Upon outputting halftone image data to an output device, density transformation is prevalently performed to improve image quality. Density transformation corresponds to linear gamma correction, and cannot correct for color fog and the like that may occur in a photo image. Hence, lightness and 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 and color-difference transform amount of the halftone image data is set on the basis of the acquired lightness and color-difference information.