1. An image processing apparatus for processing image data having attribute information for every pixel constituting the image data, comprising: a combining unit that combines component image data, which corresponds to each of a plurality of component images, into composite image data that corresponds to a single composite image; an attribute appending unit that appends identification information, which indicates the component image data to which each pixel belonged, as attribute information of pixels that constitute the composite image data; and a processing unit that applies common image processing to pixels having identical values as the identification information.