Fragile watermark for detecting printed image copies
The present invention is concerned with inclusion of a “fragile watermark” in an original printed document to aid in a determination whether a document under examination is an original or a copy. The watermark may be applied by making small tonal adjustments on a pixel block by pixel block basis in data used to generate the original document. Documents to be verified may then be examined on a block-by-block basis after scanning.
1. A method of determining whether a printed-image-under-examination (PIUE) is a original printed image, with a fragile watermark the method comprising:
(a) scanning the PIUE to generate scanned image data, the scanned image data comprising pixel data, the pixel data comprising gray scale values and representing the PIUE as a set of scanning pixels;
(b) forming a plurality of data blocks from the scanned image data, each data block consisting of pixel data which corresponds to a respective region of the PIUE;
(c) determining for each of the data blocks an average value of the pixel data in the data block;
(d) determining for each of the data blocks an index value based on the average value of the pixel data in the data block;
(e) for each data block, calculating a difference between the index value for the data block and a value that represents a target value for a corresponding block of pixel data that was used to generate the original printed image, and
(f) determining whether the PIUE is the original printer images with the fragile watermark.
2. The method according to claim 1 , wherein the index value for each data block is selected from a predetermined set of gray scale levels as a one of said set of gray scale levels to which the average value of the pixel data in the data block is closest.
3. The method according to claim 2 , further comprising:
(f) squaring the differences calculated in step (e).
4. The method according to claim 3 , further comprising:
(g) summing the squared differences.
5. The method according to claim 4 , further comprising:
(h) comparing a sum generated at step (g) with a threshold.
6. The method according to claim 5 , further comprising:
(i) providing an indication as to whether the PIUE is an original printed image on the basis of a result of step (h).
7. The method according to claim 1 , wherein the PIUE was printed with a first resolution that is less than a second resolution at which the PIUE was scanned in step (a).
8. A method of determining whether a printed-image-under-examination (PIUE) is a original printed image, with a fragile watermark the method comprising:
(a) scanning the PIUE to generate scanned image data, the scanned image data comprising pixel data, the pixel data comprising gray scale values and representing the PIUE as a set of scanning pixels;
(b) forming a plurality of data blocks from the scanned image data, each data block consisting of pixel data which corresponds to a respective region of the PIUE;
(c) determining for each of the data blocks an average value of the pixel data in the data block;
(d) determining for each of the data blocks an index value based on the average value of the pixel data in the data block;
(e) for each data block, calculating a difference between the index value for the data block and an expected value of the index value; and
(f) determining whether the PIUE is the original printed image with the fragile watermark.
9. The method according to claim 8 , wherein the index value for each data block is selected from a predetermined set of gray scale levels as a one of said set of gray scale levels to which the average value of the pixel data in the data block is closest.
10. The method according to claim 9 , further comprising:
(f) squaring the differences calculated in step (e).
11. The method according to claim 10 , further comprising:
(g) summing the squared differences.
12. The method according to claim 11 , further comprising:
(h) comparing a sum generated at step (g) with a threshold.
13. The method according to claim 12 , further comprising:
(i) providing an indication as to whether the PIUE is an original printed image on the basis of a result of step (h).
14. The method according to claim 8 , wherein the expected value of the index value for each of the data blocks is representative of a target value that was used to generate pixel values for a corresponding pixel block of the original printed image.