1. An image forming method, comprising: extracting, by a processor, a bitmap image from print data; generating edge color channel images using the extracted bitmap image; converting the extracted bitmap image to a sketch image by multiplying each of edge color channel images of the extracted bitmap image; replacing the bitmap image with the sketch image in the print data; printing the print data in which the bitmap image is replaced with the sketch image; and multiplying each of color channels of the bitmap image.