Patent US 3,940,557

A method and apparatus is described for performing controlled erosion of electrical signals relating to detected features in an image which is scanned. The erosion is performed by counting (in an up or down counter) n timing pulses in each sequence corresponding to a detected signal pulse before an overflow signal indicating that n timing pulses have been received is generated. The overflow signal is used to release subsequent timing pulses which are then analysed by counting. The timing pulses so released correspond to the portion of a detected feature left after erosion from the leading edge. The method is applied parallel, perpendicular and at non-orthogonal angles to the line scan direction. The apparatus required can be minimised by applying down-counting techniques and storing a signal to indicate complete erosion in one direction instead of recirculating large quantities of information from line to line. An overflow signal indicating that erosion has been successfully completed in a second subsequent direction together with the indicator signal from a previous erosion is used to instigate another direction of erosion or to release an output pulse for counting.