VLEGO: A Simple Two-handed Modeling Environment
Based on Toy Blocks
Kiyoshi Kiyokawa, Haruo Takemura,
Yoshiaki Katayama, Hidehiko Iwasa and Naokazu Yokoya
Nara Institute of Science and Technology (NAIST)
8916-5 Takayama, Ikoma, Nara 630-01, JAPAN
Tel: +81 7437 2 5291 Fax: +81 7437 2 5299
{kiyosi-k, takemura, katayama, iwasa, yokoya}@is.aist-nara.ac.jp
ABSTRACTThis paper describes a case study of building a prototype of
an immersive three dimensional (3-D) modeler which
supports simple two-handed operations. Designing 3-D
objects in a virtual environment has a number of advantages
for 3-D geometry creation over designing with traditional
computer aided design (CAD) tools. In order to enhance thehuman-computer interaction in a virtual workspace, twohanded spatial input has been incorporated into a few 3-D
designing applications. However, existing 3-D designing
tools do not utilize two handed interaction for enhancing theinterface sufficiently. Our prototype immersive modeler,
VLEGO, employs some features of toy blocks to give
flexible two-handed interaction for 3-D design. Features of
VLEGO can be summarized as follows: Firstly, VLEGO
supports various two-handed operations and hence it makes
design environment intuitive and efficient. Secondly,
possible location and orientation of primitives are discretely
limited so that the user can arrange objects accurately withease. Finally, the system automatically avoids collisions
among primitives and adjusts their positions. As a result,
precise design of 3-D objects can be achieved easily byusing a set of two-handed operations in intuitive way. This
paper describes the design and implementation of VLEGO
as well as an experiment for examining the effectiveness of
two-handed interaction.
Keywords
virtual reality, 3-D user interface, two-handed interaction,
collision detection.

INTRODUCTION
Designing 3-D objects in a virtual environment has threepotential advantages over designing with traditional 3-D
CAD tools: Firstly, 3-D objects can be observed through astereoscopic display from an arbitrary point of view basedon the user's head position in real-time. This feature helps
the user to understand the spatial relationships among
objects in 3-D space. Secondly, spatial input devices enabledirect manipulation of objects, and that makes 3-D design
intuitive and usable. Finally, they offer highly interactivemodeling environments; i.e., modifications in design
process are directly reflected by the appearance ofcorresponding objects. According to the rapidly accelerating
progress of virtual reality (VR) technology, a number of
researches have been made to produce a virtual workspacefor 3-D design [1, 2, 3, 4].
Two-handed interaction are known to enhance the humancomputer interaction in a virtual workspace, and it attracts
many researchers. Hinckley et al. claims that two-handed
input not only improves the efficiency of human-computerinteraction, but can also help to make spatial input
comprehensible [5]. There are some 3-D designing tools
employing two-handed interaction [3, 4]. However, twohanded interaction in such tools seems to be based on 2-D
painting or drawing rather than actual 3-D designing
activities; e.g., sculpturing, building up bricks and making aplastic models. 3-D designing activities are expected tomake better use of two-handed interaction than 2-D
designing. Therefore, for the purpose of providing moreintuitive ways for 3-D design, we employ two-handed
interaction based on actual 3-D designing methods.
In this paper, described is a case study of building a
prototype of an immersive 3-D modeler supporting simple
two-handed operations. Here, immersive style means such aviewing style that dynamically provides a pair of binocularstereoscopic images according to the user's viewing
direction, of which field of view is sufficiently wide fordesigning. Our immersive modeler, VLEGO (VirtuaL
Environment for Generating Objects) employs the metaphorof widespread toy blocks, which includes flexible twohanded manipulations. In addition to the two-handed
interaction, this metaphor leads to other significant
features : discrete placement constraints and collision
detection detailed in later section. These features reduce thedifficulty of accurate manipulation of 3-D objects, whicharises from computational delay, restricted spatial
resolutions of input/output devices, and physiologicallimitation such as depth perception and trembling of limbs.
In consequence, VLEGO offers natural and quick ways tocreate 3-D objects in the virtual workspace.
This paper is organized as follows. Firstly, previous related
works are briefly reviewed. Then, three characteristics of
VLEGO are summarized: two-handed manipulations,discrete constraints and collision avoidance. The
implementation of VLEGO is then explained, especially its
characteristics and typical manipulations. Finally, an
experiment for examining the effectiveness of the twohanded manipulation is described.

through the experiment described later, it was confirmedthat two-handed interaction improves the efficiency ofhuman-computer interaction in a number of ways as follows.


RELATED WORKSIn this section, related works are briefly reviewed. Table 1
shows the classification of a number of designing tools
including VLEGO according to their characteristics.
Butterworth et al., at the University of North Carolina, built
an immersive style tool named 3DM [1]. The system
employs the model manipulation techniques from both CAD
and 2-D drawing programs like MacDraw. The 3DM uses ahead mounted display so as to give the effect of being in thevirtual workspace.
Liang, Halliday et al., at the University of Alberta,
developed the JDCAD and the JDCAD+ [2]. These nonimmersive style systems use a single six-degrees-offreedom input device to design both geometry and behaviorof 3-D mechanical parts.
Although these tools are well designed, they do not provide
any two-handed interaction.The less-dominant hand is morethan a poor approximation of the dominant hand. Kabbash
et al. showed that the less-dominant hand performed as sameability as the dominant hand for relatively large targets andlarge distance in pointing and dragging tasks [6]. According
to the previous works [6, 7, 8, 9, 10] and our observation

Users can perform various tasks with flexible andefficient ways because he/she is able to set multiple
values simultaneously with up to 12 degrees of
freedom. That is, two-handed interaction enables users
to perform two sub-tasks in parallel, and also
encourages users to perform one task using two hands
cooperatively. Here, two-handed cooperation means
such manipulation that the less-dominant hand holds
an object and the dominant hand holds another object
or some kind of tools, then both hands perform
together an action — attaching, shaving, cutting,coloring, and so on. The passive interface props
presented by Hinckley et al. are examples of making
good use of cooperative two-handed interaction [11].
Using both hands helps users’ understanding of spatial
relationship because users know precisely where theirhands are relative to each other. In addition to thisfeature, cooperative two-handed manipulations make itfacile to select a point in a virtual workspace in spite ofscreen vacillation arisen from unstable data of position
trackers. This is because both hands vacillate in almost
the same direction so that the vacillation is canceledout and their relative positions are comparatively
stable.

As a 2-D drawing application, Toolglass developed by Bieret al. at Xerox PARC is a good example using two-handed
interaction [12]. The user moves a transparent sheet using
one hand on which various tools are located, and the other
hand clicks through the tool onto application objects.
There are several tools for creating 3-D objects using twohanded interaction. Sachs et al. at MIT developed the 3Draw as a non-immersive tool for designing 3-D shapes [3].
The system's user interface is based on a pair of sixdegrees-of-freedom input devices, one held in each hand so
as to simplify shape design. Shaw et al. at the University ofAlberta built the non-immersive style system, THRED, for
two-handed polygonal surface design [4].
There are two matters to be pointed out in 3-Draw and
THRED in the context of our goal of providing more

Table 1: Classification of designing tools.
one-handed interface

two-handed interface

2-D drawing

MacDraw

Toolglassnon-immersive style 3-D modeling

JDCAD, JDCAD+

3-Draw, THRED

immersive style 3-D modeling

3DM

VLEGO

intuitive ways for 3-D design. One is that 3-Draw and
THRED do not provide immersive style stereo viewing
facility, which enables the user to understand the virtual
workspace intuitively and to manipulate virtual objects
directly. In order to provide intuitive usability, immersive
style is one of key factors to be considered, especially whencombined with two-handed interaction, because it makes
better use of knowledge of spatial relationship in the virtual
workspace.
The other matter is that the less-dominant hand is not so
effectively utilized for enhancing the usability incomparison with its ability. That is, creating 3-D shapes isperformed mainly by the dominant hand and the lessdominant hand is used for moving the base coordinate frameof the model or selecting the status of the modeler. The
less-dominant hand has a variety of functions in actual 3-D
objects creation such as building a miniature toy model : itperforms the same functions as the dominant handconcurrently and assists the functions of the dominant handcooperatively as well. Therefore, in order to make creating
3-D objects intuitive, it is useful to employ more flexible
two-handed interaction based on actual 3-D designing
activities.
SYSTEM CHARACTERISTICS
In order to develop intuitive and quick ways for creating 3D objects, widespread Lego-like block toys attracted us. Toy
blocks provide us an easy way of creating various 3-D
objects owing to a number of features. Our immersivemodeler, VLEGO employs the features of toy blocks. The
characteristics of VLEGO are summarized as follows:
1.

2.

3.

VLEGO supports a number of flexible two-handed
manipulations such as assembly, decomposition,coloring and scaling objects, so that 3-D modeling canbe performed in intuitive and efficient ways.
Possible location and orientation of primitives are
discretely limited. Owing to the feature, the user canarrange objects accurately with ease in spite of thecomputational delay and screen vacillation arisen fromunstable data of position trackers and trembling limbs.
The system detects collisions among picked and allother objects and adjusts the locations of picked
objects so as to avoid interference of any two objects.
This feature also assists the user to manipulate virtual
objects accurately and intuitively. For instance, avirtual object can be moved over surfaces of otherobjects while continuing to touch them.

The two-handed manipulations and collision avoidance
make manipulation of virtual objects intuitive, while thediscrete constraints and the collision avoidance makeprecise manipulation facile. In consequence, VLEGO offers

CrystalEYES

21’ display

to Onyx RE2



3-D input
devicepick
copy

3SPACE

Figure 1: Hardware components.
natural and quick ways to create 3-D objects in the virtual
workspace.
IMPLEMENTATIONThis section describes the implementation of VLEGO.
Hardware components, properties of the elements thatconsist the virtual workspace, three characteristics touchedin the previous section and some typical two-handed
operations are explained in the following.
HardwareFigure 1 illustrates hardware components of VLEGO.
VLEGO is implemented on a graphics workstation Onyx
RE2 (SGI). A user views a stereoscopic virtual workspace
through a liquid crystal shuttered glasses, CrystalEYES(StereoGraphics) with head-tracking facility using a 3-D
magnetic tracker 3SPACE (Polhemus). A pair of 3-D input
devices for two-handed manipulations are build (lower rightof Figure 1). Each of them is composed of four feather touch
switches and a 3SPACE tracker.
Software
The workspace of the system is composed of a number of
elements displayed on the screen as shown in Figure 2: two
3-D cursors, three coordinates axes, object primitives, aprimitive box and a palette ball. This subsection is devoted
to explain each element.
3-D cursors
There are two arrow-shaped 3-D cursors corresponding toright and left input devices respectively, that move freely
following to the devices. 3-D cursors are used to perform all
operations in VLEGO.
Coordinate axes
The origin of the coordinate system of VLEGO is the centerof the screen. Two horizontal coordinate axes and a verticalcoordinate axis, that are perpendicular to one another, aredisplayed on the screen. Two horizontal coordinate axes canbe picked and rotated around the vertical coordinate axis soVertical Axis

4DOFC

6DOF

Figure 3: Two types of constraints.

Horizontal
Axes
Primitive Box

3-D Cursors

Palette Ball

Figure 2: Software components.
that whole workspace can be observed and operated from avariety of view points.
Primitives
There are a few kinds of box- and wedge-shaped primitives.
Height, width and depth of primitives are n (integer) cm
long.Each primitive has studs which are evenly placed on its
upper face with intervals of 1cm. Primitives are attachable
to each other at a discrete position corresponding to thestuds in the similar way as actual toy blocks. In this paper,
block is a general term which means a single primitive or acomposite of assembled primitives.

designed 3-D objects using one or two hands by properly
assembling primitives that have a few kinds of box- andwedge-shapes. The user can perform a number of functions
on primitives as shown in Table 2. In this subsection, onehanded manipulations are explained. Two-handed
manipulations are described in the later subsection after we
explain a number of constraints used in two-handed
interaction.
Selection
When a tip of a 3-D cursor is positioned in a block, it isselected. To make the selection clear, the stabbed primitiveis highlighted and the bounding box of the whole blockappears.
Picking
The selected block can be picked by pressing thecorresponding button. The picked block is thenunhighlighted. The picked block moves following to the 3D cursor until the block is released by releasing the button.
Copy
The picked block can be copied by pressing thecorresponding button.

Primitive box
Primitive box is a movable wire-frame box containing a
number of typical primitives. The user can generateprimitives unlimitedly from the primitive box simply bypicking the primitives in the box. The user can also delete apicked block by releasing it in the primitive box.

Deletion
The picked block can be deleted by releasing the block in theprimitive box as mentioned above.

Palette ball
There is a pickable colorful palette ball in the workspace, of
which hue, saturation and intensity changes gradually. The
user can pick the palette ball and use it to color primitives
with two hands.

Assembly
When the upper or lower face of the picked block contacts
with other non-picked blocks, these blocks can be joined byreleasing the picked block. Both one-handed and twohanded assembly is detailed in later subsection.

One-handed Manipulations on Primitives
In VLEGO, as with real toy blocks, the user can easily

Discrete Constraints and Operating Modes
This subsection describes how VLEGO restricts the
positions of blocks. First of all, we introduce two alternative
types of constraints put on blocks. In principle, each picked
block is bound to 4DOFC (4 Degrees Of Freedom with grid
Constraints). Being bound to 4DOFC, the block can belocated on discrete positions at intervals of 1cm and its
orientation is restricted at 0, 90, 180 and 270 degrees ofhorizontal rotation. As a result, all non-picked block is

Table 2: Functions on primitives.
One-handed

selection, picking, copy, deletion, assemblyTwo-handed

assembly, decomposition, coloring, scalingalways aligned one another with 4DOFC. Owing to thesegrid constraints, the user is able to arrange virtual objects
accurately with ease. Left side of Figure 3 illustrates
4DOFC. On the other hand, there are a number of cases
when a block is bound to 6DOF (6 Degrees Of Freedom).
Being bound to 6DOF, the position and orientation of theblock can be operated freely as shown in right side of Figure
3.

3-D cursors

4DOFC

Separative mode
Separative mode is selected when only one hand picks ablock or when two blocks picked by two hands are distantfrom each other (the latter case is shown in Figure 4, S-1 and
S-2). In this mode, the user can not attach two picked blocks
each other. Therefore, it is natural to consider that the user
would be manipulating each picked block independently andhe/she would align each picked block with non-picked
blocks placed around each picked block. In this mode,
hence, in order to assist the user to align picked blocks with
non-picked blocks, each picked block is manipulated with
4DOFC based on the global coordinate system. When eachhand picks a block and these two blocks are close enough toeach other, operating mode is smoothly changed fromseparative mode to cooperative mode mentioned below asshown in Figure 4, from S-1 to C-1.
Cooperative mode
Cooperative mode is selected when each hand picks a blockand the distance between these picked blocks is shortenough. Figure 4, C-1 and C-2 show this case. In this mode,
then, it is natural to consider that the user would align two
picked blocks each other. Therefore, to provide as naturaland easy way for aligning blocks, the system puts 6DOF and
4DOFC constraints onto firstly picked block (called base)and secondly picked one (called work), respectively. What is
important is that the 4DOFC of the work is dynamicallydetermined based on the orientation and location of thebase. Owing to these constraints, two-handed assembly or
decomposition of blocks becomes intuitive and efficient.
When one of the blocks is released or when two picked
blocks depart from each other, these blocks are forced toalign with all non-picked blocks with 4DOFC,and operating
mode changes to the separative one as shown in Figure 4,
from C-2 to S-2. In this mode transition, orientations andpositions of blocks are smoothly interpolated by animation
for a duration of about 500ms to reduce the cognitive load
arising from visual discontinuity.

4DOFC

picked blocks

In order to support natural two-handed operations, VLEGO
has two alternative operating modes, separative mode andcooperative mode, that are switched dynamically according
to the distance between two blocks picked by left and righthands. Each operating mode makes good use of two types of
constraints explained above.

base

6DOF

work

4DOFC

short

S-1.

C-1.

long
6DOF

4DOFC

with Animation

4DOFC

S-2.
Separative Mode

4DOFC

C-2.

Cooperative Mode

Figure 4: Transition of operating modes.
Collision Avoidance
In order to simplify the algorithm of the collision detection,
the subjects of the collision detection vary according to thetwo operating modes. In separative mode, all position
relationships among picked and all other blocks is checked
while only between two picked blocks is checked incooperative mode. This restriction assures that all thesubjects to be checked should be already aligned one
another with 4DOFC in each case. As a result, the algorithm
for collision detection is so simple to perform. The collision
avoidance is performed as follows.
1.

Let the picked block move toward the corresponding
3-D cursor in X direction (of the coordinate system of
the current 4DOFC) while no collision occurs.

2.

Let the picked block in Y and Z direction in the same
way as above.

3.

Draw a line between the picked block and the cursor toindicate the discrepancy of their locations.

Two-handed Manipulations on Primitives
In this subsection, a number of two-handed manipulations
are explained.
Assembly of blocks
Assembling function utilizes discrete constraints andcollision avoidance described above, and is most importantto create 3-D objects. All primitives in VLEGO can be
simply attached to one another in the same way as toy
blocks. If a picked block contacts to other blocks verticallyafter the collision avoidance, these blocks are joined when
the picked block is released. There are one-handed assembly
and two-handed assembly according to the two operating
modes.

base

work

A.

B.

C.

A.

D.

Figure 5: An example of assembly.
In separative mode, the user assembles blocks by one hand.
In this mode, the system detects collisions among picked
blocks and all other blocks, then adjusts the positions ofpicked blocks before the scene is rendered, in order to avoid
the interference among blocks. After the collision avoidance
process, if the picked blocks can be attached to any otherblock, all bounding boxes of the picked and all attachable
blocks appear. All these blocks fit together if the picked
objects are released.
In cooperative mode, the user assembles blocks by two
hands. In this mode, the system avoids the interference
between two picked blocks, however it does not check thecollision and attachability among picked and non-picked
blocks. In this case, two bounding boxes of both picked
blocks appear if they can attach mutually as shown in Figure
5-C, and the two blocks fit together when one of them isreleased as shown in Figure 5-D.
Decomposition of blocks
There are two methods to decompose assembled blocks.
First, the user can remove a primitive one by one with one
hand from an assembled primitives picked by the otherhand. This removal changes the operating mode fromseparative to cooperative. Second, the user can divide anassembled primitives into two parts using a cutting plane.
Detailed process is as follows:
1.

Pick a block (base) with one hand as shown in Figure
6, from A to B.

B.

C.

D.

Figure 6: An example of decomposition.
2.

Press the first switch of the other hand (work) at any
position where no object is selected.

3.

Then the shape of the 3-D cursor of the work changes
translucent square (namely, cutting plane) as shown in
Figure 6, from B to C. Now, both the picked block andthe cutting plane can be manipulated exceptionallywith 6DOF.

4.

Release the cutting plane at proper location andorientation, then picked block is divided into two
blocks as shown in Figure 6, from C to D.

Coloring primitives
For coloring primitives, the user can choose any color using
one hand (called work) from the palette ball picked by theother hand. Once the palette ball is picked, it can be
manipulated with 6DOF and a wire-frame cube indicating
current color appears as shown in Figure 7-b. While the ballis held, the user can select any color from a primitive or theball according to the position of the work. Figure 7-c shows
an example of selecting color from a primitive, Figure 7-dshows an example of selecting color from the palette ball.
Once the user push the picking button, the current color isdecided and the small cube colored the decided color
appears at the tip of the work. Then the user can color
primitives as shown in Figure 7-e to 7-h until the small cubeor the palette ball is released.

Y+

a.

e.

Z-

Y+

X-

X+ X+

Z+

ZY-

Z+
5cm
X8cm

8cm

Y-

Figure 8: An initial state of the block used for theexperiment.
b.

c.

f.

g.

One hand pick

Separative pick Cooperative pick

A.

B.

C.

Figure 9: Three ways for picking primitives in theexperiment.

d.

h.

Figure 7: An example of coloring primitives.
Other two-handed operations
A picked primitive can be discretely scaled using two hands.
For example, a white primitive is being scaled in Figure 2. In
addition to cooperative two-handed operations, all onehanded operations can be performed concurrently by left
and right hands.
EXPERIMENT EXAMINING THE EFFECTIVENESSOF TWO-HANDED INTERACTION
An experiment was conducted to evaluate the effectivenessof the two-handed interaction [13]. All activity in a virtualenvironment is based on a sequence of position-selecting
task. For example, assembling task is performed by selecting
a picking and a releasing points. A position selecting task is
performed by three ways in VLEGO, by one hand, by two
hands concurrently and by two hands cooperatively — acooperative assembly, a decomposition and a color selection
from the palette ball are example tasks using two-handed
cooperative point selection. As a consequence of this, inorder to validate the effectiveness of two-handed interaction
with VLEGO, the experimental task should be designed thatit can be performed with these three ways under the same
criterion.
Taking account of this consideration, the experimental task
was designed as follows: A block composed of 72 primitives
as shown in Figure 8 was presented to subjects, and 18 of 72
primitives are colored red at random location while the rest
are colored white at every trial. All 72 primitives can be seen
by moving head position at the initial location. Eachprimitive turn its color red to white or white to red whenclicked. Subjects were asked to click up all red primitives ofthe block so that the entire block turn out to be white by
three ways as shown in Figure 9:A. click up by one hand, theblock doesn't move. B. click up by two hands, the block
doesn't move (separative operation). C. click up by one
hand, and the block can be moved with 6DOF by the other
hand (cooperative operation).
Table 3 shows the mean task completion time, variance anderror rate for the experiment. Here, error rate is a ratio of thenumber of clicks coloring white to red to the least number ofclicks to be needed to perform the task (namely, 18). The
analysis of variance about task completion time have shown
the effectiveness of two-handed operations in position
selecting with VLEGO ( = 0.05 ). Separative operation
surpasses one-handed operation because each hand canmanipulate different primitives simultaneously. Cooperative
operation surpasses one-handed operation because
following three reasons: Firstly, the composed block can be
moved with 6DOF so that the red primitives can be easilyfound. Secondly, as the composed block can be moved, thesubjects have to move so slight their heads and the otherhands that they can click the targets quickly. Finally, these
two factors can be done concurrently. As a matter of fact,

Table 3: Mean task completion time, variance and error
rate for the experiment.
Manipulatingstyle
A. One-handed

Mean task completiontime (Secs.)

Variance

Error rate

31.7

11.40

0.096

B. Separative
two-handed
C. Cooperative
two-handed

27.1

9.29

0.102

26.8

14.41

0.065

most subjects reported that this manipulating style is theeasiest to perform the task among three manipulating styles.
The error rate in cooperative two-handed style is improvedin contrast to the error rate in one-handed style in certain
extent. This implies one of the advantages of the cooperative
two-handed operation, facilitation of point selecting. On theother hand, The error rate in separative two-handed style is
not improved in contrast to the error rate in one-handedstyle.
CONCLUSIONS AND FURTHER WORKVLEGO has demonstrated that flexible two-handed
interaction based on actual 3-D designing activities
enhances the usability of the virtual modeling environment.
For example, it took only twenty minutes to create theschool buildings of NAIST composed of 368 primitives
shown in Figure 2. This high level of performance results
partly from the separative and cooperative use of both hands
as shown in the experiment for examining the effectivenessof the two-handed interaction. Being combined with proper
constraints on the arrangement of virtual objects andcollision avoidance among them, VLEGO offers natural andquick ways to create 3-D objects in the virtual workspace.
Despite of the limited modeling ability, VLEGO is useful inapplications in which 3-D objects have to be created tounderstand their appearance in a short time. Possible
applications of VLEGO include architecture, education,
training and amusement.
As further studies of virtual designing workspace employing
simultaneous use of multiple hands, we focus on thefollowing two themes for development and evaluation. One
is on designing methods such as virtual clay that have highability for creating complicated or freeform 3-D objects
using two-handed interaction. The other concerns
supporting methods for manipulating virtual objects withmultiple users.
ACKOWLEDGEMENTS
This work is partly supported by Grant-in-Aid for Scientific
Research under Grant Nos. 08234218 and 08555103 by
Ministry of Education, Science and Culture. We also wish to
thank the VRST '96 paper reviewers for their thoughtfulcomments.
REFERENCES
1. Butterworth, J., Davidson, A., Hench, S. and Olano, T.
M.: 3DM: A Three Dimensional Modeler Using a
Head-Mounted Display, ACM SIGGRAPH: Proc.
ACM Sympo. on Interactive 3D Graphics (GI '92),
135-139.
2.

Halliday, S. and Green, M.: A Geometric Modelingand Animation System for Virtual Reality, Proc. ACM

Conf. on Virtual Reality Software and Technology(VRST '94), 71-84.
3.

Sachs, E., Roberts, A. and Stoops, D.: 3-Draw: A Toolfor Designing 3D Shapes, IEEE Computer Graphics &Applications, 11, No.6, 18-26, Nov. 1991.

4.

Shaw, C. and Green, M.: Two-Handed Polygonal
Surface Design, Proc. ACM Sympo. on User Interface
Software and Technology (UIST '94), 205-212.

5.

Hinckley, K., Pausch, R., Goble, J. C. and Kassell, N.
F.: A Survey of Design Issues in Spatial Input, Proc.
ACM Sympo. on User Interface Software andTechnology (UIST '94), 213-222.

6.

Kabbash, P., MacKenzie, I. S. and Buxton, W.: Human
Performance Using Computer Input Devices in the
Preferred and Non-Preferred Hands, Proc. ACM Conf.
on Human Factors in Computing Systems (INTERCHI'93), 474-481.

7.

Kabbash, P., Buxton, W. and Sellen, A.: Two-Handed
Input in a Compound Task, Proc. ACM Conf. on
Human Factors in Computing Systems (CHI '94),
417-423.

8.

Buxton, W. and Myers, B. A.: A Study in Two-Handed
Input, Proc. ACM Conf. on Human Factors in
Computing Systems (CHI '86), 321-326.

9.

Bolt, R. A. and Herranz, E.: Two-Handed Gesture In
Multi-Modal Natural Dialog, Proc. ACM Sympo. on
User Interface Software and Technology (UIST '92),
7-14.

10. Mapes, D. P. and Moshell, J. M.: A Two-HandedInterface for Object Manipulation in Virtual
Environments, PRESENCE: Teleoperators andVirtual Environments, Vol.4, No.4, 403-416, 1995.
11. Hinckley, K., Pausch, R., Goble, J. C. and Kassell, N.
F.: Passive Real-World Interface Props for
Neurosurgical Visualization, Proc. ACM Conf. on
Human Factors in Computing Systems (CHI '94),
452-458.
12. Bier, E. A., Stone, M. C., Fishkin, K., Buxton, W. and
Baudel, T.: A Taxonomy of See-Through Tools, Proc.
ACM Conf. on Human Factors in Computing Systems(CHI '94), 358-364.
13. Kiyokawa, K., Takemura, H., Katayama, Y., Iwasa, H.
and Yokoya, N.: VLEGO - VirtuaL Environment for
Generating Objects, Proc. of SICE HI Sympo., 375380, 1995 (in Japanese).