1. An image data processing method that is performed by an image data processing apparatus, comprising steps of: extracting information indicating a pixel value width of an object in an original image, wherein said pixel value width is a range within which pixels of the object in the original image can have signal values; generating a gradation conversion curve based on the pixel value width; generating a high-frequency-component based on the original image; converting the high-frequency-component based on information concerning an inclination of the gradation conversion curve; and generating a processed image based on the converted high-frequency-component.