Protecting images with an image watermark

• US 5,825,892 A
• Filed: 10/28/1996
• Issued: 10/20/1998
• Est. Priority Date: 10/28/1996
• Status: Expired due to Term

• Current Assignee: RPX Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Braudaway, Gordon Wesley, Mintzer, Frederick Cole
• Primary Examiner(s): Tarcza, Thomas H.
• Assistant Examiner(s): Laufer, Pinchus M.
• Application Number:
  US08/738,930
• Time in Patent Office: 722 Days
• Field of Search: 380/3, 380/4, 380/51, 380/54, 380/55, 399/366, 399/84, 382/100, 382/232, 382/274, 382/250, 382/280
• US Class Current: 380/51
• CPC Class Codes: G06T 1/005 Robust watermarking, e.g. average attack or collusion attack resistant
• CPC Class Codes: G06T 2201/0065 Extraction of an embedded watermark; Reliable detection
• CPC Class Codes: H04N 1/32203 Spatial or amplitude domain methods H04N1/32309 takes precedence
• CPC Class Codes: H04N 1/32208 involving changing the magnitude of selected pixels, e.g. overlay of information or super-imposition H04N1/32219, H04N1/32224 take precedence
• CPC Class Codes: H04N 1/32251 in multilevel data, e.g. greyscale or continuous tone data
• CPC Class Codes: H04N 2201/3233 of authentication information, e.g. digital signature, watermark
• CPC Class Codes: H04N 2201/327 which are undetectable to the naked eye, e.g. embedded codes

Abstract

A robust means of watermarking a digitized image with a highly random sequence of pixel brightness multipliers is presented. The random sequence is formed from `robust-watermarking-parameters` selected and known only by the marker and/or the marking entity. A watermarking plane is generated which has an element array with one-to-one element correspondence to the pixels of the digitized image being marked. Each element of the watermarking plane is assigned a random value dependent upon a robust random sequence and a specified brightness modulation strength. The so generated watermarking plane is imparted onto the digitized image by multiplying the brightness value or values of each pixel by its corresponding element value in the watermarking plane. The resulting modified brightness values impart the random and relatively invisible watermark onto the digitized image. Brightness alteration is the essence of watermark imparting. Detection of an imparted watermark requires knowing the watermarking plane with which the watermark was imparted. Regeneration of the watermarking plane requires knowledge of the robust-marking-parameters used in its formulation. This is generally only known to the marker and/or marking entity. Once regenerated the watermarking plane is used together with a verifying image located in a `visualizer` to demonstrate the existence of the watermark.

Claims

1. A method for imparting a watermark onto a digitized image comprising the steps of:

providing said digitized image comprised of a plurality of pixels, wherein each of said pixels includes brightness data that represents a brightness of at least one color; and

multiplying said brightness data associated with at least one of said pixels by a predetermined brightness multiplying factor, said brightness multiplying factor, "mf", is selected from the group consisting of (0.9<

="mf"1<

1), and (1<

"mf"<

=1).

2. A method as recited in claim 1, wherein each pixel is associated with a brightness data value that represents a brightness of a monochrome color.

3. A method as recited in claim 1, wherein each pixel is associated with a plurality of brightness data values.

4. A method as recited in claim 3, wherein said plurality of brightness data values represents brightness of a red, green and blue color.

5. A method as recited in claim 1, wherein said watermark has an invisibility classification level of undetectably invisible.

6. A method as recited in claim 1, wherein said brightness multiplying factor has a relationship with a number taken from a random number sequence.

7. A method as recited in claim 6, wherein said relationship is a linear remapping to provide a desired modulation strength.

8. A method as recited in claim 7, wherein said modulation strength is 10%.

9. A method as recited in claim 6, wherein each of said pixels has a row and a column location in an array representing said digitized image, and wherein said brightness multiplying factor employs a different sequential combination of numbers from said random number sequence in sequential correspondence to said row and column location.

10. A method as recited in claim 6, wherein said random number sequence is generated employing a National Standard Data Encryption Algorithm.

11. A method as recited in claim 6, wherein said sequence is formed from a plurality of watermarking parameters.

12. A method as recited in claim 11, wherein said watermarking parameters provide the watermark with at least one property of a robust invisible watermark.

13. A method for generating a watermarked image wherein a watermark is imparted onto a digitized image having a plurality of original elements having original brightnesses, said method comprising the steps of:

providing a digitized watermarking plane comprising a plurality of watermarking elements, each having a watermark brightness multiplying factor and having one-to-one correspondence with said original elements; and

producing a watermarked image by multiplying said original brightness of each of said original elements by said brightness multiplying factor of a corresponding one of said watermark elements wherein said watermark is invisible.

14. A method as recited in claim 13, in which said brightness multiplying factors are in a range of 0.9 to 1.1.

15. A method as recited in claim 13, in which said original image forms an original plane and said watermarking image forms a watermarking plane being smaller than said original plane, said method further comprising the step of extending said watermarking plane by tiling such that said watermarking plane at least covers said original plane.

16. A method as recited in claim 15, further comprising the step of truncating said watermarking plane such that said watermarking plane covers said original plane, upon determining that said watermarking plane extends beyond said original plane.

17. A method as recited in claim 15, wherein the watermarking plane is produced by employing an inverse discrete Fourier transform.

18. A method as recited in claim 13, wherein each of said original elements include a plurality of brightness values corresponding to a red, a green and a blue color component.

19. A method for imparting a watermark onto a digitized image comprising the steps of:

providing said digitized image comprised of a plurality of pixels, wherein each of said pixels includes brightness data that represents a brightness of at least one color; and

multiplying said brightness data associated with at least one of said pixels by a predetermined brightness multiplying factor in the range of 0.9 to 1.1,

wherein said brightness multiplying factor has a relationship with a number taken from a random number sequence, said relationship is a linear remapping to provide a desired modulation strength, and said modulation strength is less than 0.5 percent.

20. A method for imparting a watermark onto a digitized image comprising the steps of:

providing said digitized image comprised of a plurality of pixels, wherein each of said pixels includes brightness data that represents a brightness of at least one color; and

multiplying said brightness data associated with at least one of said pixels by a predetermined brightness multiplying factor in the range of 0.9 to 1.1,

wherein said brightness multiplying factor has a relationship with a number taken from a random number sequence, said relationship is a linear remapping to provide a desired modulation strength, said sequence is formed from a plurality of robust watermarking parameters, and said parameters comprise a cryptographic key, two coefficients and an initial value of said random number generator.

21. A method for forming a watermarking plane including a plurality of elements each having a brightness multiplying value, said method comprising:

generating a secure random sequence of integers having a first plurality of bits;

linearly remapping said random sequence to form a remapped sequence of brightness multiplying factors to provide a desired modulation strength;

computing a discrete Fourier transform of said remapped sequence to form a Fourier sequence having frequency coordinates;

expanding said frequency coordinates to form an expanded sequence;

computing an inverse Fourier transform of said expanded sequence to obtain a watermarking sequence of values; and

deriving said brightness multiplying values of said elements of said watermarking plane based upon said watermarking sequence of values.

22. A method as recited in claim 21, wherein said step of expanding is accomplished by zero-padding.

23. A method as recited in claim 21, wherein said watermarking elements form an array having an array size of 32 by 128.

24. A method as recited in claim 21, wherein said first plurality of bits is equal to sixteen bits and is formed from a second sequence of eight bit integers, by selecting abutted pairs of said eight bit integers.

25. A method as recited in claim 21, wherein said modulation strength is 1 percent, and said multiplying factors are within a range ranging from 0.98 to unity.

26. A method as recited in claim 21, further comprising the steps of hardclipping said watermarking sequence to form a hardclipped sequence having sequence members, and utilizing a different one of said sequence members to provide said multiplying value for each of said elements.

27. A method as recited in claim 21, further comprising:

adjusting said watermarking sequence to form a linearly remapped sequence of values having a mean and a median equal to the difference between unity and said modulation strength, and having a maximum of unity; and

employing said linearly remapped sequence to provide said multiplying value for each of said elements.

28. A method as recited in claim 21, further comprising:

providing an unmarked original image having a plurality of original pixels, each of said pixels having at least one brightness magnitude, wherein a first number of said original pixels is greater than a second number of said plurality of elements;

expanding said watermarking plane by tiling to cover said unmarked original image such that one of each of said pixels has one corresponding element from said elements; and

multiplying said at least one brightness magnitude of each of said pixels by said multiplying value of said corresponding element.