Abstract: A method for defining trajectories of fiber on a layup surface, including the definition of a transfer mesh of a transfer surface corresponding to a simplified surface, substantially continuous, of the layup surface, the direction of a fiber at an analysis point of the layup surface, being obtained by: definition of a projected point by normal projection of an analysis point on the transfer mesh, definition of a reference direction at said projected point, definition of the normal to the transfer element, definition of a cutting plane by the projected point, the reference direction and the normal, definition of the line of intersection of the cutting plane with the plane of the element, the direction of the fiber at the analysis point being obtained from this line of intersection.