1. An image data processor comprising: a data analyzing section that receives information specifying a reusable object that is an image object used repeatedly over a plurality of pages, information specifying a variable object that is an image object used once on a particular page, and page description data in page description language describing arrangement positions of image objects by each page formed by the reusable object and the variable object, and that generates object data having image objects specified by the page description data, and layout information having arrangement positions by each page of the image objects specified by the page description data; a rasterizing section that receives the object data generated in the data analyzing section and rasterizes each image object in the object data, thereby generates an object raster that is object data in a rasterized format of each image object; a layout processing section that generates, based on the layout information generated in the data analyzing section, a storage page layout for reusable objects for one page by a group of a page where same reusable objects are arranged respectively on same positions, the storage page layout for reusable objects describing IDs of the reusable objects to be arranged on the page and arrangement positions of the reusable objects, generates a storage page layout for each variable object describing an ID of the variable object and an ID of an image object overlapping the variable object at least partially, and a manner in which the image object overlaps the variable object, and thereby generates storage layout information formed by all storage page layouts including the storage page layout for reusable objects and the storage page layout for the variable object, and further generates, for each output page for printing, output page joint information describing an ID of a storage page layout for reusable objects forming the output page, an ID of a storage page layout for the variable object forming the output page, and an arrangement position of the variable object on the output page specified by the storage page layout for the variable object, thereby generating output joint information formed by output page joint information for all the output pages; an assembling section that assembles each object raster generated in the rasterizing section, based on the storage layout information generated in the layout processing section, by each storage page layout forming the storage layout information, thereby generating a storage page raster representing a rasterized image by each storage page specified by each storage page layout; and an output raster joint section that joints storage page rasters generated in the assembling section, based on the output joint information by each output page joint information forming the output joint information, thereby generating an output page raster representing a rasterized image by each output page specified by each output page joint information.