1. An image processing apparatus comprising: a pattern generating section for generating two patterns; an image input section for inputting an input image; an additional information input section for inputting additional information; a pixel position designating section for designating an image position at which the pattern is superimposed on the input image inputted by the image input section; and a pattern superimposing section for selecting one of the two patterns generated by the pattern generating section based on the additional information to superimpose the selected pattern on the image position in the input image designated by the pixel position designating section, wherein when corresponding pixels in two patterns are added to each other, all of elements become zero; wherein when all of pixels in each of patterns are added to each other, the added pixels become zero; and wherein each of patterns has two edges, which include discontinuous pixel values and pass through a center portion of each of patterns in difference directions from each other.