Optical watermark

• US 20020054680A1
• Filed: 03/16/2001
• Published: 05/09/2002
• Est. Priority Date: 09/15/2000
• Status: Active Grant

• Current Assignee: Trustcopy Pte Ltd.
• Original Assignee: Trustcopy Pte Ltd.
• Inventors: Huang, Sheng, Wu, Jian Kang
• Granted Patent: US 7,366,301 B2
• Time in Patent Office: Days
• US Class Current: 380/54
• CPC Class Codes: G06T 1/0071 using multiple or alternating watermarks
• CPC Class Codes: G06T 2201/0051 Embedding of the watermark in the spatial domain

Abstract

A multiple-layered watermark is generated to be placed on document, to protect against counterfeiting and forgery. Hidden information embedded into each of the watermark'"'"'s layers is only detectable by using a corresponding decoder. Because of the multiple-layered structure, it is difficult to reverse engineer the optical watermark. The generalized watermark structure significantly increases the “key space” of the decoder.

Claims

1. A method for producing a watermark on a document, the method including the step of:

a) determining a required plural number of watermark layers and a dot patterns for each of the plurality of watermark layers;

b) selecting at least one latent image object for each of the plural number of watermark layers and embedding each latent image object into its respective watermark layer;

c) superposing the watermark layers to form the watermark;

d) defining and generating a decoder for each of the number of watermark layers; and

e) applying the watermark to the document.

2. The method of claim 1, wherein the dot pattern for each of the number of watermark layers is a pattern defined on a two dimensional digital image plane.

3. The method of claim 2, wherein the dot pattern is a basic two-dimensional dot array.

4. The method of claim 2, wherein the dot pattern is a linear coordinate mapping of a basic two-dimensional dot array.

5. The method of claim 2, wherin the dot pattern also includes a non-linear coordinate mapping of a basic two-dimensional dot array.

6. The method of claim 2, wherein the dot pattern is a random distributed dot array

7. The method of claim 2, wherein the dot pattern is a result of a set of operations on a set of basic and other types of two-dimensional dot arrays.

8. The method of claim 1, wherein the latent image objects contain information which is critical to the application.

9. The method of claim 8, wherein the information is from the group consisting of:

copy, void, a critical name, and a number from the document.

10. The method of claim 1, wherein the latent image object is embedded into each watermark layer by a modulation method.

11. The method of claim 10, wherein the modulation method is phase modulation.

12. The method of claim 11, wherein the phase modulation includes a post-processing to smooth any abrupt phase changes along the edges of the latent image object.

13. The method of claim 1, wherein the decoder has a decoder structure related to a dot pattern structure of a carrier dot pattern of the watermark layer and in the direction where the latent image object is embedded.

14. The method of claim 13, wherein the relationship is selected from the group consisting of:

the same, and conjunct.

15. The method of claim 10, wherein after modulation the watermark layer and its decoder each carries part of the information of the latent image object which is generated based on the latent image object and a random function.

16. The method of claim 1, wherein the watermark layers are different to each other and are of sufficient difference to avoid interference between them as a result of them being superposed.

17. The method of claim 13, wherin one of the watermark layers is a counterfeit-proof layer, the decoder for which is a photocopier.

18. The method of claim 17, wherein after the superposition of the watermark layers with the counterfeit-proof layer there is included a post-processing step to remove dots which are too close to adjacent dots in a non-object area.

19. The method of claim 1 when included in a method for printing a document for protection and authentication including the steps of:

a) verifying the authenticity and copyright of the document before printing;

b) generating the watermark according to the method of claim 1;

c) controlling the printing process to protect the document and the watermark from attack; and

d) generating the decoder device and distributing the decoder device to enable verification of the authenticity of the document.

20. The method of claim 1, wherein the watermark is extended to colour documents.

21. The method of claim 20, wherein the watermark layers are in different colour channels in various colour spaces.

22. The method of claim 21, wherein colour spaces CMY/CMYK, HIS, XYA, Yuv and RGB are used.

24. The watermark of claim 23, wherein the dot pattern for each of the number of watermark layers is a dot pattern defined on a two dimensional digital image plane.

25. The watermark of claim 24, wherein the dot pattern is a basic two-dimensional dot array.

26. The watermark of claim 24, wherein said dot pattern for the watermark is a linear coordinate mapping of a basic two-dimensional dot array.

27. The watermark of claim 26, wherin the dot pattern also includes a non-linear coordinate mapping of a basic two-dimensional dot array.

28. The watermark of claim 24, wherein the dot pattern is a random distributed dot array

29. The watermark of claim 24, wherein the dot pattern is a result of a set of operations on a set of basic and other types of two-dimensional dot arrays.

30. The watermark of claim 23, wherein the latent image objects contain information which is critical to the application.

31. The watermark of claim 30, wherein the information is from the group consisting of:

copy, void, a critical name, and a number from the document.

32. The watermark of claim 23, wherein the latent image object is embedded into each watermark layer by modulation.

33. The watermark of claim 24, wherein the modulation is phase modulation.

34. The watermark of claim 33, wherein any abrupt phase changes along the edges of the latent image object are smoothed.

35. The watermark of claim 33, wherein there is included a decoder for each watermark layer which has a decoder structure related to a dot pattern structure of a carrier dot pattern of the relevant watermark layer and in the direction where the latent image object is embedded.

36. The watermark of claim 35, wherein the relationship is selected from the group consisting of:

the same, and conjunct.

37. The watermark of claim 35, wherein after modulation the watermark layer and its decoder each carries part of the information of the latent image object which is generated based on the latent image object and a random function.

38. The watermark of claim 23, wherein the watermark layers are different to each other and are of sufficient difference to avoid interference between them as a result of them being superposed.

39. The watermark of claim 35, wherein one of the watermark layers is a counterfeit-proof layer, the decoder for which is a photocopier.

40. The watermark of claim 39, wherein after superposition of the watermark layers with the counterfeit-proof layer dots which are too close to adjacent dots in a non-object area are removed.

41. The watermark of claim 23 included on or in a document for protection and authentication.

42. The watermark of claim 41, wherein the watermark is included on or in the document as a device selected from the group consisting of:

background, seal, logo, graphic device, trade mark and a word.

43. The watermark of claim 23, wherein the watermark is coloured.

44. The watermark as claimed in claim 43, wherein the watermark layers are in different colour channels in various colour spaces.

45. The watermark as claimed in claim 44, wherein colour spaces CMY/CMYK, HIS, XYZ, Yuv and RGB are used.

23. A watermark for use on a document, the watermark having:

a) a required plural number of watermark layers, each of the plural number of watermark layers each being a dot pattern;

b) at least one latent image object embedded into each watermark layer; and

c) the watermark layers being superposed to form the watermark