Vol.4 No.4 pp.120-127

芸術科学会論文誌

Visual Simulation of Hand Touching Introducing
Elastic Deformation and Color Change
Youhei ISHIGURO , Tsukasa KIKUCHI
Takushoku University
E-mail: {yishigur ,tkikuchi }@id.takushoku-u.ac.jp
†

†

††

††

Abstract: The hands are the most characteristic part of the human body from the standpoint that they are related
to other elements through actions like grasping and touching. Several elaborate animation techniques for thehand with 3DCG have recently been proposed, but they are not good enough for simulations of hand touching,which requires simulation of complicated actions and position-dependent elastic forces in the hand. In thisreport, an algorithm is proposed which the user can operate intuitively for the deformation and the color changeof the hand induced by touching. An outline of the algorithm is as follows; 1) elastic deformation, depending on
the position of the hands, is controlled by channel images, and 2) color change is expressed by synthesizing threekinds of texture images.
Key words: Human hand, Elastic deformation, Color change, 3DCG simulation
1. Introduction
With the recent progress of 3-Dimensional Computer
Graphics(3DCG) techniques, images using computer
animation have been employed in various fields. In
particular, because of its high information density, 3DCG
has been utilized multi-directionally. The production ofcomputer animations requires enormous time and labor,
as it requires composition of a scene at each moment intime and its continuous display of that scene. Thus,
various types of simulation techniques to automatically
produce object movement have long been desired. 3DCG
expression techniques for the human hand are needed inmany fields[1], for example in medicine, as simulations
for surgery, in education, as tools to learn techniques
using the hands, such as sign language or playing musicalinstruments, and in the field of equipment design, as tools
for the virtual evaluation of digital mockup-human
compatibility. In entertainment, in particular, it is necessary to build up models with a high degree of freedom
together with accurate dynamic simulations to reflect theintent of the makers.
In the hand-touching simulation proposed in this work,
the extent of the deformed region induced when objects
expressed by a spring model overlap can be visuallycontrolled, and positional variation of the elasticity in thehand can be introduced by channel images. Based on the
resulting simulation data, a color change effect can also
be introduced by synthesizing three kinds of textures,
leading to a hand-touching simulation technique whichenables intuitive control with a high degree of freedom.
2. Previous studies
As computer processing speeds continue to increase, theexpression of the human form in virtual spaces is attracting interest from many fields. In the entertainment field,
for instance, human hands are typically expressed withintricate detail in movies, games and commercial films[2].
However, because these representations do not express
skin elasticity by means of touching (as the techniquedescribed in this paper attempts to do), they look rather
like the hard hands of mannequins. One of the mostwidely used methods for expressing the elasticity ofmatter is the spring model[3]. The method proposed inthis paper also uses a computation technique that applies
the spring model. Recently, some elasticity models for use
in real-time simulations of deformation have been
proposed[4], [5], [6], [7]. Among the techniques that havebeen proposed to simulate hand-joint motions are thefinite element method[8], the anatomical skeleton andvirtual muscle model[9], the example-based deformation
technique using medical images[10] and a model thatsimulates the action of grasping[11]. However, since thesetechniques adopt bottom-up type algorithms, they cannot
be applied directly to actual graphic representations, thus
limiting the freedom of the animator. Moreover, when the
purpose is to produce high-quality images, it is not-120-

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always necessary that the elasticity model run in real time.
Therefore, the method we are proposing puts the emphasison expanding the range of expressions available to animator rather than on enabling real-time generation ofanimations.
Since animators usually employ the key frame method —which generates animations by moving objects visually on
the screen, the method proposed here functions like atouch-up effect. It is added to the user-produced animation
through a process similar to interpolation, rather thanbeing specified with parameters.
A number of methods have also been proposed for changing the surface shapes. These include a technique thatgenerates wrinkles along the vector field[12], one that generates wrinkles from a cubic Bezier curve using ageometric model[13], and one that deforming the skinmodel by means of muscles and expresses the shapes of
wrinkles as functions according to the deformation of theskin model[14]. However, while these techniques express
fine wrinkles or wrinkles on the muscular structure below
a simple skin surface such as the face, they do not consider
the mechanism of formation of wrinkles specific to thepalm.
Several techniques have also been developed to express
the colors and textures of the skin, including a techniquethat expresses texture by referring to the complicated interplay of multiplex reflections of light on the microscopic
structure of the skin surface[15]. Other techniques include
a technique for expressing colors based on the pigments
contained in skin[16], a technique for expressing bothcolors and textures using a volume model[17] and ananalysis/synthesis technique that extracts hemoglobin andmelanin information based on actual measurement
data[18]. However, there have been no attempts to simulate color changes caused by pressure on the skin such as
touching, as we are proposing. On the other hand, there arereports on the change in pigment component distribution
due to hemostasis and the results of measurement of reactions against load on the hand by measuring the variationin the oxygen saturation under anaerobic motions[19]. The
method proposed in this paper makes it possible to express
color change effects according to the intention of the user,
and is also capable of simulation using measurement results.

data input
setting of elastic force
calculation per frame (time)
calculation per vertex
detection of collision
deformation

restoring

deformation by springvertex animation output
Fig.1: Outline of deformation simulation(Fig.2). The extent of the intersecting region, indicating
the magnitude of the collision of the objects, can be controlled by the user. Also, the extent of the deformation of
the spring and the elastic force at each vertex(section 3.3)
can be arbitrarily controlled. Next, if the objects are colliding with each other as described in Fig.3, the point ofintersection of the inverse vector of the normal line vector
at the vertex i with the other object is defined as collision
point j. Using the movement vectors in the user-input animation, Mi for vertex i and Mj for the collision point j,length to the surface of target object L, a force vector applied on point i will be obtained by

(

L

)

1

.

By obtaining the point of intersection of the inverse
vector of normal line vector at vertex i with the otherobject using the vector fi obtained from eq.1, the positionof the vertex i is determined. Also, as shown in Fig.4(a),when moving approximately perpendicular to the normalline of the object at the time of data input, the user canchoose whether to move by pushing the collision peak as
in Fig.4(b), or by restoring the elastic body as in Fig.4(c).
In the case of Fig.4(c), the force vector, fi can be defined
by

(

L

3. Algorithm
3.1. Simulation of deformation
Fig.1 shows the outline of the algorithm. First, animation
data, where mesh objects (partially intersected), are input

)/|

|

,

2

where Ni represents a normal vector at the vertex i. If bothobjects are elastic, the displacement is derived from thecomparison between the elastic forces at the vertex i and-121-

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Vol.4 No.4 pp.120-127

Fig.2: A simple example of two bodies making contact without collision
detection and deformation

(a)

(b)

Fig.6: Spring model calculation technique used by the proposed
method. Which was initially in position j’, comes to position j, the force
imposed on vertex i can be calculated as fi using formula (3).

the calculations for all the vertexes, forces generated by
the springs are added to the vertexes aroundthe deformed vertex (section 3.2). By outputting thesecalculations as a vertex animation, an animation of thedeformation of elastic matter can be produced (Fig.5).
3.2. Spring Model
Spring models are widely used for motional simulationsof elastic matter [3]. The models consist of mass vertexes
and springs linking the vertexes. As shown in Fig.6, whenthe vertex that has initially been in position j' moves toposition j as a result of collision, a force fi, acting onvertex i is calculated from the following equation (3),using the relative position vector Pij between the vertex iand the collision vertex j, the natural length between thetwo vertexes Lij (length with input animation), the userset elastic coefficient Ki and the attenuation coefficient Di
at vertex i.

(c)Fig.3: The two big circles represent the objects that are colliding, andcalculations are performed on vertex i. Assuming that the point of intersection between the negative vector from vertex i to normal line Ni andthe object is point j (a), the movement vector Mi,Mj caused by the userinput animation between vertex i and intersecting point j is obtained (b).
Then, deformation vector fi due to collision at vertex i is determinedbased on formula (1) (c).

(a)

(b)

(c)

Fig.4: The hashed area represents the elastic object, the polygonal lines
represent the object edges, the dot represents the vertex, the rectangle
represents the collision target object, the thick arrow represents the
movement of the target object and the thin arrow represents the
movement of the vertex. When movement is in the direction
perpendicular to the normal line of the elastic object (a), the user
selects whether the movement pushes the colliding region onto theelastic object (b) or moves along it as if tracing its surface (c). When (c)is selected, formula (2) is used in the deformation calculation.

3
In the algorithm proposed in this paper, the user canarbitrarily set the area influenced by the spring force atthe time of data input. For example, if a load is applied to
the fingertip, the simulation can be bound to the joint, andif a load is applied to a region of the palm, the simulation
can be bound to the whole palm. Such a setting cangreatly save computation time, and can also suppress
deformations not expected by the user. However, limiting
the region affected by the spring force leads to someinconvenience since application of the force will be
discontinuous at the region’s boundary. Thus, to avoid a
gap between the two regions, a weight function of the
distance from the vertex j to each vertexes, which is 1.0 at
the vertex j and 0.0 at the longest distance within the area
influenced by the spring force, is introduced. This isdefined as the attenuation coefficient, D.
3.3. Elastic Displacement
Channel images are used to interactively control thevariation of the elastic force depending on the position in
the hand. A 2D channel image is prepared for a handmodel developed as shown in Fig.7 or a parallel-projected

(a)

(b)
Fig.5: Result of deformation simulation using the animation shown in
Fig.3(a) shows the deformation of an elastic object (the red sphere)
when it collides with a rigid body (the wire frame) (b) shows theresulting deformation when both objects colliding are elastic.

the collision point j, leading to the determination of theposition. When there is no collision and the objects are
deformed, force is applied to restore them to their original
shape. The amount of restorative force applied can be set
arbitrarily by the user during data input. After finishing-122-

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Vol.4 No.4 pp.120-127

mesh object (Fig.7(a)). In the simulation of touching, thebrightness (0.0-1.0) at the corresponding points from eachvertex is determined. This is defined as the elastic coefficient K. Currently, the most common technique forexpressing the hand in image production is animator to
draw a texture map on a 2D image that is usually
expanded by means of parallel projection or cylindricalprojection. This fact suggests that drawing the elastic
channel is also a relatively easy and intuitive task foranimators. Using this, it is possible to intuitively control
partial hardening, e.g., for the nail, and the formation of
wrinkles when external forces are applied[23]. In Fig.8,
the nail area is regarded as an image with a brightness of
0, indicating that the nail is a non-deformed element.
Fig.9 show that the wrinkles induced by external forces(Fig.9(d)) can be expressed by drawing lines on the hand

(b)

(a)

(c)

(d)
(a)
(b)
Fig.7: (a) Parallel projection of the mesh model of hand.
(b) Color map image of the hand model used in the figures.

(a)Fig.9: Examples of lines drawn on the channel image. Similarly to
Fig.8, (a) shows the channel image, (b) shows an example of mapping,(c) shows the animation without elastic displacement and (d) shows theanimation with elastic displacement. These figures show the possibilityof intuitive control of wrinkles formed by a force.

(b)

(b)

(c)
(a)
(d)
(c)

(e)

(d)
Fig.8: Examples of how the elastic channel is used. (a) shows the elastic
channel image, (b) shows an example of mapping on an object, (c)shows the animation without the elastic displacement, and (d) shows an
animation to which elastic displacement has been applied.

(f)Fig.10: Examples of comparison with actual image. (b), (c) and (d)show the cross-sectional shapes estimated at the respective positions,(e) shows the map image for representing the status of (a), and (f)shows the result of simulation using (e).

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in the channel images (Fig.9(a)). The technique of forming wrinkles by drawing lines may seem to resemble thetechnique proposed by Bando et al.[13], but thistechnique expresses the wrinkle shapes exclusively using
the shape shown in Fig.10(d). Nevertheless, sincewrinkles on the hand, particularly the palm, have a large
variety of form variations as shown in Figs.10(b) and (c),some flexibility is required for their shapes as well as
their positions and depths. This will make it possible toachieve more finely detailed expression by specifying thewrinkles by means of images rather than using line dataonly.
3.4. Color Change Simulations
The visible phenomenon caused by the touch of human
hand includes color change as well as deformation. Skin

(a)

(b)

color is determined mainly by the concentrations of melanin and carotene contained in the skin, the concentrationof hemoglobin in the capillaries, and the oxygen saturation (that is, the percentage of oxidized hemoglobin in the
total hemoglobin[16]). With regard to the effects of hemostasis on the skin, there is no reported change in melanincontent, but hemoglobin concentration and oxygen saturation vary noticeably[19]. This suggests that skin color
changes caused by a load such as touching can be classified as a change in the hemoglobin concentration due topressure on the skin. A yellowish color change would be
due to a drop in the oxygen saturation caused by a change
in the hemoglobin concentration due to pressure on the internal capillaries, while a reddish color change would be
due to a rise in oxygen saturation. Consequently, thispaper proposes a technique for expressing color changes
due to skin touching by synthesizing three kinds of textures as described below.

(c)Fig.11: Three kinds of synthesized textures used in Figs.12 and 13. (a)shows the basic texture, (b) the texture of the colliding region and (c) the
texture around the colliding region.

(a)
(a)

(b)

(b)

(c)

(d)
Fig.12 Result of color change simulation using the texture shown in
Fig.11: (a) and (b) are the same images rendered from different angles,
and similarly for (c) and (d). In (c) and (d), the texture areas of the colliding region are set more widely than those in (a) and (b), and the texture
areas of the peripheral regions are set more narrowly.

(c)Fig.13: Animation showing the recovery from the status of Fig.12 to thebasic texture. (b) is set to take longer to return to the basic texture than(a), and (c) is set to take less time.

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The outline of the algorithm is as follows. At first, threekinds of textures are prepared (Fig.11), the basic texture,
the texture of the colliding region, and the texture aroundthe colliding region. The user can arbitrarily set the area
where each texture appears. Then, the collision peak is
obtained from the simulation data, and the peak color isdetermined using the values set at the collision vertex andthe peripheral vertexes. Lastly, using these data, channel
images synthesized from the three kinds of textures areprepared to produce an animation with the color change
as illustrated in Fig.12[23]. When the colliding object isseparated from the target, the colliding region maintains
the collision texture for a certain period, but then changes
to the surrounding texture and eventually returns to theoriginal basic texture (Fig.13). This represents the phenomenon in which hemoglobin concentration and oxygen
saturation rise significantly as a result of the sudden
blocking of blood flow by pressurization. The duration ofeach texture can be set as desired by the animator.

(a)

(b)

(c)
Fig.16: Comparison of color change animation with an actual image sequence. (a) shows the actual sequence, (b) shows the three kinds oftextures use in reproduction, and (c) shows the color change animation
using textures (b).

comparison of the color change simulation sequence withan actual image. The actual color change of the hand is
not as uniform as shown in Fig.12, but the proposed
method employs the texture synthesis technique so it iscapable of producing complex color change animations as
desired by the animator.

4. Simulation Results
Fig.14 shows results of simulation using the algorithm
described above[23]. Figs.14(a), (b), and (c) show an
original animation, its deformed version, and version witha color change, respectively. These images show that the
proposed method can effectively express the reaction totouching for joint motion with a high degree of flexibility.
The number of vertexes is 2495 for these hands. Eachstep in the simulation is assumed to be 0.02[s] and computation takes about 20.0[s] using a machine with Xeon
3.2GHz with 3.0GB of memory. Since the primary goalof this method is not to create a real-time model, but toimprove the user’s freedom and control and to ensure
maximum image quality, it does not take full advantage
of increased computation speed. As a result, there is stillroom for further improvement in terms of proper utilization of computation speed. Although Fig.14(d) shows anoverstated hand, the expression of touching via the complex interplay of associated elements should be effective
not only for animation of real human characters , but also
for overstated characters. This algorithm can be applied
not only to the human hand but also to any other elastic
matter. Moreover, the expression technique introduced
here, which expresses the deformations and color changes
due to touching, can also be applied in fields other thanentertainment. For example, Fig.15 shows images as tools
to learn techniques for playing guitar. A user will be able
to better understand by seeing both the front view(Fig.15(a)) and the back view (Fig.15(b)). Fig.16 shows a

5. Future tasks
In this paper[20], an effective technique for the representation of hand touching has been proposed, and results ofthe simulation have been reported. These results show thatthe algorithm proposed enables operation with a highdegree of freedom for the user, and is effective for visual
simulations of the hand. However, improvement of the following points is necessary.
1. When the objects are excessively intersected, they dent
deeply. Thus, there exists a limit to the magnitude of thecollision in order to avoid their penetration.
2. When multiple parts collide simultaneously, it requires a
great deal of labor to individually set parameters such ascolor change.
The skeletal subspace deformation technique[21], used forvarious kinds of software at present, is not accurate anatomically, and thus not suitable for the deformation of thehand[10]. On the other hand, when hand deformation islooked from the viewpoint of collision, the skin collides toexpand inside the deformation, and the collision between
the skin and bone can emboss the shape of the bone outside
the deformation. By simulating them and applying a color
change effect due to pressing and extrusion, an algorithm
enabling intuitive control and precise expression can be expected. A study regarding restriction of the joint movement-125-

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Vol.4 No.4 pp.120-127

to actions occurring simultaneously has been reported[22],and we have examined the effect of the extreme intersection mentioned in the preceding chapter to the joints. Thus,
considering these previous studies, we are planning to develop a more advanced algorithm.

Graphics & CAD joint symposium 2001, pp.7-12, 2001, inJapanese.
[13]Y.Bando, T.Kuratate and T.Nishita, "A Simple Method
for Modeling Wrinkles on Human Skin", Pacific Conference on Computer Graphics and Applications, pp.166-175,
2002.
[14]Y.Wu, P.Karla and N.M.Thalmann ”Physically-based
Wrinkle Simulation & Skin Rendering.” Eurographics
Workshop on Computer Animation and Simulation ‘97,
pp.66-79,1997.
[15]T.Ishii, T.Yasuda, S.Yokoi, J.Toriwaki, ”Human Skin
Rendering Based on the surface Microstructures”, Proceedings of Nicograph 1991,pp.68-76, 1991, in Japanese.
[16]H.Nakai, Y.Manabe, S.Inokuchi, “Simulation and
Evaluation of Human Skin-Like Quality”, IEICE TRANSACTIONS D-II, J84-D-II No.2, pp.321-327, 2001, in Japanese.
[17]T.Kawai, Y.Kurioka, “Volumetric Visualization of
Human Skin”, The transactions of the Institute of ElectricalEngineers of Japan, C,123,11, pp.1936-1943, 2003, in Japanese[18]N.Tsumura, et. al., “Image-based skin color and texture analysis / synthesis by extracting hemoglobin and melanin information in the skin”, Proceedings of SIGGRAPH
2003, pp.770-779, 2003.
[19]D.Nakao, N.Tsumura, Y.Miyake, “Real-time multispectral image processing for mapping pigmentation inhuman skin”, Medical Imaging Technology Vol.20 No.2,
pp.120-133, 2002, in Japanese[20]Y.Ishiguro, T.Kikuchi, ”Visual Simulation of Hand
Touching Introducing Elastic Deformation and Color
Change”, Proceedings of Nicograph International 2005,
pp.55-60, 2005[21]N.M.Thalmann, R.Lperriere and D.Thalmann, "Jointdependent local deformations for hand animation andobject grasping",Graphics Interface '88,pp.26-23, 1988
[22]G.ElKoura, K.Singh, ”Handrix: Animating the HumanHand”, Proceedings of Eurographics/SIGGRAPH Symp.
on Computer Animation 2003, 2003

References[1]Foundation Machine System Promotion Society, "Investigation Research Report Concerning Getalhand Technology", System technology development investigation research, 15-R-2,
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HQLnews, No.1059, 2004, in Japanese.
[2]The ANIMATRIX http://www.intothematrix.com/, 2003.
[3]D. Terzopoulos, J. Platt, A. Barr and K. Fleisher, “Elastically Deformable Models”, Computer Graphics, 21 (4),pp.205-214, 1987.
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SIGGRAPH 99, pp.65-72, 1999.
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Materials”, Eurographics 2001 Computer Animation and
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[6]M.Kato, Y.Sakamoto,“Generation and Transformation
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Appendix[23]Sample movies~
http://www.id.takushoku-u.ac.jp/ tkikuchi/nicointer2005/-126-

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(a)

(b)
Fig.15: Animation of a handplaying a guitar. (a) shows
the frontal view and (b) shows
the rear view by turning theneck semi-transparent.

(b)
(d)
(a)
(c)
Fig.14: Examples of application of the algorithm to the hand model. (a) shows the user-input animation before
simulation, (b) shows the result of deformation simulation, (c) shows the result of color change simulation applied to (b),and (d) shows an example of application to the deformed hand model.

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