1. An image processing device comprising: first storage means for storing source image data in units of pixels; second storage means for storing destination image data in units of pixels; rendering means for performing an action of applying a stipulated pixel-unit operation to the source image data stored in said first storage means and rendering the data as destination image data in the second storage means in units of polygons repeatedly until a stipulated arithmetic result is obtained; and means for specifying an operation mode between said source image data and said destination image data, wherein said means for specifying specifies as said operation mode either a first mode wherein said source image data is added to said destination image data, or a second mode wherein said source image data is subtracted from said destination image data.