Simulation of Skin Aging and Wrinkles with CosmeticsInsight

Laurence Boissieux*, Gergo Kiss*, Nadia Magnenat Thalmann*, Prem Kalra+

* MIRALab, CUI, University of Geneva24, rue du General-Dufour, CH-1211, Geneva, Switzerland

Email:{boissieux, kiss, thalmann}@cui.unige.ch+ Visiting from Department of Computer Science and Engineering,

Indian Institute of Technology, Delhi, IndiaEmail: pkalra@cse.iitd.ernet.in

Abstract

This paper briefly reviews the existing approaches employed in computer animation for skinmodeling, wrinkle formation and aging process and proposes our approach. Two models havebeen proposed, one is image-based for rapid prototyping and instant results and another ismodel based an extension to our earlier work. In the model-based technique skin is consideredas a volumetric substance, as opposed to an elastic membrane, having layers of differentmaterials and a finite element method is used for computing the deformation.

The ultimate aim is to devise a practical system, which can enable modeling of skin of anindividual person using empirically acquired biomechanical parameters such as YoungModulus. Further, the system is capable to simulate the visual effect of external treatment onthe skin, e.g., use of the cosmetics, and exposure to the sun. An application software to thiseffect has been developed in collaboration with L’Oreal, a renowned cosmetic producer.

Keywords: Skin aging, wrinkle simulation, skin deformation, facial cosmetics.

1 Introduction

The skin is a continuous external sheet that covers the body. Due to its outsidevisibility and aesthetic value people tend to give a lot of attention to skin. It is achallenging task to accurately model skin appearance and its behavior with details.This has variety of applications from entertainment to cosmetics, to plastic surgery.Though the problematics of skin modeling and aging holds for the entire body, aparticular attention is given to the facial skin as face being the most important bodycomponent. Facial features including skin form essential elements to recognizeindividuals, interpret facial expressions, and communicate with others. Furthermoreappearance of wrinkles due to facial expressions and aging add realism to themodeling and the animation.

This paper focuses on the skin simulation concentrating on both the visual andbiomechanical aspects of the skin. First we give some background on the skin andwrinkle physiology, which we believe is necessary to study and analyze for devisingan appropriate computational skin model. Next, we give the related work done in skinmodeling and simulation of wrinkles in the domain of computer animation and

simulation. Our approach towards skin and wrinkle simulation is given in Section 4.We address both image and model based methods employed to this effect. Finally, weconclude with future work.

2 Skin and Wrinkle Physiology

Though our intention here is not to model and simulate the exact biological form andfunctions of human skin, it is, however, important to study and analyze skin’sphysiology to determine the relevant properties that are necessary for realistic skinmodeling and simulation.

2.1 Skin Composition

The skin accounts for about 16% of the body weight1. It has surface area of typically1.5 to 2.0 m2 in adults and its thickness from 0.2 mm (eye lids) to 6.0 mm (sole offoot). The skin consists of three layers: the epidermis, dermis and hypodermis. It isobserved that the general appearance of skin and the wrinkles and other lines aredetermined by the combined effect of the three layers.

2.2 Skin Surface Structure

The outer skin surface consists of a geometrical structure that manifests the form ofvisible skin. A close-up of the skin surface depicts a common micro structure with arather well defined geometrical form resembling a layered net-like pattern. On theother hand, the visible lines, wrinkles, creases and folds constitute a distinct macrostructure that may be specific to one part or region of the body.

2.3 Mechanical Properties of Skin

The important mechanical properties of the skin are extensibility, resistance tofriction, and response to lateral compressive loading1. Skin properties vary withspecies, age, exposure, hydration, obesity, disease, site and orientation. The othermaterial properties of skin are: non-linearity, anisotropy and visco-elasticity,incompressibility and plasticity2.

2.4 Wrinkle Physiology

Skin changes with age, wrinkles emerge and become more pronounced. Wrinklesdepend on nature of skin and muscle contraction. Wrinkles are most important macrostructures. Two types of wrinkles are considered: expressive wrinkles (particularlyrelevant for the face) and wrinkles due to age. Expressive wrinkles also referred to astemporary wrinkles that appear on the face during expressions at all ages and maybecome permanently visible over time. In addition to their visual effects expressivewrinkles act as an important factor for understanding and interpreting facialexpressions, and permanent visible wrinkles indicate the age of a person.

3 Related Work

Though skin simulation is not restricted to the face, we consider face to be animportant body part for its various role in identification, communication andbeautification. We give some related work in the simulation of facial skindeformation. Varied models are used to simulate facial animation and skindeformation for different purposes3. These are geometric models, physically-basedmodels and biomechanical models using either a particle system or a continuoussystem. Many geometrical models have been developed, such as parametric model,4 5

geometric operators6 and abstract muscle actions.7 There are also different kinds ofphysically-based models, such as the tension net model8 and the three layereddeformable lattice structure model.9 10 The finite element method is also employed formore accurate calculation of skin deformation, especially for potential medicalapplications such as plastic surgery.11 12 13 Some work based on finite element methodhas also been reported on the internet,14 15 however, not much details are given.

Many research efforts have been undertaken for generating textures for animal skin aswell as human skin. Bump and color mapping techniques,16 texture synthesislanguage,17 face data recording18 and a micro geometrical model19 are used tosimulate different skin patterns by texture.

There is a few facial animation models with dynamic wrinkles. Viaud et al.20 havepresented a geometric hybrid model for the formation of expressive and agedwrinkles, where bulges are modeled as spline segments and determined by ageparameter. There are also physically-based facial animation models, where somewrinkles appear as the outcome of the skin deformation.9 21

4 Our Approach

One can envisage two main categories of modeling skin with wrinkle formation andaging: image based method and model based method. In image based method, theimage is transformed using image-warping and other image processing operations forchanging luminance and coloration to give impression for the wrinkles and otheraging artifacts. The evolution of such transformation would generate the effects ofaging and formation of wrinkles on the image. This image can be texture mapped on a3D model. In the second approach a 3D model is deformed using an appropriatetechnique –geometric or physically based. This method though computationallyexpensive can give more accurate results in terms of deformations to thegeometry/structure of the skin. As follows we give our approach for skin simulationused in these two categories at MIRALab.

4.1 Image Based Method

Here, some generic masks of pre-computed wrinkles are applied as textures on a 3Dmodel of a face. The idea is to darken with a certain amount the color of initial skintexture to give the impression of wrinkle depth, i.e., the luminance of pixels ismodified. The pixel intensity and color are associated with the aging parameters (age

and cosmetics). The motivation here was to be able to visualize instantly the effect ofthe use of skin care products on the facial model of a particular person. This however,does not include the other morphological changes on the face as a consequence ofaging. The method is based on image/texture fitting and mapping on the facial model.

4.1.1 Definition of Generic MasksThe generic masks constitute different types of wrinkle sets to customize for aparticular person with different features. After the analysis of the qualitative data fromL’Oreal, 8 basic masks are generated corresponding to the following criteria:

• gender (male or female)• shape of the face (round or long)• expression (often smiling or not)

The gender determines wrinkles specific to a particular gender, for example, femaleshave vertical wrinkles above the mouth region. The shape of the face also plays a rolefor wrinkle features: for a round face, the wrinkles are deeper, shorter, and less innumber compared to a long face. Concerning expressions, a very smiley person willhave more pronounced wrinkles around the eyes and the mouth than an expressionlessface. The design of these 8 basic masks is done on the basis of the extreme case, i.e.,they correspond to the maximum age (80 years) and without any anti-age cosmeticcorrection. Figure 1 shows the 8 masks. These masks have been generated usingsamples from the real photos of the aged people.

Figure 1: The eight generic masks.

4.1.2 Data CorrelationL’Oreal provided both the qualitative and quantitative data, which has been used inour simulation process. For example, the data consists of the relative change in thebiomechanical parameters like Young modulus, and skin viscosity. In addition, thedata is provided in terms of change in wrinkle intensity when a particular cosmetic

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

(e) (f) (g) (h)

product is employed. The qualitative data about the amount, shape and intensity ofwrinkles with respect to the gender, facial shape and expression was used for thedesign of the eight generic masks described in the section above. Data was alsoprovided for the depth of wrinkles with age,22 a least-square fitting is used forobtaining a linear relation. Thus, a simple relation as follows is derived between theage and the wrinkle depth.

Wrinkle Depth = 2.74 * Age

The wrinkle’s depth changes with the use of cosmetic products such as Mexoryl andRetinol. This data is also provided and used for the simulation process.

4.1.3 AgingThe process of aging is simulated considering the linear relationship between the ageand the wrinkle’s intensity as derived above. For relative measurements, at age 80, theextreme case with maximum intensity of wrinkles is conceived. Thus, if a face is to begenerated which has the age of 55, it will be deduced by using the wrinkle parametersfor the age of 55. Figure 2 (see color plates) shows faces of the age 25, 55 and 80.This is applicable to all the eight generic masks.

Wrinkles seem to delineate with the use of cosmetic products. As mentioned above, arelationship between the wrinkle’s depth and a particular cosmetic product isestablished based on the data provided by L’Oreal. This can be visually simulated in astraightforward manner, a method of visual inspection can be employed to validatethe simulation. Figure 3 (see color plates) shows the result of applying Mexoryl to theface of the age 55.

4.1.4 Customizing to a specific person (cloning and aging)One of the interesting applications of this approach is to apply the simulation processto a specific person. This requires first to obtain the model of the particular personusing the pictures or otherwise. A method has been developed at MIRALab for virtualcloning of people using two orthogonal views (front and side) of the pictures.23 Toapply the wrinkles on the specific model, we use local coordinate system defined oneach triangle of the mesh, which gives the location of point within a triangle. As thetriangle mesh has the same topology, getting the wrinkles on the modified face fromthe generic face is straight forward using the barycentric coordinates.24 Once it isidentified to which generic class the particular face matches, the wrinkles aregenerated on the selected generic face. The process of cloning and texture fitting arethen employed to obtain the wrinkles on the specific face. A convivial user interfaceis provided to perform these tasks. Figure 4 (see color plates) gives an example ofaging on a cloned face.

4.1.5 LimitationsThe image-based approach has some inherent limitations. The perceptible geometricaland structural changes cannot be modeled using this approach. Thus, for certainapplications this approach is not realistic. The current system simulates only thepermanent wrinkles due to aging, however, extension to the temporary wrinkles is

possible. The impact of biomechanical parameters cannot be incorporated directly inthe simulation process.

4.2 Model Based Approach

4.2.1 Elastic Membrane ModelIn an earlier approach for facial animation, a three-layered structure is employedconsisting of a skin layer, a connective tissue layer and a muscle layer25. Thedeformation of skin is activated by the simulated muscle layer, constrained byconnective tissue layer and decided by a biomechanical model. The skin representedas a triangular mesh, follows a linear, isotropic, elastic behavior. During the processof a muscle contraction, for every predefined wrinkle, the system constantly measuresthe shrinking of the skin surface along each wrinkle line. Considering one wrinkle, thedirection of measurement is locally perpendicular to its line at every point. Principalstrains are not computed, instead, plain and shear strains are measured along aspecific coordinate system for every triangle. This coordinate system is aligned withthe connected muscle's contraction direction. Skin's incompressibility is not modeledexplicitly, however, its effect is shown by increasing the amplitude of the nearbywrinkles. The wrinkle formation and rendering details are provided through color,bump or displacement texture mapping using a layered rendering process ofRenderMan.26 The texture images consist of synthetic patterns as well as real photos.The dynamics of wrinkle simulation is computed using the strain measures of skindeformation of the 3D facial model. Figure 5 illustrates the results of the facialsimulation and wrinkle generation.

Figure 5: Facial animation with wrinkle generation.

4.2.2 Proposed ModelThe previous model is relatively simple and adequately fast to be used for a practicalapplication. However, there are some limitations of the model, which may be relevantfor realistic simulations. For example, in the previous model, skin has no realthickness, it is basically modeled as an elastic membrane. The incompressibility is

treated in a 'loose' manner and the system relies on user inputs at many instances. Theproposed model is devised taking into account some of these issues. In the proposedmodel, we consider different layers in skin with given thickness and their mechanicalproperties such as elastic modulus and Poisson ratio. The model is intended to providethe different characteristics of wrinkles --location, number, density, cross-sectionalshape, and amplitude-- as a consequence of skin deformation caused by a muscleaction. The previous approach required specification of wrinkle lines with theirlocations. However, for a realistic simulation, wrinkles arise with all of theirproperties contributing to the equilibrium state.

Layered StructureSkin is considered as layers of different type of tissues having different properties asshown as a cross section in Figure 6. The multi-layer notion corresponds to thereality. For simplicity only two layers have been considered, where, the upper mostlayer modeling epidermis has more stiffness than the layer underneath. The layeredstructure give the notion of having substance to each layer and thus allows volumepreservation.

epidermis

dermis

subcutaneoustissue

fascia

Thin and stiff

(upper) surface layer

Thick and soft lower layer

(a) (b)

Figure 6: The structure of the proposed model

The behavior of tissue is controlled by elastic deformation. For a 2D cross section, thetriangle mesh is deformed using a similar mechanism as the surface membrane usedin the previous model. That is, each layer here is considered as a linear, isotropic,elastic material.

Wrinkle SimulationThe model allows simulation of both the temporary and permanent wrinkles. Asfollows we provide the basic concept for the simulation of the two types.

a) Temporary WrinklesAt present, simulations are performed on an abstract, simplified piece of skin. Theprocess of deformation does not use explicit definition of a muscle in the currentsimulation. The positions of the two ends, which may be achieved as the consequenceof a muscle action, act as input to the simulation. The upper surface layer responds to

this compression with bulging out of its original line, whereas, the underlying layerregulates this deformation. In other words, where the surface bulges up, theunderlying tissue stretches (extends vertically, shortens horizontally), and where thesurface bulges down, the underlying tissue squeezes (shortens vertically, extendshorizontally). These deformations appear in a periodic pattern, ending up with asinusoid like line of the surface as illustrated in Figure 7.

Figure 7: The concept of temporary wrinkle generation

Such a sinusoidal pattern shows not enough similarity with a wrinkle. The cross-sectional curve of real wrinkles has similar hills, but sharp valleys in contrast to thesesmooth ones. We achieve this more realistic type of wrinkle cross-section by using asinusoidal interface between the two layers (Figure 8). It is also observed in the realstructure of skin that the interface between epidermis and dermis is not flat, rather it isclose to a sinusoidal curve, see Figure 6(a).

Figure 8: Simulation result using sinusoid interface between the two tissues

b) Permanent WrinklesEvery triangle that the tissues consist of has a shape memory, i.e., its rest shape. Wemay introduce plastic effects in this model by constantly adjusting the rest shapes ofthe triangles based on the current deformations. This causes a slow adaptation todeformations. As a result, the overall shape of skin reflects its history. In addition, it isobserved that the wrinkles formed, naturally guide the location of future wrinkles.Figure 9 illustrates the influence of the plastic factor.

(a): Remnant wrinkles after contraction-decontraction when using a plastic factor

(b): Recontracting the plastic skin piece

Figure 9: Effect of plastic factor in the formation of wrinkles.

Experimental ResultsSeveral simulations have been performed with different parameters. Figure 10 givesan overview of the effect of the different parameters. Three parameters are consideredhere: Young_surf, the Young modulus of the surface (upper) layer, Young_under, theYoung modulus of the underlying layer, and thickness_surf, the thickness of thesurface layer. The thickness of the underlying layer is given a constant value of 1.3mm, and the Poission ratio of both layers is taken as 0.5. It was observed that changein the thickness of underying layer and the Poission ration do not give noticeabledifference in the results.

Smaller wrinkles emerge with lower values of the Young_surf. This is in accordancewith the real-life experience: hydrating cosmetics, decrease the elastic modulus ofepidermis and thus can lessen the wrinkles, whereas, skin having a higher Young_surfvalue (dry epidermis) wrinkles are more accentuated. Increasing the Young_underparameter flattens the wrinkles, while, its lower values give rise to wavy folds (rippleeffect) on the skin. The thickness_surf parameter seems to influence the number andsize of the wrinkles. A thinner surface layer produces denser and smaller wrinkles,because such a surface layer tends to bend more. In a thicker surface layer fewer foldsappear owing to low bend-ability.

Simulations verify that the model used is independent of the definition of theresolution and the total length of the skin.

Figure 10: Experimental results with various parameters.

5 Conclusion

In this paper we have presented our approach for skin simulation, wrinkleformulation, and aging on facial skin. Two methods have been presented: imagebased and model based. Image based method employs eight generic mask astemplates, which characterize wrinkles. A process of cloning is involved to customizethe template to a particular person. The image based method enables instantaneousdisplay of simulation results. In the model-based technique first we consider skin asan elastic membrane. Further, we extend it and consider skin as a volumetricsubstance having layers of different materials and a finite element method is used forcomputing the deformation.

The system allows simulating the visual effect of external treatment on the skin, e.g.,use of the cosmetics, and exposure to the sun.

For the proposed layered skin model, where the experiments have been performed onan abstract skin, extensions are being made to use the model the face. Computationallimitations of the method suggest to using hybrid approach of generating wrinkleswhere, rendering can be enhanced with texture mapping techniques.

Acknowledgements

Authors would like to thank L’Oreal, Paris for furnishing the data, which has beenused for the simulation.

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