1. An image forming apparatus which discharges ink onto a recording medium to form an image on a surface of the recording medium, comprising: a preprocessing unit configured to apply preprocess liquid to the surface of the recording medium before the image is formed; a postprocessing unit configured to discharge postprocess liquid on the recording medium after the image is formed the postprocess liquid being different from the preprocess liquid, and the postprocess liquid being discharged in a spot or stripe shape having a predetermined height on the image; and a control unit configured to determine an amount of the preprocess liquid to apply based on a type of ink to form the image, and to determine an amount of the postprocess liquid to discharge based on the amount of the preprocess liquid to apply.