Detecting printed image copies using phase-space-encoded fragile watermark
The present invention is concerned with detection of a “fragile watermark” in a printed image to aid in a determination whether a document under examination is an original or a copy. The watermark may be applied by phase-space encoding data to be included in the watermark. The image may be examined on a block-by-block basis after scanning. Correlation of detected watermark strength with block brightness and/or with wave vectors used for encoding may be used to detect that the image is a copy.
1. A method of determining whether a printed-image-under-examination (PIUE) is a copy of an original printed image, 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) transforming the pixel data in at least some of the data blocks to obtain transform domain data by applying at least one of a Fourier transform, a fast Fourier transform, a discrete cosine transform (DCT) and a wavelet transform to the pixel data in the at least some of the data blocks to obtain the transform domain data;
(d) applying a watermark detecting operation to the transform domain data for respective ones of the data blocks to generate recovered watermark data; and
(e) determining a correlation between the recovered watermark data for at least some of the data blocks and a brightness of said data blocks.
2. The method according to claim 1 , further comprising:
(f) determining that the PIUE is a copy of the original printed image if a strength of a brightness level of the recovered watermark data is negatively correlated with the brightness of said data blocks.
3. The method according to claim 1 , wherein the watermark detecting operation includes multiplying the transform domain data with a detecting function.
4. The method according to claim 3 , wherein the detecting function is e ikr , where k and r are phase space indices applicable to the transform domain data.
5. The method according to claim 3 , wherein the detecting operation further includes applying an envelope function to the transform domain data that has been multiplied by the detecting function.
6. The method according to claim 5 , wherein the detecting operation further includes applying an inverse transform to the transform domain data that has been multiplied by the detecting function and to which the envelope function has been applied.
7. The method according to claim 1 , wherein the PIUE is part of a postal indicium.
8. The method according to claim 1 , wherein at least one of the regions of the PIUE overlap with one or more other regions of the PIUE to which the data blocks correspond are overlapping with each other.
9. The method according to claim 1 , wherein the brightness of said data blocks is determined by calculating an average gray scale value of said data blocks.
10. A method of determining whether a printed-image-under-examination (PIUE) is a copy of an original printed image, the original printed image including a watermark applied to the image using a plurality of wave vectors, 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) transforming the pixel data in at least some of the data blocks to obtain transform domain data by applying at least one of a Fourier transform, a fast Fourier transform, a discrete cosine transform (DCT) and a wavelet transform to the pixel data in the at least some of the data blocks to obtain the transform domain data;
(d) applying a watermark detecting operation to the transform domain data for respective ones of the data blocks to generate recovered watermark data; and
(e) determining at least one of (i) a correlation between the recovered watermark data for at least some of the data blocks and a brightness of said data blocks, and (ii) a correlation between the recovered watermark data and the wave vectors.
11. The method according to claim 10 , further comprising:
(f) determining that the PIUE is a copy of the original printed image if a signal level of the recovered watermark data decreases with the brightness of said data blocks.
12. The method according to claim 10 , further comprising:
(f) determining that the PIUE is a copy of the original printed image if a signal level of the recovered watermark data increases with wavelengths of the wave vectors.
13. The method according to claim 10 , wherein the watermark detecting operation includes multiplying the transform domain data with a detecting function.
14. The method according to claim 13 , wherein the detecting function is e ikr , where k and r are phase space indices applicable to the transform domain data.
15. The method according to claim 13 , wherein the detecting operation further includes applying an envelope function to the transform domain data that has been multiplied by the detecting function.
16. The method according to claim 15 , wherein the detecting operation further includes applying an inverse transform to the transform domain data that has been multiplied by the detecting function and to which the envelope function has been applied.
17. The method according to claim 10 , wherein the PIUE is part of a postal indicium.
18. The method according to claim 10 , wherein at least one of the regions of the PIUE overlap with one or more other regions of the PIUE to which the data blocks correspond are overlapping with each other.