Image analysis – Applications
Reexamination Certificate
2000-10-23
2004-07-13
Patel, Jayanti K. (Department: 2625)
Image analysis
Applications
C382S162000, C382S163000, C382S166000, C715S252000, C358S003280
Reexamination Certificate
active
06763122
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to steganography and more particularly to inserting digital watermarks in images and to detecting and reading such watermarks.
BACKGROUND AND SUMMARY OF THE INVENTION
The technology for inserting digital watermarks into images is well developed. For example see issued U.S. Pat. No. 5,862,260 (Rhoads), U.S. Pat. No. 5,930,369 (Cox), U.S. Pat. No. 5,905,800 (Moskowitz), and U.S. Pat. No. 6,122,403 (Rhoads) and co-pending application Ser. No. 09/553,084 filed Apr. 19, 2000 (now U.S. Pat. No. 6,590,996), all of which are hereby incorporated herein by reference. Watermarking technology is also included in some commercially available image editing programs such as “Adobe Photoshop” which is marketed by Adobe Corporation of San Jose Calif. and “Corel Draw” which is marked by Corel Corporation of Ontario Canada.
Colored Images are generally stored in computers using a RGB (Red, Green, Blue) format. The above referenced commercially available programs insert a watermark into an RGB image by modifying the luminance value of pixels in the image. The value of each color plane in the image is modified by the known relationship between overall luminance and the value of each color plane. For example, it is known that the ratio between overall luminance and the value of the colors in an RGB image is 0.3 for Red, 0.6 for Green and 0.1 for Blue.
In many situations better results can be achieved if the watermark is inserted adaptively. That is, if the intensity of the watermark inserted in each particular area of the image is adjusted in accordance with the data hiding attributes associated with that particular area of the imager. The above referenced commercial image editing programs insert watermarks into images adaptively. U.S. Pat. No. 6,590,996 filed Apr. 19, 2000 (which is incorporated herein by reference) describes adapting the watermarking process to the color of an image.
Colored images are printed using multiple printing plates. The image is divided into color planes corresponding to the colors of ink used to print the image. Each color is printed using a separate plate which prints that color. For example an image may be separated into Cyan, Magenta, Yellow and Black (CMYK) color planes. A separate plate is used to print each color. The different plates must be precisely aligned. Any misalignment of the plates will cause blurring in the image and may make it difficult or impossible to read a watermark that was embedded in the image.
That is, when an image contains digital watermark data in each color plane, misalignment of the plates used to print different colors can cause the watermark data in one color plane to, in effect, cancel the watermark data in a different color plane.
FIG. 1
illustrates the main steps in an image watermarking process. The process begins with an image
101
which has RGB (red, green blue) values for each pixel in the image. The object of the process is to insert watermark payload data
102
into image
101
. The change (or tweak) for the luminance of each pixel in image
101
which will insert the payload
102
into the image is calculated. An example of how tweak values can be calculated is shown in the above referenced issued U.S. patents and in other publicly available literature. The tweak values for the luminance are changed into changes in RGB values as indicated by block
105
. The transformation is done according to the known relationship between color values and luminance. Generally the luminance of a pixel can be approximated as 0.3 times the red value plus 0.6 times the green value plus 0.1 times the blue value.
The color values of each pixel in the image
101
are changed by the calculated amounts as shown in
FIG. 1B
in order to watermark the image. Watermark data tile
116
specifies the amount of change in luminosity for each pixel in a square array of pixels. The pixels in the image
101
are divided into an array of squares that have the same size as the tile
116
. The amount that the pixels in image
101
are changed is adapted to the characteristics of the image. For example consider two squares
118
a
and
118
b
in the image
101
. If the characteristics in square
118
a
are such that it can carry less watermark signal than the characteristics of the image in square
118
b
, the pixels in square
118
a
may only be changed by one half of the amount specified in tile
116
and the pixels in square
118
b
may be changed by the full amount specified in the tile
116
. The technology for adaptively inserting watermark signals in an image using various techniques is known.
Finally, as indicated by block
107
, the RGB colors are changed into CMYK (cyan, magenta, yellow, and black) values for printing, and each color is printed with a separate plate as indicated by block
108
. If the plates used to print the different colors are misaligned, the watermark in one color can effectively cancel the watermark in another color.
Printing with misaligned plates is illustrated in
FIG. 2
, which shows (greatly exaggerated and simplified) the areas printed by different plates for two pixels in the image. To facilitate illustration and explanation, in
FIG. 2
, rectangles are used to designate the area printed by a first color plate and the circles are used to designate areas printed by a different color plate. The areas are the size of one pixel in the image. It is noted that in a typical printing process the areas would have the same shape. They are shown here as having different shapes for ease of illustration and explanation. It is also noted that each pixel area would normally contain multiple ink dots. The actual ink dots are not shown in FIG.
2
. As illustrated in
FIG. 2
, square
201
C represents the cyan printing from one pixel in the image and circle
201
Y represents the yellow printing from this same pixel. The same applies to square
202
C and circle
202
Y. If the plates where aligned, the circles and squares would be directly on top of each other. However, in the illustration shown in
FIG. 2
, due to misalignment of the printing plates, the circles and squares are not aligned. Let us assume that the watermarking process increased the luminance of the pixel from which square
201
C and circle
201
Y originated and decreased the luminance of the adjacent pixel. Since the plates were misaligned, in the area where circle
201
Y overlaps square
202
C, the plus increment in circle
201
Y would cancel the negative decrement in square
202
C. Thus, misalignment of printing plates can have the effect of at least partially canceling watermark data in an image. In most situations the misalignment would not completely cancel the watermark data; however, it would weakens the watermark signal.
One aspect of the present invention is directed to minimizing the effect of plate misalignment on the detectability and readability of watermark data in an image. The effect of misalignment of the printing plates is minimized by detecting the dominant color in an image and inserting a watermark only into that color plane of the image. In another embodiment of the invention, the image is divided into regions and the dominant color in each region is determined. In each region the watermark data is inserted into the dominant color in that region. In still another embodiment of the invention, a first watermark is inserted into the dominant color plane of an image and a second watermark is inserted in one of the other color planes of the image. In still another embodiment of the invention the dominant color of the entire image (or of a region of the image) is detected and the watermark is inserted into the dominant color if that color is on the yellow-blue axis. If the dominant color is not on the yellow-blue axis, the watermark is inserted into the strongest of the secondary colors.
In a situation where an image is watermarked by first separating the image into color planes and inserting a watermark into one or more color plane, it is easier to detect and read the watermark if the image is first separate
Gustafson Ammon
Miller Marc
Reed Alastair M.
Rhoads Geoffrey B.
Rodriguez Tony
Choobin Barry
Digimarc Corporation
Patel Jayanti K.
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