1. A computer-implemented method for designing a three-dimensional modeled object using processing circuitry, comprising: obtaining a sketch, from a user, of 3D curves generated from scratch or on the basis of physical measurements; obtaining, at the processing circuitry, a data structure representing sample points of the 3D curves sketched by the user, the data structure including data representing an association between sample points of a same 3D curve; determining, by the processing circuitry, a volumetric function, within a predetermined class of volumetric functions, the volumetric functions of the predetermined class each being a continuous scalar function defined over a volume containing the sample points and whose isovalue surfaces define closed shapes, as the optimum of an optimization program that explores orientation vectors defined at the sample points under the constraint that the explored orientation vectors be normal to the 3D curves and respect a minimal rotation propagation condition over each 3D curve, wherein the optimization program penalizes a distance from the explored orientation vectors; fitting, by the processing circuitry, the sample points with an isovalue surface of the volumetric function; and generating, by the processing circuitry, a closed shape for the three-dimensional modeled object based on the fitting.