1. An image processing method comprising the steps of: transforming image data into spatial frequency components; quantizing the transformed spatial frequency components; performing a modification process of changing spatial frequency components in a predetermined frequency domain on the quantized spatial frequency components; inverse-quantizing the spatial frequency components on which the modification process is performed; inverse-transforming the inverse-quantized spatial frequency components into image data; and reducing the number of gray levels in the inverse-transformed image data, based on a threshold value.