Virtual Character Definition and Animation within the MPEG-4 Standard

Besides being one of the well-known audio/video coding techniques, MPEG-4 provides additional coding tools dedicated to virtual character animation. The motivation of considering virtual character definition and animation issues within MPEG-4 is first presented. Then, it is shown how MPEG-4, Amendment 1 offers an appropriate framework for virtual human animation and compression/transmission. It is shown how this framework is extended within the new MPEG-4 standardization process by: 1) allowing the animation of any kind of articulated model, and 2) addressing advanced modeling and animation concepts, such as "Skeleton, Muscle and Skin"-based approaches. The new syntax for node definition and animation stream is presented and discussed in terms of a generic representation and additional functionalities. The biomechanical properties, modeled by means of the character skeleton that defines the bone influence on the skin region, as well as the local spatial deformations simulating muscles, are supported by specific nodes. Animating the virtual character consists in instantiating bone transformations and muscle control curves. Interpolation techniques, inverse kinematics, discrete cosine transform and arithmetic encoding techniques make it possible to provide a highly compressed animation stream. Within a dedicated modeling approach the so-called MESHGRID we show how the bone and muscle-based animation mechanism is applied to deform the 3D space around a humanoid.