1. An image processing method that performs an image transformation operation on input image data using a computer to generate output image data for display, the image processing method comprising: acquiring input image data; generating transformed image data corresponding to a transformation process by performing each of a plurality of the transformation processes which are set corresponding to a transformation parameter representing a transformation operation on the acquired input image data; generating inverse-transformed image data by inverse-transforming the generated transformed image data into a shape corresponding to the input image data; determining deterioration of an image for each transformation process by comparing the generated inverse-transformed image data to the input image data; selecting an optimal transformation process among the plurality of transformation processes based on a result of the deterioration determination; and generating the output image data by performing a transformation operation on the input image data based on the optimal transformation process.