To assign angle-dependent information to an input image in order to highlight and display one-dimensional patterns. First, data of an input image is inputted to a computer (Sa1). An arbitrarily defined constant m is then inputted as a contrast intensity (Sa2). Next, a closed region (x, y) made up of a plurality of pixels in the input image is demarcated as a target region D, degrees of inclination between respective two pixels, namely, a pixel of interest in the target region D and arbitrarily defined neighboring pixels thereof, are summed over a whole circumference around the pixel of interest, and the sum is divided by the number of pixels within the target region to calculate a mean value thereof (Sa3). Furthermore, an amplitude value β of a gradient obtained in step (Sa3) is multiplied by the contrast intensity m (where m is a positive constant) inputted in step (Sa2) and an arbitrarily defined offset value γ is added to the obtained value (Sa4) to obtain a final output image (Sa5).
