1. An image transform method, for transforming original input image data into image data expanded by a ratio represented by a rational number or an integer, comprising the steps of: forming an image by linearly expanding original image data in the vertical and horizontal directions; and reducing the vertical and horizontal directional correlation of said image through a rank order processing to generate a final expanded image, wherein the rank order processing includes: raster-scanning a window enclosing a target pixel and one or more of its neighboring pixels; and computing an output value of the target pixel by performing an averaging operation on pixels enclosed within the window; determining, for said expanded image, whether the contrast of said original image data can be maintained at a predetermined level; and extracting a high frequency component from said expanded image, when said contrast can not be maintained at said predetermined level, and adding said frequency component multiplied by a constant to said expanded image, or subtracting said frequency component multiplied by a constant from said expanded image.