U.S. patent application number 09/829206 was filed with the patent office on 2002-02-07 for method and apparatus for superposing a digital watermark and method and apparatus for detecting a digital watermark.
Invention is credited to Moriyama, Yoshiaki, Nakamura, Takeshi, Sugaya, Kazumi.
Application Number | 20020015509 09/829206 |
Document ID | / |
Family ID | 17790727 |
Filed Date | 2002-02-07 |
United States Patent
Application |
20020015509 |
Kind Code |
A1 |
Nakamura, Takeshi ; et
al. |
February 7, 2002 |
Method and apparatus for superposing a digital watermark and method
and apparatus for detecting a digital watermark
Abstract
An apparatus for superposing a digital watermark for superposing
digital watermark information on an information data signal bearing
original information superposes digital watermark information on an
information data signal bearing original information, for each unit
block consisting of a group of pieces of information data. This
apparatus is provided with an auxiliary embedding circuit 10 for
embedding second digital watermark information in the information
data; a blocking circuit 12 for dividing into the unit blocks the
changed information data signal obtained by the auxiliary embedding
circuit; and a main embedding circuit 20 for embedding first
digital watermark information in the changed information data
signal for each of the divided blocks. The second digital watermark
information bears information for identifying the position of the
divided unit block in a predetermined section of the information
data signal. On the basis of the second watermark information, a
detecting apparatus effects proper blocking with respect to the
information data signal with the watermark embedded therein, so as
to detect the first watermark information.
Inventors: |
Nakamura, Takeshi; (Saitama,
JP) ; Sugaya, Kazumi; (Saitama, JP) ;
Moriyama, Yoshiaki; (Saitama, JP) |
Correspondence
Address: |
SUGHRUE, MION, ZINN, MACPEAK & SEAS, PLLC
2100 Pennsylvania Avenue, N.W.
Washington
DC
20037-3213
US
|
Family ID: |
17790727 |
Appl. No.: |
09/829206 |
Filed: |
April 9, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09829206 |
Apr 9, 2001 |
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09152293 |
Sep 14, 1998 |
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6246775 |
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Current U.S.
Class: |
382/100 ;
375/E7.089; 375/E7.226 |
Current CPC
Class: |
H04N 1/32352 20130101;
H04N 1/32208 20130101; H04N 21/23892 20130101; G06T 2201/0061
20130101; H04N 1/32149 20130101; G06T 1/0071 20130101; H04N
2201/3233 20130101; G06T 2201/0065 20130101; H04N 19/467 20141101;
H04N 1/32165 20130101; G06T 2201/0052 20130101; H04N 19/60
20141101; H04N 2201/327 20130101; H04N 1/32203 20130101; G06T
1/0064 20130101; H04N 21/8358 20130101 |
Class at
Publication: |
382/100 |
International
Class: |
G06K 009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 17, 1997 |
JP |
P. HEI.9-293123 |
Claims
What is claimed is:
1. A method for superposing a digital watermark for superposing
digital watermark information on an information data signal bearing
original information, for each unit block consisting of a group of
pieces of information data, comprising the steps of: embedding
second digital watermark information in the information data;
dividing the changed information data signal thereby obtained into
the unit blocks; and embedding the first digital watermark
information in the changed information data signal for each of the
divided blocks, wherein the second digital watermark information
bears information for identifying a position of the divided unit
block in a predetermined section of the information data
signal.
2. The method for superposing a digital watermark according to
claim 1, wherein the information data signal is a video signal.
3. An apparatus for superposing a digital watermark for superposing
digital watermark information on an information data signal bearing
original information, for each unit block consisting of a group of
pieces of information data, comprising: auxiliary embedding means
for embedding second digital watermark information in the
information data; blocking means for dividing into the unit blocks
the changed information data signal obtained by said auxiliary
embedding means; and main embedding means for embedding first
digital watermark information in the changed information data
signal for each of the divided blocks, wherein the second digital
watermark information bears information for identifying a position
of the divided unit block in a predetermined section of the
information data signal.
4. The apparatus for superposing a digital watermark according to
claim 3, wherein the information data signal is a video signal.
5. The apparatus for superposing a digital watermark according to
claim 3 or 4, wherein the second watermark information is
information of a random pattern generated on the basis of an
M-sequence.
6. A method for detecting a digital watermark for detecting digital
watermark information from a changed information data signal
generated by superposing the digital watermark information on an
information data signal, for each unit block consisting of a group
of pieces of information data, comprising the steps of: detecting
second digital watermark information from the changed information
data signal, and recognizing a position of a divided unit block in
a predetermined section of the information data signal; dividing
the changed information data signal on the basis of the recognized
position into the unit blocks; and detecting first digital
watermark information for each of the divided unit blocks from the
changed information data signal.
7. The method for detecting a digital watermark according to claim
6, wherein the information data signal is a video signal.
8. An apparatus for detecting a digital watermark for detecting
digital watermark information from a changed information data
signal generated by superposing the digital watermark information
on an information data signal, for each unit block consisting of a
group of pieces of information data, comprising the steps of:
position-identifying-informati- on detecting means for detecting
second digital watermark information from the changed information
data signal, and recognizing a position of a divided unit block in
a predetermined section of the information data signal; blocking
means for dividing the changed information data signal on the basis
of the recognized position into the unit blocks; and information
detecting means for detecting from the changed information data
signal first digital watermark information for each of the unit
blocks divided by said blocking means.
9. The apparatus for detecting a digital watermark according to
claim 8, wherein the information data signal is a video signal.
10. The apparatus for detecting a digital watermark according to
claim 9, wherein the video signal bears an image of a letter box
type.
11. The apparatus for detecting a digital watermark according to
claim 9, wherein the video signal bears an image of a pan scan
type.
12. The apparatus for detecting a digital watermark according to
any one of claims 8 to 11, wherein said
position-identifying-information detecting means detects the second
digital watermark information from the changed information data
signal on the basis of information of a random pattern generated in
accordance with an M-sequence.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a technology in which
digital watermark information (hereafter referred to as the
watermark) is superposed on, or embedded in hidden form in, data on
image or sound data, and more particularly to a method for
embedding a watermark on an information data signal in block units
each consisting of a group of a small number of pieces of
information data, a method of detecting the watermark from the
information data signal with the watermark embedded therein, and a
method in accordance with these methods.
[0003] 2. Description of the Related Art
[0004] Currently, as optical recording media on which information
data can be written, DVD-Rs and DVD-RAMs are being put to practical
use, but in putting the DVD-Rs and DVD-RAMs to practical use, it is
necessary to devise measures for preventing illegal copying of
video and audio software and the like. Accordingly, attention is
being focused on a digital watermark technology in which copyright
information or information indicating a copy guard is expressed by
a watermark which is a visually or audibly inconspicuous image
pattern, and this watermark is superposed on the image data or the
sound data.
[0005] The basic system of such a digital watermark technology is
broadly classified into type types. One is a type in which sampled
values of waveform and pixels are provided with processing, and the
watermark is embedded therein. For instance, a technique in which
the watermark is added to luminance values of the image corresponds
to this type. As another type, there is a system in which the image
data and the sound data are converted to frequency components, and
the watermark is embedded in particular frequency components. The
fast Fourier transform (FFT), the discrete cosine transform (DCT),
and the like are used in such frequency conversion.
[0006] On the other hand, a technique is also known in which data
is divided into a plurality of small pixel blocks, and watermark
information is embedded (hereafter referred to as the block
division method) (see FIG.l). In this technique, image data is
divided into a plurality of small unit pixel blocks each having a
size of N.times.N pixels, and a watermark having the same block
size of this unit pixel block is added to the pixel block.
According to this technique, even if a portion of image data of one
frame is extracted, the watermark is left insofar as its size is
not smaller than that of the unit pixel block. On the reproduction
side, the embedded data is divided into similar unit pixel blocks,
and the watermark is detected for each unit pixel block and is
decrypted.
[0007] Here, a form is considered in which, for example, image data
in which a watermark is embedded is recorded on a disk such as a
DVD, and the image data is read from the disk and is
reproduced.
[0008] There are cases where a disk player for reading such a disk
is required to output a first image signal having an aspect ratio
of 16:9 for displaying an image on a so-called wide-screen
television, as well as a second image signal having an aspect ratio
of 4:3 for displaying an image on ann ordinary television. If it is
assumed that image data for forming a squeezed-type original image
with 480 [dots] high and 720 [dots] wide has been recorded on the
DVD, the disk player must perform aspect ratio conversion with
respect to the image data obtained from the DVD in order to
generate the first image signal. The same also applies to the
second image signal, and the disk player must perform the aspect
ratio conversion with respect to the image data obtained from the
DVD in order to generate the second image signal.
[0009] The aspect ratio conversion into the first image signal can
be basically accomplished by effecting interpolation of pixels in
the horizontal direction of the screen, as shown in FIG. 2A. Two
forms such as those shown in FIGS. 2B and 2C are conceivable as the
aspect ratio conversion into the second image signal.
[0010] As shown in FIG. 2B, the squeezed-type original image with
480 [dots] high and 720 [dots] wide recorded on the DVD is
converted into image data by being upsampled in such a manner that
the aspect ratio becomes 4:3. This is a converted image which is a
so-called pan scan, and assumes a form in which left- and
right-hand side end portions of the image based on the image data
obtained from the DVD are cut off so that the image assumes the
aspect ratio of 4:3. Meanwhile, as shown in FIG. 2C, the original
image recorded on the DVD is converted into image data by being
downsampled in such a manner that the aspect ratio becomes 4:3.
This is a converted image which is a so-called letter box, and
assumes a form in which predetermined images (e.g., strip-like
images of a black color) are pasted on upper and lower end portions
of the image based on the image data obtained from the DVD so that
the image assumes the aspect ratio of 4:3.
[0011] In the same way as the first image signal, the second image
signal converted into such a pan scan or letter box image can be
recorded on a recording medium such as the aforementioned DVD-R or
DVD-RAM by the DVD- recorder, for example. At this time, however,
in the DVD recorder, the watermark embedded in the second image
signal to prevent an act of infringement of the copyright is
detected and decrypted. If it is detected that the relevant
watermark bears, for instance, information indicating prohibition
of copying, the DVD recorder prohibits its own recording operation,
and does not record the second image signal even if a recordable
DVD is supplied thereto. On the other hand, only when it is
detected that the relevant watermark bears information indicating
that copying is allowed, the DVD recorder is able to record the
second image signal on the DVD supplied thereto.
[0012] In the case of FIG. 2A, since the DVD player effects
conversion into the information of the 16:9 image by using all the
information of the squeezed-type original image and by expanding
the information, in the block division method, the upper leftmost
unit pixel block in the original image, for example, is positioned
in the converted 16:9 image at the same upper leftmost position by
changing its size. Therefore, the DVD recorder is capable of
properly detecting and decrypting the watermark consecutively
starting with, for example, this upper leftmost unit pixel block of
the 16:9 image data supplied thereto.
[0013] However, as can be seen from what is shown in FIGS. 2B and
2C, in the case of the images such as the pan scan and the letter
box, their pixel positions and sizes have changed from those of the
original images, so that the recognition of the unit pixel blocks
in the DVD recorder needs to be made different from that in the
case of FIG. 2A.
[0014] To give a more detailed description, in the case of FIG. 2B,
since the DVD player effects conversion into the information of the
4:3 image by eliminating the information of left- and right-hand
side portions of the squeezed-type original image and by expanding
the remaining information, in the block division method, the upper
leftmost unit pixel block in the original image is already missing
in the converted 4:3 image. Further, even if, for example, the
third unit pixel block from left in the uppermost row is present in
the converted 4:3 image, there is a high possibility that the 4:3
image starts midway in the relevant block, as illustrated in the
drawing.
[0015] In such a case, if the DVD recorder starts detection of the
watermark without recognizing that, for example, the third unit
pixel block from the left in the 4:3 image data supplied thereto is
fragmentary, the DVD recorder fails to properly detect not only
that third unit pixel block but also ensuing unit pixel blocks.
[0016] In addition, in the case of FIG. 2C, since the DVD player
effects conversion into the information of the 4:3 image by
synthesizing predetermined images on upper and lower sides of the
squeezed-type original image by expanding the synthesized image
information, in the block division method, the leading upper
leftmost unit pixel block in the original image does not correspond
to the upper leftmost portion of the converted 4:3 image, and is
positioned by being moved to the leftmost position in the row
immediately below the upper predetermined image used in the
synthesis.
[0017] In such a case, if the DVD recorder starts detection of the
watermark without recognizing that the relevant leading unit pixel
block of the supplied 4:3 image data has shifted from the leftmost
position to the moved position in the image (frame image), the DVD
recorder naturally cannot detect the watermark from the
predetermined image where the watermark is not embedded, and this
state will continue for a long period. Depending on the watermark
detection process, if the watermark-undetected state continues for
a long period of time, a error may be outputted, or the image may
be handled as one in which the watermark is not present in this
image.
SUMMARY OF THE INVENTION
[0018] The present invention has been devised in view of the
above-described problems, and an object of the invention is to
provide a method and apparatus for superposing a digital watermark
on image data and a method and apparatus for detecting a digital
watermark which make it possible to properly recognize a unit pixel
block in the block division method and reliably detect watermark
information.
[0019] Another object of the present invention is to provide a
method and apparatus for superposing a digital watermark on image
data and a method and apparatus for detecting a digital watermark
which make it possible to properly recognize a unit pixel block and
reliably detect watermark information even after image data is
subjected to aspect ratio conversion.
[0020] In accordance with the present invention, there is provided
a method for superposing a digital watermark for superposing
digital watermark information on an information data signal bearing
original information, for each unit block consisting of a group of
a small number of pieces of information data, comprising the steps
of: embedding second digital watermark information in the
information data; dividing the changed information data signal
thereby obtained into the unit blocks; and embedding first digital
watermark information in the changed information data signal for
each of the divided blocks, wherein the second digital watermark
information bears information for identifying a position of the
divided unit block in a predetermined section of the information
data signal.
[0021] In such a method, the information data signal may be a video
signal.
[0022] In accordance with the present invention, there is provided
an apparatus for superposing a digital watermark for superposing
digital watermark information on an information data signal bearing
original information, for each unit block consisting of a group of
a small number of pieces of information data, comprising: auxiliary
embedding means for embedding second digital watermark information
in the information data; blocking means for dividing into the unit
blocks the changed information data signal obtained by the
auxiliary embedding means; and main embedding means for embedding
first digital watermark information in the changed information data
signal for each of the divided blocks, wherein the second digital
watermark information bears information for identifying a position
of the divided unit block in a predetermined section of the
information data signal.
[0023] In such an apparatus, the information data signal may be a
video signal.
[0024] In addition, the second watermark information may be
information of a random pattern generated on the basis of an
M-sequence.
[0025] In accordance with the present invention, there is provided
a method for detecting a digital watermark for detecting digital
watermark information from a changed information data signal
generated by superposing the digital watermark information on an
information data signal, for each unit block consisting of a group
of a small number of pieces of information data, comprising the
steps of: detecting second digital watermark information from the
changed information data signal, and recognizing a position of a
divided unit block in a predetermined section of the information
data signal; dividing the changed information data signal on the
basis of the recognized position into the unit blocks; and
detecting first digital watermark information for each of the
divided unit blocks from the changed information data signal.
[0026] In such a method, the information data signal may be a video
signal.
[0027] In accordance with the present invention, there is provided
an apparatus for detecting a digital watermark for detecting
digital watermark information from a changed information data
signal generated by superposing the digital watermark information
on an information data signal, for each unit block consisting of a
group of a small number of pieces of information data, comprising
the steps of: position-identifying-information detecting means for
detecting second digital watermark information from the changed
information data signal, and recognizing a position of a divided
unit block in a predetermined section of the information data
signal; blocking means for dividing the changed information data
signal on the basis of the recognized position into the unit
blocks; and information detecting means for detecting from the
changed information data signal first digital watermark information
for each of the unit blocks divided by the blocking means.
[0028] In such an apparatus, the information data signal may be a
video signal.
[0029] Further, the video signal may bear an image of a letter box
type or an image of a pan scan type.
[0030] Furthermore, in each form of the apparatus for detecting a
digital watermark, the position-identifying-information detecting
means may detect the second digital watermark information from the
changed information data signal on the basis of information of a
random pattern generated in accordance with an M-sequence.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a schematic diagram illustrating a form in which
digital watermark information is embedded in image data by a block
division method;
[0032] FIGS. 2A to 3C are schematic diagrams illustrating various
forms of aspect ratio conversion from a squeezed-type image
recorded on a DVD;
[0033] FIG. 3 is a block diagram illustrating a schematic
configuration of a watermark superposing apparatus in accordance
with an embodiment of the present invention;
[0034] FIG. 4 is a block diagram illustrating a schematic
configuration of a DVD recorder in which the watermark detecting
apparatus in accordance with an embodiment of the present invention
is adopted;
[0035] FIG. 5 is a time chart illustrating by way of example the
relationship among an image f' in which the watermark information
is embedded, a cross-correlation function R.sub.f',n(t) of random
noise n, and an auto-correlation function R.sub.n(t) of the random
noise n;
[0036] FIG. 6 is a characteristic diagram of an auto-correlation
function Rm(t) of a sequence m.sub.k in which "0" and "1" in an
M-sequence a.sub.k with a period N are made to correspond to -1 and
+1, respectively;
[0037] FIG. 7 is a flowchart illustrating a
position-identifying-watermark embedding operation effected by the
apparatus shown in FIG. 3;
[0038] FIG. 8 is a flowchart illustrating the characteristic
operation effected by the blocking circuit and the
watermark-information detecting circuit shown in FIG. 4;
[0039] FIG. 9 is a flowchart illustrating the procedure of
letter-box determination processing in Step S3 shown in FIG. 8;
and
[0040] FIG. 10 is a flowchart illustrating the procedure of
position identification processing in Step S6 shown in FIG. 8.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0041] Referring now to the drawings, a detailed description will
be given of the embodiments of the present invention.
[0042] FIG. 3 shows a schematic configuration of a watermark
superposing apparatus for generating a watermark in accordance with
a water mark superposing method based on the present invention and
for superposing the watermark on an input video signal.
[0043] In FIG. 3, the input vide signal which is a digital signal
of a sequence of sampled values is supplied to an
auxiliary-watermark-informat- ion embedding circuit 10 where an
auxiliary (second) watermark is embedded in the overall frame image
so as to be provided in hidden form. This auxiliary embedding
circuit 10 can be realized by a configuration including a level
adder. This embedded video signal is supplied to a blocking circuit
12.
[0044] The blocking circuit 12 divides the supplied video signal
into groups of a small number of pieces of information data, e.g.,
pixel blocks each consisting of data of 8 pixels wide.times.8
pixels high in a frame image, and supplies them to a discrete
cosine transform (DCT) circuit 13. The DCT circuit 13 performs
two-dimensional DCT operations with respect to such pixel data
blocks each consisting of 8.times.8 pixel data so as to obtain DCT
coefficients DC1 to DC64 corresponding to 64-system frequency
components, and supplies these DCT coefficients DC1 to DC64 to a
main-watermark-information embedding circuit 20. This
main-watermark-information embedding circuit 20 can be realized by
a coefficient adder.
[0045] Values which bear a copy control watermark, i.e., a main
first watermark different from a position identifying (positioning)
watermark, i.e., the aforementioned second watermark, is also
supplied to the embedding circuit 20. The copy control watermark
indicates whether or not, after an output video signal from this
apparatus has been recorded on a disk, the video signal obtained by
reading that disk can be copied, i.e., recorded. The position
identifying watermark embedded in the preceding stage of the
blocking circuit 12 is for allowing the position of a unit pixel
block in a predetermined section, e.g., a frame, of the video
signal to be recognized correctly, and its details will be
clarified by a description which will be given later.
[0046] The embedding circuit 20 adds the values bearing the copy
control watermark to particular ones of the DCT coefficients DCl to
DC64, and outputs values of the result of the addition. As a
result, the first and second watermarks are embedded in particular
frequency components in the supplied video signal. Since the
embedding circuit 20 embeds the first watermark with respect to the
video signal in which the second watermark has been embedded, the
video signal in which both the first and second watermarks have
been embedded is consequently obtained from that output.
[0047] The 64 DCT coefficients in which the watermarks are thus
embedded are supplied to an inverse DCT circuit 27. The inverse DCT
circuit 27 obtains image data blocked for each 8.times.8 pixels by
performing a two-dimensional inverse DCT operation for each
8.times.8 pixel data block with respect to the supplied 64 DCT
coefficients. At this time, the first and second watermarks are
embedded in the image data obtained by the inverse DCT circuit
27.
[0048] A format converting circuit 29, which has the function of
resynthesizing the video signal and to which the image data is
supplied from the inverse DCT circuit 27, rearranges the image data
of each block to a position corresponding to the horizontal
scanning line of the screen (frame), is restored to a digital video
signal format, and is outputted.
[0049] Such an output video signal is subjected to predetermined
coding by an unillustrated MPEG (Moving Picture Experts Group)
encoder, and is recorded on the DVD after undergoing various signal
processing.
[0050] The DVD recorded in the above-described manner is read by an
unillustrated DVD player. The DVD player performs demodulation and
decoding with respect to the relevant DVD read signal, and effects
aspect ratio conversion such as the one described in the prior art
(FIGS. 2A to 2C) to obtain the first or second image data of the
wide-screen television, pan scan, or letter box type, and outputs
the same as an analog video signal.
[0051] Such an analog video signal is supplied to a DVD recorder,
and serves as a signal subject to recording on a disk such as
DVD-RAM or a DVD-R by the DVD recorder.
[0052] In the DVD recorder, the watermark is detected from the
supplied video signal in accordance with the watermark detecting
method of the present invention. THE DVD recorder controls the
operation of prohibiting the recording operation in accordance with
the watermark thus detected. FIG. 4 shows a schematic configuration
of such a DVD recorder.
[0053] In FIG. 4, an A/D converter 50 generates a digital video
signal by digitizing the video signal bearing the first or second
image data, and supplies it to a blocking circuit 51 and a
position-identifying-informati- on detecting circuit 6A. The
blocking circuit 51 divides the inputted digital video signal into
pixel blocks each consisting of 8.times.8 pixel data, and supplies
them to a discrete cosine transform (DCT) circuit 52. The
position-identifying-information detecting circuit 6A detects the
first watermark, i.e., the position identifying watermark, from the
inputted digital video signal, and sends in-frame position
information in unit blocks based on that watermark to the blocking
circuit 51.
[0054] In such division into blocks, the blocking circuit 51
confirms the leading position of the unit pixel blocks based on the
position identifying watermark. Namely, the blocking circuit 51
effects the division into blocks while properly recognizing the
blocks on the basis of the position identifying watermark. The
details thereof will be described later.
[0055] The DCT circuit 52 performs two-dimensional DCT operations
with respect to the pixel blocks in units of 8.times.8 pixel data
determined by the blocking circuit 51 so as to obtain the DCT
coefficients DC1 to DC64 corresponding to the 64-system frequency
components, and supplies these DCT coefficients DC1 to DC64 to a
watermark-information detecting circuit 60. The
watermark-information detecting circuit 60 detects the
aforementioned copy control watermark for each pixel block in units
of the 8.times.8 pixel data and decrypts it, and supplies a signal
corresponding to the contents of information indicated by the
relevant watermark to a video-signal recording system 70.
[0056] The video signal, which is a digital output from the A/D
converter 50, is being supplied to the video-signal recording
system 70. Such a video signal is subjected to code conversion
processing similar to that of the aforementioned MPEG encoder, and
is also subjected to various signal processing for recording on the
recordable DVD 8, thereby generating a recording video signal.
Recording on the DVD 8 is made correspondingly. In response to the
watermark signal supplied from the watermark-information detecting
circuit 60, the video-signal recording system 70 prohibits the
recording operation if the watermark indicates that copying to the
DVD 8 is prohibited. On the other hand, if the watermark indicates
that copying is allowed, the video-signal recording system 70
enables the recording operation.
[0057] Next, a detailed description will be given of the basic
principle of embedding and detecting the position identifying
watermark based on the configurations shown in FIGS. 3 and 4.
[0058] [A] Basic Principle 1 (Position Identification Using
Correlation Function
[0059] This basic principle is based on the position identification
using a correlation function.
[0060] It is now assumed that random noise n which is independent
as the position identifying watermark in the
auxiliary-watermark-information embedding circuit 10 is added to
the image inputted to the auxiliary-watermark-information embedding
circuit 10, i.e., an original image f, and that a new image
obtained from the output of the inverse DCT circuit 27, for
example, is f'. That is, it is assumed that there is a relationship
of f'=f+n.
[0061] In this case, the cross-correlation function R.sub.f',.sub.n
between f' and n is as follows: 1 R f , n ' ( t ) = ( 1 / N ) k = 0
N - 1 f k ' n k + t = ( 1 / N ) k = 0 N - 1 ( f k + n k ) n k + t =
( 1 / N ) k = 0 N - 1 f k n k + t + ( 1 / N ) k = 0 N - 1 n k n k +
t = R f , n ( t ) + R n ( t ) ( 1 )
[0062] Thus, because f'=f+n, it can be understood that
R.sub.f'/n(t) is the sum of the cross-correlation function
R.sub.f,n(t) between the original image f and the random noise n
and the auto-correlation function R.sub.n(t) of the random noise n,
and that R.sub.f'/n(t)=R.sub.f,n(t)+R.s- ub.n(t) is derived.
[0063] Here, since the original image f and the random noise n are
mutually independent,
R.sub.f,n(t)=0 (2)
[0064] Therefore,
R.sub.f'.sub.,n(t)=R.sub.n(t) (3)
[0065] Accordingly, it can be understood that if the
auto-correlation function R.sub.n of the noise is known, it is
possible to identify the position by providing its matching in the
position-identifying-informatio- n detecting circuit 6A.
[0066] Incidentally, an illustration of the relationship between
R.sub.n(t) and R.sub.f',n(t) is shown in FIG. 5.
[0067] [B] Basic Principle 2 (Use of M-sequence)
[0068] More specifically, the M-sequence can be applied to the
aforementioned random noise, and the recurrence of the M-sequence
with a period N can be used as a random pattern of the noise.
[0069] The M-sequence referred to herein is one of pseudorandom
signals which is the so-called maximum length sequence. As for the
M-sequence, a detailed description is given in a publication
entitled "M-sequence and its applications" (author: Hiroshi
Kashiwagi; published by Shoukoudou Co. Ltd. on Mar. 25, 1996).
[0070] The auto-correlation function Rm(t) of a sequence m.sub.k in
which "0" and "1" in an M-sequence a.sub.k with a period N are made
to correspond to -1 and +1, respectively, has the characteristic
that the following formula holds: 2 Rm ( t ) = ( 1 / N ) i = 0 N -
1 m i m i + t = 1 ( t = kN where k is an integer ) or = - 1 / N ( t
kN where k is an integer ) ( 4 )
[0071] The M-sequence has a sharp auto-correlation which is
advantageous in the identification of the position. As can be
appreciated from the above Formulae, the M-sequence exhibits a
sharp peak only when the position has matched, and assumes a value
which is substantially close to zero at other locations. By
detecting this peak, it is possible to effect the identification of
the position.
[0072] To improve such a detecting capability, the M-sequence
a.sub.k which is used should preferably have as long a period N as
possible, and it is preferable to select an M-sequence a.sub.k
which has good independence with respect to the original image.
[0073] Incidentally, FIG. 6 shows a characteristic diagram of
Rm(t).
[0074] [C] Embedding Method
[0075] Next, a description will be given of an actual method of
embedding the position identifying watermark.
[0076] First, the position identifying watermark is fixed between
frames, and is set as a random pattern m.sub.x,y extended
two-dimensionally on the basis of the M-sequence. In addition, as
the period N of the M-sequence a.sub.k used, a value greater than
the number of pixels of one frame is selected. Further, the
generation of the M-sequence a.sub.k which forms the basis in the
random pattern generation is effected by a shift register which
makes use of primitive polynomials. In other words, it is
unnecessary for the random pattern generator to be provided in the
form of a ROM. Further, an initial value of the shift register at
this time is fixed, so that the random pattern which is generated
is made identical between the frames.
[0077] The random pattern m.sub.x,y used is changed as shown in the
formula below through the value of the M-sequence a.sub.k
generated. In this formula, H and V denote the numbers of pixels in
the horizontal direction and the vertical direction in the image
(frame image), and B denotes a parameter of strength.
m.sub.x,y=B (a.sub.Hy+=1
[0078] or
=(-B+1) or (-B) (a.sub.Hy+x=0) (5)
[0079] The random pattern m.sub.x,y thus generated is added to the
overall original image.
[0080] At this juncture, an optimum strength B is selected by
taking the detecting capability and image quality into
consideration. Namely, since the strength B exerts a large
influence on the image quality and the detecting capability, it
suffices if an optimum value is determined after attempting the
embedding with a number of values.
[0081] [D] Detecting Method
[0082] Meanwhile, an actual method of detecting the position
identifying watermark is as follows.
[0083] The M-sequence a.sub.k used herein is used as being known,
and a sequence I.sub.i,j(x,j) of the following formula is
obtained.
I.sub.(i,j)x,y=1 (a.sub.H(y+j)+(x+i)=1)
[0084] or
=-1 (a.sub.H(y+j)+(X+i)=0 (6)
[0085] where it is assumed that, as for the size of the original
image, the number of pixels in the horizontal direction is H, and
the number of pixels in the vertical direction is V.
[0086] Then, the cross-correlation between this sequence
I.sub.i,j(x,j) and the original image f.sub.x,y is calculated.
[0087] The range of deviation between the original image and the
input image (the image with the watermark supplied to the blocking
circuit 51 and the position-identifying-information detecting
circuit 6A) is already known in many cases. Accordingly, (i, j) is
changed within such a range, and the aforementioned calculation,
i.e., the calculation of the cross-correlation between this
sequence I.sub.i,j(x,j) and the original image f.sub.x,y, is
repeated.
[0088] The combination of (i, j) at he time when the peak value of
the cross-correlation is assumed in the result of such calculation
indicates the position which corresponds to the upper left pixel in
the original image frame (the leading-end pixel of the leading unit
pixel block) in the input image frame.
[0089] By adopting such a procedure, it becomes possible to
identify the final position of the block in the image data after
the aspect ratio conversion. Subsequently, if detection of the copy
control watermark is effected for each block on the basis of the
position information thus obtained, proper detection can be
effected.
[0090] FIG. 7 shows the principal characteristic operation of the
auxiliary-watermark-information embedding circuit 10 shown in FIG.
3.
[0091] In FIG. 7, the embedding circuit 10 initializes the
aforementioned shift register (Step S01), and stores 0 in x and y,
respectively (Step S02).
[0092] Then, the embedding circuit 10 determines the aforementioned
M-sequence a.sub.k (Step S03), and determines the aforementioned
random pattern m.sub.x,y (Step S04). Further, the embedding circuit
10 accomplishes the embedding of the position identifying watermark
by adding m.sub.x,y to the original image f.sub.x,y (Step S05).
Then, the embedding circuit 10 changes the values of x, y (Step
S06), and repeats the processing in Steps S03 to S07 until the
setting of x, y for a predetermined image region and the embedding
of the watermark based thereon is completed (Step S07). If the
processing for the predetermined image region is completed, the
embedding circuit 10 executes the embedding processing of the
position identifying watermark with respect to an ensuing frame
image.
[0093] FIG. 8 is a flowchart illustrating the principal
characteristic operation of the blocking circuit 51, the
position-identifying-informatio- n detecting circuit 6A, and the
watermark-information detecting circuit 60 which are shown in FIG.
4.
[0094] In FIG. 8, if a digitized video signal is inputted from the
A/D converter 50 (Step S1), the blocking circuit 51 determines
whether or not the copy control watermark has been detected (Step
S2). In this determination, a flag signal (watermark detection
flag) is used which indicates whether or not it has been possible
to detect the copy control watermark and which is issued from the
watermark-information detecting circuit 60.
[0095] If the video signal from the A/D converter 50 bears the
aforementioned first image data, the matching characteristic of the
unit pixel block in the frame image, which is formed by the video
signal, with respect to the original image is not destroyed in many
cases. Hence, the division into blocks which is effected by the
blocking circuit 51 is effected on the basis of predetermined
position coordinates, with the result that the
watermark-information detecting circuit 60 is capable of properly
detecting and encrypting the copy control watermark with respect to
the divided unit pixel blocks (the watermark is successfully
detected).
[0096] On the other hand, if the video signal from the A/D
converter 50 bears the aforementioned second image data, the
matching characteristic of the unit pixel block in the frame image,
which is formed by the video signal as described earlier with
reference to FIGS. 2A and 2C, with respect to the original image is
destroyed. To cope with this situation, the
position-identifying-information detecting circuit 6A determines
whether or not the relevant video signal is of the letter body type
(Step S3). The video signal which is not determined to be of the
letter box type can be determined to be of the pan scan type.
[0097] If the video signal is of the letter box type, the blocking
circuit 51 upsamples the video signal and effects aspect conversion
in the direction opposite to the converting direction shown in FIG.
2C so as to return the image to the original image (Step S4). On
the other hand, if the video signal is of the pan scan type, the
blocking circuit 51 downsamples the video signal and effects aspect
conversion in the direction opposite to the converting direction
shown in FIG. 2B so as to return the image to the original image
(Step S5).
[0098] After the image is thus returned to the original image, the
position-identifying-information detecting circuit 6A effects the
identification of the image position by using the position
identifying watermark embedded in the video signal, more
specifically the proper positioning of the unit pixel blocks to be
divided with respect to the video signal (Step S6). The details of
the image position identification processing in this step will be
described later.
[0099] After the identification of the image position, the blocking
circuit 51 effects the division of the video signal into blocks on
the basis of the position of the unit pixel blocks thus determined,
and the watermark-information detecting circuit 60 detects and
encrypts the copy control watermark with respect to the divided
unit pixel blocks (Step S7). If proper block division has been
effected, the aforementioned watermark detection flag outputted
from the watermark-information detecting circuit 60 is set in a set
state (the watermark is successfully detected), and the
video-signal recording system 70 prohibits/allows the recording
operation with respect to the DVD 8 in accordance with the contents
of the copy control watermark detected. If, to the contrary, on the
grounds that proper block division has not been effected, the
watermark detection flag outputted from the watermark-information
detecting circuit 60 is not set in the set state (the watermark is
not detected), the video-signal recording system 70 effects
processing corresponding to it, e.g., prohibits the recording
operation with respect to the DVD 8.
[0100] If the watermark is not detected, similar processing is
effected with respect to a predetermined number of other frames as
well. If, here again, the watermark cannot be detected from the
predetermined number of frames, a final determination is made that
the watermark detection is unsuccessful.
[0101] The procedure of the processing in Step S3 is shown in FIG.
9.
[0102] In FIG. 9, the position-identifying-information detecting
circuit 6A first sets the value of a threshold th (Step S31). Then,
the position-identifying-information detecting circuit 6A binarizes
the input vide signal on the basis of the value of this threshold
th (Step S32).
[0103] With respect to the binarized data, a determination is made
by the position-identifying-information detecting circuit 6A as to
whether or not strip-like images with values 0 are present in
predetermined image regions, i.e., in upper and lower portions of
the frame image (Step S33). Such strip-like images are peculiar to
the letter box image shown in FIG. 2C, and correspond to the images
provided at the upper and lower portions of the original image.
[0104] Accordingly, if these strip-like images are present, it can
be determined that the relevant video signal bears the letter box
image, and if not, it can be determined that the video signal bears
the pan scan image.
[0105] The procedure of the processing in Step S6 shown in FIG. 8
is shown in FIG. 10.
[0106] In FIG. 10, the position-identifying-information detecting
circuit 6A first stores 0s in i, j, respectively, so as to
initialize i, j (Step S61), and stores 0s in s, y, and S as well
(Step S62).
[0107] Next, the position-identifying-information detecting circuit
6A determines the M-sequence a.sub.k (Step S63), and determines the
random pattern I.sub.(i,j)x,y in accordance with the aforementioned
formula (6) (Step S64 ). The position-identifying-information
detecting circuit 6A calculates the product of the thus-determined
sequence I.sub.(i,j)x,y and the original image f.sub.x,y, and adds
the result of the calculation to S (Step S65). Then, the
position-identifying-information detecting circuit 6A changes x, y
(Step S66), and repeats the processing in Steps S63 to S66 until
the calculation with respect to the predetermined image region is
completed (Step 67).
[0108] If the summing calculation with respect to the predetermined
image region is completed, the correlation coefficient becomes
definite. The position-identifying-information detecting circuit 6A
replaces the value of S as the definite cross-correlation R.sub.i,j
(Step S68). Then, the blocking circuit 51 changes i, j (Step S69),
and repeats the processing in Steps S62 to S69 until the setting of
i, j with respect to the predetermined search region and the
calculation based thereon are completed (Step S6A).
[0109] Upon completion of the calculation processing of i, j with
respect to the predetermined search region, the
position-identifying-information detecting circuit 6A detects a
maximum value among the values of the cross-correlations R.sub.i,j
stored in Step S68, and recognizes that (i, j) corresponding to
that detected maximum value designates the identified position
(Step S6B). After the recognition, the processing of this flowchart
ends.
[0110] Thus, when the data of the pan scan image is inputted, the
position-identifying-information detecting circuit 6A is capable of
recognizing that the third unit pixel block from the left in the
4:3 image, such as the one shown in FIG. 2B, is fragmentary, and of
correctly starting the detection of the copy control watermark
beginning with the ensuing unit pixel block which is not
fragmentary. As a result, it is possible to effect the correct
detection of the watermark with respect to the unit pixel blocks
following the third unit pixel block, so that the drawback which is
conventionally encountered does not occur.
[0111] In addition, in a case where the data of the letter box
image is inputted, the position-identifying-information detecting
circuit 6A is capable of recognizing that the relevant leading unit
pixel block in the 4:3 image, such as the one shown in FIG. 2C, has
shifted from the leftmost position to the moved position in the
image (frame image), and is capable of correctly starting the
detection of the copy control watermark beginning with the moved
position. As a result, it is possible to effect the correct
detection of the watermark with respect to the unit pixel blocks
following the moved position, so that the drawback which is
conventionally encountered does not occur.
[0112] Although, in the above-described embodiment, a description
has been given of the identification of the position which is
effected in the DVD recorder, the present invention is not
necessarily limited to this form. For instance, it is possible to
effect the identification of the position with respect to an
information data signal which is transmitted and received in, for
example, Internet and other communications or transmission forms in
which various media are handled.
[0113] In addition, although, in the above-described embodiment, as
the main first watermark a description has been given of the copy
control watermark which bears information as to whether or not
copying is allowed, the present invention is not necessarily
limited to the same. For instance, ID information on a person to
whom the use of a relevant copyrighted item is allowed may be set
as the first watermark information, and various other information
may also be applied.
[0114] Furthermore, although, in the above-described embodiment, a
description has been given of the embedding of the watermark using
the DCT and the inverse DCT, the block division method is also
applicable to the method in which the watermark information is
embedded in sampled values as described above, and the present
invention is effective in such block division as well. In brief,
the present invention is widely applicable to forms in which
information data to be watermarked is divided into blocks and the
main watermark is embedded.
[0115] In addition, although, in the above-described embodiment, a
description has been given of the embedding and detection of
watermark information with respect to image data, it goes without
saying that the present invention is applicable to sound data and
the like.
[0116] In addition, although, in the above-described embodiment,
the 8.times.8 pixel data is cited as the divided unit block of the
image data signal, it goes without saying that unit blocks other
than the same may be formed.
[0117] Although, in the above-described embodiment, various means
are described in a limiting manner, the above-described means may
be modified, as required, within a range designable by those
skilled in the art.
[0118] As described above, in accordance with the present
invention, it is possible to provide a method and apparatus for
superposing a digital watermark on image data and a method and
apparatus for detecting a digital watermark which make it possible
to properly recognize a unit pixel block in the block division
method and reliably detect watermark information.
[0119] Furthermore, after image data is subjected to aspect ratio
conversion, it is possible to properly recognize a unit pixel block
and reliably detect watermark information.
* * * * *