1. A method for converting a document into a full-page image using a graphical device interface (GDI) of an operating system, the operating system being installed on a computer system for controlling operations of the computer system, and the GDI capable of managing graphical processes of the computer system, the method comprising: providing objects of the document and coordinates of the objects in the document using the GDI; dividing the document into a plurality of blocks; establishing an address array having a plurality of address units, wherein each of the address units corresponds to one of the blocks for recording positions of the objects in the block; updating the positions recorded in the address units according to the coordinates of the objects provided by the GDI; generating a corresponding image piece for each block according to the updated positions recorded in the corresponding address unit; and merging the image pieces into the full-page image.