Patent Publication Number: US-8121339-B2

Title: Adaptive mark placement

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application claims the right of priority under 35 U.S.C. §119 based on Australian Patent Application No. 2005209707, filed 13 Sep. 2006, which is incorporated by reference herein in its entirety as if fully set forth herein. 
     FIELD OF THE INVENTION 
     The current invention generally relates to steganographically storing data onto printed documents by superimposing symbols in the form of marks onto the document. Steganography refers to hiding a secret message within another message. 
     BACKGROUND 
     Keeping track of printed documents where additional data, such as date of printing or copying, is steganographically stored within the printed document is an ongoing area of investigation. For a technique to be applicable to a greater number of applications, it is further desired that the technique is performed without affecting the visible quality of the original document along with the conflicting goal of being able to recover the additional data even from subsequent photocopies of the document. 
     Existing techniques which encode additional data onto printed matter superimpose a pattern of marks, typically dots, which contains the additional data, onto the printed matter. However, superimposing a pattern containing additional data over the entire original printed matter has many disadvantages. Some of these include substantial degradation of the quality of the document as well as difficulty in identifying dots within images due to little or no contrast between the dot and the region of the printed matter surrounding the location where the dot was placed. The situation worsens greatly when photocopies are made. 
     Other existing techniques encode additional data onto a page of printed matter selectively by identifying allowable encoding locations in the printed matter, usually the blank spaces near text, and place encoding dots at these locations. Although this avoids the problem of degradation of quality by encoding the additional data in selected locations on the printed matter, it will only work when there is sufficient white space in the printed matter. 
     Both modes of encoding additional data onto a page of printed matter have their limitations. 
     SUMMARY 
     It is an object of the present invention to substantially overcome, or at least ameliorate, one or more disadvantages of existing arrangements. 
     Disclosed are arrangements, referred to as adaptive mark methods, or techniques using adaptive marks, which seek to address the above problems by performing the encoding according to prioritisation of logical content regions of the document, and by use of adaptive marks which provide good local contrast for subsequent decoding. 
     According to a first aspect of the present invention, there is provided a method of encoding a message into a document containing known information, the method comprising the steps of: 
     identifying a plurality of logical information content categories associated with the known information; 
     establishing a priority order in which said message is to be added to each of the categories; 
     determining an amount of said message to be added to each of the categories; and encoding the message into the document according to the priority order of the establishing step and the amount of the determining step. 
     According to another aspect of the present invention, there is provided an apparatus for encoding a message into a document containing known information, the apparatus comprising: 
     a memory for storing a program; and 
     a processor for executing the program, said program comprising: 
     code for identifying a plurality of logical information content categories associated with the known information; 
     code for establishing a priority order in which said message is to be added to each of the categories; 
     code for determining an amount of said message to be added to each of the categories according; and 
     code for encoding the message into the document according to the established priority order and the determined amount. 
     According to another aspect of the present invention, there is provided a computer program product including a computer readable medium having recorded thereon a computer program for directing a processor to execute a method for encoding a message into a document containing known information, said program comprising: 
     code for identifying a plurality of logical information content categories associated with the known information; 
     code for establishing a priority order in which said message is to be added to each of the categories; 
     code for determining an amount of said message to be added to each of the categories according; and 
     code for encoding the message into the document according to the established priority order and the determined amount. 
     According to another aspect of the present invention, there is provided a method of adaptively choosing encoding marks to superimpose onto the face of a document, the method comprising the steps of:
         (a) determining the nature of the digital image representation of the original document in the vicinity of where the mark is to be placed; and   (b) judging which type of mark to superimpose from a set of predefined marks based on information obtained from (a).       

     According to another aspect of the present invention, there is provided a method of deciding whether an encoding mark is superimposed onto the face of a document the method comprising the steps of:
         (a) obtaining metadata about the logical content present in the printed matter of the document at the location where the mark is to be placed; and   (b) superimposing the mark onto the document if the logical content at the location where the mark is to be placed is of the type which is intended to be superimposed with encoding marks.       

     Other aspects of the invention are also disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One or more embodiments of the present invention will now be described with reference to the drawings and appendices, in which: 
         FIG. 1  is a functional block diagram of a general-purpose computer upon which described adaptive mark methods can be practiced; 
         FIG. 2A  shows a flow chart of a process for encoding information into a document which can make use of the disclosed adaptive mark approach; 
         FIG. 2B  shows a grid used in the process of  FIG. 2A ; 
         FIG. 3  shows a flow chart of a process for decoding information that has been encoded into a document using the process of  FIG. 2 ; 
         FIG. 4  shows an example of a mark, where each layer can have different colours or patterns; 
         FIG. 5  shows another example of a mark, where each layer can have different colours or patterns; 
         FIG. 6  is an example of a document indicating the type of logical information content in different regions of the printed matter; 
         FIG. 7  shows a white mark used in a first arrangement; 
         FIG. 8  shows a flow chart of a process for incorporating steganographic data into a document containing known information; 
         FIG. 9  is a flowchart illustrating the mark placement process involved in the first arrangement; 
         FIG. 10  shows the document divided up into a grid of cells, with regions containing different logical content; 
         FIG. 11  shows a cell as described in the first arrangement, where the cell is located in image content; 
         FIG. 12  shows a black mark placed in the centre of the cell indicated in  FIG. 11 ; 
         FIG. 13  contains another cell described in the first arrangement, where the cell is located in vector content; 
         FIG. 14  shows a white mark placed in the centre of the cell indicated in  FIG. 13 ; and 
         FIG. 15  shows how the adaptive marks of  FIGS. 4 and 5  are incorporated as steganographic information into a document having known information. 
     
    
    
     DETAILED DESCRIPTION INCLUDING BEST MODE 
     Where reference is made in any one or more of the accompanying drawings to steps and/or features, which have the same reference numerals, those steps and/or features have for the purposes of this description the same function(s) or operation(s), unless the contrary intention appears. 
     From a terminology perspective, the term “document” refers to a medium upon which information is written, and includes commonly referred to documents, printed or electronic, images, multimedia images including printed material, graphic material and the like. The terms “adaptive mark” and “mark” are used interchangeably unless the contrary intention is made clear from the context. 
     As noted, the current invention generally relates to steganographically storing data onto printed documents by superimposing symbols in the form of marks onto the document. From a terminology perspective, the secret message to be added to the document is referred to simply as a message, or as steganographic information, and the information that is on the document prior to addition of the steganographic information is referred to as “known information”. 
       FIG. 1  is a functional block diagram of a general-purpose computer system  1100  upon which described adaptive mark method can be practiced. The processes of  FIGS. 2A ,  3  and  9  may be implemented as software, such as an application program executing within the computer system  1100 . In particular, the steps of adaptive marking are effected by instructions in software (such as  1127  and or  1124 ) that are carried out by respective computers  1101  and  1122 . The aforementioned software can be arranged in different operational configurations. Thus in one arrangement, encoding of data using the disclosed adaptive marking methods can be performed using the software  1124  on the computer  1122 , and decoding of the encoded data can be performed using the software  1127  on the computer  1101  after the encoded data is communicated from the computer  1122  to the computer  1101  over a network  1120 . Alternately both the encoding and decoding of data can be performed on one of the aforementioned computers  1122 ,  1101 . 
     The instructions may be formed as one or more code modules, each for performing one or more particular tasks. The software may also be divided into two separate parts, in which a first part performs the adaptive marking methods and a second part manages a user interface between the first part and the user. The software may be stored in a computer readable medium, including the storage devices described below, for example. The software is loaded into the computer(s) from the computer readable media, and then executed by the computer(s). A computer readable medium having such software or computer program recorded on it is a computer program product. The use of the computer program product in the computer preferably effects an advantageous apparatus for adaptive marking. 
     The computer system  1100  includes the computer module  1101  and  1122 . The remainder of the description directed to  FIG. 1  relates primarily to the computer module  1101 , however clearly the description applies equivalently to the computer module  1122 . The system  1100  also includes input devices such as a keyboard  1102 , a scanner  1128  and a mouse  1103 , output devices including a printer  1115 , a display device  1114  and loudspeakers  1117 . A Modulator-Demodulator (Modem) transceiver device  1116  is used by the computer module  1101  for communicating to and from a communications network  1120 , for example connectable via a telephone line  1121  or other functional medium. The modem  1116  can be used to establish communications between the computer  1101  and the computer  1122  across the Internet and other network systems such as Local Area Networks (LAN) or Wide Area Networks (WAN), and may be incorporated into the computer module  1101  in some implementations. 
     The computer module  1101  typically includes at least one processor unit  1105 , and a memory unit  1106 , for example formed from semiconductor random access memory (RAM) and read only memory (ROM). Similarly the computer module  1122  typically includes at least one processor unit  1123 , and a memory unit  1125 , for example formed from semiconductor random access memory (RAM) and read only memory (ROM). The module  1101  also includes an number of input/output (I/O) interfaces including an audio-video interface  1107  that couples to the video display  1114  and loudspeakers  1117 , an I/O interface  1113  for the keyboard  1102  and mouse  1103  and optionally a joystick (not illustrated), and an interface  1108  for the modem  1116  and printer  1115 . In some implementations, the modem  11116  may be incorporated within the computer module  1101 , for example within the interface  1108 . A storage device  1109  is provided and typically includes a hard disk drive  1110  and a floppy disk drive  1111 . A magnetic tape drive (not illustrated) may also be used. A CD-ROM drive  1112  is typically provided as a non-volatile source of data. 
     The components  1105  to  1113  of the computer module  1101  typically communicate via an interconnected bus  1104  and in a manner which results in a conventional mode of operation of the computer system  1100  known to those in the relevant art. Examples of computers on which the described arrangements can be practised include IBM-PC&#39;s and compatibles, Sun Sparcstations or alike computer systems evolved therefrom. 
     Typically, the adaptive mark software application program is resident on the hard disk drive  1110  and read and controlled in its execution by the processor  1105 . Intermediate storage of the program and any data fetched from the network  1120  may be accomplished using the semiconductor memory  1106 , possibly in concert with the hard disk drive  1110 . In some instances, the application program may be supplied to the user encoded on a CD-ROM or floppy disk and read via the corresponding drive  1112  or  1111 , or alternatively may be read by the user from the network  1120  via the modem device  1116 . Still further, the software can also be loaded into the computer system  1100  from other computer readable media. The term “computer readable medium” as used herein refers to any storage or transmission medium that participates in providing instructions and/or data to the computer system  1100  for execution and/or processing. Examples of storage media include floppy disks, magnetic tape, CD-ROM, a hard disk drive, a ROM or integrated circuit, a magneto-optical disk, or a computer readable card such as a PCMCIA card and the like, whether or not such devices are internal or external of the computer module  1101 . Examples of transmission media include radio or infra-red transmission channels as well as a network connection to another computer or networked device, and the Internet or Intranets including e-mail transmissions and information recorded on Websites and the like. 
     The method of adaptive marking may alternatively be implemented in dedicated hardware such as one or more integrated circuits performing the functions or sub functions of adaptive marking. Such dedicated hardware may include graphic processors, digital signal processors, or one or more microprocessors and associated memories. 
     One method in which adaptive marks are used to encode information is via their presence or absence in a regular grid. This regular grid is set up such that the presence or absence of a dot at each grid point is used to store data. In one implementation, the presence of a dot at a grid point indicates a binary one (1), and the absence of a dot at the grid point indicates a binary zero (0). A grid with “a ” grid points in one dimension, and “b ” grid points in another dimension, is thus able to store a×b bits of data. This grid, of dimensions a and b, can then be repeated a number of times on a document on which information is to be encoded. These repeated instances of the grid provide redundancy of data, such that if a bit read from one grid is interpreted incorrectly (i.e. if a one (1) is interpreted as a zero (0), or a zero (0) is interpreted as a one (1)), there are other copies of the same bit on the document. By taking the most common value found for the bit, the correct bit value can generally be recovered. In the disclosed adaptive mark approach, the ‘dot’ which is either present or absent at each grid point is an adaptive mark. 
     To encode a piece of data of length a×b bits in the document, the data is expressed in binary form. The bits of the binarised data are then progressively written in a predetermined order (such as horizontally and then vertically in scanline order) into a regular grid of dimensions a and b. An adaptive mark is placed at a grid point if the bit is a binary one (1) and no adaptive mark is placed at a grid point if the bit is a binary zero (0). This regular grid is then incorporated into the document, and other copies of the same grid are also incorporated into the document for redundancy. Because of the redundancy, some areas of the document may be protected from the placement of adaptive marks because of a desire for visual quality. 
     To decode the information that has been encoded into the document, the grids on the document are first detected and each grid point in each grid is located. Next, a bit is progressively extracted in a predetermined order from each grid point (eg horizontally and then vertically in scanline order) for each grid. If an adaptive mark is found at a particular grid point in most of the grids on the page, then the bit extracted for that grid point is a one (1). If however no adaptive mark is found at a particular grid point in most of the grids on the page, then the bit extracted for that grid point is a zero (0). The original piece of data of length a×b bits is thus obtained. The aforementioned encoding and decoding processes are described in relation to  FIGS. 2A and 3  respectively. 
       FIG. 2A  shows a flow chart of a process  1200 , performed by the computer  1122 , for encoding information into a document which can make use of the disclosed adaptive mark approach. The encoding process  1200  is performed by the processor  1123  under direction of the adaptive mark software application  1124  on the computer  1122 . 
     The process  1200  commences with a start step  1201  in which the processor  1123  is provided with parameters of the document into which information is to be encoded, and with the information to be encoded into the document. Thereafter in a step  1202  the processor defines a regular grid  1207  of size a×b, seen in  FIG. 2B . The example grid  1207  has grid points at positions depicted by a reference numeral  1208 . The example grid  1207  is 7 grid points wide and 6 grid points high and can thus accommodate 7×6=42 bits. 
     In a following step  1203  the processor  1123  determines locations of multiple instances of the grid on the document in question. The step  1203  is subject to constraints as will be described in relation to  FIG. 9 . In a following step  1204  the processor  1123  expresses the information to be encoded as a binary number of length a×b. Thereafter in a step  1205  the processor progressively writes, as depicted by an exemplary arrow  1209  in relation to the example grid  1207 , the bits of the binary number at successive grid points such as  1208  in at least some of the multiple instances of the grid that have been defined in the step  1203 . The encoding process  1200  then terminates at a stop step  1206 . 
       FIG. 3  shows a flow chart of a process  1300 , performed by the computer  1101 , for decoding information that has been encoded into a document by the computer  1122  using the process  1200  of  FIG. 2A . The process  1300  commences with a start step  1301  at which time the document encoded according to the process  1200  has been communicated over the network  1120  from the computer  1122  to the computer  1101 . In a following step  1302  the processor  1105 , under the control of the adaptive mark software application  1127 , detects locations of the multiple instances of the grid (eg.  1207 ) that have been incorporated into the received document. 
     In a following step  1303 , the processor  1105  determines the locations of the grid points of the aforementioned grids. In a following step  1304  the processor  1105  progressively reads the encoded information from successive grid points of the grids in a predetermined pattern corresponding to  1209  in  FIG. 2B . Thereafter in a step  1305  the processor  1105  performs a comparison of each detected bit from each grid with corresponding bits from corresponding grid positions of the other grids. 
     In a following step the processor determines valid data for each grid point based upon the most commonly read bit values from each grid point for the plurality of grids, after which the process  1300  terminates in a stop step  1307 . 
     The disclosed adaptive mark technique adaptively chooses the characteristics and the placement of marks steganographically onto printed matter in such a way that the adaptive marks are detectable even on photocopies, but at the same time, the disclosed method can in many cases maintain a high quality of the printed matter. 
     An adaptive mark consists of a first layer superimposed onto a second layer wherein the second layer consists of a filled geometric shape. The concept of “layers” is introduced to more easily describe the adaptive mark examples, however the printing of adaptive marks may be implemented using other methods. The first layer can be any geometric arrangement of one or more geometric shapes so long as their collective extent is encompassed by the geometric shape in the second layer. The use of two layers enables the design of suitable fill colours, textures or arbitrary images for each layer to enhance detectability of the mark even after photocopying. In  FIG. 4 ,  100  is an example of an adaptive mark which is composed of a filled circle first layer  120  superimposed onto a filled circle second layer  110  of a larger diameter. Similarly, in  FIG. 5 ,  200  is another example of an adaptive mark which is composed of a first layer made up of a filled triangle  210  and a filled rectangle  220 , superimposed onto a second layer which is composed of a filled square  230 . Depending on the choice of fills, the contrast between the two layers facilitates detection of the adaptive mark, and may allow the adaptive mark to survive photocopying of the encoded document, regardless of the content of the background printed matter. However, when choosing suitable fills, minimising the contrast of the adaptive mark with the background printed matter is taken into consideration in order to minimise the impact on visual quality. 
     It can be seen that there are many combinations of parameters that result in a correspondingly wide variety of adaptive marks. For example, changing the geometry and fill of one or both layers will result in many possible adaptive marks. Thus, rather than making use of a single type of adaptive mark, a set of suitably designed adaptive marks can be created and used. Decisions can be made as to which mark within the set is chosen to be superimposed onto the printed matter depending on the local characteristics of the printed matter, such that the impact on visual quality is minimised. 
     To further promote visual quality, whether or not an adaptive mark is to be superimposed at all can be decided based on the logical content of the printed matter at the adaptive mark location under consideration. For example, information regarding the logical content of the information in the document can be divided into categories such as text, photo content, line art, flesh tones, etc. in increasing impact on visual quality when adaptive marks are added.  FIG. 6  shows a document  300  which divides the printed matter into logical content categories of white space  330 , text  310 , images  320  and vector content  340 . 
     There are many ways of obtaining such logical content descriptions of the information in the document. One approach is by considering documents containing printed matter in vector digital format such as Postscript™ or Portable Document Format™. Raster image processing techniques are applied to vector digital formatted documents and in the process, information can be obtained so that the printed matter can be divided into logical content categories such as those described above (in relation to  FIG. 6 ) at pixel level granularity. 
     Another approach takes the entire document as a raster image and performs whole page analysis on the printed matter. Such analyses may be used to identify skin tones or regions of high noise at pixel level granularity, for example. 
     Another approach for deriving the logical content information for a document uses meta-data descriptions for the document. 
     The additional encoded data using adaptive marks is typically not encoded directly as raw data but rather, in a form where controlled redundancy is introduced through the use of error correction codes. When error correction codes are used, the entire encoded steganographic raw data can be fully recovered even when a certain percentage of the encoded adaptive marks are lost. A desired robustness can generally be achieved for the added steganographic encoded data when a certain percentage of the page is encoded. Then, it is possible to decide which categories of logical content of the page are to be encoded with marks and which categories can be skipped, to encode at least the required percentage of the document. 
     Documents can be partitioned into regions containing corresponding categories of logical content. These regions can be prioritised according to defined (first) criteria, and steganographic data thereby encoded into each of the aforementioned categories according to the priority of the category. The first criterion may be determined according to the region type where the priority is based on the content of the region. The region type is based on the contents of the region and includes vector image, raster image, text or white space. An example of a priority order would be, from high to low priority, white space, vector image, raster image and then text. 
     Furthermore, the amount, either relative or absolute, of steganographic data per logical content category can also be determined according to defined (second) criteria. This is akin to steganographic density and may vary according to the document resolution or the resolution of a component in which the message information is to be included. The second criterion can also be based on the region type. For example, a raster image may be given a low priority based on the priority of the first criteria. However, if data is to be encoded in to the raster image region the data density would be low. Alternatively the second criterion can be based on the properties of the region such as the density of the text in the region, the line spacing for a text region, the amount of skin tones used in images or the extent of white space in the region. Where the density of the text or the amount of skin tones used in an image is high, the amount of steganographic data can be reduced. Where the amount of white space is high or the line spacing is large, then the amount of steganographic data can be increased. 
     The term “relative amount of steganographic data per logical content category” refers to the amount of steganographic data to be written into the logical content category in question as a proportion of the total amount of steganographic data to be incorporated into the document. The term “absolute amount of steganographic data per logical content category” refers to the amount of steganographic data to be written into the logical content category in question without reference to the total amount of steganographic data to be incorporated into the document. 
       FIG. 8  shows a flow chart of a process  1500  for incorporating steganographic data into a document containing known information. The process commences with a start step  1501  after which in a step  1502  the processor  1105  identifies logical information categories in the document. In another arrangement the various available logical information categories are pre-defined. A following step  1503  establishes a priority according to which steganographic information is to be encoded into each of the aforementioned categories according to a first criterion (eg. the first criteria mentioned above). Thereafter a step  1504  determines how much steganographic information is to be added to each of the aforementioned categories according to a second criterion (eg. the second criteria mentioned above). Subsequently a step  1505  partitions the document into regions each of which comprises information in one of the aforementioned categories. A following step  1506  encodes the steganographic data into each region according to the priorities determined in the step  1503  and the amounts determined in step  1504 . The process  1500  then terminates with a step  1507 . 
     In the first arrangement, original documents containing black and white printed matter are considered. A set of two distinct adaptive marks are employed, namely, a white adaptive mark and a black adaptive mark. Referring to  FIG. 7 , the white adaptive mark is shown in  400  which is made of a first layer consisting of a filled white circle  420  superimposed onto the centre of a second layer consisting of a filled black circle  410  of a diameter larger than that of the white circle, i.e. diameter  430 &gt;diameter  440 . The corresponding black adaptive mark is the same as the white adaptive mark except with the colours interchanged. Thus, the black adaptive mark contains a black centre circle and the white adaptive mark contains a white centre circle. 
     The process of the first arrangement for superimposing a single mark will be described below with reference to process  800  in  FIG. 9 , which starts at a step  810  and ends at a step  880 . For the purpose of illustration, it is assumed that the data is encoded by placing adaptive marks located at the centre of cells of a regular grid. 
     To illustrate, consider  FIG. 10  which shows a document  500  where printed matter is divided into logical content categories. Specifically, a circular region  510  indicates vector content and a triangular region  520  contains image content. Furthermore, the page has been divided up into cells on a regular grid of cells. 
     Returning to  FIG. 9 , firstly consider a particular adaptive mark location where an adaptive mark is to be placed. Depending on the logical content of the printed matter located in the cell containing the proposed adaptive mark location, as determined by a step  820 , a decision is made whether the adaptive mark is actually to be superimposed onto the printed matter or skipped as shown in a following decision  830 . If the adaptive mark is to be skipped, then the process  800  follows a YES arrow and ends at a step  880 . 
     As an example, if it is decided that all image contents are to be skipped for mark encoding, then only regions of white space and regions of vector content within the circle  510  will be encoded. Although this results in the loss of encoding marks in some regions of the document  500 , in many instances the additional (redundant) encoded data will have sufficient redundancy to remain robust against this loss. 
     Once the decision to superimpose a dot is made in the step  830 , a following step  840  in  FIG. 9  is executed and a greyscale digital image representation of the original document is obtained. From this digital image, the local average greyscale value around the location where a mark is to be placed is determined. For example, if the data is encoded by placing marks located at the centre of cells of a regular grid, the average greyscale value for a given mark location could be calculated by finding the average greyscale value within the cell containing the mark location. These values are recorded in a greyscale map. 
     Consider a cell  610  which is highlighted by a bold perimeter in  FIG. 10  and magnified in  FIG. 11 . When an adaptive mark is superimposed onto the printed matter of this cell, the average greyscale value of the cell is obtained and compared to a threshold. This is depicted by the step  840  and the decision step  850  in  FIG. 9 . If the greyscale value is less than the threshold, then the background is considered dark coloured and so the process  800  follows a NO arrow and a white mark is added as shown in a step  860 . Otherwise, the background is considered light coloured and so the process  800  follows a YES arrow and a black mark is added instead, as shown in process  870 . 
     For example, assuming that the average greyscale value of cell  610  in  FIG. 10  and  FIG. 11  is greater than the threshold, then a black mark  620  is chosen to be placed in the centre of the cell, as shown in  FIG. 12 . Similarly, assuming that the average greyscale value of cell  710  in  FIG. 10  and  FIG. 13  is less than the threshold, then a white mark  720  is chosen to be placed in the centre of the cell, as shown in  FIG. 14 . Referring to a mark  400  in  FIG. 7 , the optimal approach uses a black mark which has diameter  430  of 11 pixels and diameter  440  of 5 pixels, both at a resolution of 600 dpi. The white mark has diameter  430  of 11 pixels and diameter  440  of 8 pixels, both at resolution of 600 dpi. 
     It is clear that this approach exemplified by the process  800  in  FIG. 9  can be extended to colour documents. In a second arrangement, one such approach is described as follows. The essence of this second arrangement is the same as that of the previous first arrangement so the details that are similar will not be repeated. The main difference is rather than obtaining a greyscale digital image representation of the original document in the step  840 , a colour digital image representation is obtained and stored in the well known “Lab” colour space. The local average “ab” value around the location where a mark is to be placed is determined. Having obtained this average ab value, the first layer is filled with a colour which has the same ab value but a higher L value. The second layer is filled with a colour which also has the same ab value but a lower L value. Thus, contrast within the mark is maximised whilst contrast with the background printed matter is minimised. 
       FIG. 15  shows how the adaptive marks of  FIGS. 4 and 5  are incorporated as steganographic information into a document  1400  having known information. The document  1400  has known information in the form of a background  1401 , a shape  1402 , a black coloured letter  1403  and a white coloured letter  1404 . An adaptive mark in the form of a dot at a boundary of the shape  1402  is depicted by an arrow  1405  in enlarged form. A fragment  1409  of the background  1401  has superimposed thereon an adaptive dot  1408  of the form shown in  FIG. 4 . An adaptive mark in the form of a dot at a boundary of the black coloured letter  1403  is depicted by an arrow  1406  in enlarged form. A fragment  1411  of the dark coloured letter  1403  has superimposed thereon an adaptive dot  1410  of the form shown in  FIG. 4 . An adaptive mark in the form of a dot at a boundary of the white coloured letter  1404  is depicted by an arrow  1407  in enlarged form. A fragment  1412  of the white coloured letter  1404  has superimposed thereon an adaptive dot  1413  of the form shown in  FIG. 5 . 
     INDUSTRIAL APPLICABILITY 
     It is apparent from the above that the arrangements described are applicable to the computer and data processing industries. 
     The foregoing describes only some embodiments of the present invention, and modifications and/or changes can be made thereto without departing from the scope and spirit of the invention, the embodiments being illustrative and not restrictive. 
     [Delete this as it is not for Australia] 
     In the context of this specification, the word “comprising” means “including principally but not necessarily solely” or “having” or “including”, and not “consisting only of”. Variations of the word “comprising”, such as “comprise” and “comprises” have correspondingly varied meanings.