Yin Wu

Biomechanical Models for Soft Tissue Simulation

An overview of biomechanical modeling of human soft tissue using nonlinear theoretical mechanics and incremental finite element methods, useful for computer simulation of the human musculoskeletal system.

Simulating wrinkles and skin aging

We describe a methodology and framework to simulate facial animation and skin aging, taking into account skin texture and wrinkle dynamics. We split the facial simulation process into facial surface deformation and wrinkle generation. The first of these is based on a three-layered facial structure (muscle, connective tissue, and skin layers). The layer of B-spline patch muscles provides the relevant contraction forces to enable the skin deformation, while the layer of connective tissues constrains the range of skin movement. The wrinkle generation uses a synthetic texture, and wrinkles are formed dynamically, thanks to a linear plastic model. The wrinkle generation and rendering can be incorporated into real-time facial animation systems.

A dynamic wrinkle model in facial animation and skin ageing

The paper describes a dynamic model to simulate expressive wrinkles in 3D facial animation and skin ageing. A skin surface is defined that can slide over an underlying layer, which constrains the surface by a spring force that simulates the connective fat tissue between them. Muscle masks are constructed to characterize the muscular contractions that provide the facial movement. Skin deformation is simulated through an elastic process assembled with visco and plastic units. By adjusting parameters for this physically based model, distinctive wrinkles for different faces can be generated

Simulation of static and dynamic wrinkles of skin

Wrinkles are an extremely important contribution for enhancing the realism of human figure models. We present an approach to generate static and dynamic wrinkles on human skin. For the static model, we consider micro and macro structures of the skin surface geometry. For the wrinkle dynamics, an approach using a biomechanical skin model is employed. The tile texture patterns in the micro structure of skin surface are created using planar Delaunay triangulation. Functions of barycentric coordinates are applied to simulate the curved ridges. The visible (macro) flexure lines which may form wrinkles are predefined edges on the micro structure. These lines act as constraints for the hierarchical triangulation process. Furthermore, the dynamics of expressive wrinkles-controlling their depth and fold-is modeled according to the principal strain of the deformed skin surface. Bump texture mapping is used for skin rendering.

Virtual clothes, hair and skin for beautiful top models

Since 1986, we have led extensive research on simulating realistic looking humans. We have created Marilyn Monroe and Humphrey Bogart that met in a Cafe in Montreal. At that time, they did not wear any dress as such. Humphreys body was made out of a plaster model that has the shape of a suit. Colours on Marilyns body looked like a dress. Hairs were simulated as a global shape and skin was a colour. Since then, we have developed extensive research to simulate real virtual deformable clothes wearing by virtual humans. We also needed to have appropriate simulation of a skin and recently, we have developed new research on skin in order to decrease the plastic colour of our synthetic actors. Also there was a need to simulate hair in an efficient way. New methods have been developed, both for design and animation purpose that are compatible with the clothes module. In this paper, we introduce our most recent research results on these topics. We are now able to simulate top models that start to look like real ones. Our latest work, that shows Marilyn receiving a golden camera Award in Berlin, Germany, demonstrates the results of our research. This sequence has been shown in a ZDF television program that was seen by more than 15 million viewers.

Physically-based Wrinkle Simulation & Skin Rendering

Wrinkle simulation with skin modeling and deformation are extremely important in enhancing the realism of human figure models. In this paper we present a model which simulates the skin deformation based on a biomechanic model and generates dynamic wrinkles rendered with texture image. The biomechanical model considers the skin as a membrane undergoing large deformations and puts the local coordinate system of each triangle element to its principal strain directions. Synthetic micro and macro structure patterns combined with real photos are employed as texture images to mimic the skin surface details. Furthermore, the potential wrinkle lines are defined, the dynamics of expressive wrinkles and aging wrinkles --their depth and fold-- is modeled based on the principal strain of the deformed skin surface and duration of deformation. Multi-layered color and bump texture mapping is used for skin rendering.

Cloning and aging in a VR family

Face cloning and animation considering wrinkle formation and aging are an aspiring goal and a challenging task. This paper describes a cloning method and an aging simulation in a family. We reconstruct a father, mother, son and daughter of one family and mix their shapes and textures in 3D to get virtual persons with some variation. The idea of reconstruction of a head is to detect features from two orthogonal pictures, modify a generic model with an animation structure and use an automatic texture mapping method. It is followed by a simple method to do 3D-shape interpolation and 2D morphing based on triangulation for experiments of mixing 3D heads between family members. Finally, wrinkles within facial animation and aging are generated based on detected feature points. Experiments are made to generate aging wrinkles on the faces of the son and the daughter.

Skin aging estimation by facial simulation

We propose a layered facial simulation model for skin aging with wrinkles, which includes muscle, connective tissue and skin layer. Our aim is to simulate relevant facial animation and aging with the guidance of general facial tissue anatomy, so that the model can be extended to medical and cosmetic applications. B-spline muscle patches are automatically adapted to each individual face by mapping the anatomical facial muscle image. Connective tissues are simulated as simple springs with the length of hypodermis thickness that constrain skin movement. Facial skin deformation and aging are estimated based on an elaborated biomechanical model considering large strain deformation and wrinkle formation. Finally, multi-layered color and bump texture mapping are used to represent wrinkle forms and to render an aged face.

Muscle Contraction Modeling

Soft tissue constitutive modeling requires a particular investigation into muscle contraction since muscles also exhibit an active behavior. In this area, there are three major approaches corresponding to different purposes. Most models are devoted to muscle force prediction, and only provide models for the global uniaxial output force of given muscles in defined conditions and experiments. These models don’t take into account the local mechanics involved inside the fibers. Conversely, some models are devoted instead to the understanding of the contractile mechanism, and describe the chemico-mechanical aspects of the contraction process at the sarcomeral level, but have hardly been related to a realistic global output force involving the 3D anatomical and passive properties of muscle. Only few studies attempt to provide a model of muscle including anatomical and mechanical, active and passive properties, allowing a realistic simulation of its contractile behavior in relation with its deformation and its global output force. These different aspects are overviewed in the following.

Soft Tissue Physiology

From a physiological point of view, any solid component of the organism from bones to cells may be considered as a living tissue. Soft tissues may be distinguished from other tissues like bones for their flexibility, their soft mechanical properties. This concerns the connective tissues, the muscles, the organs and the brain (Lee 82). A more accurate distinction may be made in considering their respective functions in the organism. Bones are dedicated to building the rigid skeletal structure of the body, cartilage to lubricating the articulations, skeletal muscles to producing strength and to moving the skeleton through the tendons, and organs and brain play physiological functions to maintain and control the organism. In this report, we are mainly concerned with the mechanical properties of the soft tissues involved in body motion and deformation, i.e., skeletal muscles, tendons, ligaments, and skin. Their respective mechanical behavior may be expected to be related to their specific composition, structure, location, and function in the organism. However, for dynamic modeling, they may be satisfactorily approximated by macroscopic properties, as described in the next chapter.