Patent US 5,748,801

A method of setting threshold values for extracting image data includes the steps of determining (a) that a pixel of an image to be extracted is to be displayed in color if (1) a density value of the pixel is between a first threshold value and a second threshold value, and (2) a density gradient of the pixel is between a third threshold value and a fourth threshold value, and (b) that the pixel is to be displayed in monochrome if (1) the density value of the pixel is not between the first threshold value and the second threshold value, or (2) the density gradient of the pixel is not between the third threshold value and the fourth threshold value, and changing at least one of the first threshold value and the second threshold value to enable a desired image region of the image to be extracted to be displayed in color.