To convert N-valued input image data to output image data, where N is an integer equal to or greater than two, a concentration value is determined for each pixel in a set of pixels including a pixel of interest and its neighboring pixels. Ranks are assigned to each of these pixels on the basis of the concentration values. The pixel values of the pixel of interest and its neighboring pixels are then redistributed according to the assigned ranks. This scheme enables pixel clustering to be manipulated in a natural way.