Abstract: A method for defining trajectories of fibers on a layup surface for producing at least one ply having a given theoretical orientation, by laying up fibers, including the steps of providing the definition of the constraint curves and/or the definition of at least one constraint grid with association of at least one constraint vector to each node of the constraint grid, the direction of a fiber at an analysis point of the layup surface, being obtained by calculation of the normalized weights of the constraint vectors of the constraint curves and/or by calculation by of the normalized weights of the constraint vectors of the constraint grid, and by weighting by the normalized weights of the constraint vectors.