-
age [SD04,CDF
∗
06,LTJD08]. The presented system extends
these works to (1) enable smooth transitions between levels of
structural abstraction with (2) a context-dependent selection
of LoAs using saliency metrics defined per feature type, and
(3) their dynamic parametrization at run-time. In addition, the
system provides cartography-oriented, thematic visualization
using different LoAs for selected model entities or RoIs.
Further relevant work visualized 3D geovirtual en-
vironments with high detail and applied deformation
techniques [MDWK08,DK09] or focus+context zooom-
ing [QWC
∗
09] to magnify RoIs and scale landmarks along
routes to increase visibility of important information. The sys-
tem presented here, by contrast, maintains cartographic rela-
tions in stylized foci and instead visualizes with cartography-
oriented design to reduce visual clutter in context regions
and support saliency-guided visualization. Because the sys-
tem seamlessly integrates into a visualization pipeline, it can
be used to implement these techniques to increase visibility
in RoIs. In addition, it generalizes context regions and can
therefore enhance focus+context zooming.
Semantics-based Visualization.
One approach to para-
metrize visualization of model contents is a semantics-based
image abstraction [YLL
∗
10]. To adapt a visualization to
model contents, CityGML [Kol09] introduced a semantics-
driven classification and exchange format that has been stan-
dardized by the OGC and is accepted by a growing number of
GIS software vendors. In the system presented here, semantic
information is derived from material and texture informa-
tion, or defined explicitly at run-time to enable a customized
parametrization of visual attributes. Brewer [Bre94] proposed
conventions for using colors in cartography-oriented design.
The system presented here uses qualitative color schemes to
represent entity types of city models.
2.3. Transitions for Level-of-Abstraction
Alpha blending, animation, and geomorphing are common
visualization techniques to enable smooth transitions between
graphic styles in context-aware abstraction.
Alpha Blending.
A well-known method for image composit-
ing is alpha blending [PD84], which is used in multiperspec-
tive rendering to enable a “quasi”-continuous transition be-
tween focus and context regions [MDWK08,LTJD08]. The
system presented in this paper uses cumulative alpha blending
to blend multiple RoIs with varying LoA.
Animation.
An alternative approach for smooth transitions
is to animate visual and structural changes. Previous work in
information visualization showed that animated transitions
ease orientation and guidance [RCM93,TMB02,HR07], and
aid the reconstruction of information spaces [BB99]. More-
over, animated transitions “improve graphical perception of
changes between statistical data graphics” [HR07], facilitate
understanding and increase engagement. In the system pre-
sented here, global deformations of 3D building models are
animated to enable predictable transitions between detailed
and cartographic visualization of landmarks [EPK05].
Morphing.
Morphing is a visual effect that enhances anima-
tions by smooth transitions between models with varying reso-
lution [LDSS99,SD96]. For instance, geomorphing was used
in continuous LoD of digital terrain models [Hop98,Wag03]
to provide smooth transitions and temporal coherence. How-
ever, morphing is based on assumptions about the geometric
representations and can only be applied to 3D objects with
a suitable geometry. By contrast, virtual 3D city models, in
general, cannot fulfill such assumptions. Moreover, morph-
ing of 3D city models has to take cartographic generaliza-
tion [Mac95] into account. Previous work on continuous LoD
exemplified how smooth, view-dependent transitions can be
achieved using collapsing as the pre-dominant generalization
operator [LKR∗96,Hop98].
© 2012 The Author(s)
© 2012 The Eurographics Association and Blackwell Publishing Ltd.

A. Semmo et al. / Interactive Visualization of Generalized Virtual 3D City Models using Level-of-Abstraction Transitions
Original
Quantized
FDoG
rgb
α
Feature ID Type
... ...
438
439
440
441 Building
Building
Street
Water Surface
... ...
w0= 0.9 w1= 0.2 w2= 0.5
Figure 3:
Geospatial data processed by the system: multires-
olution models (A), semantic information (B), landmarks with
interest values wiand best-views (C), stylized textures (D).
3. Method
An overview of the system presented in this paper is shown
in Figure 2. The input data consists of textured multiresolu-
tion 3D models (Figure 3A) and task-dependent transition
configurations. These models are typically defined as trian-
gular irregular networks (e.g., acquired by remote sensing,
procedural generation, or manual modeling). The 3D mod-
els are composed of features (i.e., abstractions of real-world
phenomena [ISO]) that are categorized using a feature taxon-
omy, and grouped according to their appearance (Section 3.1).
Based on this information, the system performs visual abstrac-
tion (Section 3.2):
•by geometric transformation of features (LoD),
•
and by cartographic shading (LoA), such as waterlining,
signatures for green spaces, and abstract building facades.
To perform context-dependent visualization, features are
rendered multiple times using different graphic styles that
are continuously blended. To each graphic style of a certain
feature used for visualization, interest values are assigned
that are computed at rendering time using saliency metrics,
such as viewing distance, view angle, or region interest (Sec-
tion 3.2.1). These interest values are computed for all visible
and non-visible (i.e., occluded) fragments of a feature. After
a normalization, these are used as blend values to compose
the final image by order-independent image blending [PD84].
The remainder of this section describes the stages of the
transition pipeline and its architecture in detail.
3.1. Pre-processing Geospatial Data
To enable LoA transitions for complex scenes, such as virtual
3D city models, global information about a model’s features
is required: a feature type, location in the 3D scene, global
interest, and how visual attributes adapt to user interaction
(intelligence of objects [MEH
∗
99]). The pre-processing of
this information is explained in the remainder of this section.
Scenario Definition.
The system presented in this paper is
based on usage scenarios that define how a 3D scene is vi-
sualized for a given task and how graphics are dynamically
adapted to a user’s context. A scenario consists of a set of
Figure 4:
Continuous LoA for textured green spaces: near
distance (A), mid-range distance (B), far distance (C).
features with unique interest values and a set of transition
configurations that define rules and constraints for LoA tran-
sitions. To enable a parametrization of graphic styles for
each feature, information about a feature’s type (i.e., building,
green space, street, water, or terrain) and sub-type (e.g., conif-
erous forest, deciduous forest) is stored (Figure 3B). Thereby,
the system enables cartography-oriented design, leading to
improved perception of context information [JD08]. The re-
quired semantic information can be derived automatically
from texture and material information by grouping features
with similar appearance. Alternatively, semantic information
can be provided manually at run-time or as part of the model
data (e.g., CityGML [Kol09]).
Parts of the input data are best-view directions of build-
ings and sites to enable a cartographic visualization of land-
marks [EPK05,GASP08]. The definition of landmarks is
context-dependent [GASP08], using interest values defined
per feature (Figure 3C). The computation of these records is
not limited to pre-processing, but can be updated at runtime if
models are added or removed from a 3D scene, or if a user’s
interest in a specific feature type changes. Thereby, the system
maintains interactivity and context-dependent visualization.
Transition Configuration.
Transitions between graphic
styles are implemented by rendering features multiple times
and compositing the intermediate results using image blend-
ing [PD84]. This approach was chosen because it is generic
and simplifies the extension of the system with new graphic
styles. The sequence of graphic styles can be configured at
three levels:
•
Ascope defines if a graphic style applies to a certain inter-
est in a feature.
•
The transition is parametrized with a fragment-, object-
or group-based interpolation. For this purpose, the axis-
aligned bounding box of each feature is stored.
•
The parametrization of the LoD and LoA, such as by color,
texture abstraction, and edge enhancement.
Thereby, the system enables a user-defined visual abstraction
of features. Figure 2(top left corner) exemplifies a transition
configuration for tree models.
Image Abstraction.
A bilateral and difference of Gaussians
(DoG) filter [KD08] is utilized to automatically stylize tex-
tures in a pre-processing stage. The input textures are first
converted to mip maps [Wil83], and then processed for each
level separately. This provides a continuous LoA of textured
© 2012 The Author(s)
© 2012 The Eurographics Association and Blackwell Publishing Ltd.

A. Semmo et al. / Interactive Visualization of Generalized Virtual 3D City Models using Level-of-Abstraction Transitions
Figure 5:
Exemplary saliency metrics defined by the system: view distance (A), view angle (B) and region interest (C). The debug
outputs show areas of a 3D scene to be visualized with high detail (black) and low detail (white), respectively.
surfaces (Figure 4), while using standard capabilities of graph-
ics hardware [Wil83]. In contrast to [KD08], the output of the
edge enhancement is not combined with the quantized color
output (Figure 3D). Instead, color and outline are blended
at rendering time for individual parametrization. The image
abstraction is performed once per model. For a 3D city model
(CityGML LoD3 [Kol09]) with
1,520
unique texture maps,
each with an average resolution of
128 ×128
pixels, this
process takes
∼20
minutes (using the first hardware configu-
ration and Chemnitz model described in Section 4).
3.2. Rendering
The rendering comprises the following steps: (1) computing
the interest of features for a user’s task and context, such
as defined by the viewing perspective and region interest,
(2) visual abstraction depending on the features’ interest, and
(3) image compositing.
3.2.1. Computing Interest using Saliency Metrics
The thematic categorization (Section 3.1) is used to stylize in-
formation with high interest (high salience) differently from
information with low interest (low salience). The present
system interprets a high interest in a feature by selecting
photorealistic graphics, and a low interest by selecting ab-
stract graphics for rendering respectively, where interest val-
ues in-between yield a mix of graphic styles. To identify
areas to be visualized with high detail, the interest value
for each visible feature is computed using saliency metrics,
such as view distance, vertical view-angle, and region interest
(Figure 5). Other metrics can be added as long as they are
normalized. For instance, view metrics can be defined by
normalized Euclidean distances and angles as is shown in
Figure 5. The region interest is represented by a distance map
that is computed using the jump-flooding algorithm [RT06],
and is used to visualize RoIs or routes through a virtual 3D
city model [TGBD08]. The computation of distance maps is
T0
T1
T2
1
0
1
0
0
1
b1
b2
Smooth Transition
A B C D
Hard Transition
b1
b0
Photorealistic
Silhouettes
Generalized
Figure 6: Examplary transition states for tree models.
based on the assumption that the terrain in the locality of the
camera can be approximated by a plane (Figure 5). In contrast
to previous techniques [CDF
∗
06,MDWK08,LTJD08], the
system presented here enables multivariable transitions based
on interest values and saliency metrics, resulting in increased
flexibility. For instance, a weighted blending between view
distance and view angle can be defined to prevent high detail
presentations in bird’s eye views with a near viewing distance.
A transition between graphic styles is based on image
blending [PD84]. Blend values are computed for each transi-
tion configuration, and features with matching feature types
and LoD. This procedure is performed during scene graph
traversal on the CPU (Algorithm 1), and by using linear or
smooth blend functions on the GPU (Figure 2C). The va-
lidity range of each transition configuration determines the
blend value of a graphic style for a feature of certain interest.
Depending on how the threshold values for two successive
transition configurations are defined, two general cases can
be identified (Figure 6):
1. Smooth transition.
A smooth transition from one graphic
style to another can be defined by the fade-out interval and
fade-in interval of two successive transition configurations.
A smooth transition between two graphic styles is enabled
© 2012 The Author(s)
© 2012 The Eurographics Association and Blackwell Publishing Ltd.

A. Semmo et al. / Interactive Visualization of Generalized Virtual 3D City Models using Level-of-Abstraction Transitions
AB C D E
Figure 7: Transformation of landmarks: scaling (A), rotation to best-view (B), flattening (C), billboard transformation (D).
Algorithm 1: Extended scene graph traversal (CPU)
Input: A usage scenario Swith features {Fi}and transition
configurations {Tj}
1forall the t∈ {Tj}do
2forall the f∈ {Fi}do
3if feature type of f and t match and LoD of f and t
match and f is inside validity range of t (=not culled)
then
4Render fand apply graphic style of t
5end
6end
7end
using the smoothstep function within these two intervals
(Figure 6A-C).
2. Hard transition.
For certain configurations, discrete LoA
transitions are appropriate – for instance, if two graphic
styles lead to distorted color tones [GW07]. Hard transi-
tions are enabled if the overlap of a fade-in and fade-out
interval is set to zero (Figure 6C-D).
For the computation of blend values, three interpolation
modes are distinguished: (1) a fragment-based interpolation
for smooth transitions within 3D features, (2) an object-based
interpolation with blend values applied uniformly to a feature
by using the center of a feature’s axis-aligned bounding box
as focus point, and (3) a group-based interpolation with a
shared focus point among features – for instance, to replace
tree instances by a coarse geometry representing woodland.
3.2.2. Global Transformations
Certain buildings and sites within a virtual environment serve
as landmarks (i.e., reference points with a characteristic ap-
pearance or location, or user’s interest). The visualization of
landmarks is essential for localization, orientation and nav-
igation [GASP08]. To this end, the system presented here
provides a map-like visualization of landmarks, using global
deformation applied prior to rasterization. The flattened land-
marks are rotated to face the user’s viewing direction ac-
cording to their best-views. For buildings, best-views often
face the street or main entrance and are approximated using
viewpoint entropy [VFSH04]. To obtain a deformed land-
mark in world space coordinates, the following four steps are
performed on a per-vertex basis during rendering:
1. Landmark scaling
. Landmarks are scaled to improve their
visibility in far view distances (Figure 7A-B). A weighted
smoothstep function is used to compute the scale factor. To
avoid over cluttering, landmarks are smoothly faded-out
according to their interest values.
2. Rotation to best-view
. Landmarks are pitched so that their
best-view direction horizontally coincides with the virtual
camera’s view direction (Figure 7B-C).
3. Object flattening
. Landmarks are flattened in depth; that
is, their vertices are projected to the plane facing the hori-
zontal view direction (Figure 7C-D).
4. Cylindrical billboard transformation
. The flattened land-
mark is yawed by the camera elevation to vertically face
the view direction (Figure 7D-E).
The system linearly blends original and transformed vertices
based on a feature’s interest using shader technology (GPU).
Thus, further transformation techniques can be seamlessly
integrated, such as global deformation to increase visibility
in RoIs [MDWK08,DK09,QWC
∗
09] or terrain geomorphing
[Wag03]. Furthermore, the system smoothly shrinks non-
landmarks and translates them below the terrain to remove
extraneous information for map-like visualization (Figure 1).
3.2.3. Cartographic 3D City Presentations
To demonstrate the system’s ability to integrate customized
2D and 3D graphic styles, several abstraction techniques are
authored to achieve thematic visualization in context regions.
The thematic categorization (of Section 3.1) is used to stylize
features by non-photorealistic rendering techniques:
Building models
. Stylized texture maps, as discussed in Sec-
tion 3.1, are used to provide a continuous LoA of building
facades. With increasing distance or viewing angle, subtle
details are smoothly coarsened.
Street networks
. Street networks are stylized using carto-
graphic color schemes [Bre94]. Important edges are enhanced
using an image-space edge enhancement technique [ND03].
In general, the system seamlessly integrates street labels using
distance maps that are blended on graphics hardware. For this,
the authors enhanced Green’s [Gre07] rendering technique to
align and scale street labels in a view-dependent way.
Water surfaces
. Water surfaces are visualized using a novel
waterlining shading technique that is based on distance maps
[RT06]. The technique visualizes waterlines with non-linear
intervals to propagate distance information (Figure 8A).
Green spaces and trees
. As for buildings, stylized tex-
ture maps are used for continuous LoA (Figure 4). In
addition, signatures are visualized using texture bombing
© 2012 The Author(s)
© 2012 The Eurographics Association and Blackwell Publishing Ltd.

A. Semmo et al. / Interactive Visualization of Generalized Virtual 3D City Models using Level-of-Abstraction Transitions
Mixed ForestConiferous Forest
CDA B
Figure 8: Stylization techniques for water surfaces (A), green spaces (B), tree models (C) and digital terrain models (D).
and parametrized to represent tree species (Figure 8B).
For this, the authors developed an enhanced variant of
Glanville’s [Gla04] texture bombing algorithm that ensures
signatures always face the viewing direction.
Tree models are stylized using view-dependent enhance-
ment of silhouettes [DS00]. Technically, point sprites were
used and the differences of depth values thresholded to en-
hance silhouettes in image-space (Figure 8C).
Digital terrain models
. Digital terrain models are visual-
ized as relief presentations using loose lines, slope lines, and
shadowed hachures [BSD∗04] (Figure 8D).
Generic models
. Generic stylization is applied to miscella-
neous features, such as city furniture. A combination of styl-
ized texture maps, edge enhancement in image-space [ND03]
and object-space [DW03], and thematic colorization [Bre94]
is used.
3.2.4. Image Compositing
The final image is composed using alpha blending [PD84].
Technically, a stencil routed A-buffer [MB07] is used, since
it is able to buffer fragments in depth at real-time frame rates.
Because certain features have a high complexity in depth (e.g.,
foliage of trees), the system provides the capability to render
features off-screen into a Ping-Pong buffer. It comprises two
render textures, uses a depth test, and switches its render
texture for successive graphic styles. For image compositing,
the output of both textures is blended and combined with the
information of the A-buffer.
To improve the rendering performance, the A-buffer sort-
ing [MB07] is enhanced using a dynamic image composi-
tion (Algorithm 2). Fragments with depth values
d=1
are
excluded so that only routed samples are blended (see Algo-
rithm 2, lines 4-7). The system then improves shape and depth
perception by unsharp masking the depth buffer [LCD06]. To
reduce ringing artifacts, the method is enhanced by locally
weighting depth differences according to the alpha values.
4. Applications and Evaluation
The system presented here was implemented using C++,
OpenGL and GLSL. Two platforms were used for perfor-
mance evaluation: (1) an Intel
®
Xeon
™ 4×
3.06 GHz with
6 GByte RAM and NVidia
®
GTX 560 Ti GPU with 2 GByte
VRAM, and (2) an Intel
®
Core2Duo
™ 2×
3.0 GHz with
4 GByte RAM and NVidia
®
GTX 460 GPU with 1 GByte
VRAM. To show the effectiveness of the system, usage sce-
narios were authored for the virtual 3D city model of Chem-
nitz (Germany) with
458
features,
223,743
vertices,
176,601
Algorithm 2: Image compositing using shaders (GPU)
Input: Buffered color, alpha and depth values per pixel
Output: Blended color values per pixel
1begin
2color ←background color;
3count ←#SAMPLES;
4for n←0,#SAMPLES −1do
5depths[n]←fetchABufferDepth(n);
6count ←count − bdepths[n]c;
7end
8for n←0,count do
9colors[n]←fetchABufferColor(n);
10 end
11 blendPingPongFBO(colors[count],depths[count]);
12 count ←count +1;
13 SortColorByDepth(colors,depths,count);
14 for n←count −1,0do
15 color ←mix(color.rgb,colors[n].rgb,colors[n].a);
16 end
17 end
faces, and
1,520
texture maps (Figure 1and Figure 9A-D);
another 3D city model with
532
objects,
63,630
vertices, and
40,993
faces (Figure 9E); and a virtual landscape model of
Mount St. Helens (Figure 9F). To improve the rendering per-
formance, view-frustum and back-face culling were enabled,
and geometry instancing applied for the vegetation objects.
For order-independent blending, an A-buffer with 8 samples
was used.
4.1. Usage Scenarios
This section demonstrates the benefits of the system in ap-
plications of 3D geovirtual environments, such as map view-
ing, business locating, navigation, and wayfinding. Further,
thematic visualization was used to provide a cartographic
presentation (Section 3.2.3).
Despite the manual selection of the LoA (Figure 9A), view-
distance-based transitions were authored to visualize features
near the virtual camera at high detail, and distant features in
an abstracted way (Figure 9B). This approach can be of in-
terest in highly dynamic and ubiquitous information systems.
For instance, mobile navigation systems could use this visual-
ization to present places close to a viewer’s position with high
detail for local orientation assistance, and places far away
with less detail and emphasized landmarks for navigational
© 2012 The Author(s)
© 2012 The Eurographics Association and Blackwell Publishing Ltd.

A. Semmo et al. / Interactive Visualization of Generalized Virtual 3D City Models using Level-of-Abstraction Transitions
BB C
DE F
A
Figure 9:
Exemplary applications authored with the visualization system: manual LoA selection (A), distance-based transitions
(B), route highlighting (C), and circular RoIs (D-F).
assistance. Thereby, the exploration of complex virtual envi-
ronments can be improved, in general, since the viewer is not
required to switch between a 3D perspective view and a map
view, as common in map view services like Google Maps.
Because of emphasized features in the foreground, it further
facilitates direction guidance of a viewer’s gaze [CDF
∗
06]
while preserving context information in the background.
Combined with the vertical viewing angle as saliency met-
ric (Section 3.2.1), detailed and map-like visualization can be
seamlessly blended. The present system used a map-like visu-
alization to highlight thematic information (e.g., landmarks,
labeled roads, green spaces, and water surfaces), leveraging
the capability of the system to implement cartographic color
schemes [Bre94] and non-photorealistic rendering techniques
(Section 3.2.3). This is useful for 3D car navigation systems
because only the most relevant information is communicated
in areas of high information compression (e.g., in the back-
ground of perspective views [JD08]). Further, orientation as-
sistance is provided by seamlessly integrating 3D illustrations
of landmarks. Furthermore, the driving speed could be used as
metric to dynamically select the LoA used for visualization.
Wayfinding is an important task for virtual environments
and can be improved by the proposed system using saliency
metrics for LoA transitions (Section 3.2.1). Features can be
highlighted along routes to attract and direct a viewer’s fo-
cus – for instance, as a navigational aid to guide a user to
a destination or RoI (Figure 9C). Within this application
domain, the system can provide improvements over previ-
ous techniques designed for occlusion-free route visualiza-
tion [QWC
∗
09,MDWK08] because these neglect informa-
tion abstraction in context regions. In addition, the system
parametrizes LoA transitions at run-time. This can be used to
Table 1:
Performance evaluation measured in frames-per-
second for three virtual environments and screen resolutions.
The evaluation was performed on two platforms (Section 4).
Model / Screen Res. 1920 ×1080 1280 ×720 800 ×600
Chemnitz 5.7 5.0 5.8 5.2 6.1 5.3
MegaCity 14.2 13.1 17.2 16.6 20.4 20.1
Mt. St. Helens 46.5 36.8 72.2 57.6 87.8 78.2
highlight selected information of database queries for analy-
sis purposes. Moreover, there is potential to use the system
for the visualization of (time-based) model variants. Further-
more, the system is feasible to visualize RoIs as blue prints
and seamlessly combine these with high-detail graphic styles
in the context area (Figure 9E). Applications designed for ur-
ban planning could use this visualization to highlight complex
structures or architectural features of 3D building models.
4.2. Saliency-guided Visualization
To demonstrate the advantage of saliency-guided visualiza-
tion as provided by the system, the authors compared saliency
maps of the system’s output and homogeneous high-detail
visualization typical for mass-market systems (e.g., Google
Earth). As can be seen in Figure 10, visual saliency of homo-
geneous graphic styles is distributed across focus and context
regions. By contrast, visualization of the system presented in
this paper yields concentrated high saliency within a circular
RoI and for single landmarks in the context area. In case of
saliency-guided route visualization, the saliency follows the
route due to high frequencies in color, orientation, and depth.
4.3. Performance Evaluation
The performance tests were conducted for the aforementioned
platforms, virtual environments, and usage scenarios. The
test results in Table 1show that the system provides inter-
active frame rates in HD resolution. It was observed that
the performance depends on the total number of transition
configurations defined for a usage scenario. For instance,
a view-distance-based transition performs, in mean,
27.5
%
slower than a visualization with a homogeneous graphic style.
Further, it was observed that the system is fill-limited, with a
performance increase of
75
% (in mean) when using a reso-
lution of
800 ×600
pixels over
1920 ×1080
pixels. For the
city of Chemnitz, it was observed that the system is CPU-
limited because of the rendering engine being limited to a
single-threaded traversal of the scene graph. Moreover, the
results for Mount St. Helens show that the system can handle
3D scenes with high visual complexity in full HD resolution
at real-time frame-rates.
© 2012 The Author(s)
© 2012 The Eurographics Association and Blackwell Publishing Ltd.

A. Semmo et al. / Interactive Visualization of Generalized Virtual 3D City Models using Level-of-Abstraction Transitions
0.2
0.4
0.6
0.8
1.0
0.0
Saliency
Detailed visualization (no highlighting) Highlighted circular region-of-interest Highlighted route
OutputSaliency Map
Figure 10:
Examples showing a circular RoI and a highlighted route within the city of Chemnitz, compared to a detailed version.
The bottom row shows the respective saliency maps using the algorithm of graph-based visual saliency [HKP07].
The memory consumption (VRAM) of our system mainly
depends on the A-buffer (32bit color, 24bit depth, and 8bit
stencil values per sample), the Ping-Pong buffer (32bit color
and 32bit depth values per pixel), and the geometry buffer
(32bit edge map, 32bit ID map, and 32bit normal map):
M=W·H·(2N+7)
262,144 MB,
where
W
and
H
refer to screen resolutions in pixels, and
N
to the number of samples used by the A-buffer.
4.4. Lessons Learned
The system is currently used in the authors’ research group as
a platform for implementing novel visualization techniques
designed for 3D geovirtual environments. During develop-
ment and authoring of usage scenarios, it was observed that
the parametrization of the system can be cumbersome. Moti-
vated by this, functionality to serialize transition configura-
tions, feature classifications, and parametrizations of graphic
styles was added to the system, to be able to maintain de-
signs in libraries and easily deploy usage scenarios. It was
further observed that development of new graphic styles can
become time consuming. Therefore, a shader editor was inte-
grated into the system, which facilitates modification of ver-
tex, geometry, and fragment shaders at run-time. Moreover,
the performance evaluation indicates that a single-threaded
rendering engine has impact on the system’s interactivity.
Therefore, the authors plan to port the system to a rendering
engine that supports multi-threading. Finally, the approach
to buffer fragments in depth is memory consuming and re-
quires sufficient samples, or additional rendering passes using
occlusion queries [MB07] to avoid visual artifacts. The pro-
posed Ping-Pong buffer only resolves this issue if features
are rendered opaquely.
5. Conclusions and Future Work
This paper presents a concept and an implementation of a sys-
tem that visualizes virtual 3D city models with parametrized
level-of-abstraction transitions for a seamless combination
of various graphic styles in a single view. The system pro-
vides interactive, saliency-guided visualization by coupling
saliency metrics with cartographic rendering techniques. It
is extensible by custom 2D and 3D graphic styles, integrates
into a visualization pipeline, and can be used to improve ex-
isting visualization techniques (e.g., based on focus+context
zooming [QWC
∗
09] and deformation [MDWK08]). Usage
scenarios based on the system’s capability for thematic vi-
sualization demonstrate the system’s benefits for typical ap-
plications of geovirtual environments – in particular, map
viewing, business locating, navigation, and wayfinding.
Since the system operates in 3D space, it can be used
to enhance the x-ray volumetric lens effect [VCWP96] for
indoor visualization. Visualization on mobile devices also
has high potential to benefit from the system because level-of-
abstraction can reduce information compression on displays
with limited size. Finally, saliency maps of the presented
results show that the system is feasible to draw attention to
important information, though this requires further validation.
Therefore, the authors plan to conduct a user study to confirm
this hypothesis.
Acknowledgments
The authors would like to thank the anonymous reviewers for
their valuable comments.
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