1. A method comprising: using one or more computers to perform: accessing data defining a continuous curve; generating a model of the continuous curve comprising a plurality of mass points, each mass point having an assigned mass that is a sum of scaled masses of segments of the curve adjacent to the mass point, and a plurality of springs whose forces act on the mass points; receiving data indicating an editing operation to be performed on the continuous curve; determining a modified shape of the continuous curve resulting from the editing operation, wherein said determining comprises performing a physical simulation that is dependent on spring forces acting on one or more of the mass points; and storing data representing the continuous curve in its modified shape.