a) An example of the result of the Laplace filtering of the difference of two following NAC images. b) Boolean mask of the grain track identified by the definition of an adequate threshold. c) Result of the dilatation process. Finally, d) shows the result (after erosion filtering). In red is the isocurve of the border of the grain and in green the ellipsoid fit (expanded for visibility) used to distinguish the grain from other features by eccentricity.