A hollow piston has an end wall that receives pressure in a cylinder bore of a compressor. In a one embodiment, the sender 100 computes an identity-based encryption key from an identifier ID associated with the receiver 110. Electrodes, spaced apart from the proof mass, are connected to the support structure and are suspended above the substrate. The digital picture frame device includes a display screen, a frame, an input, memory and control circuitry. The multiresolution representation requires little storage overhead, and the algorithms using the representation are both simple and efficient.