Abstract:
Visible digital watermarks enable a robust tracking method to be associated with one or more input images. Specifically, by modifying a halftone screen within a certain area by setting a gray-scale range within that area to a threshold, and halftoning an input image based on this modified halftone screen, visible watermarks can be embedded into an input image.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention is directed to systems and methods for embedding visible watermarks. 
     2. Description of Related Art 
     Methods for protecting copyrightable works have evolved from a simple designation below the works to highly complex methods for embedding watermarks in the work. Watermarking can take two basic forms: visible and invisible. Examples of visible watermarks are the commonly-seen copyright logos or symbols that are generally affixed to the work before sales or distribution. Especially in the case of images, the presence of the watermark is very visible, and is generally difficult to remove without damaging the image. Generally speaking, visible watermarks do not harm the image, while providing a very robust form for encoding information in the image. 
     Invisible watermarks can encompass the standard and commonly used copyright logos or symbols, as well as company logos, serial numbers, or general identification marks and/or encrypted data. These invisible watermarks are embedded into the work in a way which is not generally discernible without the aid of visualization device such as a key or computer. Theoretically, these embedded images can be retrieved from the work at any time in the works&#39; history or from any other form or embodiment into which the work may have been translated. This allows the owner to track the work and clearly establish ownership rights when those rights are in dispute. Furthermore, since the embedded watermark image is essentially invisible to the unaided eye, the likelihood of tampering with or removal of the watermark is reduced. 
     Invisible watermarks, by definition, are essentially invisible to the unaided eye. Therefore, to retrieve such an invisible watermark, usually a hardware or a software technique needs to be applied to the watermarked image in order to retrieve the embedded information. 
     SUMMARY OF THE INVENTION 
     This invention provides systems and methods for embedding visible watermarks in images or documents. 
     This invention further provides systems and methods that allow visible watermarks to be embedded in images through a halftoning process. 
     This invention also provides systems and methods for modifying a halftone screen when the halftone screen contains the watermark to be embedded in a halftone image. 
     Embedding of visible watermarks is accomplished by altering a halftone screen prior to halftoning of an input image. Specifically, by modifying all threshold values within a specified area of halftone screen, and setting the gray-scale range to a constant value within this range, any existing halftone screen, e.g., clustered halftone screens or dispersed halftone screens, can be converted to a halftone screen that produces visible digital watermarks in the output halftoned image. The visible watermarks will appear in the output image as symbols defined by the specified area where the halftone screen was modified through the selected gray-scale range. Thus, the watermark is visually apparent within the output image and is very difficult to remove. 
     In particular, using an existing halftone screen, a portion of the screen is identified as an area which will correspond to the visible watermarks in the output image. The area can correspond to a letter, a word, a logo, a symbol, a signature, a pattern, a texture, or any other indicia a user desires to reproduce in the output image. In addition to selecting the area in which the visible watermarks will appear, a gray-scale range is selected such that the watermarks will only be shown when the values of the input-image fall into this range. For example, to convert an existing halftone screen to a screen for visible watermarks, the threshold values of the halftone screen within the specified area and within the selected gray-scale range are replaced by, for example, the upper limit of the selected range, for example in a 256 gray level image,  255 , Those threshold values of the halftone screen within the specified area, that are not within the selected gray-scale range, are not changed. Therefore the specified area changes brightness but does not change to a solid, uniform output gray level. When the input gray values lie within the selected range, for example between 160 and 200, the halftone output by the modified screen inside the specified area will show no difference from the constant input equal to, for example, 161. Thus, the background, which is outside the specified area, varies as the halftone output by the unmodified screen. Therefore, the contrast between the inside and the outside areas makes the watermark visible. 
     These and other features and advantages of this invention are described in or are apparent from the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments of this invention will be described in detail, with reference to the following figures wherein: 
     FIG. 1 is an exemplary halftoned image containing a visible watermark; 
     FIG. 2 is a functional block diagram of a watermark embedding device according to this invention; 
     FIG. 3 is a flowchart outlining an exemplary watermark embedding process according to this invention; and 
     FIG. 4 is a flowchart outlining in greater detail the halftone input image step of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an exemplary image  100  containing the visible watermarks  110  formed in accordance with the systems and methods of this invention. Here, the visible watermarks  110  illustratively appear as an “X logo” throughout a portion of the image  100 . These visible watermarks  110  illustratively appear lighter than their adjacent background. 
     FIG. 2 shows a watermark embedding device  200  for embedding watermarks in an input image according to this invention. As shown in FIG. 2, an input image is input from an image input device  300  over a link  205  to the watermark embedding device  200 . It should be appreciated that the image input device  300  can be any device that stores and/or generates an electronic version of an image. 
     Thus, the image can be a printed hard copy version of the image, and the image input device  300  can be a scanner that scans and outputs an electronic version of the image over the link  205  to the watermark embedding device  200 . Furthermore, the image input device  300  and the watermark embedding device  200  can be elements integrated into a photocopier. 
     Similarly, the image input device  300  can be a server or other node on a local area network, a wide area network, an intranet, the Internet or any other distributed network. In this case, the image is already stored on the network in electronic form. 
     Finally, the link  205  can be a wired or wireless link to the image input device  300 . Thus the image input device  300  and the link  205  can be any known or later developed elements that are capable of supplying an electronic image to the watermark embedding device  200 . 
     As discussed above, the system and methods of this invention work equally well on images that have not been transferred to hard copy. In this case, the image is already in digital form and the image is ready for processing by the watermark embedding device  200 . 
     The watermark embedding device  200  includes an I/O interface  210 , a controller  220 , a memory  230 , a watermark storage device  240 , a halftone screen modifier  250  and a halftoning device  260 . Connected to the watermark embedding device  200  is an image input device  300 , a user input device  310  and an image output device  320 . 
     In operation, an image is received from the image input device  300 , via link  205  and the I/O interface  210 , in the watermark embedding system  200 . The I/O interface  220  forwards the input image data received from the image input device, under the direction of the controller  220 , to the memory  230 . Upon selection of a watermark by a user with the input device  310 , the criteria for the watermark are retrieved from the watermark storage device  240  and forwarded to the halftone screen modifier  250 . The halftone screen modifier  250  modifies a halftone screen based on the received watermark criteria from the watermark storage device  240 . The halftoning device  260 , using the modified halftone screen produced by the halftone screen modifier, then halftones the input image embedding the visible watermark. The image containing the visible watermark is then output, via I/O interface  210  and link  205 , to the image output device  320 . 
     It should be appreciated that the image output device  320  can be any device or combination of devices that outputs or displays the resulting image data. Thus, the resulting image can be a printed version of the input image, and the output device  320  can be a printer. Similarly, the output device  320  can be a monitor which is capable of displaying the electronic version of the resulting image for viewing. Furthermore, the image input device  300 , the watermark embedding device  200 , and the image output-device  320  can be elements integrated into a single device, such as a photocopier. 
     Similarly, the image output device  320  can be a server or other node on a local area network, a wide area network, an intranet, the Internet, or any other distributed network. In this case, the resulting image is transferred and stored on the network in electronic form. Finally, the link  205  can be a wired or wireless link to the image output device  320  or any other image output device or display device or to the network. Thus, the image output device  320  and the link  205  can be any known or later developed systems and methods that are capable of receiving and outputting or storing the resulting electronic image generated by the watermark embedding device  200 . 
     Specifically, having received the electronic version of the input image from the image input device  300 , the image is ready for embedding with the visible watermark. In particular, a user selects, via the user input device  310 , a watermark stored in the watermark storage device  240 . Alternatively, a user can create or import a watermark, for example, based on a corporate logo or input texture or pattern, which is then stored in the watermark storage device  240 . Having chosen a particular watermark, or introduced a new watermark into the watermark embedding system  200 , a user determines which portion of the input image should be embedded with the visible watermark. Secondly, a determination is made as to what gray-scale range within the specified area should be modified in accordance with a correlation to the visible watermark. For example, in a 256 gray level image, the selected range of the gray value of the input image can be chosen, for example, between 160 and 200. Then, a determination is made as to what value to set the area within the range. For example, if the color value of the input image varies between 160 and 200, the values within the range can be replaced by, for example, the upper limit value of the range, say, 255. Alternatively, the values within the range can be replaced by the upper limit value of the input range. If the input range is chosen, for example, between 160 and 200, then the values within the range would be replaced with the upper limit value within that range, or, 200. In another alternative, the values within the range can be replaced by the lower limit value of the range. If the input range is chosen, for example, between 160 and 200, then the values within the range would be replaced with the lower limit value of the range, or, 0. In yet another alternative, if the input range is chosen, for example, between 160 and 200, then the values within the range would be replaced with the lower limit value of the input range, or, 160. These different alternatives will affect the appearance of the visible watermark in the output halftoned image, determining whether the visible watermark will appear to be brighter or darker than the background value of the image. 
     The halftone screen modifier  250 , knowing the area and the range in which to embed the visible watermark, modifies the halftone screen in accordance with these parameters. Specifically, the halftone screen modifier  250  replaces the values within the selected range within a predetermined value. Thus, a modified halftone screen is generated such that when an input image value varies between the specified range, the area inside the specified area will show no difference from a constant input equal to the selected value. The background, or input image, which is outside the specified area, varies as to halftone output by the unmodified screen. The halftoning device  260  then halftones the input image based on the modified halftone screen. The modified halftone imagine, containing the embedded watermark can then be output to the image output device  320  over link  205  via the I/O interface  210 . 
     The modification of halftone screens to incorporate a visible watermark also applies to color halftone screens used for color images. Such halftone screens may be designed for different separations. Some examples of such separations include three-color, red, green, and blue separations. Alternatively, these separations might include four-color, cyan, magenta, yellow and black separations. The number of separations is not limited to these examples and may include more than four or fewer than three separations. 
     In the case of a color image, there can be at least one halftone screen per separation. In such a case, this method of incorporating a visible watermark is applied to each halftone screen for each separation. The specified areas and selected ranges may be the same for each separation or they may vary from screen to screen. For example, if the same watermark area is specified in each halftone screen for each separation, but a different selected range is chosen, then the watermark will appear in different regions of the image in different colors depending on the manner in which background values of the different separations vary across the image. On the other hand, if the selected area is also different for different halftone screens for different separations, then the visible watermark will appear as different shapes in different colors depending on the manner in which background values of the different separations vary across the image. 
     FIG. 3 outlines the method according to this invention for embedding visible watermark into received images. Control begins in step S 100  and continues to step S 110 . In step S 110 , an image is received. Next, in step S 120 , the input image is halftoned. Then, in step S 130 , the modified halftone image containing the visible watermarks is output. Control then continues to step S 140  where the control sequence ends. 
     FIG. 4 illustrates in greater detail the halftone image step of FIG.  3 . Specifically, control begins in step S 200  and continues to step S 210 . In step S 210 , the halftone screen is selected. Next, in step S 220 , the range within the halftone screen in which to embed the watermark is determined. Next, in step S 230 , the area within the range is set to a predetermined value. Then, in step S 240 , the modified halftone screen is applied to the input image. Control then continues to step S 250  where the control sequence ends. 
     As shown in FIG. 2, the watermark embedding system is preferably implemented on a single program general purpose computer or separate programs general purpose computers. However, the watermark embedding system can also be implemented on a special purpose computer, a programmed microprocessor or microcontroller and peripheral integrated circuit element, an ASIC, or other integrated circuit, a digital signal processor, a hard-wired electronic or logic circuit such as a discrete element circuit, a programmable logic device such as a PLD, PLA, FPGA, PAL or the like. In general, any device capable of implementing a finite state machine that is in turn capable of implementing the flow charts shown in FIGS. 3-4 can be used to implement the watermark embedding system. 
     Furthermore, the disclosed method may be readily implemented in software using object or object-oriented software development environments that provide portable source code that can be used on a variety of computer or workstation hardware platforms. Alternatively, the disclosed watermark embedding system may be implemented partially or fully in hardware using standard logic circuits or a VLSI design. Whether software or hardware is used to implement the systems in accordance with this invention is dependent on the speed and/or efficiency requirements of the system, the particular function, and the particular software or hardware systems or microprocessor or microcomputer systems being utilized. The watermark embedding systems and methods described above, however, can be readily implemented in hardware or software using any known or later developed systems or structures, devices and/or software by those skilled in the applicable art without undue experimentation from the functional description provided herein together with a general knowledge of the computer arts. 
     Moreover, the disclosed methods may be readily implemented as software executed on a programmed general purpose computer, a special purpose computer, a microprocessor, or the like. In this case, the methods and systems of this invention can be implemented as a routine embedded in a personal computer, as a resource residing on a server or graphics workstation, as a routine embedded in a dedicated watermark embedding system, or the like. The watermark embedding system can also be implemented by physically incorporating the system and method into a software and/or hardware system, such as the hardware and software systems of a graphics workstation or dedicated watermark embedding system. 
     It is, therefore, apparent that there has been provided, in accordance with the present invention, systems and methods for embedding visible watermarks. While this invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, Applicants intend to embrace all such alternatives, modifications and variations that follow in the spirit and scope of this invention.