1. An image forming apparatus comprising: a storage unit configured to store a rendering image having a plurality of tones; a communication interface unit configured to receive print data; a rendering unit configured to render the received print data to a bitmap image; a mergence unit configured to merge the bitmap image and the rendering image into a single mergence image; a binary coding unit configured to perform halftoning for the generated mergence image to generate binary data; and a print engine unit configured to form an image on a paper by using the generated binary data.