Title:
Applying digital watermarks using printing process correction
Kind Code:
A1


Abstract:
Image color values are modified in accordance with printing process characteristics. Digital watermark signal representations are determined and modified in accordance with the characteristics. The modified signal representations are combined with the original image color values. The image is then printed by the printing process. The resulting printed image includes a watermark that is not materially affected by the printing process characteristics.



Inventors:
Reed, Alastair M. (Lake Oswego, OR, US)
Application Number:
10/791213
Publication Date:
12/02/2004
Filing Date:
03/01/2004
Assignee:
REED ALASTAIR M.
Primary Class:
Other Classes:
382/162
International Classes:
G06T1/00; H04N1/32; H04N7/16; H04N7/26; H04N21/2389; H04N21/44; H04N21/4627; H04N21/8358; H04H20/31; (IPC1-7): G06K9/00
View Patent Images:
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Primary Examiner:
PERUNGAVOOR, SATHYANARAYA V
Attorney, Agent or Firm:
DIGIMARC CORPORATION (9405 SW GEMINI DRIVE, BEAVERTON, OR, 97008, US)
Claims:

What is claimed is:



1. A method of digital watermarking an image comprising: adjusting the image in accordance with values in a first representation associated with a printing process; determining values to convey a digital watermark in the adjusted image; adjusting the values in accordance with a second representation associated with the printing process; and combining the adjusted change values and the image to produce a digital watermarked image.

2. The method of claim 1, wherein the first representation comprises a forward dot gain curve.

3. The method of claim 2, wherein the second representation comprises a backward dot gain curve.

4. The method of claim 3 wherein the backward dot gain curve comprises an inverse of the forward dot gain curve.

5. The method of claim 1 wherein the printing process comprises an offset printing press.

6. The method of claim 1 wherein the image is watermarked using a scale to black technique.

7. The method of claim 1 wherein said image is watermarked using a scale to white technique.

8. A method of steganographically hiding a signal in an image comprising: determining change values to represent the signal in the image; and altering color values of the image by an amount to achieve the change values, wherein the amount includes a compensation for a variation in a relationship of an input color value and at least one of ink and dye provided by a printing process to represent the input color value, and wherein the image includes the signal steganographically embedded therein when printed with the printing process.

9. The method of claim 8, wherein the printing process comprises an offset printing process.

10. The method of claim 8, wherein the steganographically hiding comprises digital watermarking.

11. The method of claim 8, further comprising printing the image, wherein the printed image includes the signal steganographically embedded therein.

12. A method of processing an image to compensate for variation in a printing process, wherein the image includes a plurality of color values, said method comprising: receiving a representation of a variation in a relationship of an input color value and at least one of ink and dye provided by the printing process to represent the input color value; determining change values needed to alter the image to accommodate a digital watermark embedded therein; adjusting the change values with the representation; and modifying the image with the adjusted change values to accommodate the digital watermark and to compensate for the variation.

13. The method of claim 12 wherein the printing process comprises an offset printing press.

Description:

RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent application Ser. No. 10/209,053, filed Jul. 30, 2002 (now U.S. Pat. No. 6,700,995). The Ser. No. 10/209,053 application is a continuation in part of co-pending U.S. patent application Ser. No. 09/553,084, filed Apr. 19, 2000 (now U.S. Pat. No. 6,590,996). Each of these patent documents is herein incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates steganography and more particularly to the digital watermarks.

BACKGROUND AND SUMMARY OF THE INVENTION

[0003] The technology for applying digital watermarks to images and to other types of data is well developed. For example see issued U.S. Pat. No. 5,748,783, issued U.S. Pat. No. 5,768,426 issued U.S. Pat. No. 5,822,435 and the references cited in these patents. Also various commercially available products (such as the widely used image editing program Photoshop™ marketed by Adobe Corporation) have image watermarking capability. There are many other patents and much technical literature available relating to the application of digital watermarks to images and to other types of data.

[0004] Co-pending application Ser. No. 09/553,084 (now U.S. Pat. No. 6,590,996) describes a technique of color adaptive watermarking. With the technique described in application Ser. No. 09/553,084 a change in an image attribute such as luminance (or chrominance) is mapped to a change in color components such that the change is less visible application Ser. No. 09/553,084 describes the “scale to black” and the “scale to white” techniques for applying watermarks. By using the scale to white method for colors with a high yellow content such as yellow, red and green, and by using the scale to black for blue, cyan and magenta a watermark with a lower visibility and the same detect ability can be embedded in an image.

[0005] It is known that when an image is printed on a standard offset press, the relationship between the digital value of a color and the amount of ink actually applied by the press is not linear. FIGS. 1 illustrates the dot gain curve for a typical standard offset printing press. The horizontal axis gives a digital value of a color and the vertical axis indicates the amount of ink actually transferred by the press. The shape of the dot gain curve of offset printing presses is well known.

[0006] As a result of the dot gain curve illustrated in FIG. 1, when an image containing a watermark is printed on an offset press, a watermark signal in the shadows (i.e. in an area with more ink) is reduced and a watermark signal in the highlights (i.e. in an area with less ink) is amplified. Note that the slope of the dot gain curve is different in the shadow area and in the highlight area. Thus, the same amount of change in color value produces a different amount of change in the ink applied in the two different areas. The present invention provides a technique which insures that a watermark signal is preserved in an printed image as accurately as possible not withstanding the fact that the dot gain curve of the printing press is not linear.

[0007] With the present invention, the image data is first modified in accordance with the forward dot gain curve of a printing press, next the watermark “tweak” values (i.e. the watermark change values) are calculated for this modified image data. The calculated “tweak” values are then modified in accordance with the backward dot gain curve of the printing press. The modified tweak values are then added to the original image data values to produce a watermarked image. The watermark image is then printed on the printing press. The result is that the “effective” tweak on printed paper is not materially affected by the dot gain curve of the printing press.

BRIEF DESCRIPTION OF FIGURES

[0008] FIG. 1A shows a forward dot gain curve.

[0009] FIG. 1B shows a backward dot gain curve.

[0010] FIG. 2 illustrates scaling to black.

[0011] FIG. 3 illustrates scaling to white.

[0012] FIG. 4 is a program block flow diagram of the operation of the preferred embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

[0013] Co-pending application Ser. No. 09/553,084, filed Apr. 19, 2000 (Now U.S. Pat. No. 6,590,996) describes a system for watermarking images. The system described in application Ser. No. 09/553,084 inserts watermarks in images by selecting and modifying colors to obtain approximately equal visibility for all colors. The preferred embodiment of present invention, as described herein, is described as a modification of the system described in application Ser. No. 09/553,084. The object of the modifications is to compensate for the dot gain curve of a printer. The entire specification of application Ser. No. 09/553,084 is hereby incorporated herein by reference.

[0014] It is desirable that a watermark embedding algorithm produce luminance changes with approximately equal visibility through color space. Adaptive color embedding as described in application Ser. No. 09/553,084, selects the colors that are modified to produce a required luminance change, in a way that obtain approximately equal visibility for all colors. The dot gain correction provided by the preferred embodiment described herein approximately compensates for the non-linear effect of the printing process, so that a desired percentage change is achieved on press (that is, in the amount of ink applied to create the image). It is noted that the slope of the dot gain curve is different in the shadow area and in the highlight area. Thus, the same amount of change in color value produces a different amount of change in the ink applied in the two different areas. The preferred embodiment insures that a watermark signal (i.e. a change value) is preserved in a printed image as accurately as possible not withstanding the fact that the dot gain curve of the printing press is not linear.

[0015] As explained in application Ser. No. 09/553,084 a watermark can be applied to images using either a scale to black or a using a scale to white technique. With the scale to black technique, the image pixel is like a vector between black and the pixel color value. The vector is increased or decreased as shown in FIG. 2. That is, FIG. 2 illustrates the color changes for a luminance change utilizing the scale to black technique. The following table lists for each color, the colors that are modified as a result of a luminance change. The table also indicates the degree to which the modification is visible.

[0016] For Scale to Black: 1

ColorColors ModifiedVisibility of the change
yellowcyan/magentahigh
redcyanhigh
greenmagentamedium
BlueYellowlow
CyanMagenta/yellowlow
MagentaCyan/yellowlow

[0017] FIG. 3 illustrates the color changes that occur with a scale to white technique. The scale to white technique obtains the same luminance change as the scale to black technique; however, when scaling to white the image pixel is a vector between white and the pixel color value as shown in FIG. 2. The following table lists for each color, the colors modified as the result of a luminance change. The table also indicates the degree to which the modification is visible.

[0018] For Scale to White 2

ColorColors ModifiedVisibility of change
yellowyellowlow
redmagenta/yellowlow
greencyan/yellowmedium
BlueCyan/magentahigh
CyanCyanhigh
MagentaMagentamedium

[0019] By using the scale to white method for colors with high yellow content such as yellow and red, and scale to black for blue, cyan, magenta and green a lower visibility mark can be made with the same detectability. Scaling to white results in the watermark being applied mainly to the dominant colors, and scaling to black implies that the watermark is mainly in the secondary colors.

[0020] When images are printed on an offset press, it is known that there is not a straight line relationship between the digital value of the color at any point in the image and the corresponding amount of ink applied to the paper at that point. This is known as dot gain. FIG. 1A shows the forward dot gain curve. That is the relationship between the digital value of a color and the amount of ink actually applied. FIG. 2B shows a backward dot gain curve. That is, FIG. 2 indicates the value needed in order to get a particular amount of ink on the paper.

[0021] The following is a list of 256 values that generate a curve as shown in FIGS. 1A. That is, the following is a list of 256 positions on the vertical axis for 256 positions (i.e. for 0 to 255) on the horizontal axis. 3

0 7 12 18 22 26 29 32 34 37 3942
 44 46 48 50 52 54 55 57 59 60 6264
 65 67 68 70 71 73 74 76 77 78 8081
 83 84 85 86 88 89 90 91 93 94 9596
 97 99100101102103104105106108109
110111112113114115116117118119120
121122123124125126127128129130131
132133134135135136137138139140141
142143144144145146147148149150150
151152153154155155156157158159160
160161162163164164165166167168168
169170171171172173174175175176177
178178179180181181182183184184185
186186187188189189190191191192193
194194195196196197198198199200201
201202203203204205205206207207208
209209210211211212213213214215215
216216217218218219220220221222222
223224224225225226227227228229229
230230231232232233234234235235236
237237238238239240240241241242243
243244244245246246247247248249249
250250251251252253253254254255

[0022] The following is a list of 256 values that generate the curve shown in FIG. 1B. That is, the following are the vertical values for 256 positions (i.e. 0 to 255) on the horizontal axis. 4

0 1 1 1 1 1 1 1 2 2 2 2
2 3 3 3 3 3 3 4 4 4 4 5
5 5 5 6 6 6 7 7 7 8 8 9
9 9 10 10 11 11 11 12 12 13 1314
 14 15 15 16 16 17 17 18 19 19 2020
 21 22 22 23 23 24 25 25 26 27 2728
 29 29 30 31 31 32 33 34 34 35 3636
 37 38 39 40 40 41 42 43 44 44 4546
 47 48 49 49 50 51 52 53 54 55 5657
 57 58 59 60 61 62 63 64 65 66 6768
 69 70 71 72 73 74 75 76 77 78 7980
 81 82 83 84 86 87 88 89 90 91 9293
 94 96 97 98 99100101103104105106
107109110111112113115116117118120
121122123125126127129130131132134
135136138139140142143144146147149
150151153154156157158160161163164
166167168170171173174176177179180
182183185186188189191193194196197
199200202203205207208210211213215
216218219221223224226228229231233
234236238239241243244246248250251
253255

[0023] It is noted that different offset processes produce different amounts of dot gain; however, with most offset processes, the dot gain curve has the shape shown. For some particular offset processes, the actual values may to 50 or 75 percent of the values given above. The values used in any particular application should be the values appropriate for the particular printing process that will be used to print a particular image.

[0024] FIG. 4 is a block program flow diagram of a program for the preferred embodiment of the invention. The process begins with an image 401 which is in the CYMK color space. As indicated by block 402, the values for each color in the image are first modified in accordance with the values of the forward dot gain curve. This generates a modified image.

[0025] Next as indicated by block 403 calculations are made using the modified image to determine the “tweak” (i.e. the change) values needed to embed a particular watermark in the modified image. This calculation can be done using known watermarking techniques. In the preferred embodiment, the tweak values are calculated using the technique available in the commercially available Photoshop image editing program. However, in other embodiments, other watermarking techniques can be used.

[0026] The tweak values are next modified in accordance with the backward dot gain curve values as indicated by block 404. Next as indicated by block 405, the modified tweak values are added to the values in the original image 401, thereby producing a watermarked image. Finally as indicated by block 406 the watermarked image is printed using an offset press which has the forward and backward dot gain values used in blocks 402 and 404.

[0027] The watermark can then be read from the printed image using known watermarks reading techniques.

[0028] In an alternate embodiment of the invention, the tweak values are added to the modified image values and then the resultant image is modified in accordance with the backward dot gain curve values; however, it has been found that in most instances, the process described in FIG. 4 eliminates some rounding errors.

[0029] In some applications, it has been found desirable to add back a constant that controls the amount of the scale to black signal when a color with high yellow-blue saturation is being embedded. This is sometime necessary, since some cameras are insensitive in the blue channel, so changes in yellow are not detected very well.

[0030] In general to dot gain correction is only applied to the CMY channels, and not to K channel. However, if desired the dot gain correction can be applied to all the channels.

[0031] The preferred embodiments described above relate to the dot gain curve for offset printing processes. It is noted that other processes such as ink jet printing have a different type of dot gain curve. The invention can be applied to most types of printing processes by merely using a dot gain curve appropriate to the particular process.

[0032] Images watermarked using the embodiments described above can be read with conventional watermark reading techniques. Naturally as is conventional the watermark reading technique used should coincide with the particular technique used to generate the change values, that is, with the technique used to watermark the image.

[0033] While the invention has been described with respect to watermarking images it should be understood that the principle is applicable to other types of data.

[0034] The preferred embodiment relates to an image in the CYMK color space. Other embodiments using the same principles can operate on images in various other color spaces.

[0035] While the invention has been shown and described with respect to preferred embodiments, it should be understood that various changes in form and detail may be make without departing from the spirit and scope to the invention. The scope of the invention is limited only by the appended claims.