SMI 2004 – Paper #86
Abstract — This paper introduces an immersive styling
environment, which aims at closing the gap between 2D drawing
and 3D modeling in the design process. The main goal of the
styling system is to provide the user an easy to use interface
hiding the inherent mathematic nature of CAD. Creating rough
3D sketches should literally be as intuitive as 2D sketching with
pen and paper. To achieve this, the tools developed in our system
benefit from the stylists’ skills, acquired through training over
time. This paper focuses on the stroke-input methods of our
styling system. We present different techniques for creating and
modifying 3D curves: stroke splitting, oversketching and taping.
In addition we report on the viability of using input constraints
in immersive environments to overcome inherent weaknesses.
Index Terms — computer aided styling (CAS), immersive
environments, stroke input, interaction with shapes.
OWADAYS designers most preferred tool still is pen
and paper. Though a lot of mainly young designers
already started using computers, the interaction paradigm has
not changed. Whether pen or tablet, or paper or paint
software is used, the first sketches in the automobile design
process are always perspective sketches on a 2D medium.
Only after pre-selecting first concept drawings, a 3D model by
using computer aided styling tools is created. Current CAS
systems do not support direct 3D input. To use 2D input and
output devices to create 3D models, highly trained engineers
with good mathematical foundations are needed. The 3D
model evolves in collaboration between engineers and
designers, which is an immensely time consuming task.
Our aim is to reduce the gap between perspective 2D
drawings and 3D modeling by developing a 3D styling tool,
which can be used by designers as intuitively as pen and
paper. In general to reach this goal, there are two possible
approaches. One is to create 3D curves and surfaces using 2D
interfaces. The other is to create a model directly in 3D space
using an immersive environment. In this work we follow the
latter approach and do not consider the first.
Regardless of which approach, one main drawback of CAD
is the inherent mathematic nature of its operations and
representations. Making the system accessible to designers
means, that this characteristic has to be hidden by interaction
tools as much as possible, without loosing control over the
results. Though this seems contradictory, we focus on
presenting a viable solution to this challenge in our work.
Unlike paper or canvas, the media air, as free space, does
not create any feedback while sketching, which makes control
over a sketch more difficult. Although employment of haptic
devices could help to reduce this weakness, we do not
consider such devices here. This is mainly due to the fact, that
wide area haptic devices are still not commercially available.
Our approach to overcome this limitation is to constrain the
user input on the software side.
The presented immersive modeler is developed in close co-
operation with automotive
functionality tends to be tailored according to their specific
needs, meeting the criteria of their domain. Nevertheless, most
methods are very generic and results should partly be valid for
other design processes as well.
2. State of the Art
In the past few years design and modeling in virtual
environments has been more and more a subject of research.
Starting with simple visualization systems and point-grab-and-
move interaction, more complex applications are now ported
to and developed for VR.
One of the first modeling applications using immersive VR
technology was 3DM by Butterworth et al . Stereoscopic
output was realized by a head-mounted display (HMD). The
modeling functionality was limited to the creation of
compound objects from standard primitives like spheres and
cylinders. Free-form modeling functionality was not
supported. In non-immersive applications, Steed and Slater 
have evaluated different metaphors for 3D interaction with a
desktop bat (a 5 DOF device). They focused on picking and
moving objects as well as on navigation through space; object
creation and modification were not addressed.
designers. Therefore its
Stroke-Input Methods for
Immersive Styling Environments
Timo Fleisch, Gino Brunetti, Pedro Santos, André Stork
Fraunhofer IGD, Darmstadt, Germany
SMI 2004 – Paper #86
Chu et al.  are working on a multi-modal VR-CAD
system. This application can be used at a Virtual Table (VT)
driven by data gloves. Forsberg et al. have extended their
SKETCH  system towards 3D and use two magnetic
trackers at a VT for object transformation with the non-
dominant hand and 3D sketching with the dominant hand.
Fröhlich  et al. at Stanford University has developed an
architectural application that has assembly and disassembly
features and snapping capability.
An approach to define and alter free-form surfaces within
an immersive virtual environment by hand gestures was
described by Usoh et al . The deformations follow a
physical model, which results in an elastic behavior of
surfaces. The drawback of this approach is the complexity of
the calculations, which cannot be performed in real-time. This
heavily reduces the interactivity of the system.
Hummels et al.  examine the working situations of car
designers and suggest a gesture-based virtual environment.
The so-called “fish-tank” serves as desktop on which the
content of a monitor is mirrored. Most of the gestures should
be two-handed. No implementation details and results are
provided. In the same application area, i.e. car design, Fontana
 et al. suggest a classification scheme for so-called “detail
features” for an aesthetic and/or functional characterization of
predefined free-form features.
Further research and implementation was done by Dani et
al. . Their Conceptual Virtual Design System (COVIRDS)
explores the multi-modal use of different input streams, like
speech-input, gesture-recognition and 3D I/O devices for
modeling 3D objects. The proposed techniques for free-form
surface design are presented more detailed in Dani et al. .
They describe a creation and modification technique where
the user working at a projection wall is faced with a planar
default face. Indirect 3D interaction with a ray allows him to
select a set of control points, which can be a rectangular
subset of all control points, a row or a column. By moving the
set of control points the user alters the shape of the surface.
Liverani, G. Piraccini  reported on their development
on VISM (Virtual Surface Modeler). Addressing designers
their system allows working on full-scale models using 3 rear-
projected translucent screens (2.5mx1.9m each) to modify and
manipulate surfaces. The user uses a glove-like input device
moving over the surface to deform and shape it.
Wesche in  has described an interesting system for
modeling surfaces in a virtual environment. Using 3D input
devices his system allows to create and modify curves,
surfaces are derived from. The most interesting aspects are the
“smoothener” and “sharpener” operations, that provide local
modification of curves and – to a certain extent – also
surfaces. In the latter case, real-time behavior can only be
ensured for small patches. There is no immediate visual
feedback provided by the system when sketching surfaces.
In  Fiorentino et al. presented Spacedesign, which is the
predecessor of our system presented here. Spacedesign used a
similar setup to the one described here and offered functions
to create curves and surfaces in an immersive environment.
Some of the listed references already describe functionality
to create and modify geometry in an immersive environment.
However all of them merely investigated the technical aspect
of immersive modeling. Our system considers the stylist to be
at the center of the development process. Based on user tests
with partner from the car styling industry we studied the
typical skills of designers and developed our interaction
techniques to benefit from these skills. Our main conclusion is
that stylists work strongly stroke orientated. Though one-
gesture free form surface input methods, like the coon patch
function described by Fiorentino, create a 3D surface in a very
fast and convenient way, designers complain of not having
enough control over the results.
In our system we focus on methods to create and manipulate
strokes, which afterwards can be used to create surfaces. How
surfaces are created, is not subject of this paper.
3. System Overview
The immersive styling system setup basically consists of
The prototype can either run in virtual reality or in
augmented reality. In virtual reality a virtual table or
projection wall for augmented reality video or optical see-
through glasses is used.
To track the user’s head and hand positions we use an
optical tracking system from ART, which provides high
accuracy and low latency. To overcome the disadvantage of
keeping the camera’s line of sight clear, a redundant amount
of cameras and markers is used. In addition optical tracking
allows for untethered objects, such as a wireless pen.
Various tracked objects are used as input devices. They are
shown in Figure 1.
Depending on the setup, shutter glasses, linear polarization
glasses or a head mounted display is used. The virtual images
are rendered according to the user’s head position / viewpoint.
A wireless pen with 3 buttons is used as main input device.
SMI 2004 – Paper #86
Tape Finger L-Axis Tangible Plane
Figure 1: Input Devices
PIP-Sheet (Personal Interaction Panel)
The PIP (Personal Interaction Panel) is a transparent
plexiglas panel on which the application menu is projected.
The menus on the panel are operated with the pen.
Navigator axis (L shape, cube shape)
The navigator axis is used to navigate the model in 3DOF.
The user can choose between an “L”-shaped device and a
Tangible planes (mirror/projection)
These tracked artifacts are used by the mirror and
projection plane function. Both functions operate on a virtual
plane, which is moved according to the artifact.
This device is only used for the virtual taping function,
which is controlled by two input devices, the taping finger
representing the tape-fixing finger (see below) and the pen
applying a tangent to the curve.
Virtual Reality Setup
For styling, the output device should provide high quality
rendering, and immersion. We decided to use a semi-
immersive virtual table with a diagonal of 1.7 meters, which
allows creating e.g. parts of a car body in scale by appropriate
hand gestures and arm movements.
In stereoscopic mode the user wears a pair of tracked
shutter glasses, and the scene is rendered according to the
user’s point of view. In this way the virtual objects appear
floating in space above the table. The setup is displayed in
Figure 2: Virtual Reality Setup
A second scenario uses an upright projection wall to display
the picture. In this case the tracked artifacts are placed on a
table and the user operates the pen in front of the wall. The
disadvantage of this setup is that the tracked artifacts cannot
be placed anywhere conveniently.
4. Stroke input Methods
4.1. Stroke input
Our approach to stroke input processing is very straight
forward. While moving the 3D pen device in space the tracker
continuously delivers position events. These positions are
used as sample points for the stroke. The shape of the created
NURBS curve depends on the distances between the sample
points. The result is therefore always a trade off between
complexity of the curve and the distances between sample
points. Well-suited 3D curve approximation methods
including algorithmic details have been described by L. Piegl
and W. Tiller in .
Figure 3: Automatic Stroke Splitting
SMI 2004 – Paper #86
4.2. Automatic Stroke Splitting
High quality curves should have as few control points as
possible. But one problem with curves built of few control
points is, that it is not possible for them to have sharp edges.
We therefore developed a function that analyses the drawn
curve according to its curvature. It detects points of high
curvature and cuts the curve at these points. The resulting
curve then consists of partial curves with few control points
and possible sharp edges at the junctions. In
Figure 3 two curves are shown. The upper curve is drawn
with the conventional freehand spline function. It can clearly
be seen that the curve’s tip is rounded. The lower curve is
automatically split using our method, therefore the tip remains
as sharp as the user drew it.
The splitting algorithm uses the stroke input sample points.
These sample points can be regarded as a polyline. For each
point of the line the angles to neighbouring segments are
calculated. If these angles are small then the curvature of the
curve at this point is high. Along the list of per-point angles
the maxima are searched. If an angle maxima lies above a
defined curvature limit, the point becomes a splitting point.
Afterwards for each segment defined by the splitting points a
NURBS curve is generated according to the sample points and
the distance between sample points as described in the
4.3. Stroke Oversketching
In the task analysis phase we studied how stylists do their
pen and paper drawings. A common technique to create a
curve is to repetitively draw over – this technique is called
oversketching. To mimic this curve modification method, we
implemented virtual oversketching. With this function a curve
can be changed by sketching a new curve over the existing
The main tasks of the oversketching algorithm are:
• Find the part of the original stroke that has been
• Substitute the oversketched part with the new stroke.
• Smoothening the transition
Reshaping the original stroke according to the oversketch
can either be done on the base of control points or sample
Operation on control points
In this method the oversketch stroke is first converted to a
NURBS curve. In the following just the control points of the
original and the oversketch curve are considered. For the
oversketch curve’s first and last control point we search the
corresponding control point of the original curve. Then we
replace the control points in this interval with the
oversketched curve’s control points.
The disadvantage of this method is that the resulting control
points get unequally distributed over the curve, especially if
the operation is performed multiple times. Although the curve
itself still looks nice, surfaces created with these curves are of
Operation on sample points
As opposed to the above approach, not the control points
but the sample points are considered. Therefore the original
curve has to be re-sampled first, because it is internally stored
as NURBS curve. Then the interval of the original curve to be
replaced has to be found. Therefore the closest point to the
oversketched curve’s first and last point is identified. After
replacing this interval, the resulting curve is converted to a
NURBS curve again.
Because of the recalculation of the curve’s parameters, the
original curve’s character is lost. On the other hand the
recalculation always creates a good quality NURBS curve that
can be used to create surfaces from. Another disadvantage of
this method is that the transition area at the replace interval
start and end points appears to be harsher.
Both methods can be controlled by the user with two
parameters. One parameter defines the amount of influence of
the oversketched curve in the replacement interval. If the
value is 1, the curve is just replaced like described before. If
the value is below 1, the oversketched curve sample points or
control points are calculated as to lie on a perpendicular line
to the original curve’s sample point. If the scale value is 0 the
curve remains unchanged.
The second parameter defines a transition interval for the
replacement interval end and start points. If the transition
interval is zero the resulting curve might have harsh changes
at the replacement interval borders. This applies even more to
the method operating on sample points. Within the user
defined interval the replacement therefore is done according to
the above described factor of influence of the oversketched
curve. Also the scaling is defined similarly. However over the
transition interval scaling is a function from 0 to 1 to fade in
and 1 to 0 to fade out. For better results the transition is not
linear but according to a sine function.
Figure 4: Stroke Oversketching
SMI 2004 – Paper #86
In Figure 4 two samples of oversketching are shown. In a),
the scaling parameter of the oversketched curve is 0.5.
Therefore the resulting curve is about between the original
and the oversketched. In b) the influence parameter is 1.0,
therefore the result follows the oversketched curve.
Like in most CAD applications we also provide the
possibility to edit a curve by directly dragging its control
points. User tests showed that designers prefer our
oversketching method at early stages, where changes to apply
to curves are still quite big. For posterior refinement designers
still prefer control point modification, because it gives more
accurate control over the result.
We also extended our approach to surface oversketching,
where the shape of a surface is modified according to a curve
sketched close to it. (see
Figure 5: Surface Oversketching
Another one-stroke curve modeling method that has been
adapted from a traditional styling method is 3D tape drawing.
Tape drawing is a wide-spread modeling technique in
industrial design that is used to define characteristic lines,
typically on 1:1 sized perspective 2-dimenisional drawings of
the product model. In tape drawing instead of using pencils, a
special adhesive tape is used, which the stylist unrolls with
one hand (typically the right hand), sliding over the tape with
the other hand applying gentle pressure to fix it on the
drawing. In this way smooth 2-dimensional curves are
designed, where the first hand defines the tangent line with
respect to the curve point currently fixed by the other hand as
illustrated in Figure 6.
Experienced stylists define such taped curves with a single
two-handed stroke, where the tape easily allows undoing or
correcting the curve by un-taping it back to any point of the
curve, as required.
A first approach to develop a computer aided 2D tape
drawing has been presented by T. Grossman et al. ,
Figure 6: Traditional 2D-tape drawing
allowing to define a curve by using a 2D device (mouse) and
defining two subsequently clicked points on the projection
plane as a pair being the first point, a point on the curve and
the second point defining a tangent line (the vector between
the two points). To define the final 2D curve, the curve points
are interpolated applying a Hermite interpolation using the
tangent (direction and value of the first derivative)
information as boundary conditions.
The approach presented in this paper differs from previous
approaches, and actually extends digital tape drawing in four
1. It maps the 2-dimensional tape drawing into a
corresponding 3D metaphor as has been introduced by
De Amicis et al. ;
2. It extends 2D tape drawing to a 3D curve modeling
method, enhancing the modeling power of this styling
3. It provides direct manipulative two-handed input with
3D devices, which naturally builds on existing well
developed skills of stylists, allowing to define 3D curves
in a single stroke as it is the case in the traditional 2D
4. Finally, the approach integrates tape drawing into
allowing stylists to immediately inspect and validate
resulting curves from any viewpoint, with any scaling
factor, and related to the 3-dimensional presentation of
the product model, instead of a perspective 2D drawing,
The 3D devices applied for 3D tape drawing are shown in
Figure 7 and Figure 1. Those devices are a “tape finger”
device (left hand), which is tracked for the position of the
hand and finger, respectively, defining the curve points, and a
pen device (right hand) corresponding to the roll of tape. The
curve modeling starts when pushing the pen button and stops
when releasing it. All optically tracked positions of these two
devices are used to define the final shape of the curve. In
addition the head position and view orientation of the user as
well as the “navigator axis” are tracked to define the current
view with respect to the model.
SMI 2004 – Paper #86
Figure 7: Virtual 3D tape drawing at the Virtual Table
with the optically tracked finger (left hand), pen
(right hand), head position, pip position, an the
“navigator axis” (on the right).
When starting the 3D tape drawing, the method is
initialized with the current position of the finger Pf1, which
also defines the starting point P1 of the curve, and the current
position of the pen Pp1. New positions of the finger or the pen
are considered, if their current positions have a minimal
displacement with respect to their last position. A threshold of
10-5 to 10-4 units displacement has proven to provide an
accuracy well accepted by users at user tests.
For the iterative definition of the sample points Pi of the
curve the following approach has shown to work well in
Given the last point Pi two vectors are defined, the relative
. Now the current finger position is projected onto the
displacement of the finger
and the vector defined by
has the following characteristics:
1. Any subsequent point is defined such that the vector
between the new Pi+1 and the pen position defines a
tangent line on this point Pi+1, as it is the case in the
traditional 2D tape drawing;
(see Figure 1 for details). The procedure
2. Finger displacements in depth are corrected avoiding
unwanted oscillations in that direction. Remember that
stylists have a physical feedback constraining the finger
movement to a given plane when taping on drawings,
which they do not have in virtual space;
3. The orientation, which in contrast to 2D tape drawing is
now a 3D vector, is defined by the leading pen (taking
the place of the tape roll) exactly as it is the case of the
original tape drawing process. Hence it provides a means
of adding degrees of freedom to tape drawing, benefiting
from the already well developed skills of stylists.
Once the input is finalized the sample points Pi are
approximated by a cubic NURBS curve.
Ideas to extend the 3D tape drawing metaphor to an
approach for freeform surface modeling, i.e. working with a
tangent plane instead of a tangent line, have been rejected,
because it turned out that the power of such an approach
would be limited to the type of surfaces that can better be
modeled as linearly extruded 3D curves.
Figure 8: Calculation of the taped 3D curve with
points Pi derived from the current and last finger
and pen positions.
4.5. Strokes as gestures for input control
The immersive modeller currently supports two functions
via gesture input. The gesture is performed using a 3rd pen
button. Drawing a circle over a surface will delete the surface.
Drawing a rectangle over a surface will select it (see Figure
9). The gesture interface is implemented in a way, that it can
easily be expanded to support further functions. After further
user evaluation we shall decide which other functions will be
For gesture recognition CALI  was integrated into the
prototype. The 3D input is projected onto a regression plane,
mapped to 2D and sent to CALI, which returns the recognised
SMI 2004 – Paper #86
Figure 9: Stroke gestures to delete and select
Rotate plane on table surface
Figure 10: The stylist positions a plane arbitrarily in
the immersive space. Then the scene is rotated, so
that the plane is equivalent to the table (work bench)
5. Immersive Constraints
5.1. Tangible planes
Our immersive modeler has two drawing tools based on
planes, namely a mirror plane and a projection plane. With an
active mirror plane, all user interaction is mirrored according
to the position of the mirror plane. The purpose of the
projection plane is to constrain the user input to an arbitrary
positioned plane. To handle these planes we make use of the
immersive environment and the tracking system. The virtual
planes have a physical counterpart, the tangible plane (see
Figure 1). Because of their transparent Plexiglas they merge
seamlessly with the virtual rederings in immersive space. We
attached marker balls to the plane, so it is tracked by the
tracking system. The user can now use these physical objects
to position the virtual planes in immersive space. The virtual
plane is projected on the Plexiglas.
5.2. Projection plane to table
Stylists are used to work with media that returns haptic
feedback. In free space no feedback is available, which makes
accurate drawing a difficult task. Haptic devices might be a
promising solution, but are not conveniently available for an
area as large as the virtual table we use.
We found a solution to overcome this problem at least for
drawing on 2D planes by using the table itself as a drawing
surface. By using the tangible plane, we arbitrarily position a
projection plane somewhere inside the scene. When activating
the “projection-plane-to-table”-function, the projection plane
is rotated to the table surface, so the designer can directly
draw on the table surface. Furthermore all geometry on the top
of the table is clipped away. On the upper part of
Figure 10 the projection plane is positioned somewhere in
the scene. On the lower part the rotated view clipped at the
plane position in orthographic view mode can be seen.
5.3. Constraint stroke manipulation
To manipulate a stroke in our immersive modeller we
provide the already described oversketching and control point
modification. A problem with modelling in free space is, that
effects of oversketching in direction of the user’s view vector
cannot be controlled intuitively. We therefore added the
option, that when manipulating a stroke, changes in this
direction are suppressed. For simplification the suppressed
direction is one of the dimensions of the coordinate system.
For this purpose the axis most parallel to the user view vector
is calculated. If then the stylist moves a control point, the
parameters affecting the suppressed axis are not changed.
SMI 2004 – Paper #86
6. User Tests and Results
With our immersive modeler we enter a new domain in
CAS. The quality of the results we can expect from our
system is not yet comparable to commercial software used in
this field nowadays. We do not compete with them in the
sense of precision and surface quality. Our goal is to create
models comparable to quick paper sketches, giving the stylist
a tool to create a 3D model and to express his design ideas as
fast as he would do with pen and paper. Therefore our user
tests did not compare our system to software used in the
styling phase today, but questioned, if immersive modeling
had the potential to serve as an additional tool to reach our
goal of quick sketches.
The main user testing has been done in multiple sessions
with car stylists from major automobile manufactors and auto
styling companies. Apart from this we also made some minor
testing with product designers and media artists. In the first
round of user testing with an earlier prototype each user had
about two hours, starting with a tutorial and a free sketch
phase at the end. All users had computer background
knowledge, some working with CAS tools, others with paint
tools. At this time we tested 20 persons to get a more general
The second user test was done with our latest prototype and
included only 3 stylists, each of them having two days time to
test the system and discuss their views.
In general it can be said that in principle all users accepted
the system, although everybody could imagine an immersive
workplace only as part time addition to its usual working
environment. More than two hours in a row did not appear to
be convenient due to the working stance and the stereo
display. For more detailed comments it is necessary to
distinguish between the different backgrounds of the users:
• Junior Car Stylists
A group of people already used to CAS due to their
education. They tend to accept the immersive
environment pretty fast. But, because of their
background in CAS software, they demand for similar
handling, functions and precision, as they are used to.
Though the 3D environment is well liked for immediate
result review they prefer 2D views and planes for the
construction phase. Strokes are preferably drawn on a
plane and not in free space.
• Senior Car Stylists
Educated with pen and paper and using a computer
mainly with paint software, they have a different
approach in using the software. 2D modes and planes
seem to them even more important than to the juniors,
additionally they get irritated by head tracking. Another
drawback to them was the fact, that calibration of 3D
devices is not as precise as the physical counterparts. The
fact that the virtual pen and the physical pen device are
never perfectly aligned irritated them.
• Media and Fine Artists
Though actually media and fine artists are a very
different group, they have in common that they are not as
limited to 2D planes as the car stylists. Fine artists are
practised to draw on canvas with rather free and wide
arm movements whereas media artist often do not come
with special drawing customs. Their impression was that
though they have difficulties to make drawings in free
space right now, after a training period working with 3D
strokes should be possible and usable.
Figure 11: Results the immersive modeler. The
surfaces were created from curves.
Altogether all users have already been able to create 3D
sketches expressing their intended design.
Figure 11 shows two cars created with the modeller in just
two to five minutes. All surfaces were created from curves
using the described stroke input. Though surface connection
and detailed modelling is still not supported efficiently, the
stylists were able to create models that represent the character
and idea of the intended design in a short amount of time.
SMI 2004 – Paper #86
7. Conclusion and Future Work
The results show our immersive modeler already enables
stylists to sketch characteristic 3D models within a short time.
But to be able to model in more detail and for improved
interaction a lot of work is still to be done. Stroke input has to
be improved by giving users more influence on how the input
sample points are converted to NURBS curves. Having
control over the complexity of the curve is important for high
quality results. Furthermore we work on integrating features
to easily trim parts of the curve, as well as a more controlled
oversketching, where the user can exactly define the starting
point from where the curve should be modified.
Furthermore the functions to create surfaces from strokes
have to be improved and extended. Finally surfaces should be
connectable without gaps and modifications and changes of a
surface should affect connected surfaces accordingly.
This work was funded in part by the European Commission
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