An image forming apparatus comprising: a communication interface unit to receive print data, such as a page description language postscript, PCL, XPS, PDF; an extraction unit to extract a bitmap image from the received print data, the bitmap image being a contone color bitmap image; a sketch image generating unit to generate a sketch image by using the extracted bitmap image; characterized in that the sketch image generating unit comprises: a contrast enhancing part to enhance a contrast of the extracted bitmap image; a color converting part to convert the bitmap image with the enhanced contrast to a grayscale image; a mask generating part to generate a mask by detecting edges of the grayscale image; and a multiplication part to multiply the bitmap image with the enhanced contrast by the mask to generate the sketch image; and the image forming apparatus further comprises: an image forming unit to replace the bitmap image by the generated sketch image in the received print data and to print the print data with the replaced sketch image, wherein the color converting part converts the generated sketch image to a grayscale sketch image, and the image forming unit replaces the bitmap image by the grayscale sketch image in the print data and prints the print data with the replaced sketch image. a communication interface unit to receive print data, such as a page description language postscript, PCL, XPS, PDF; an extraction unit to extract a bitmap image from the received print data, the bitmap image being a contone color bitmap image; a sketch image generating unit to generate a sketch image by using the extracted bitmap image; characterized in that the sketch image generating unit comprises: a contrast enhancing part to enhance a contrast of the extracted bitmap image; a color converting part to convert the bitmap image with the enhanced contrast to a grayscale image; a mask generating part to generate a mask by detecting edges of the grayscale image; and a multiplication part to multiply the bitmap image with the enhanced contrast by the mask to generate the sketch image; and a contrast enhancing part to enhance a contrast of the extracted bitmap image; a color converting part to convert the bitmap image with the enhanced contrast to a grayscale image; a mask generating part to generate a mask by detecting edges of the grayscale image; and a multiplication part to multiply the bitmap image with the enhanced contrast by the mask to generate the sketch image; and the image forming apparatus further comprises: an image forming unit to replace the bitmap image by the generated sketch image in the received print data and to print the print data with the replaced sketch image, wherein the color converting part converts the generated sketch image to a grayscale sketch image, and the image forming unit replaces the bitmap image by the grayscale sketch image in the print data and prints the print data with the replaced sketch image. an image forming unit to replace the bitmap image by the generated sketch image in the received print data and to print the print data with the replaced sketch image, wherein the color converting part converts the generated sketch image to a grayscale sketch image, and the image forming unit replaces the bitmap image by the grayscale sketch image in the print data and prints the print data with the replaced sketch image.