1. A method for generating image data, comprising the steps of: receiving an input of image data, and receiving information of width and/or thickness of a recording medium, the recording medium receiving an image representing the image data to be recorded thereon; obtaining a preset pressing pressure value based on the information of width and/or thickness, the preset pressing pressure value being used when the image is recorded on the recording medium; obtaining information of image density in a reference area, the reference area being a part of the image data; obtaining converted image density information by converting the information of image density in the reference area based on the pressing pressure value and the information of image density; and generating image data according to the obtained converted image density information.