A magnetic-field-generating device for producing an optical effect layer made of a hardened coating, said magnetic-field-generating device being configured for receiving a supporting surface carrying a coating composition comprising a plurality of non-spherical magnetic or magnetizable pigment particles and a binder material, and being configured for forming concave magnetic field lines for orienting at least a part of the plurality of non-spherical magnetic or magnetizable pigment particles in an orientation forming a positive rolling bar effect, wherein the magnetic-field generating device is located on the side of the supporting surface opposite to the side carrying the coating composition, wherein said magnetic-field-generating device is a) a bar dipole magnet and a pair of bar dipole magnets and, said bar dipole magnets, and having their North-South axis substantially parallel to the supporting surface and the same magnetic North-South direction, a1) said bar dipole magnet is disposed below the supporting surface and said pair of bar dipole magnets and are disposed below the bar dipole magnet apart from each other; or a2) said pair of bar dipole magnets and are disposed below the supporting surface and apart from each other, and said bar dipole magnet is disposed below said pair of bar dipole magnets and; or b) a pair of bar dipole magnets and and a pole piece, said pair of bar dipole magnets and having their North-South axis substantially parallel to the supporting surface and the same magnetic North-South direction, said pole piece being disposed between said bar dipole magnet and said bar dipole magnet; or c) a pair of bar dipole magnets and, a pole piece and a magnetic plate, said pair of bar dipole magnets and having their North-South axis substantially parallel to the supporting surface and the same magnetic North-South direction, said magnetic plate having its North-South axis substantially perpendicular to the supporting surface, said pole piece being disposed between said bar dipole magnet and said bar dipole magnet; or d) wherein said magnetic field generating device comprises a pair of spaced apart bar dipole magnets and a third element, preferably a third dipole magnet or a pole piece, wherein the dipole magnets have north to south axes that are aligned with each other, that are substantially parallel to the supporting surface and that have a same magnetic North-South direction, wherein the dipole magnets are spaced apart along the north south axes so as to provide a gap region between the dipole magnets in which magnetic field lines are such that the magnetic or magnetizable pigment particles are oriented in the gap region to form the positive rolling bar effect and wherein the third element is arranged with the pair of spaced dipole bar magnets to disturb the magnetic field in the gap region between the spaced apart bar dipole magnets. a) a bar dipole magnet and a pair of bar dipole magnets and, said bar dipole magnets, and having their North-South axis substantially parallel to the supporting surface and the same magnetic North-South direction, a1) said bar dipole magnet is disposed below the supporting surface and said pair of bar dipole magnets and are disposed below the bar dipole magnet apart from each other; or a2) said pair of bar dipole magnets and are disposed below the supporting surface and apart from each other, and said bar dipole magnet is disposed below said pair of bar dipole magnets and; or a1) said bar dipole magnet is disposed below the supporting surface and said pair of bar dipole magnets and are disposed below the bar dipole magnet apart from each other; or a2) said pair of bar dipole magnets and are disposed below the supporting surface and apart from each other, and said bar dipole magnet is disposed below said pair of bar dipole magnets and; or b) a pair of bar dipole magnets and and a pole piece, said pair of bar dipole magnets and having their North-South axis substantially parallel to the supporting surface and the same magnetic North-South direction, said pole piece being disposed between said bar dipole magnet and said bar dipole magnet; or c) a pair of bar dipole magnets and, a pole piece and a magnetic plate, said pair of bar dipole magnets and having their North-South axis substantially parallel to the supporting surface and the same magnetic North-South direction, said magnetic plate having its North-South axis substantially perpendicular to the supporting surface, said pole piece being disposed between said bar dipole magnet and said bar dipole magnet; or d) wherein said magnetic field generating device comprises a pair of spaced apart bar dipole magnets and a third element, preferably a third dipole magnet or a pole piece, wherein the dipole magnets have north to south axes that are aligned with each other, that are substantially parallel to the supporting surface and that have a same magnetic North-South direction, wherein the dipole magnets are spaced apart along the north south axes so as to provide a gap region between the dipole magnets in which magnetic field lines are such that the magnetic or magnetizable pigment particles are oriented in the gap region to form the positive rolling bar effect and wherein the third element is arranged with the pair of spaced dipole bar magnets to disturb the magnetic field in the gap region between the spaced apart bar dipole magnets.