Physically Based Real-time Human Skin Deformation

As an important part of the human figure animation, human skin deformation can be found inwide applications in many realms such as entertainment and virtual reality. The scope of thestudies is to build the mechanism of skin deformation based on a particular human model, and toobtain the variations in shape changes of human skin when human figure is in motion. Today thereare much more demands for realism and real-time in the real-time applications such as 3D games.Realism includes visual and physical realism. Traditional technology of human skin deformationcould hardly achieve projected realism and real-time at the same time. Geometry based skindeformation is quick engouh to achieve real-time so it is widely used. But it is lack of physicalrealism. By building physical model according to the physical properties of human, physicallybased skin deformation obtain good visual and physical realism. But it is too slow to achievereal-time because of complexitY of physical computing.In order to obtain enough realism and high real-time, this thesis propose new human modeland the mechanism of skin deformation. Quick algorithm of skin deformation is also proposed.The main contributes of this thesis are as follows:First, a new three-layered human model and the mechanism of skin deformation are proposed.The human model is built according to a special human skin mesh. The model is composed ofskeleton layer, middle layer and volumetric mesh, and geometric mapping relation betweenvolumetric mesh and skin is built. The driving mechanism of the model is as follows: (1) themiddle layer is driven by the skeleton layer. The deformation of the middle layer is obtainedaccording to the contour deformation method; (2) the volumetric mesh is driven by the middlelayer. The physical model is built for volumetric mesh and finite element method is used tocalculate the deformation of the volumetric mesh; (3) obtain the skin deformation according to thegeometric mapping relation between volumetric mesh and skin. Utilizing the middle layer, thebend and twist of arm can be realized conveniently, and the distortional deformation is avoided.Utilizing the volumetric mesh where the physical model is built, the dynamic and staticdeformations are obtained. High-resolution skin mesh with any topology can be applied to themodel so the detailed appearance of skin is maintained. Moreover, most tasks of modeling arefinished automatically thus simplifies the work of designers.Second, quick algorithm is proposed so that the skin deformation based on the three-layered model can obtain high real-time. Physical computation cost most time within the wholecomputation, and the resolution of volumetric mesh and numerical algorithm are two main factorsaffect the speed of physical computation. The thesis introduces low-resolution volumetric mesh toreduce the DOF (Freedom of Degree) of physical equation so as to accelerate the computation ofphysical equation. For the numerical algorithm, there are three steps to accelerate the computationof physical equation: presumes that the volumetric mesh makes small deformation so the nonlinearproblem is translated into linear problem; implicit difference method is used to translate ordinarydifferential equations into linear systems so as to stably solve the equations in large timestep;preconditioned conjugate gradient method is used to quickly solve the linear systems.Experimental results indicate that high real-time can be achieved using above described algorithm.Last, muscle model is added to three-layered human model so that it can realize the skindeformation under the muscle control. Based on the mechanical properties of skeletal muscle,muscle model is built using EFFD (Extended Free Form Deformation) technology. The musclemodel includes flexor-extensor EFFD and tendon EFFD. Flexor-extensor EFFD is used tosimulate the muscle deformation of upper arm. Tendon EFFD is used to simulate the muscledeformation near the elbow joint. Besides the deformations of the three-layer model, skindeformation under the muscle control is obtained using the muscle model so that realism of skindeformation is enhanced. Moreover, above described quick algorithm is used to accelerate thecomputation so as to achieve high real-time.