Capacitive sensor and biometric image generating method

A biometric image generating method using a capacitive sensor, the capacitive sensor comprising a plurality of electrodes each of which outputs an electrical signal corresponding to a capacitance determined by a distance between a surface of the sensor and an electrical conductor, wherein the plurality of electrodes include electrodes having a first parasitic capacitance and electrodes having a second parasitic capacitance different from the first parasitic capacitance and are arranged in a prescribed pattern different than a pattern of biometric information of a body part to be read, the method comprising:

• generating an image in which each of the plurality of electrodes corresponds to one pixel and each pixel has a pixel value proportional to an electrical signal output from a corresponding one of the plurality of electrodes;
• extracting from the image a foreground region which is a region that contains pixels at each of which an electrical conductor is located at a distance capable of forming a capacitor with a corresponding one of the plurality of electrodes;
• calculating correlation between the foreground region and a filter that matches the prescribed arrangement pattern, and thereby detecting a non-biometric region formed by a non-biometric electrical conductor adhering to a sensor surface of the capacitive sensor;
• and
• calculating a metric that decreases as the area that the non-biometric region occupies in the foreground region increases, and outputting the image if the metric is higher than a predefined threshold value, but discarding the image if the metric is not higher than the predefined threshold value.