1. An image forming apparatus comprising: a sheet-type determining unit that determines a sheet type of a recording sheet; a sheet-characteristic measuring unit that measures a sheet characteristic of the recording sheet; an image processing unit that performs an image processing on image data of an image for adjusting a consumption amount of color material that is used for forming the image on the recording sheet based on the sheet type and the sheet characteristic; and an image forming unit that forms the image of the image data processed by the image processing unit on the recording sheet.