1. An image processor comprising: a raster processing unit that converts image data input from an application program to raster data; a color converter that converts a color format of the image data converted to the raster data; an effective image area detector that detects an effective image area in a page image of the image data after the color conversion; and an image processing unit that applies image processing to only image data of the effective image area, and transfers the processed image data of the effective image area together with position and size information of the effective image area to a print engine that forms an image on a print sheet.