Upon outputting halftone image data to an output device, density transformation is prevalently done to improve image quality. Density transformation corresponds to linear gamma correction, and cannot correct any color fog and the like of a photo image. Hence, lightness/color difference information of halftone image data is acquired on the basis of low-frequency components of frequency data obtained by transforming halftone image data into spatial frequency components, and a lightness/color difference transform amount of the halftone image data is set on the basis of the acquired lightness/color difference information.