1. An image forming apparatus, comprising: a scanner that reads an original document; an image processor that processes a high-lightness region of the original document so as to be viewed, the high-lightness region being lighter than a base of the original document; and an output unit that outputs the image data of the original document subjected to image processing, wherein the image processor includes: a lightness correcting unit that corrects the lightness of the image data of the original document read by the scanner by ensuring a wide dynamic range that does not saturate the lightness of the high-lightness region; and a contrast enhancing unit that enhances the contrast of a local area of the image data whose lightness has been corrected, and wherein the contrast enhancing unit includes a smoothing filter that smoothes a specified area of the lightness-corrected original image data, whereby enhancing the contrast of the local area by subtracting a smoothing output that is obtained by multiplying the output of the smoothing filter by a first coefficient from enhanced data that is obtained by multiplying the input of the smoothing filter by a second coefficient.