With recent improvements in image processing and image forming technologies, it has become possible that a high-tech digital color copying apparatus, for example, can reproduce a monetary document with high fidelity such that the reproduced image is hardly distinguishable from the original. Monetary documents include paper currency, securities, and the like, for example, and cannot normally be copied since unauthorized monetary document reproduction is illegal. Therefore, the high-tech digital color copying apparatus requires a feature that prevents high fidelity reproduction of an anti-copy document such as a monetary document, a confidential document, etc. With this feature, reproduction of an anti-copy document may be prevented entirely or can be affected such that the resulting copy is unreadable, for example.
Many confidential documents besides monetary documents are not intended for unrestricted reproduction or copying, such as documents that include trade secrets. These documents require copy protection to prevent unauthorized reproduction using high-tech digital color copying apparatus.
Under these circumstances, various techniques have been used with the above-mentioned high-tech digital color copying apparatus in an attempt to restrict unauthorized image reproduction with high fidelity.
Japanese Laid-Open Unexamined Patent Application Publication, No. 06-125459 describes a technique for recognizing a special document such as currency, a security, and so on by comparing input image data with a previously stored specific mark using pattern matching and determining that the input image data is a special, or copy protected, document when the input image data is found to contain the previously stored specific mark. Japanese Laid-Open Unexamined Patent Application Publication, No. 2001-086330 also describes a similar technique. If an input original is determined to be a special document in this way, an unauthorized reproduction of this document can readily be prevented.
The above technique requires reference pattern data to be applied to or associated with the anti-copy protection for a specific document. However, it would be difficult to apply anti-copy protection using fixed reference pattern data to an indefinite number of copy protected or confidential documents.
As another example, Japanese Laid-Open Unexamined Patent Application Publication, No. 07-036317 describes a technique for recognizing a confidential document as a copy-prohibited document by detecting a specific mark such as “CONFIDENTIAL,” for example, printed on the confidential document and indicative of its confidentiality. A print of such a specific mark indicating a document's confidentiality is a common practice used in the above-mentioned publication. When an input original is determined to be a special document in this way, unauthorized reproduction of the document can be prevented. Japanese Laid-Open Unexamined Patent Application Publication, No. 07-087309 also describes a similar method.
However, when a confidential document is marked with or includes a specific confidential mark such as “CONFIDENTIAL,” for example, a copy of the confidential document can easily be made by concealing the specific confidential mark by covering the mark with a piece of paper, for example. Thus, the anti-copy protection fails to protect the confidential document from copying.
Japanese Laid-Open Unexamined Patent Application Publication, No. 09-164739 also describes a similar technique for restricting document reproduction by embedding a watermark in an original image requiring copy protection. This technique uses a paper document having a specific background image to which an original image is attached. The specific background image includes a watermark pattern having a base area and a message area. From its nature, the watermark pattern is inconspicuous in comparison with the original image and therefore it does not render the original image unreadable. This technique, however, causes the watermark pattern to appear when the original image having this watermark pattern is copied. The watermark pattern of this technique includes a warning such as “anti-copy,” for example, so that a copy of this original document is recognized easily at a glance as a confidential document which is intended to be protected from copying. Therefore, this technique has a psychological effect that tends to inhibit a person from copying a confidential document.
However, this technique produces the above-mentioned psychological effect only after a copy is made. Therefore, anti-copy protection that uses a watermark of this nature does not inhibit persons who do not care about the appearance of the watermark pattern.