1. An image transform method comprising: transforming original input image data into image data expanded by a ratio represented by a rational number or an integer, including the steps of: reducing correlation in the vertical and horizontal directions of an image that is linearly expanded in the vertical and horizontal directions to generate first expanded image data; performing linear interpolation, based on correlation with a target pixel constituting said original image data and neighboring pixels arranged in oblique directions, using said neighboring pixels to generate second expanded image data; and employing said first expanded image data and said second expanded image data to generate a final image.