we have found that the arrangement of objects within an image can be used to convey information — which is otherwise imperceptible to a human viewer . we arrange so - called “ message objects ” to convey or represent a steganographic signal ( e . g ., a digital watermark ). we define a message object broadly herein as including an information carrier , an image object , a shape , an object or collection of objects , a pixel or group of pixels , a contrast or color / gray - scale area , etc . a set of message objects is arranged within an image or area to form a steganographic message . a few examples are provided below . consider a drawing illustrating a dalmatian puppy . the puppy has a white coat complimented with black spots . an artist ( or digital editor ) can arrange the spots — an example of a message object — so as to convey a hidden or steganographic message . more practical , however , is to align the spots according to a predetermined steganographic signal and then sculpt or design the puppy around the aligned spots . now consider an image or picture depicting hundreds of marbles strewn across a surface . certain of the marbles have an offsetting color ( or luminance , gray - scale level , contrast , etc .) when compared to the surface or to other marbles . we call these offset color marbles our “ message marbles .” the message marbles are arranged to represent ( or to form ) a steganographic signal . more practical , perhaps , is to initially arrange the message marbles according to a predetermined signal ( e . g ., a digital watermark signal , an orientation signal or various combinations of both ) and then “ fill - in ” non - message marbles to complete the image . another example is particularly appealing to our astronomy friends . consider an image or graphic depicting a nighttime sky . the sky is populated with “ message stars .” the message stars are arranged in the nighttime sky in a predetermined manner according to a steganographic signal . the sky is further populated with non - message stars . the message stars are preferably distinguishable from the non - message stars . the distinguishing characteristics need not be visibly perceptible , and may be based on subtle differences , e . g ., as measured in luminance , color levels , brightness , contrast , etc . a steganographic decoder , analyzing a digital version ( e . g ., an optically captured image ) of the dalmatian puppy , marble image or populated nighttime sky , decodes the steganographic message . while the above signal - conveying techniques rely on the arrangement of message objects in a spatial domain ( e . g ., in an image ), message formation or detection can be based in either a spatial or transform ( e . g ., fourier or frequency ) domain . for example , the arrangement of the message objects in the spatial domain may have significance in a frequency domain ( e . g ., may correspond to a pattern of peaks , etc .). message detection can be accordingly facilitated , e . g ., as discussed in assignee &# 39 ; s u . s . patent application ser . nos . 09 / 940 , 872 ( published as us 2003 - 0039376 a1 ) and 09 / 503 , 881 ( now u . s . pat . no . 6 , 614 , 914 ), each of which is herein incorporated by reference . the incorporated by reference patent documents detail many techniques for signal hiding and message detection . ( applicant notes that since the fourier transform is a linear operation , adjustments to pixels in a region may be made in the frequency or spatial domain . for example , a digital watermark embedder can adjust the frequency domain representation of the host signal according to the watermark definition to form a frequency domain representation of the watermarked signal region , and then take the inverse fourier transform of the watermarked regions to produce the watermarked signal in the spatial domain . alternatively , the embedder can compute a difference signal to effect the desired changes to the region in the frequency domain , and then compute the inverse transform of the difference signal into the spatial domain , where corresponding pixels of the difference signal and host signal region are summed . either way , the result is a watermarked signal in the original domain of the host signal .). we now describe our preferred image construction with reference to fig1 - 4 . fig1 illustrates a flow diagram depicting method steps for a first embodiment of the present disclosure . a digital watermark signal is provided in step 10 . the watermark signal preferably includes a message component , e . g ., a payload or identifier , and / or an orientation signal . an orientation signal is helpful to resolve image distortion such as scale and rotation . fig2 illustrates an example of a watermark signal shown in a spatial domain . ( we note that the fig2 representation is exaggerated to help simplify the discussion .). although not required , the digital watermark signal is preferably a pure ( or “ raw ”) signal in that it does not include image data . thresholding is applied to the fig2 watermark signal ( step 12 ). thresholding preferable identifies ( or reduces the watermark signal to ) a set of relevant points . the relevant points may correspond to or represent a wide range of features , such as signal or frequency peak levels , magnitude peaks , watermark message components , watermark orientation references , spatial domain signal characteristics , etc . regardless of the relevant features used to determine a relevant point , the set of relevant points is preferably sufficient to represent ( or convey ) the watermark signal . ( we use the term “ thresholding ” generally herein to include a process to identify a set and location of spatial points for placement of message objects . alternatively , the thresholding may identify relevant frequency domain points , which can be mapped or transformed into a spatial domain representation .) the thresholding procedure can also be adjusted to provide a more or less robust watermark signal . for example , the spacing of relevant points can be increased , but at a cost of robustness . while the term “ thresholding ” is used as an illustrative technique , the present invention is not so limited . indeed there are other ways to refine a watermark signal into a set or map of relevant points . for example , a frequency analysis can be used to identify significant frequency characteristics ( e . g ., peaks ). these characteristics can be mapped to the spatial domain to identify a placement point ( e . g ., a spatial location ). or , as in another implementation , the digital watermark signal is quantitized , e . g ., via a root mean square measurement . of course other techniques can be used to reduce a watermark signal to a set of relevant spatial points sufficient to convey the signal . the set of relevant points comprises a placement map as shown in fig3 . ( it should be appreciated that there are many , many possible placement map patterns , including many more or less significant positions . fig3 illustrates but one such possible placement map .). the fig3 placement map includes a plurality of spatial positions or points ( e . g ., 20 and 21 ). these points guide the placement of message objects . in one alternative implementation , the placement map includes a relative contrast indicator . for example , a placement map point may include a relatively darker or larger point ( 20 ), indicating a need for a stronger contrast level ( or color , gray - scale , etc .) of a message object , in comparison to a relatively lighter or smaller point ( 21 ). a higher contrast may signify a predetermined frequency domain characteristic , such as peak magnitude or frequency response , etc . returning to fig1 , message objects are arranged according to the placement mapping ( step 14 ). for example , message marbles ( or message stars , black dalmatian spots , etc .) are placed on or over ( or otherwise placed corresponding to ) the placement map points . the message objects thus convey the steganographic signal . other image objects can be placed in the image , e . g ., to fill in or otherwise populate the image . there are many alternative ways to implement our inventive steganographic techniques . consider the following illustrative examples . off - the - shelf digital image editing tools ( e . g ., as provided by adobe ) can be used to place message objects in an image . the operator selects a message object , adjusts the object contrast ( or color / gray - level , etc .) to sufficiently set the object apart from other image objects or the image background , and then places the objects according to a digital watermark placement map . of course this placement process can be automated . a farmer &# 39 ; s field can be plowed or crops can be planted or cut to represent message objects , all arranged according to a steganographic placement map . an aerial image of the farmer &# 39 ; s field then includes the steganographic message . different color tulips ( or other flowers ) can be planted according to a placement map . for example , groups of white tulips ( e . g ., message tulips ) can be planted in an otherwise purple tulip field . an aerial image captures the field — including the steganographic message . captured images can be used in advertisements , e . g ., when the steganographic message carries a link ( e . g ., a url or an identifier used to obtain a link ). assignee &# 39 ; s u . s . patent application ser . no . 09 / 571 , 422 , filed may 15 , 2000 ( u . s . pat . no . 6 , 947 , 571 ), discloses many suitable linking techniques that are expressly contemplated as being combined with the present invention . this patent application is herein incorporated by reference . our techniques can even be advantageously employed in the photo - mosaic field . ( photo - mosaic processes are even further discussed , e . g ., in u . s . pat . nos . 6 , 137 , 498 and 5 , 649 , 032 , which are each incorporated herein by reference .). as disclosed in u . s . pat . no . 6 , 137 , 498 , a mosaic image is formed from a database ( or collection ) of source images . source images are analyzed , selected and organized to produce the mosaic image . a target image is divided into tile regions , each of which is compared with individual source image portions to determine the best available matching source image . positioning respective best - matching source images at the respective tile regions forms the mosaic image . an improvement to a photo - mosaic process is to arrange message source photos ( e . g ., representing message objects ) according to a watermark placement map . preferably , the message source photos are subtly distinguishable from other mosaic photos via a gray - scale value , a color value , contrast or luminance , etc . the message source photos form ( or convey ) a steganographic signal . in one implementation , the arrangement of message source photos is carried out via the “ best available matching ” discussed above with respect to the u . s . pat . no . 6 , 137 , 498 . in a first implementation , the process determines whether a selected best available photo is to be titled over a placement map position . if so , the photo characteristics ( luminance , contrast , gray - scale , etc .) are subtly altered to create a message source photo . in a second implementation , the “ best available matching ” algorithm includes selection criteria , e . g ., if selecting a photo for a placement map position , the algorithm selects a photo with sufficient distinguishing characteristics to qualify as a message object . the distinguishing characteristics can be measured in terms of its neighbors ( e . g ., a message photograph may include an overall different contrast , color or gray - scale level from its neighboring photographs ) or in terms of non - message photographs . in a third implementation , message source photos are arranged according to a placement map , and then other source photos are used to fill in or complete the photo mosaic . in a variation of this third implementation , the other , non - message source photos are selected and arranged according to a best available matching technique . similar embedded designs can be made using coins , bills , umbrellas , coffee mugs , opened or closed blinds in an apartment building , shapes , snow flakes , groups of pixels , etc ., etc . with reference to fig4 , message objects of different colors ( or gray - levels , tones , contrasts , luminance , etc .) can be used in the same image . for example , the fig4 image includes a gray background ( shown by the diagonal hashes ). a message object can be distinguished from the background if it is either lighter 30 or darker 31 than the background . or if the background is blue , message objects can be red or green , etc . an image created according to our inventive techniques can be read using steganographic or digital watermarking decoding techniques , e . g ., as described in assignee &# 39 ; s ser . no . 09 / 571 , 422 ( u . s . pat . no . 6 , 947 , 571 ) and / or ser . no . 09 / 503 , 881 ( u . s . pat . no . 6 , 614 , 914 ) applications . in one implementation , digimarc mediabridge watermark reading software , available from digimarc corporation headquartered in tualatin , oreg ., is used to read an image including a corresponding mediabridge digital watermark signal represented through our message object arranging techniques . of course , other decoding techniques can be used , particularly when they correspond to the techniques used to generate the original watermark signal . ( for example , when using a digimarc mediabridge reader , the watermark signal is preferably created using a mediabridge signal generator or embedder .). most commonly , the reader identifies the message objects from the different levels of contrast ( or color , gray - scale , luminance , etc .) between a message object and other objects or background . the foregoing are just exemplary implementations of the present disclosure . it will be recognized that there are a great number of variations on these basic themes . the foregoing illustrates but a few applications of the detailed technology . there are many others . to provide a comprehensive disclosure without unduly lengthening this specification , applicants incorporate by reference , in their entireties , the disclosures of the above - cited patents and applications . the particular combinations of elements and features in the above - detailed embodiments are exemplary only ; the interchanging and substitution of these teachings with other teachings in this application and the incorporated - by - reference patents / applications are expressly contemplated . one application uses our inventive embedding techniques for “ long - distance ” or aerial digital watermark reading , e . g ., for some of the traffic monitoring examples disclosed in assignee &# 39 ; s u . s . provisional patent application no . 60 / 327 , 687 , filed oct . 5 , 2001 . ( in one experiment we created a digitally watermarked image using our inventive object placement techniques disclosed herein . we then optically captured an image of the watermarked image with a digital camera augmented with a telescope . the watermarked image was about 100 feet away from the camera / telescope . the watermark was successfully detected .). although not belabored herein , artisans will understand that the systems and methods described above can be implemented using a variety of hardware and software systems . alternatively , dedicated hardware , or programmable logic circuits , can be employed for such operations . the various section headings in this application are provided for the reader &# 39 ; s convenience and provide no substantive limitations . the features found in one section may be readily combined with those features in another section . in view of the wide variety of embodiments to which the principles and features discussed above can be applied , it should be apparent that the detailed embodiments are illustrative only and should not be taken as limiting the scope of the invention . rather , we claim as our invention all such modifications as may come within the scope and spirit of the following claims and equivalents thereof .