Patent ID: 8169634

Claim:
An image forming apparatus comprising: an image input section configured to input an image; a memory section configured to store a visible watermark image, which is visible on a recording medium and invisible on a copied paper sheet; a generating section configured to draw an invisible watermark image in the visible watermark image read from the memory section to generate a visible/invisible watermark image; an overlaying section configured to overlay the visible/invisible watermark image generated by the generating section on the image input by the image input section by a predetermined overlay ratio and generate an overlaid image; and a forming section configured to form an image on the recording medium based on the overlaid image; wherein the predetermined overlay ratio is a value configured to make the visible/invisible watermark image visible on the copied paper sheet, and invisible when displayed on a display device based on the image overlaid by the overlaying section, wherein the value is greater than or equal to A/M and smaller than B/M, where A is a minimum gradation of allowing the image to be distinguished as white in a printer visual characteristic, B is a minimum gradation of allowing the image to be distinguished as white in a display visual characteristic, and M is the maximum gradation.