A stimulus locally affects a functional layer (1, 1a, 1b) comprising a solution, in its optical-, aggregation-, cross-linking- and/or swelling properties. The resultant change persists until further stimulation occurs. Surplus solution is removed or transformed reversibly into hydrogel structures. Local regions of the functional layer undergo change in optical properties, in accordance with shape memory effect. The layer is transformed reversibly from an oil, paraffin or wax, into a solid state.