Automatic adjustment of image watermark strength based on computed image texture

• US 6,222,932 B1
• Filed: 11/26/1997
• Issued: 04/24/2001
• Est. Priority Date: 06/27/1997
• Status: Expired due to Fees

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Rao, Ravishankar, Mintzer, Frederick Cole
• Primary Examiner(s): Johns, Andrew W.
• Assistant Examiner(s): Dastouri, Mehrdad
• Application Number:
  US08/978,847
• Time in Patent Office: 1,245 Days
• Field of Search: 382/135, 382/137, 382/138, 382/100, 382/250, 382/280, 283/113, 162/110, 380/4, 380/51, 380/54, 713/176, 705/57
• US Class Current: 382/100
• CPC Class Codes: G06T 1/0028 Adaptive watermarking, e.g. Human Visual System [HVS]-based watermarking
• CPC Class Codes: G06T 1/0092 Payload characteristic determination in a watermarking scheme, e.g. number of bits to be embedded
• CPC Class Codes: G06T 2201/0202 whereby the quality of watermarked images is measured; Measuring quality or performance of watermarking methods; Balancing between quality and robustness
• 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 1/32229 with selective or adaptive application of the additional information, e.g. in selected regions of the image H04N1/32213 takes precedence
• CPC Class Codes: H04N 1/32251 in multilevel data, e.g. greyscale or continuous tone data
• CPC Class Codes: H04N 1/32309 in colour image data

Abstract

A system and method of automatically adjusts watermark strength of a watermarked image based on the content of the image. The described system includes a process by which the correct watermark strength is estimated with a model of preferred watermark strength based upon human perception as measured with a test group. Preferred watermark strength of a watermark within a watermarked reference image is determined and parameter constants associated with measured image texture values for the reference image are then determined. Image texture values are measured for a particular image to be watermarked, and these determined parameter constants are used to predict the strength of the watermark for the particular image. Images are watermarked by modifying the luminance of those pixels that fall within a mask specifying the watermark design. In this way, the chromaticity of the pixel remains constant but the luminance changes. The amount by which the luminance is changed is dependent on the desired strength of the watermark: a stronger watermark demands a larger change in the luminance.

Claims

1. A method of determining a watermark strength of a watermarked image, the watermarked image formed from an image and a watermark, the watermark strength corresponding to a reference watermark strength of a reference image and the reference image having a set of parameters associated with a measured texture value of the reference image, the method comprising the steps of:

a) changing the reference watermark strength until a human observer indicates that the watermark is visible;

b) measuring a texture value of at least a portion of the image; and

c) calculating the watermark strength for the watermarked image based on the measured texture value of the image and the set of parameters of the reference image.

2. The method of determining the watermark strength for a watermarked image as recited in claim 1, wherein the reference image is one of a set of original images, the reference watermark strength is determined from a plurality of appropriate watermark strengths for each of the plurality of original images, and the set of parameters are further associated with at least one measured texture value of each of the plurality of original images.

3. The method of determining the watermark strength for a watermarked image as recited in claim 2, wherein the set of parameters comprises a set of coefficients of a model, the set of coefficients in combination with the measured texture value of each of the plurality of original images providing the reference watermark strength.

4. The method of determining the watermark strength for a watermarked image as recited in claim 1, further comprising the step of c) adjusting a current strength of at least one selected portion of the watermark so as to provide the calculated watermark strength for the watermarked image.

5. The method of determining the watermark strength for a watermarked image as recited in claim 1, wherein the set of parameters is determined by (i) setting the reference watermark strength as a linear combination of at least one measured texture value of the reference image and the set of parameters, and (ii) determining each parameter value of the set of parameters using an estimation measure.

6. The method of determining the watermark strength for a watermarked image as recited in claim 5, wherein the step of measuring at least one texture value measures at least one of image inertia, image energy, image homogeneity, and image entropy for the watermarked image using a co-occurance matrix.

7. The method of determining the watermark strength for a watermarked image as recited in claim 5, wherein the step of measuring a texture value measures a local image energy of at least a portion of the image using at least one energy mask.

8. The method of determining the watermark strength for a watermarked image as recited in claim 4, wherein the image includes a plurality of image pixel values each having a corresponding pixel strength, the watermark includes a plurality of watermark pixel values each having a corresponding watermark pixel strength, the watermarked image is a combination of the plurality of image pixels with the plurality of watermark pixel values, and the adjusting step adjusts the watermark pixel strength of selected ones of the plurality of watermark pixel values combined with corresponding ones of the plurality of image pixel values so as to provide the calculated watermark strength of the watermarked image.

9. The method of determining the watermark strength for a watermarked image as recited in claim 4, wherein the image includes a plurality of frequency-domain transformed image coefficient values, the watermark includes a plurality of watermark values, the watermarked image is a combination of the plurality of frequency-domain transformed image coefficient values with the plurality of watermark values, and the adjusting step adjusts selected ones of the plurality of watermark values so as to provide the calculated watermark strength of the watermarked image.

10. An apparatus for setting a watermark strength of a watermark within a watermarked image, the watermarked image formed from an image and the watermark, the watermark strength corresponding to a reference watermark strength of a reference image and having a set of parameters associated with a measured texture value of the reference image, comprising:

receiving means for receiving from at least one human subject, an indication that the watermark is visible in said reference image which corresponds to said image;

measuring means for measuring a texture value of at least a portion of the image;

calculating means for calculating the watermark strength for the watermarked image based on the measured texture value of the image and the set of parameters of the reference image; and

adjusting means for adjusting a current strength of at least one portion of the watermarked image so as to provide the strength level for the watermarked image.

11. The apparatus for setting the watermark strength as recited in claim 10, wherein the set of parameters associated with the measured texture value of the reference image is determined by (i) setting the reference watermark strength as a linear combination of at least one measured texture value of the reference image and the set of parameters, and (ii) determining each parameter value of the set of parameters using an estimation measure.

12. The apparatus for setting the watermark strength as recited in claim 11, wherein the measuring means measures at least one texture value corresponding to at least one of image inertia, image energy, image homogeneity, and image entropy using a co-occurance matrix and local image energy using at least one energy mask.

13. The apparatus for setting the watermark strength as recited in claim 11, wherein the image includes a plurality of image pixel values each having a corresponding pixel strength, the watermark includes a plurality of watermark pixel values each having a corresponding watermark pixel strength, and the adjusting means adjusts the watermark pixel strength of selected ones of the plurality of watermark pixel values combined with corresponding ones of the plurality of image pixel values so as to provide the calculated watermark strength.

14. Apparatus for watermarking a plurality of images, each watermarked one of the plurality of images having a watermark strength, the apparatus comprising:

receiving means for receiving from at least one human subject, an indication that the watermark is visible in a reference image which corresponds to said image;

storage means for storing a plurality of watermarks and for storing a plurality of sets of parameters, each of the sets of parameters associated with at least one measured texture value of said reference image; and

processing means for processing a received one of the plurality of images with a selected one of the plurality of watermarks, the processing means including;

measuring means for measuring at least one texture value of at least a portion of the received one of the plurality of images;

selecting means for selecting one set of the plurality of sets of parameters based on the selected one of the plurality of watermarks and the received one of the plurality of images;

calculating means for calculating the watermark strength based on the at least one measured texture value and the selected set of the plurality of sets of parameters;

watermarking means for applying the selected one of the plurality of watermarks to the received one of the plurality of images to obtain a watermarked image having a current strength; and

adjusting means for adjusting the current strength level of the watermarked image to the calculated watermark strength to provide the one watermarked image.

15. The apparatus for applying a watermark to an image as recited in claim 14, wherein each set of parameters associated with at least one measured texture value of a corresponding reference image is determined by (i) setting the reference watermark strength as a linear combination of at least one measured texture value of the reference image and the corresponding set of parameters, and (ii) determining each parameter value of the set of parameters using an estimation measure.

16. A method of determining a watermark strength of a watermarked source image, the watermarked source image formed from a source image and a watermark, the watermark strength based on the source image and a reference watermark strength of a reference image and the reference image having a set of parameters associated with a measured texture value of the reference image, the method comprising the steps of:

a) selecting an image for use as a reference image;

b) determining the reference watermark strength by adjusting the strength of the watermark applied to said reference image until a human observer indicates that the watermark has been seen;

c) measuring the texture of said reference image; and

developing a set of parameters of the reference image that describe the texture for said reference image;

d) measuring a texture value of at least a portion of the source image; and

e) calculating the watermark strength for the watermarked image based on the measured texture value of the source image, the reference watermark strength, and the set of parameters of the reference image.

17. A method of determining a watermark strength of a watermarked source image, the watermarked source image formed from a source image and a watermark, the watermark strength based on the source image and a reference watermark strength of a reference image and a set of parameters associated with a measured texture value of said reference image, the method comprising the steps of:

a) selecting a set of prototype images;

b) for each of the prototype images;

determining a just-visible watermark strength by adjusting the strength of the watermark applied to said prototype image until a human observer indicates that the watermark has been seen; and

measuring the texture of said prototype image and developing a set of parameters that describe the texture for said prototype image;

c) selecting one of the prototype images as a reference image wherein the reference watermark strength is the just-visible strength of the selected prototype image and the set of reference parameters is the set of parameters describing the texture of said selected prototype image;

d) measuring a texture value of at least a portion of the source image; and

e) calculating the watermark strength for the watermarked image based on the measured texture value of the source image, the reference watermark strength, and the set of parameters of the reference image.