1. An image forming apparatus comprising: an input part to receive image data; an image processor to image-process the input image data; an output part to output data processed at the image processor; and a controller to control the image processor to process the image using one of a plurality of image processing modes having a toner consumption lower than a toner density to use when the toner density to use to output the input image data exceeds a preset reference density, wherein the plurality of the image processing modes comprises a boundary image processing mode which outputs only boundaries of objects in the image data.