a pair of target still images or between frames from sequences of video. These morphing techniques usually consist of a combination of a geometric deformation technique, which aligns the target images, and a cross-fade which creates the smooth transition in the image texture. An early example of image morphing can be seen in Michael Jackson's video for "Black Or White". In 3D animation texture based animation can be achieved by animating the texture itself or the UV mapping. In the latter case a texture map of all the facial expression is created and the UV map animation is used to transition from one expression to the next. Physiological models, such as skeletal muscle systems and physically based head models, form another approach in modeling the head and face. Here, the physical and anatomical characteristics of bones, tissues, and skin are simulated to provide a realistic appearance (e.g. spring-like elasticity). Such methods can be very powerful for creating realism but the complexity of facial structures make them computationally expensive, and difficult to create. Considering the effectiveness of parameterized models for communicative purposes (as explained in the next section), it may be argued that physically based models are not a very efficient choice in many applications. This does not deny the advantages of physically based models and the fact that they can even be used within the context of parameterized models to provide local details when needed. Face animation languages Many face animation languages are used to describe the content of facial animation. They can be input to a compatible "player" software which then creates the requested actions. Face animation languages are closely related to other multimedia presentation languages such as SMIL and VRML. Due to the popularity and effectiveness of XML as a data representation mechanism, most face animation languages are XML-based. For instance, this is a sample from Virtual Human Markup Language (VHML): <vhml> <person disposition="angry"> First I speak with an angry voice and look very angry, <surprised intensity="50"> but suddenly I change to look more surprised. </surprised> </person> </vhml> More advanced languages allow decision-making, event handling, and parallel and sequential actions. The Face Modeling Language (FML) is an XML-based language for describing face animation. FML supports MPEG-4 Face Animation Parameters (FAPS), decision-making and dynamic event handling, and typical programming constructs such as loops. It is part of the iFACE system. The following is an example from FML: <fml> <act>