Electronic watermark insertion device

Details Electronic watermark insertion device Electronic watermark insertion device

1999-05-03

2003-01-21

Boudreau, Leo (Department: 2721)

Image analysis

Applications

C713S176000

Reexamination Certificate

active

06510233

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an electronic watermark insertion device suitable for digital images, and particularly to an electronic watermark insertion device for inserting electronic watermark into digital images.
Recently, illegal copies of digital images have become an important problem. Unlike analog images, since digital images are recognized by 0s and 1s, they can be repeatedly replicated without any degradation in image quality. This inherent feature has caused great damage to digital image copyright holders.
In order to prevent such illegal replication, it is considered to prepare a reproduction system that encrypts digital image data and has a valid secret decryption key whereby encrypted digital image data can be reproduced. However, after the encryption is once decoded, this system cannot prevent subsequent replication.
In order to prevent digital images from being illegally used or replicated, the method has been considered of burying special information (hereinafter referred to as “electronic watermark data”) in a digital image itself.
Two types of data including visible electronic watermark data and invisible electronic watermark data are considered as electronic watermark data for digital images.
The visible electronic watermark data, which contains special characters or symbols combined with an image, can be visually sensed. This electronic watermark data may degrade the image quality but has the advantage of visually warning users to prevent misappropriation of digital images.
An example of burying such visible electronic watermark data is disclosed in JP-A-241403/1996. This patent publication discloses a method of placing a visible watermark on a digital image. This method consists of the steps of supplying an original digital image, supplying a digital watermark image, and superimposing the watermark on the original image, without changing the chromaticity of pixels of the original image upon watermarking, to create a watermarked image. This method is characterized in that the image-creating step includes the step of varying the brightness (not color) of respective opaque pixels in a watermarked image to correct pixels corresponding to the original image. In this method, only the brightness of pixels corresponding to opaque portions of electronic watermark data is varied so that visible electronic watermark data is synthesized with the original image without changing the color components. The scaling value of varying the pixel brightness component depends on color components, random numbers, pixel values of electronic watermark data, or others. The prominence of the watermark is determined by the scaling value.
In invisible electronic watermark data, electronic watermark data is buried in an image, in consideration of degradation of image quality. Since the image quality degradation is not substantially negligible, the watermark cannot be visually recognized.
As described above, since special information, which can be recognized by an author, is buried as the electronic watermark data, the author can be specified by detecting the electronic watermark data even after illegal replication. Moreover, information about replication disapproval may be buried in an image. In such a case, when the reproduction unit, for example, detects the replication disapproval information, the reproduction by a VTR or the equivalent can be restricted by informing the user that the detected information is reproduction prohibited data, or by operating the replication preventing mechanism within the reproduction unit.
As one method of burying invisible electronic watermark data into digital images, special information is buried as electronic watermark data in portions not substantially affecting the image quality, such as LSBs (least significant bits) of pixel data. However, according to this method, the electronic watermark data can be easily removed from images. For example, information regarding LSBs of pixels will be missed using a low-pass filter. The image compression process discards the volume of information not adversely affecting the image quality, thus reducing the volume of data. This means that the electronic watermark data is lost. As a result, the problem is that it is difficult to re-detect the electronic watermark data.
JP-A-No. 315131/1994 shows another example of the electronic watermark burying method. This publication discloses the method of burying specific information by using the correlation between continuous frame images and detecting the area where degradation in image quality does not occur even when substitution is performed in peripheral areas upon reproduction. According to this method, an image is reconstituted by specifying an identification data buried area using the signal dropout portion and conversion information upon reproduction and then by correcting the corresponding portion.
As further another example, JP-A-No. 30466/1993 discloses the method of converting the frequency of a video signal and then burying information with signals of frequencies lower than the frequency band of the converted video signal. In this method, a broad band-pass filter extracts the original video signal while a low-pass filter extracts the buried identification data.
In another example, the method of frequency-converting images and then burying electronic watermark data into portions with strong frequency components of a video signal after the frequency conversion (see “NIKKEI Electronics”, 1996, 4.22 (no. 660), page 13). In this method, since electronic watermark data is buried into frequency components, the electronic watermark data is not lost through the compression process or filtering image process. Moreover, using the random numbers with a normal distribution as electronic watermark data makes it difficult to prevent interference between electronic watermark data and to destroy the electronic watermark data without significantly affecting the entire image.
In the electronic watermark data burying method, the original image is first transformed into frequency components by the DCT (discrete cosine transformation)
703
. n pieces of data with high values over high frequency range are selected as f(l), f(
2
), . . . , f(n). The electronic watermark data, w(l), w(
2
), . . . w(n), are selected from a normal distribution having an average of 0 and a dispersion of 1. The formula, F(i)=f(i)+&agr;×|f(i)|×w(i), where &agr; is a scaling factor, is calculated to obtain respective (i)s. Finally, the image in which the electronic watermark data is buried is obtained based on the frequency component in which f(i) is substituted for F(i).
Moreover, the electronic watermark data is detected according to the following method. In this detection method, both the original image and electronic watermark data candidate w(i) (where i=1, 2, . . . , n) must be known.
First, the image containing electronic watermark data is converted into frequency components through, for example, DCT. Values corresponding to factor values, f(
1
), f(
2
), . . . , f(n), each containing an electronic watermark, are set as F(
1
), F(
2
), . . . , F(n), respectively. The formula, W(i)=(F(i)−f(i))/f(i), is solved using f(i) and F(i) to extract the electronic watermark data W(i). Next, the statistical similarity C between w(i) and W(i) is obtained by the following formula including a vector inner product.
C=W•w
/(
WD×wD
)
where W=(W(
1
), W(
2
), . . . , W(n); w=(w(
1
), w(
2
), . . . , w(n)); WD=the absolute value of a vector W; wD=the absolute value of a vector w; and the symbol • represents an inner product.
When the statistical similarity C is more than a specific value, it is judged that the electronic watermark data is in a buried state.
The above-mentioned method, where the electronic watermark data is buried into an image, is effective when an author holding an original image detects digital image data suspected as an illegal replicate.
In the above-mentioned

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