Applying digital watermarks using dot gain correction

[0002] This application is a continuation in part of U.S. patent application Ser. No. 09/553,084, filed Apr. 19, 2000.

[0003] The present invention relates steganography and more particularly to the digital watermarks.

[0004] 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.

[0005] Co-pending application Ser. No. 09/553,084 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.

[0006] 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. FIG. 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.

[0007] 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.

[0008] 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.

[0009] FIG. 1A shows a forward dot gain curve.

[0010] FIG. 1B shows a backward dot gain curve.

[0011] FIG. 2 illustrates scaling to black.

[0012] FIG. 3 illustrates scaling to white.

[0013] FIG. 4 is a program block flow diagram of the operation of the preferred embodiment.

[0014] Co-pending application Ser. No. 09/553,084, filed Apr. 19, 2000 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.

[0015] 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.

[0016] 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.

[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] 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.

[0019] 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.

[0020] The following is a list of 256 values that generate a curve as shown in FIG. 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] The watermark can then be read from the printed image using known watermarks reading techniques.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.