Method for describing a thread as a geometric object in a three dimensional computer graphics system. The method comprises generating a set of fixture points defining points in space through which the thread passes, generating a set of one or more segments representing the thread between adjacent fixture points, linking each segment to a pair of fixture points, one at each end of the respective segment, and describing the geometrical properties of each segment. The segments and fixture points are preferably defined as objects containing functions and variables allowing the exchange of data between the various objects. The method allows for the calculation of the shape of the thread based on a calculation of the shape of each individual segment. A simulation system utilizing the method receives update information such as control signals, over an input interface and includes instructions on a storage media and a processor for calculating the shape of the thread and outputting the result over an output interface.