Muscle-based models, or abstract-muscle models, mimic at a simple level the action of three primary muscle groups of the face: (1) the linear, such as the zygomatic major, (2) the sphincter, such as the obicularis oculi, and (3) the sheet, such as the frontalis major [106][87][145][144]. This approach has also been extended to B-spline surfaces [143][141]. There are two distinct advantages for these models: they are independent of particular facial geometry and they map directly into muscle-based coding systems.