1. A method comprising: deforming, by a data processing apparatus, a skin representation relative to a muscle representation in response to simulated motion of the muscle representation, the skin representation including a plurality of skin-vertices, the simulated muscle representation motion determined by controlling respective geometries of rings of the muscle representation in accordance with first motion-rules; for each of a portion of the skin representation skin-vertices, displacing, by the data processing apparatus, the skin-vertex by controlling respective geometries of two or more selected rings of the muscle representation in accordance with second motion-rules; identifying, by the data processing apparatus, two rings of the muscle representation adjacent to the displaced skin-vertex, said identifying based on the respective geometries of the rings of the muscle representation, wherein said identifying the two rings of the muscle representation adjacent to the displaced skin-vertex comprises for each ring of the muscle representation, calculating a scalar product between a normal to a center of the ring and a vector drawn from the displaced skin-vertex to the center of the ring, and selecting two consecutive rings for which the respective scalar products have opposite signs as the two rings of the muscle representation adjacent to the displaced skin-vertex; responsive to determining that a relative position of the displaced skin-vertex with respect to a portion of the muscle representation corresponding to the identified two rings of the muscle representation fails to satisfy a predetermined criterion, adjusting, by the data processing apparatus, the determined relative position of the skin-vertex with respect to the portion of the muscle representation to satisfy the predetermined criterion, said determining and said adjusting based on the respective geometries of the identified two rings of the muscle representation; and encoding, by the data processing apparatus on non-transitory computer-readable medium, the adjusted relative position of the skin-vertex with respect to the portion of the muscle representation.