A spring unit comprises a continuous coil spring contained within a sleeve of strong, yet flexible, material. The feedback circuit includes a feedback resistor and a feedback capacitor to provide a controllable increase in an order of a transfer function of the filter circuit along with non-dominant poles of the differential amplifier circuit coupled in parallel with the feedback resistor. A multiplicity of features is extracted from a found blob. If the two adjacent frames are determined to be similar, the disparity map of the next frame can be obtained according to the disparity map of the previous frame.