Rotation, scale, and translation resilient public watermarking for images using a log-polar fourier transform

• US 6,282,300 B1
• Filed: 01/21/2000
• Issued: 08/28/2001
• Est. Priority Date: 01/21/2000
• Status: Expired due to Term

• Current Assignee: NEC Corporation
• Original Assignee: Signafy Incorporated
• Inventors: Lin, Ching-Yung, Lui, Yui Man, Bloom, Jeffrey A., Cox, Ingemar J., Miller, Matthew L., Wu, Min
• Primary Examiner(s): Johns, Andrew W.
• Application Number:
  US09/489,752
• Time in Patent Office: 585 Days
• Field of Search: 382/100, 382/232, 382/280, 380/210, 380/252, 380/287, 380/54, 713/176, 370/527, 370/529
• US Class Current: 382/100
• CPC Class Codes: G06T 1/0064 Geometric transfor invariant watermarking, e.g. affine transform invariant
• CPC Class Codes: H04N 1/00005 relating to image data
• CPC Class Codes: H04N 1/00037 Detecting, i.e. determining the occurrence of a predetermined state H04N1/00031 takes precedence
• CPC Class Codes: H04N 1/00074 Indicating or reporting details of user-machine interface H04N1/0035
• CPC Class Codes: H04N 1/32154 Transform domain methods H04N1/32309 takes precedence
• CPC Class Codes: H04N 1/3216 using Fourier transforms
• CPC Class Codes: H04N 1/32267 combined with processing of the image
• CPC Class Codes: H04N 1/3232 Robust embedding or watermarking
• 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 method for detecting a watermark signal in digital image data. The detecting method includes the steps of: computing a log-polar Fourier transform of the image data to obtain a log-polar Fourier spectrum; projecting the log-polar Fourier spectrum down to a lower dimensional space to obtain an extracted signal; comparing the extracted signal to a target watermark signal; and declaring the presence or absence of the target watermark signal in the image data based on the comparison. Also provided is a method for inserting a watermark signal in digital image data to obtain a watermarked image. The inserting method includes the steps of: computing a log-polar Fourier transform of the image data to obtain a log-polar Fourier spectrum; projecting the log-polar Fourier spectrum down to a lower dimensional space to obtain an extracted signal; modifying the extracted signal such that it is similar to a target watermark; performing a one-to-many mapping of the modified signal back to log-polar Fourier transform space to obtain a set of watermarked coefficients; and performing an inverse log-polar Fourier transform on the set of watermarked coefficients to obtain a watermarked image.

Claims

1. A method for inserting a watermark signal in digital image data to obtain a watermarked image, the method comprising the steps of:

computing a log-polar Fourier transform of the image data to obtain a log-polar Fourier spectrum;

projecting the log-polar Fourier spectrum down to a lower dimensional space to obtain an extracted signal;

modifying the extracted signal such that it is similar to a target watermark;

performing a one-to-many mapping of the modified signal back to log-polar Fourier transform space to obtain a set of watermarked coefficients; and

performing an inverse log-polar Fourier transform on the set of watermarked coefficients to obtain a watermarked image.

2. The method of claim 1, wherein the projecting step comprises projecting the log-polar Fourier spectrum down to a one dimensional space.

3. The method of claim 1, further comprising the steps of:

computing a mixed signal between the extracted signal and the target watermark signal; and

modifying the image data such that the mixed signal becomes the extracted signal.

4. The method of claim 3, wherein mixed signal is computed by a weighted average of the target watermark signal and the extracted signal.

5. The method of claim 3, wherein the step of modifying the image data such that the mixed signal is the extracted signal comprises:

modifying all values in column j of the log-polar Fourier transform such that their logs sum to s_j instead of v_j, where s is the target watermark signal and v is the extracted signal;

inverting a log-polar resampling of the Fourier magnitudes, to obtain a modified Cartesian Fourier magnitude;

scaling the complex terms of the original Fourier transform to have the new magnitudes found in the modified Fourier transform; and

applying an inverse Fourier transform to obtain the watermarked image.

6. The method of claim 1, wherein the step of computing a log-polar Fourier transform of the image data to obtain a log-polar Fourier spectrum is approximated by the steps of:

padding the image data with black to obtain a larger padded image;

computing the discrete Fourier transform of the padded image; and

resampling the discrete Fourier transform in a log-polar grid using an interpolation technique.

7. The method of claim 6, wherein the interpolation technique comprises the step of linearly interpolating magnitudes of coefficients of the discrete Fourier transform.

8. A method for detecting a watermark signal in digital image data, the method comprising the steps of:

computing a log-polar Fourier transform of the image data to obtain a log-polar Fourier spectrum;

projrcting the log-polar Fourier spectrum down to a lower dimensional space to obtain an extracted signal;

comparing the extracted signal to a target watermark signal; and

declaring the presence or absence of the target watermark signal in the image data based on the comparison.

9. The method of claim 8, further comprising the step of applying a whitening filter to both the extracted signal and the target watermark signal prior to the comparing step.

10. The method of claim 8, wherein the comparing step further comprises the step of computing a correlation coefficient between the target watermark signal and the extracted signal.

11. The method of claim 10, further comprising the step of applying a whitening filter to both the extracted signal and the target watermark signal prior to computing the correlation coefficient.

12. The method of claim 10, wherein the declaring step comprises comparing the correlation coefficient to a detection threshold.

13. The method of claim 12, wherein the image data is declared to contain the target watermark if the correlation coefficient is above the detection threshold.

14. The method of claim 13, wherein the image data is declared to not contain the target watermark if the correlation coefficient is below the detection threshold.