1. A method of converting resolution of an image, comprising using a processor to perform the steps of: specifying an interpolated pixel to be added to the image; selecting a plurality of reference pixels from a vicinity of the interpolated pixel; obtaining a distance value for each of the reference pixels; extracting a pixel value for each of the reference pixels; generating a weighting factor for a target reference pixel selected from the plurality of reference pixels, using the distance value and the pixel value of the target reference pixel; and adding, to the image, the interpolated pixel having a pixel value determined by the weighting factor of the target reference pixel, wherein the generating step comprises the step of: calculating, for the target reference pixel, a pixel value difference between the pixel value of the target reference pixel and a first predetermined value; and normalizing the pixel value difference with a normalization factor.