In an X-Y conversion chart, a threshold Th is defined as Y=0, that is, defined on the X axis, a range from −Th to +Th is defined as a range in which smoothing processing is performed, and a range outside −Th to +Th is defined as a range in which sharpening processing is performed. Among equations denoted by Y1 and Y2 expressed by two linear straight lines, intersections of both the equations Y1 and Y2 are set to the maximum value and the minimum value of an X-Y conversion equation in this case. The threshold is expressed by an intersection of Y2 and the X axis. This algorithm is formed by three factors of: a coefficient α indicating the strength of smoothing; a coefficient β indicating the strength of sharpening; and a threshold Th on the X axis for determining switching between the smoothing processing and the sharpening processing.