GB2369944A - Apparatus for detecting and recovering data - Google Patents

Abstract

Apparatus operable to reduce the likelihood of falsely detecting and recovering embedded data, for example, watermark data. The apparatus comprises an error correction encoder operable to encode the data in accordance with an error correction code, and a data embedding processor operable to combine the encoded data with the information material. The apparatus further comprises an embedded data detector 350 operable to recover a representation of the error correction encoded data from the information material, and a false positive detector 400 operable to determine a number of errors in the recovered encoded data, in accordance with the error correction code, and to compare the number of detected errors with a threshold number of errors 412. If the number of detected errors is greater than or equal to the threshold, the recovered data is declared falsely detected.

Description

APPARATUS FOR DETECTING AND RECOVERING DATA Field of Invention The present invention relates to methods and apparatus for detecting and recovering data embedded in information material.

Information material as used herein refers to and includes one or more of\ideo material, audio material and data material. Video material in this context may be still images or moving images.

Background of Invention Steganography is a technical field relating to the embedding of data into material such as video material, audio material and data material in such a way that the data is imperceptible in the material.

Watermarks are data embedded in material such as video material, audio material and data material. A watermark may be imperceptible or perceptible in the material.

A watermark may be used for various purposes. It is known to use watermarks for the purpose of protecting the material against, or trace, infringement of the intellectual property rights of the owner (s) of the material. For example a watermark may identify the owner of the material.

Watermarks may be"robust"in that they are difficult to remove from the material. Robust watermarks are useful to trace the provenance of material which is processed in some way either in an attempt to remove the mark or to effect legitimate processing such as video editing or compression for storage and/or transmission.

Watermarks may be"fragile"in that they are easily damaged by processing which is useful to detect attempts to remove the mark or process the material.

Visible watermarks are useful to allow, for example, a customer to view an image via, for example, the Internet to determine whether they wish to buy it but without allowing the customer access to the unmarked image they would buy. The watermark degrades the image and the mark is preferably not removable by the customer. Visible watermarks are also used to determine the provenance of the material into which they are embedded.

In US patent 5, 930, 369 (Cox et al) it has been proposed to embed data into material such as images to form a watermark by converting the material into the I z : l I transform domain and adding the data to the image in the transform domain. For the example of images and the Discrete Wavelet Transform of these images into the transform domain, the data to be added can be combined with the wavelet coefficients of one of a plurality of sub-bands which are formed in the transform domain.

Generally, the data to be embedded is arranged to modulate a predetermined data sequence such as a Pseudo Random Bit Sequence (PRBS). For example. each bit of the data to be embedded is arranged to modulate a copy of the PRBS, and this copy is then added, for example into one of the sub-bands of the image in the transform domain. The image is then converted back to the spatial domain.

If it is desired to detect and recover the embedded data from the image. the image is converted back to the transform domain and the embedded data is recovered from the sub-band in the transform domain by cross-correlating the transform coefficients in the sub-band with the Pseudo Random Bit Sequence which is known to the detecting apparatus. The data is detected and recovered from the result of the correlation.

Summary of Invention According to the present invention there is provided an apparatus for detecting and recovering data embedded in information material, the data having been encoded in accordance with an error correction code to produce encoded data symbols which are embedded in the information material, the apparatus comprising an embedded data detector operable to recover a representation of the error correction encoded data from the information material, a false positive detector operable to determine a number of errors in the recovered encoded data in accordance with the error correction code, and to compare the number of detected errors with a threshold number of errors, and if the number of detected errors is greater than or equal to the threshold, declaring the recovered data as falsely detected.

Data may be embedded into information material for several reasons. One reason is to provide an indication which can be uniquely identified by the owner of the

information material, so that, for example, intellectual property rights vesting in the tn

information material can be asserted. However, it will be appreciated that it is very undesirable for data to be detected from information material, when in fact no such data has been embedded, because a producer of the information material may wrongly assume ownership of the material from the data recovered. Effectively, this can be viewed as a technical problem of reducing the likelihood of falsely recovering data which may be produced by noise within the information material or the information material itself, when in fact no data has been embedded in the information material.

This false recovery of embedded data from information material is referred to herein as falsely detecting data or producing a false positive.

Embodiments of the present invention address the technical problem of reducing the likelihood of falsely detecting data by utilising the error detection properties of error correction or detection codes. By detecting a number of errors in the error correction encoded data items and comparing the number of errors with a threshold number errors, an encoded data item can be declared as falsely detected if the number of detected errors is greater than the threshold. The threshold may be chosen such that the probability of falsely detecting and recovering a data item from information material at random can be made acceptably small. To this end the false positive detector assumes that the embedded data has been encoded with an error correction code and then detects the number of symbol errors for this code. Wrongly detected embedded data is declared if the number of errors is greater than the thresold.

The term error correction code as used herein refers to any error correction code, error detection code or error detection and/or correction code. The term error correction code therefore also implies a code which only provides the facility for detecting errors, as well as the majority of codes which provide error correction as well as error detection. Examples of such error correction codes are Hamming Codes, Reed-Solomon codes and convolutional codes.

In preferred embodiments the data items are meta data items such as Unique Material Identifiers.

Although embodiments of the invention find application in detecting and recovering data from any information material, a particular application of the invention is in detecting and recovering data embedded in video image or audio signals.

Various further aspects and features of the present invention are defined in the appendedCteu'.

Brief Description of the Drawings Figure 1 is a schematic block diagram of a watermarking system ; Figure 2 is a schematic block diagram of a watermark embedder appearing in Figure 1 ; Figure 3 is a schematic representation of a UMID encoded by the error correction encoder shown in Figure 2 ;

Figure 4 is a schematic block diagram of a combiner forming part of the tt, I' ZL watermark embedder shown in Figure 2 ; Figure 5 provides an illustrative representation of a transform domain image with which data is combined;

Figure 6 is a schematic block diagram of a watermark decoder appearing in Z7 Figure 1; Figure 7 is a schematic block diagram of an error correction decoder shown in Figure 6, including a false positive detector; Figure 8 is a flow diagram illustrating the operation of the false positive detector shown in Figure 7; and Figures 9A and 9B are schematic block diagrams of the structure of an extended and a basic UMID respectively.

Description of Preferred Embodiments An example embodiment of the present invention will be described with reference to a watermarking system in which data is embedded into a video image. Any type of data can be embedded into the image. However, advantageously the data embedded into the image may be meta data which describes the image or identifies some attributes of the content of the image itself. An example of meta data is the Unique Material Identifier (UMID). A proposed structure for the UMID is disclosed in SMPTE Journal March 2000. A more detailed explanation of the structure of the UMID will be described later.

Watermarking System

Figure 1 illustrates a watermarking system. generally 10, for embedding a 9 watermark into a video image 115, and recovering and removing a watermark from the video image 115. The watermarking system 10 in Figure 1 comprises an image v I t : z processor 100 for embedding the watermark into the video image. and a decoding image processor 102 for detecting and recovering the watermark, and for removing or 'washing'the watermark from the video image.

The image processor 100 for embedding the watermark into the video image comprises a strength adapter 180, and a watermark embedder 120. The watermark embedder 120 is arranged to embed the watermark into the video image 115, produced from the source 110, to form a watermarked image 125. The watermark to be embedded into the video image is formed from data 175 representing a UMID.

Generally. the UMID identifies the content of the video image. although it will be appreciated that other types of meta data which identify the content or other attributes

of the image can be used to form the watermark. In preferred embodiments the zn watermark embedder 120 embeds the UMID into the video image 115 in accordance with a particular application strength 185 from the strength adapter 180. The strength adapter 180 determines the magnitude of the watermark in relation to the video image 115, the application strength being determined such that the watermark may be recovered whilst minimising any effects which may be perceivable to a viewer of the watermarked image 125. After embedding the watermark, the image may be transmitted, stored or further processed in some way, such as for example, compression encoding the image. This subsequent processing and transmitting is represented generally in Figure 1 as line 122.

In Figure 1 the decoding image processor 102 for detecting and removing the watermark is shown as comprising a watermark decoder 140, a data store 150 and a watermark washer 130 which removes the watermark from the watermarked image 125.

The watermark decoder 140 detects the watermark from the watermarked video image and in the present example embodiment, generates a restored UMID 145 from the watermarked image 125. The watermark washer 130 generates a restored image 135, by removing as far as possible the watermark from the watermarked image 125. In some embodiments, the watermark washer 130 is operable to remove the watermark

from the image substantially without leaving a trace. The restored image 125 may incy a trace. then be stored in a store 150. transmitted or routed for further processing.

The Watermark Embedder The watermark embedder will now be described in more detail with reference to Figure 2, where parts also appearing in Figure 1 have the same numerical references. In Figure 2 the watermark embedder 120 comprises a pseudo-random sequence generator 220. an error correction encoder 200, a wavelet transformer 210. an inverse wavelet transformer 250. a modulator 230 and a combiner 240.

The error correction encoder 200 receives the UMID 175 and generates an error correction encoded UMID comprising redundant data in combination with the UMID. in accordance with an error correction encoding scheme. It will be appreciated that various error correction coding schemes could be used to encode the UMID. For the example embodiment the error correction encoder 200 uses a Bose-Chaudhuri Hocquenghem (BCH) systematic code providing 511 bit codewords comprising 248 source bits of the UMID and 263 bits of redundant parity bits. This is represented in Figure 3.

It will be appreciated that the present invention is not limited to any particular error correction encoding scheme, so that other BCH codes, or for example Reed Solomon codes or convolutional codes could be used to encode the UMIDs.

As shown in Figure 2 the error correction encoded UMID 205 is received at a first input to the modulator 230. The pseudo-random sequence generator 220 outputs a PRBS 225 which is received at a second input to the modulator 230. The modulator 230 is operable to modulate each copy of a PRBS, generated by the pseudo-random sequence generator 220, with each bit of the error correction encoded UMID. In preferred embodiments the data is modulated by representing the values of each bit of the PRBS in bipolar form ('1'as +1, and'0'as-1) and then reversing the polarity of each bit of the PRBS, if the corresponding bit of the encoded UMID is a'0'and not reversing the polarity if the corresponding bit is a'1'. The modulated PRBS is then received at a first input of the combiner 240. The combiner 240 receives at a second input the image in which the PRBS modulated data is to be embedded. However the data is combined with the image in the transform domain.

The use of a pseudo-random sequence 225 to generate the spread spectrum signal representing the watermark data allows a reduction to be made in the strength of the data to be embedded in the image. By cross-correlating the data in the transform domain image to which the modulated PRBS has been added. a correlation output signal is produced with a so called correlation coding gain which allows the modulated data bit to be detected and determined. As such, the strength of the data added to the image can be reduced, thereby reducing any percevable effect on the spatial domain image. The use of a spread spectrum signal also provides an inherent improvement in robustness of the image because the data is spread across a larger number of transform domain data symbols.

As shown in Figure 2, the wavelet transformer 210 receives the video image 115 from the source 110 and outputs a wavelet image 215 to the combiner 240. The

image is thus converted from the spatial to the transform domain. The combiner 240 is operable to combine the PRBS modulated data with the image in the transform domain, in accordance with the application strength, provided by the strength adapter 180. The watermarked wavelet image 245 is then transformed into the spatial domain by the inverse wavelet transformer 250 to produce the watermarked image 125. The operation of the combiner 240 will be explained in more detail shortly.

The skilled purpose will be acquainted with the wavelet transform and variants.

A more detailed description of the wavelet transform is provided in for example"A Really Friendly Guide to Wavelets"by C Valens, 1999 (c. \aknsI mindlcss. com).

Although in the example embodiment of the present invention the data is embedded in the image in the wavelet transform domain, it will be appreciated that the present invention is not limited to the wavelet transform and could be added to the image using any transform such the Discrete Cosine Transform or the Fourier Transform. Furthermore the data could be combined with the image in the spatial domain without forming a transform of the image.

Combiner The operation of the combiner 240 will now be explained in more detail. The combiner 240 receives the wavelet image 215 from the wavelet transformer 210, and the modulated PRBS from the modulator 230 and the application strength 185 from the

strength adapter 180. The combiner 240 embeds the watermark 235 onto the wavelet image 215. by adding, for each bit of the modulated PRBS a factor α scaled by ~1. in dependence upon the value of the bit. Selected parts of the wavelet image 215 are used to embed the watermark 235. Each pixel of the predetermined region of the wavelet image 215 is encoded according to the following equation: X't=Xt+αnWn (1) Where Xi is the i-th wavelet coefficient, an is the strength for the n-th PRBS and W n is the n-th bit of the watermark to be embedded in bipolar form, which for the example embodiment is the bit of the error correction encoded UMID.

An example of the combiner and the operation of the combiner will now be described with reference to Figures 4 and 5. In Figure 4 the combiner 240 is shown to receive the transform domain image from the connecting channel 215. The transform domain image is received at a frame store 236. The frame store 236 is arranged to store a frame of transform domain data. The combiner 240 is also arranged to receive the spread spectrum encoded and error correction encoded UMID after it has been spread using the PRBS (modulated PRBS data). For this example embodiment one UMID in this error correction and spread spectrum encoded form is to be embedded in the frame of image data within the frame store 236. Thus, each encoded UMID forms an item of data which is to be embedded into each frame of image data. To this end. the data to be embedded is received at a combining processor 237 which combines the data to be embedded into selected parts of the wavelet transform domain image stored

in the frame store 236. The combiner 240 is also provided with a control processor 238 which is coupled to the combining processor 237.

In Figure 5 an illustrative representation of a first order wavelet transform is presented. This wavelet transform is representative of a frame of the image transformed into the wavelet domain and stored in the frame store 236. The wavelet transform image WTIMG is shown to comprise four wavelet domains representative of sub-bands into which the image has been divided. The wavelets comprise a low

horizontal, low vertical frequencies sub-band IHIV, the high horizontal, low vertical frequencies sub-band hHIV, the low horizontal, high vertical frequencies sub-band IHhVi and the high horizontal, high vertical frequencies sub-band hHhV 1.

In the example embodiment of the present invention, the data to be embedded is only written into the low vertical, high horizontal frequencies sub-band hH and the low horizontal, high vertical frequencies sub-bands labelled hHlV.

By embedding the data in only the two sub-bands hH1IV1, 1H1hV1. the likelihood of detecting the embedded data is improved whilst the effects that the

embedded data will have on the resulting image are reduced. This is because the I wavelet coefficients of the high horizontal, high vertical frequencies sub-bands hHlhV are more likely to disturbed, by for example compression encoding.

Compression encoding processes such as JPEG (Joint Photographic Experts Group) z : l operate to compression encode images by reducing the high frequency components of p ZD the image. Therefore, writing the data into this sub-band hHlhVI would reduce the L-t likelihood of being able to recover the embedded data. Conversely, data is also not written into the low vertical, low horizontal frequencies sub-band IHIIVI. This is because the human eye is more sensitive to the low frequency components of the image. Therefore, writing the data in the low vertical, low horizontal frequencies sub

band would have a more disturbing effect on the image. As a compromise the data is t : l CP added into the high horizontal, low vertical frequencies sub-band hI-Fv and the low horizontal, high vertical frequencies sub-bands IHhV.

The detection and recovery of the embedded data is performed by the watermark decoder 140 forming part of the decoding image processor 102 shown in Figure 1.

Decoder The operation of the watermark decoder 140 in the decoding image processor, will now be explained in more detail, with reference to Figure 7, where parts also appearing in Figure 1, bear identical reference numerals. The watermark decoder 140 receives the watermarked image 125 and outputs a restored version of the UMID 145.

The watermark decoder 140 comprises a wavelet transformer 310, a pseudo-random sequence generator 320, a correlator 330, and an error correction decoder 350.

The wavelet transformer 310 converts the watermarked image 125 into the transform domain so that the watermark data can be recovered. The wavelet

coefficients to which the PRBS modulated data were added by the combiner 240 are then read from the two wavelet sub-bands hH IIV 1, lH 1 h V I in the reverse direction to the direction in which the data was added in the combiner 240. These wavelet

coefficients are then correlated with respect to the corresponding PRBS used in the z watermark embedder. Generally, this correlation is expressed as equation (2), below, where Xn is the n-th wavelet coefficient and Rn is the n-th bit of the PRBS generated by the Pseudo Random Sequence Generator 320.

The relative sign of the result of the correlation Cn then gives an indication of the value of the bit of the embed data in correspondence with the sign used to represent this bit in the watermark embedder. The data bits recovered in this way represent the error correction encoded UMID which is subsequently decoded by the error correction

decoder 350 using a decoding algorithm for the error correction code used by the ZD z :- t : l encoder 200. Having recovered the UMID, the watermark can be removed from the v video image by the watermark washer 130. by performing the reverse of the operations I performed by the embedder.

False Positive Detector A false positive detector which forms part of the decoder 350 is illustrated in Figure 7 where parts appearing in Figure 6 bear the same numerical references. In Figure 8 the encoded UMIDs are received via the channel 345 at a false positive detector 400 and in parallel by an error correction decoder 414. As will be familiar to those skilled in the art, an error correction code can be used either to identify the presence of errors in code word, and to identify and correct a number of errors within an encoded code word. Generally, the number of errors which can be detected is greater than the number which can be corrected. For the present example embodiment, the number of errors which can be detected within the 511-bit BCH code word is 31bits, although the presence of a greater number of errors can be detected.

In Figure 7 the false positive detector 400 receives the encoded UMID and generates an estimate of the number of errors within the encoded UMID. The false positive detector also receives a signal representing a threshold in the form of a

number of errors via a connecting channel 412. The threshold may be set to a predetermined value or may be set by a user in accordance with a particular application. The false positive detector 400 operates to compare the number of errors detected with the threshold and generates a false detection indication signal output channel 145.2, if the number of errors detected is greater than the threshold.

In parallel, the encoded UMID is fed to an error correction decoder 414 which operates to decode the UMID in accordance with the BCH error correction code applied by the encoder. The estimated UMID is then presented at the output channel 145.1.

Effectively the threshold number of errors received on the channel 412 provides a level which determines whether the recovered UMID is treated as a genuine UMID or a UMID which has effectively been generated at random from noise within the image and therefore caused a UMID to be falsely detected.

The threshold can be set to a level which provides a low probability of falsely detecting a UMID. As explained earlier, the UMID according to the example embodiment has been encoded using the BCH code generating 511 bit code words. For this example, the number of possible genuine encoded UMIDs which can have up to 31 errors within each of these possible UMIDs, which can still be corrected is expressed by the left hand side of equation (3).

In equation (3), the maximum number of bits which can be corrected by the BCH is 31-bits, which therefore sets the size of the summation term. The right hand side of equation (3) represents the number of possible 511 bit code words, which can be generated at random. Therefore, this represents the total number of words which can be randomly generated having 511 bits, which is much greater than the total number of genuine UMIDs having up to 31-errors. Rearranging equation (3), equation (4) represents the probability Pip of a falsely positive result so that with the threshold set at 31 bits this probability is very much less than 1.

In other embodiments the error correction decoder may be disabled if the false positive detector indicates that data recovered from the information material is not a genuine encoded UMID.

A flow diagram representing the operation of the decoder 350 of Figure 7 is represented in Figure 8. In Figure 8. at step Sl the number of errors in the UMID is estimated. At decision step S2 this number of errors is compared with the threshold and if this is less than the threshold then the UMID is decoded at step S3. If however the number of errors is greater than the threshold then at step S4 the UMID is declared as falsely detected. The false detection indication can be attached to the UMID or could be provided as a separate user recognisable output signal.

The Unique Material Identifier (UMID) A brief explanation will now be given of the structure of the UMID, with reference to Figure 9A and 9B. The UMID is described in SMPTE Journal March 2000. Referring to Figures 9A an extended UMID is shown to comprise a first set of 32 bytes of a basic UMID, shown in Figure 9B and a second set of 32 bytes referred to as signature meta data. Thus the first set of 32 bytes of the extended UMID is the basic UMID. The components are : sA 12-byte Unique Label to identify this as a SMPTE UMID. It defines the type of material which the UMID identifies and also defines the methods by which the globally unique Material and locally unique Instance numbers are created.

*A 1-byte length value to define the length of the remaining part of the UMID.

. A 3-byte Instance number which is used to distinguish between different 'instances'of material with the same Material number.

*A 16-byte Material number which is used to identify each clip. Each Material number is the same for related instances of the same material.

The second set of 32 bytes of the signature metadata as a set of packed 9 metadata items used to create an extended UMID. The extended UMID comprises the basic UMID followed immediately by signature metadata which comprises: 'An 8-byte time/date code identifying the time and date of the Content Unit creation.

- A 12-byte value which defines the spatial co-ordinates at the time of Content Unit creation.

. 3 groups of 4-byte codes which register the country, organisation and user codes.

More explanation of the UMID structure is provided in co-pending UK patent z : l application number 0008432.7.

Various modifications may be made to the embodiments herein before described without departing from the scope of the present invention. Although in this example embodiment, the data to be embedded is added to the image in the transform domain, in alternative embodiments the data could be represented in the transform domain, inverse transformed into the spatial domain, and added to the data in the transform domain.

Claims (19)

1. An apparatus for detecting and recovering data embedded in information material, said data having been encoded in accordance with an error correction code to produce encoded data which are embedded in said information material said apparatus comprising an embedded data detector operable to recover a representation of said error correction encoded data from said information material, a false positive detector operable to determine a number of errors in said recovered encoded data in accordance with said error correction code, and to compare said number of detected errors with a threshold number of errors, and if said number of detected errors is greater than or equal to said threshold,

declaring said recovered data as falsely detected. c

2. An apparatus as claimed in Claim 1, wherein said false positive detector is operable to generate a user recognisable signal to represent said false detection declaration.

3. An apparatus as claimed in Claim 1, wherein said false positive detector is operable to attach to said data an indication that said data is falsely detected.

4. An apparatus as claimed in any preceding Claim, wherein said dataincludes meta data describing the content of said information material.

5. An apparatus as claimed in any preceding Claim, wherein said embedded data is a plurality of data items each of which has been encoded and embedded in said information material, said false positive detector being operable to determine the number of errors in each of said encoded data items, and to compare each of said encoded data items with said threshold and consequent upon said comparison with said threshold to declare said data items as falsely detected.

6. An apparatus as claimed in Claim 5, wherein each data item includes a Unique Material Identifier (UMID).

7. An apparatus as claimed in any preceding Claim, comprising a decoding processor operable to error correction decode said encoded data to generate recovered data.

8. An apparatus as claimed in any preceding Claim, wherein said false positive detector is coupled to said error correction decoder and operable to inhibit said error correction decoding consequent upon said false positive declaration.

9. An apparatus for reducing the likelihood of falsely detecting and recovering embedded data, said embedded data being encoded in accordance with an error correction code, to produce encoded data symbols which are embedded in said information material, said apparatus comprising an embedded data detector operable to recover a representation of said error correction encoded data from said information material, a false positive detector operable to determine a number of errors in said recovered encoded data, in accordance with said error correction code, and to compare said number of detected errors with a threshold number of errors, and if said number of detected errors is greater than or equal to said threshold, declaring said recovered data as falsely detected.

10. An apparatus as claimed in Claim 9, comprising a decoding processor operable to error correction decode said encoded data to generate recovered data.

11. An apparatus for reducing the likelihood of falsely detecting and recovering embedded data, said apparatus comprising an error correction encoder operable to encode said data in accordance with an error correction code, a data embedding processor operable to combine the encoded data with said information material,

an embedded data detector operable to recover a representation of said error correction encoded data from said information material, and a false positive detection processor operable to determine a number of errors in said recovered encoded data, in accordance with said error correction code. and to compare said number of detected errors with a threshold number of errors. and if said number of detected errors is greater than or equal to said threshold, declaring said recovered data as falsely detected.

12. A method of reducing the likelihood of falsely detecting and recovering embedded data. comprising the steps of assuming said embedded data has been encoded in accordance with an error correction code, said method comprising the steps of recovering a representation of said error correction encoded data from said information material, and determining a number of errors in said recovered encoded data. in accordance with said error correction code, and comparing said number of detected errors with a threshold number of errors, and if said number of detected errors is greater than or equal to said threshold, declaring said recovered data as falsely detected.

13. A method of detecting and recovering data embedded in information material, said data having been encoded in accordance with an error correction code to produce encoded data symbols which are embedded in said information material, said method comprising recovering a representation of said error correction encoded data from said information material, determining a number of errors in said recovered encoded data, in accordance with said error correction code, error correction decoding said encoded data to generate recovered data, and comparing said number of detected errors with a threshold representative of a

maximum number of errors, and if said number of detected errors is greater than or equal to said threshold, declaring said recovered data as falsely detected.

14. Use of error correction encoding to facilitate the detection of data falsely recovered from information material.

15. A computer program providing computer executable instructions,

which when loaded on to a data processor configures said data processor to operate as z : l an apparatus according to any of Claims 1 to 10 and said system according to Claim 11.

16. A computer program having computer executable instructions, which when loaded on to a data processor causes the data processor to perform the method according to Claim 12 or 13.

17. A computer program product having a computer readable medium having recorded thereon information signals representative of the computer program claimed in any of Claims 15 or 16.

18. An apparatus as herein before described with reference to the accompanying drawings.

19. A method of embedding data in an image as herein before described with reference to the accompanying drawings.