MULTIPLE WATERMARKING TECHNIQUES

Abstract: Multiple digital watermarks, each of which has different characteristics, are embedded in a document. The characteristics of the various watermarks are chosen so that each of the watermarks will be affected in a different manner if the document is subsequently copied and reproduced. The detection process or mechanism reads each of the watermarks and compares their characteristics. While wear and handling may change the characteristics of the digital watermarks in a document, the relationship between the characteristic of multiple digital watermarks in a document will nevertheless give an indication as to whether a document is an original or a copy of an original.

Description: 1 MULTIPLE WATERMARKING TECHNIQUES 2 Field of the Invention: 3 The present invention relates to steganography and more particularly to the use of 4 watermarks to determine the authenticity and history of a particular document or 5 image 6 7 Background of the Invention: 8 Steganographic and digital watermarking technologies are well know For example 9 see U S Patent 5,636,292 and the extensive references cited therein Also see 10 copending patent applications serial number 08/327,426 which was filed 10/21/94 1 and copending application serial number 08/436, 134 which was filed 5/8/95 12 3 The technology for inserting digital watermarks in images and the technology for 14 reading or detecting digital watermarks in images is well developed, well known and 15 described in detail in public literature Furthermore, there are commercially 16 available products which include programs or mechanisms for inserting digital 17 watermarks into images For example the commercially available and widely used 18 products "Adobe Photoshop" which is marketed by Adobe Corporation of San Jose 19 California and "Corel Draw" program which is marked by Corel Corporation of 70 Ontario Canada, include a facility for inserting digital watermarks into images 21 11 The technology for making high quality copies of documents is widely available 3 The technical quality of scanners and color printers has been increasing rapidly 24 Today for a relatively low cost one can purchase a high quality scanner and a high - 5 quality color printer Thus it is becoming increasingly easy to duplicate documents _6 The ability to create high quality copies has created a need for technology which 27 can differentiate between original documents and copies of the original 1 2 It is known that watermarks can be used to help differentiate genuine documents 3 from copies However, the prior art techniques for using digital watermarks to 4 differentiate genuine documents from copies have serious limitations The present 5 invention is directed to an improved technique for using steganography and digital 6 watermark technology to facilitate differentiating original documents from copies of 7 the original 8 9 The present invention can also be used for various other purposes such as to 10 embed multiple types of information in a single document or to provide watermarks 11 which enable documents to perform special functions »2 13 Summary of the Invention: 14 With the present invention multiple digital watermarks, each of which has different 15 characteristics are embedded in a document The characteristics of the two 16 watermarks are chosen so that each of the watermarks will be affected in a different 17 manner by what may subsequently happen to the document 18 19 The detection process or mechanism reads the two digital watermarks and 20 compares their characteristics While wear and handling may change the :i characteristics of the individual watermarks, the relationship between the '2 characteristic of the two watermarks will never-the-less give an indication as to 23 whether a document is an original or a copy of an original 24 25 For example according to the present invention two digital watermarks in a 26 document may have different energy levels The absolute energy level of a digital 27 watermark in an original image may be decreased if a document is subject to wear i Likewise the energy level of the digital watermark in an image may be decreased if 2 an image is scanned and reprinted on a color printer However, the relationship 3 between the energy level of the two digital watermarks will be different in an image 4 that has been subject to weat and in a reproduced image Likewise if two digital 5 watermarks are introduced into an image where the bit pattern used to construct the 6 digital watermarks have different patterns, the ratio between the signal to noise ratio 7 of the watermarks will be different in an original subject to wear and in a copy 8 generated by scanning the original and printing the scanned image Other 9 characteristics of multiple digital watermarks can also be used to differentiate -o original documents from copies 11 i 2 Brief Description of the Figures: J 3 Figure 1 shows the paths that a document and a copy may follow 14 Figures 2A and 2B show a fine grain and a course gram watermark )5 Figure 3A and 3B show a geometrically linear and a geometrically random 16 assignment of pixels to a bit in a digital watermark 17 Figure 4 illustrates a fourth embodiment of the invention ! 8 19 Detailed Description of preferred embodiments: 20 The problem of differentiating an original document from a copy is made more 21 difficult in situations where the original document is subject to being handled, worn, 11 folded and otherwise damaged Many original documents such as identification 23 documents and currency are extensively handled The wear to which such 24 documents are subjected reduces the quality of images on the document and 15 therefore reduces the quality of any information embedded in the document using 26 conventional steganographic techniques 27 1 With the present invention a number of different watermarks are embedded in a 2 document Each of the wateimarks embedded in the document, has different 3 characteristics All watermarks are somewhat affected when a document is 4 subjected to wear, and all watermarks are somewhat affected when a document is 5 duplicated by being scanned and reprinted However, the magnitude of the effect 6 caused by being scanned and reprinted on watermarks with certain characteristics 7 is much greater than the effect on watermarks with different characteristics 8 Likewise, wear and handling of a document affects watermarks with certain 9 characteristics much more than it affects watermarks with different characteristics 10 11 Thus, if multiple watermarks with different characteristics are inserted into a 12 document, it is possible to differentiate a copy from an original document that has 13 been subjected to wear by examining the ratios of characteristics of the watermarks 14 in the image being examined 16 In order to print a document on a color printer, the document is put through a ) 7 transformation from a color space such as the RGB color space to a different color 18 space such as the CMYK (cyan, magenta, yellow, black) color space Such 19 transformations are well know For example see chapter 3 entitled "Color Spaces" :o in a book entitled "Video Demystified, A handbook for the Digital Engineer", Second " Edition, by Keith Jack, published by Harris Semiconductor and Hightext Publications 22 of San Diego, California r. 24 When an image is transformed from one color space to another color space, noise 25 is introduced into the image Among the reasons for this is the fact that each color 26 space has its own distinctive gamut (or range) of colors Where the gamut of two 27 color spaces overlap, the conversion from one color space to another color space 1 can in theory be precise However, there will be some areas which are in the gamut 2 of one color space not in the gamut of another color space Such situations 3 definitely introduce noise into the conversion process Even in areas that are in the 4 gamut of two color spaces, conversion from one color space to another color space 5 introduces noise because of such things as round off errors The present invention 6 takes advantage of the fact that if an original is copied and then a copy is printed, 7 the image on the printed copy will have gone through several conversions to which 8 the original will not have been subjected For example, the conversions to which a 9 copy may be subjected u) are i l 1 ) a document to RGB conversion (i e scanning the document into the computer), 12 2) a RGB to CMYK conversion, 13 3) a CMYK to copy conversion (i e printing the document) 14 Any characteristics of the two digital watermarks that will be affected differently by 15 the additional conversion process to which copies are subjected can be used to 16 differentiate copies from an original Since the two watermarks with different 17 characteristics are affected in a different manner by the additional conversion step, 18 a comparison of the characteristics of the two watermarks in a document being ' 9 examined will indicate if the document is an original (which has not gone through 20 the additional conversions) or a copy which has gone through the additional 21 conversions While the characteristics of each watermark will have been changed 22 by wear and by the copying process, the comparison between the characteristics of 23 the two watermarks will still be able to differential a copy from an original 24 25 Four embodiments of the invention are described below Each of the embodiments ,'6 utilizes two watermarks in a document The differences between the two 27 watermarks in the document are as follows 1 In the first embodiment 2 First watermark Has fine gram 3 Second watermark Has a course gram 4 In the second embodiment 5 First watermεtk Has geometrically linear assignment of pixels 6 Second watermark Has geometrically random assignment of pixels 7 In the third embodiment 8 First watermark Has low power 9 Second watermark Has higher power 10 In the fourth embodiment 11 Fist watermark uses standard RGB to HSI and HSI to RGB transformations 12 Second watermark is oiased before being transformed from HSI to RGB 13 14 Figure 1 shows the steps to which documents and copies are typically subjected In '5 the normal course, a document 10 may be subjected to handling and wear 11 .6 resulting in a worn document 10A Document 10 may also be scanned as illustrated 17 by box 12 The scanning produces a digital image which can be printed as 18 illustrated by box 13 The printed image may be subjected to handling and wear 14 19 resulting in a copy 10B It is noted that the document 10 may also be subject to 20 handling and wear prior to tne scanning operation 12 The task to which this 21 invention is directed is the task of differentiating the worn document 10A from the 22 copy 10B 23 24 The document 10 includes an image (not explicitly shown) which has two digital -.5 watermarks inserted therein In the first embodiment of the invention, the first 26 watermark has a fine gram and the second watermark has a course gram The ~>7 gram of the two watermarks is illustrated in Figure 2 Figure 2A shows the gram of the first watermark and figure 2B shows the grain of the second watermark The first watermark uses blocks of 9 pixels (a 3 by 3 block) Each of the pixels in each 9 pixel block has its gray value changed by the same amount For example Figure 2A shows that the first 9 pixel block has its gray value increase and the second 9 pixel block has its gray value decreased The amount of increase and the selection of blocks that is increased and decreased is conventional As shown in Figure 2B, the grain of the second watermark is in blocks that are 6 pixels by 6 pixels or 36 pixels All of the pixels in each 36 pixel block are changed by the same amount In the original document 10, the two watermarks have a power ratios of 1 to 1 After wear and handling, the power of the first watermark will be degraded somewhat more than the power of the second watermark For example, as illustrated in Figure 1 , after document 10 is subjected to handling and wear, a detector which reads the watermarks might find that the power ratio of the water marks is 1 to 2 If the document 10 is scanned and the resulting digital image is printed to make a copy of the document 10, the ratio of the power of the watermarks will be affected much more than the effect of handling and wear For example as illustrated in Figure 1 , the power ratio of the watermarks may be 1 to 10, thereby allowing one to differentiate the worn original document 10A from the copy 10B It is noted that the mechanism for inserting watermarks into an image is well known as is the technique for reading a watermark and using correlation techniques to determine the signal to noise ratio (i e the power) of a watermark 1 Figures 3A and 3B shown an alternative technique for implementing the present 2 invention In the second embodiment of the invention, the two watermarks inserted 3 into the image on a document have different patterns of assigning pixels to the bits 4 of the number which the watermark represents The first watermark utilizes a 5 geometrically linear assignment of pixels to each bit For example Figure 3A shows 6 an image that has 500 by 500 pixels Considering a watermark with 50 bits, each 7 bit of the watermark would have 5000 pixels assigned to represent that bit A linear 8 assignment could have each fifth bit in each row (100 bits per row) and each fifth 9 row (50 rows) assigned to each bit of the watermark Thus 5000 pixels would be 10 assigned to each bit in a very orderly or linear manner 1 1 12 In the second watermark the pixels would be assigned to each bit in a random 13 manner as shown in Figure 3B Each bit in the watermark would still have 5000 14 assigned bits, however, the pixels would be a random location over the image 15 Naturally it should be understood that Figure 3A and 3B illustrate how pixels are 16 assigned to one bit of the watermark The other bits of the watermarks would have 17 pixels assigned in a similar manner 18 19 Similar to the first embodiment of the invention, the watermark with a linear 20 assignment of pixels and the watermark with a random assignment of pixels would 21 be affected differently by handling and wear on the original document than they 22 would be by being scanned and reprinted 23 2-1 The third embodiment of the invention described herein utilizes watermarks which X5 have different power levels Handling and wear as contrasted to scanning and υ printing would affect a watermark with a low power level differently than a water 27 mark with a high power leve 1 Watermarks with different power levels can be 1 inserted into a document in order to practice the present invention utilizing 2 commercially available prog 'ams such as Adobe Photoshop or Corel Draw In the 3 Adobe Photoshop and Corel Draw programs, the power or intensity of the 4 watermark can be adjusted by setting a simple control setting in the program 5 6 The fourth embodiment of the invention introduces different characteristics into two 7 watermarks by modifications made to one of the watermarks during the initial step 8 during which the watermarks are introduced into an image The operation of the 9 fourth embodiment .-.an be explained as shown in Figure 4 First as illustrated by 10 equation 1 there is a convers'on from RGB to HSI as is conventional This is 11 illustrated by equatior 1 As illustrated by equation 2, the first watermark is inserted 12 into the image in a conventional manner by modifying the I value in the HSI 13 representation of the image using the first watermark values (designated as WM1 t4 Δ) A first RGB value designated RGB(1 ) is then calculated using a conventional 15 transformation designated T As indicated by equation 3, the second watermark ;6 WM2 is then biased toward a particular color and the biased watermark is then 17 combined with the HSI values and transformed to a second set of RGB values 18 designated RGB(2) Finally as indicated by equation 4, the values RGB(1 ) and 19 RGB(2) are combined to form the watermarked image designated RGB(F) 20 21 The transform used to go from RGB to HSI color space (indicated in equation 1 in 11 Figure 4) can be anyone of a variety of known other techniques For example, the 23 RGB to HSI conversion can be one of the techniques explained in the above 24 referenced text book such as the following (which assumes that RGB and Intensity 25 have a value range ot 0 to I and that Red equals 0°) 26 First calculate 27 M = max (R,G,B) 1 m = mm (R,G,B) 2 r = (M-R)/(M-m) g = (M-G) /M-n) 4 b = (M-B) / (M-m) 5 Then calculate 1, S, and H as follows 6 a) l = (M + M) /2 7 b) if M = m then S = 0 and H = 180 8 if l< or = 0 5 then S = (M-m)/(M+m) 9 if I > 0 5 then S = (M-m) / (2-M-m) !0 c) if R = M then H = 60 (b-g) 11 if G = M then H = 60 (2 +r - b) 12 if B = M then H = 60(4 + g - r) 13 if H > or = 360 then H = h 360 14 if H < 0 then H = H + 360 15 The first watermark in inserted into the RGB values in a conventional manner by 16 modifying the I value of appropriate pixels so as to combine the watermark Δ values 17 with HSI values This is indicated by equation 2 in Figure 4 Next as indicated by 18 equation 3 in Figure 4, the HSI values are converted to RGB values using a 19 transform "T" The transform "T" can be conventional and it can for example be 20 done as follows 21 First calculate 22 if I < or = 0 5 then M - I (I + S) 23 if I > 0 5 then M = I + S - IS 24 m = 21 - M '5 if S = 0 then R = G = B = I and H = 180° 26 Then calculate R, G and B as follows a) if H < 60 then R = M if H < 120 then R = m + ((M-m) / ((120- H) / 60)) if H < 240 then R = m if H < 300 then R = m + ((M - m) / ((H - 240 / 60)) otherwise R = M b) if H < 60 then G = m + ((M-m)/ ( H/60)) if H < 180 then G = M if H < 240 then G = m + ((M - m) / ((240 - H_ / 60)) otherwise G = m c) if H < 120 then B = m if H < 180 then B = m + ((M -m) / ((H-120/60)) if H < 300 then B = M otherwise B = m + ((M - m) / ((360 - H) / 60)) Next the values which represent a second watermark are used to calculate a second set of RGB values designated RGB2 In order to calculate RGB2, the values of H and S are modified so that they are slightly biased toward a particular color designated H1 and S1 New values for H and S are calculated as follows (Note, H1 must be between 0 and 360, S1 must be non-negative and can be between 0 and 1 and X is a value between 0 and 1 ) Calculate new values for H and S as follows If H > H1 then H = H - (H - H1 ) x else H = H + (H 1 -H)x If S > S1 then S= S - (S- S1 )X else S = S +(S1 -S) x 1 Next add the second watermark to the values of HSI and transform these values to 2 the RGB color space as indicated by equation 3 The transformation from HSI color 3 space to RGB color space is done as previously indicated 4 5 Finally as indicated by equation 4 in Figure 4, the final RGB value (designated 6 RGBF) is calculated by combining the values of RGB1 and RGB2 This 7 combination can be done in variety of known ways 8 > It is noted that in the .-- oove example the difference between the transformation Ό used for the first and the second watermarks involves biasing the values of H and S I Alternatively a wide variety of different changes could also be made The key to 12 this fourth embodimeri. of the invention is that in effect a different transformation is 13 used for the first and the second watermarks ',4 ' 5 While four embodiments of the invention have been shown herein , it should be 16 understood that many other characteristics and attributes of a digital watermark π could be used to practice th = present invention in addition to the characteristics and s attributes described herein Furthermore other known digital watermarking 9 techniques can be used to r-ther with and applied to the digital watermarks used for » the present invention It is also noted that while in the above examples only two 21 watermarks were used, in some situations one could use three, four five or more • 2 watermarks That is, the enbodiments of the invention specifically described herein 23 utilize two watermarks It should be understood that any number of watermarks 4 could be utilized in lu. manner Furthermore while the embodiments shown herein 25 utilize two separate w termsrks, the two watermarks used to practice the present 2o invention could be combined into one watermark which has a plurality of separate 27 identifiable and measurable characteristics It is noted that while the present invention utilizes multiple watermarks with different characteristics to differentiate original documents from copies of the original, one can also utilizes multiple watermarks with different characteristics for other reasons Documents may include multiple similar watermarks in addition to the watermarks which have different characteristics according to the present invention As used herein, in general, the term "document" refers to a physical entity While the present invention has been described with respect to four specific embodiments of the invention, it should be understood that various changes in forma and detail could be made without departing from the spirit and scope of the invention The scope of the present invention is limited only by the appended claims

Claims: 1 I claim 2 1 ) A document which has embedded therein a first digital watermark having a first 3 set of characteristics and a second watermark having a set of characteristics which 4 differ from said first set of characteristics 5 6 2) The document recited in claim 1 wherein said first watermark has a different 7 energy level from said second watermark 8 9 3) The document recited in claim 1 wherein said first and second watermarks have 10 bit patterns and wherein the bit patterns which comprise said first watermark are 1 1 different from the bit patterns which comprise said second watermark ι2 13 4) A method of creating a watermarked image which comprises the steps of 14 modifying an HSI (hue, saturation, Intensity) representation of an image to imbed a 15 first watermarked in said image and create a first watermarked image, 16 transforming said first watermarked image to the RGB (red, green, blue) color 17 space, 18 biasing the values which represent a second watermark toward a particular color, 19 modifying a said HSI values to imbed said biased second watermarked in said 20 image and create a second watermarked image, 1 transforming said second watermarked image to the RGB color space, and 22 combining the values first and second watermarked images to create a final 23 watermarked image 24 25 5) The method recited in claim 4 wherein said biasing is toward a particular color 26 1 6) A method of differentiating copies of an original document from the original 2 document comprising, said document containing a first digital watermark which has 3 a first set of characteristics and a second digital watermark which has a second set 4 of characteristics, 5 reading said first and second watermarks from said original document and 6 comparing the resultant values to generate a first set of results, 7 reading said first and second watermarks from said copy of said original document 8 and comparing the resulting values to generate a second set of results, and 9 using differences between said first and second sets of results to differentiate an 10 original document from a copy of said original document 1 1 12 7 ) The method recited in claim 6 wherein said first digital watermark has a first 13 energy level and said second watermark has a second energy level 14 1 8) The method recited in claim 6 wherein said first watermark has a first bit pattern 16 and said second watermark has a different bit pattern ι7 8 9) The method recited in claim 6 where both said original document and said copy 19 have been subjected to wear 20 1 10) The document recited in claim 1 wherein said second watermark was been 22 biased toward a particular color before it was inserted in said document 23 n 1 1) A method of differentiating an original document from a copy of said document, 25 said document having first and second watermarks imbedded therein, comprising 26 the steps of comparing the characteristics of the said first and second watermarks in 1 said original document and comparing the characteristics of said first and second 2 watermarks in said copy. -. 4 12) the method recited in claim 11 wherein said first and second watermarks have 5 different grain structures. 6 7 13) The method recited in claim 11 wherein said first and second watermarks have 8 different intensity levels. 9 10 14) The method recited in claim 12 wherein said first and second watermarks have 11 different characteristics. 12 - 3 15) A document which contains an image which has embedded therein a plurality of -4 watermarks, each of said watermarks having characteristics which differ from each 15 other.

Inventors: GUSTAFSON AMMON, RHOADS GEOFFREY, GUSTAFSON, AMMON, RHOADS, GEOFFREY
Applicants: DIGIMARC CORP, DIGIMARC CORPORATION
Application year: 1999
Main classification: G06K
IPC codes: G06T1, H04N1