WO2004079642A1

WO2004079642A1 - Symmetric tags

Info

Publication number: WO2004079642A1
Application number: PCT/AU2004/000246
Authority: WO
Inventors: Paul Lapstun; Kia Silverbrook
Original assignee: Silverbrook Research Pty Ltd
Priority date: 2003-03-04
Filing date: 2004-02-27
Publication date: 2004-09-16
Also published as: US20110226862A1; US7564605B2; US7982929B2; US20090236423A1; US20040173687A1; US20040174577A1; AU2003900983A0; US7111791B2

Abstract

Coded data for disposal on or in a surface, the coded data being arranged in accordance with at least one layout having n-fold rotational symmetry about a center of rotation, where n is at least two, and having mirror symmetry, the layout including: (a) n identical first sub-layouts rotated 1/n revolutions apart about the center of rotation, and, (b) n identical second sub-layouts rotated 1/n revolutions apart about the center of rotation, each second sub-layout being a reflection of a corresponding one of the first sub-layouts, at least one sub-layout including: (i) rotation-indicating data that distinguishes that sub-layout from at least one other sub-layout; and, (ii) reflection-indicating data that distinguishes the reflection of that sub-layout from the reflection of a corresponding reflected sub-layout.

Description

SYMMETRIC TAGS

FIELD OF INVENTION

This invention relates to coded data for disposal on or in a surface and in particular to coded data formed from symmetric tags. In use the symmetric tags can be used for providing identity-coding and position- coding of surfaces.

CROSS-REFERENCES

Various methods, systems and apparatus relating to the present invention are disclosed in the following co- pending applications and granted patents filed by the applicant or assignee of the present invention. The disclosures of all of these co-pending applications are incorporated herein by cross-reference.

10/409,876 10/409,848 10/409,845 09/575,197 09/575,195 09/575,159 09/575,132 09/575,123 09/575,148 09/575,130 09/575,165 09/575,153 09/693,415 09/575,118 09/609,139 09/608,970 09/575,116 09/575,144 09/575,139 09/575,186 09/575,185 09/609,039 09/663,579 09/663,599 09/607,852 09/575,191 09/693,219 09/575,145 09/607,656 09/693,280 09/609/132 09/693,515 09/663,701 09/575,192 09/663,640 09/609,303 09/610,095 09/609,596 09/693,705 09/693,647 09/721,895 09/721,894 09/607,843 09/693,690 09/607,605 09/608,178 09/609,553 09/609,233 09/609,149 09/608,022 09/575,181 09/722,174 09/72 1,896 0/29 ,522 10/29 1,517 471 0/29 1,470 0/29 ,819 10/29 1,481 825 0/29 1,519 0/29 ,575 10/29 1,557 558 0/29 1,587 0/29 ,818 10/29 1,576 526 6,644, 545 6,609,' 653 6,651, 879 510 9/29 1,592 0/29 ,542 10/29 1,820 363 0/29 1,487 0/29 ,520 10/29 1,521 821 0/29 1,525 0/29 ,586 10/29 1,822 553 0/29 1,511 0/29 ,585 10/29 1,374

583 0/685,455 0/685,584 NPA133US

09/575,193 09/575,156 09/609,232 09/607,844 09/607,657 09/693,593 NPB008US 09/928,055 09/927,684 09/928,108 09/927,685 09/927,809 09/575,183 09/575,160 09/575,150 09/575,169 6,644,642 6,502,614 6,622,999 09/575,149 10/322,450 6,549,935 NPN004US 09/575,187 09/575,155 6,591,884 6,439,706 09/575,196 09/575,198 09/722,148 09/722,146 09/721,861 6,290,349 6,428,155 09/575,146 09/608,920 09/721,892 09/722,171 09/721,858 09/722,142 10/171,987 10/202,021 10/291,724 10/291,512 10/291,554 10/659,027 10/659,026 09/693,301 09/575,174 09/575,163 09/693,216 09/693,341 09/693,473 09/722,087 09/722,141 09/722,175 09/722,147 09/575,168 09/722,172 09/693,514 09/721,893 09/722,088 10/291,578 10/291,823 10/291,560 10/291,366 10/291,503 10/291,469 10/274,817 09/575,154 09/575,129 09/575,124 09/575,188 09/721,862 10/120,441 10/291,577 10/291,718 10/291,719 10/291,543 10/291,494 10/292,608 10/291,715 10/291,559 10/291,660 10/409,864 10/309,358 10/410,484 NPW008US 10/683,040 09/575,189 09/575,162 09/575,172 09/575,170 09/575,171 09/575,161 10/291,716 10/291,547 10/291,538 10/291,717 10/291,827 10/291,548 10/291,714 10/291,544 10/291,541 10/291,584 10/291,579 10/291,824 10/291,713 10/291,545 10/291,546 09/693,388 09/693,704 09/693,510 09/693,336 09/693,335 10/181,496 10/274,199 10/309,185 10/309,066 PW014US NPS047US NPS048US NPS049US NPS050US NPS051US NPS052US NPS053US NPS054US NPS045US NPS046US

Some applications are temporarily identified by their docket number. This will be replaced by the corresponding application number when available.

BACKGROUND ART

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that such prior art forms part of the common general knowledge.

It is known to provide one or more coded data structures on a surface that can be read and decoded by a suitable sensing device. Various embodiments of such a device incorporating an optical sensor are described in many of the documents incorporated into the present application by cross-reference.

The coded data structures disclosed in these documents include target features that enable the sensing device to identify the position of each structure. The relative positions of the features within each structure can also be inteφreted to determine perspective distortion of the structure as sensed, enabling perspective correction to be performed on the sensed data. However, to enable the sensing device to decode the data in the structure, it is necessary that the position and orientation of the data relative to the structure and the position and orientation of the structure relative to the viewpoint be determined. Typically, this is achieved by providing at least one feature in the structure that is asymmetric in some way. For example, in one embodiment, a keyhole-shaped feature is provided that can be located with respect to the other features, and then recognized to ascertain the rotational orientation of the structure in relation to the sensing device. The actual data that is encoded in the data structure can then be decoded, since its position and orientation relative to the structure can be inferred.

Disadvantages with this arrangement include the need to dedicate space to one or more orientation features, and the difficulty of including redundancy in such features for the purposes of allowing orientation deteimination in the presence of damage to the features. It is desirable, therefore, to encode orientation information both more space-efficiently and in an error-detectable and/or error-correctable fashion.

SUMMARY OF THE INVENTION

In a first aspect the present invention provides coded data for disposal on or in a surface, the coded data being arranged in accordance with at least one layout having n-fold rotational symmetry about a center of rotation, where n is at least two, and having mirror symmetry, the layout including:

(a) n identical first sub-layouts rotated 1/n revolutions apart about the center of rotation, and,

(b) n identical second sub-layouts rotated 1/n revolutions apart about the center of rotation, each second sub-layout being a reflection of a corresponding one of the first sub-layouts, at least one sub-layout including:

(i) rotation-indicating data that distinguishes that sub-layout from at least one other sub- layout; and, (ii) reflection-indicating data that distinguishes the reflection of that sub-layout from the reflection of a corresponding reflected sub-layout.

In another aspect the present invention provides a surface bearing machine-readable coded data, the coded data being arranged in accordance with at least one layout having n-fold rotational symmetry about a center of rotation, where n is at least two, and having mirror symmetry, the layout including:

(c) n identical first sub-layouts rotated 1/n revolutions apart about the center of rotation, and, (d) n identical second sub-layouts rotated 1/n revolutions apart about the center of rotation, each second sub-layout being a reflection of a corresponding one of the first sub-layouts, at least one sub-layout including:

In a further aspect the present invention provides a method of generating an interface surface, the method including the steps of: (e) receiving user data in a printer;

(f) generating machine-readable coded data incoφorating the user data, the coded data being arranged in accordance with at least one layout having n-fold rotational symmetry about a center of rotation, where n is at least two, and having mirror symmetry, the layout including: (i) n identical first sub-layouts rotated 1/n revolutions apart about the center of rotation, and,

(ii) n identical second sub-layouts rotated 1/n revolutions apart about the center of rotation, each second sub-layout being a reflection of a corresponding one of the first sub-layouts, at least one sub-layout including:

(1) rotation-indicating data that distinguishes that sub-layout from at least one other sub- layout; and,

(2) reflection-indicating data that distinguishes the reflection of that sub-layout from the reflection of a corresponding reflected sub-layout.

(g) printing the coded data onto a surface.

In another aspect the present invention provides a method of using a sensing device to read machine-readable coded data, the coded data being arranged in accordance with at least one layout having n-fold rotational symmetry about a center of rotation, where n is at least two, and having mirror symmetry, the layout including:

(i) n identical first sub-layouts rotated 1/n revolutions apart about the center of rotation, and, (ii) n identical second sub-layouts rotated 1/n revolutions apart about the center of rotation, each second sub-layout being a reflection of a corresponding one of the first sub-layouts, at least one sub-layout including: (1) rotation-indicating data that distinguishes that sub-layout from at least one other sub- layout; and, (2) reflection-indicating data that distinguishes the reflection of that sub-layout from the reflection of a corresponding reflected sub-layout, (h) sensing at least one layout using the sensing device;

(i) decoding the coded data of at least one of the sub-layouts of the sensed layout, thereby determining at least the rotation-indicating data of that sub-layout; and (j) using the rotation-indicating data to inteφret the meaning of at least some of the coded data.

BRffiF DESCRD7TION OF THE DRAWINGS

Preferred and other embodiments of the invention will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Figure 1 shows a triangular macrodot packing with a four-bit symbol unit outlined, for use with an embodiment of the invention;

Figure 2 shows a square macrodot packing with a four-bit symbol unit outlined, for use with an embodiment of the invention;

Figure 3 shows a hexagonal macrodot packing with a four-bit symbol unit outlined, for use with an embodiment of the invention; Figure 4 shows a one-sixth segment of an hexagonal tag, with the segment containing a maximum of 11 four- bit symbols with the triangular macrodot packing shown in Figure 1 ;

Figure 5 shows a one-sixth segment of another hexagonal tag, with the segment containing a maximum of 17 four-bit symbols with the triangular macrodot packing shown in Figure 1 ; Figure 6 shows a one-quarter segment of a square tag, with the segment containing a maximum of 15 four-bit symbols with the square macrodot packing shown in Figure 2;

Figure 7 shows a one-sixth segment of an hexagonal tag, with the segment containing a maximum of 14 four- bit symbols with the hexagonal macrodot packing shown in Figure 3;

Figure 8 shows a logical layout of a hexagonal tag using the tag segment of Figure 4, with six interleaved 2⁴ - ary codewords ;

Figure 9 shows the macrodot layout of the hexagonal tag of Figure 8;

Figure 10 shows an arrangement of seven abutting tags of the design of Figure 8 and Figure 9, with shared targets;

Figure 11 shows a logical layout of a square tag using the tag segment of Figure 6, with four interleaved 2⁴ - ary (15, k) codewords;

Figure 12 shows the macrodot layout of the square tag of Figure 11;

Figure 13 shows an arrangement of nine abutting tags of the design of Figure 11 and Figure 12, with shared targets;

Figure 14 shows a logical layout of an alternative square tag, with four interleaved 2³ -ary ( ,k) codewords; Figure 15 shows the macrodot layout of the square tag of Figure 14;

Figure 16 shows a logical layout of a triangular tag, with three interleaved 2³ -ary (7, k) codewords;

Figure 17 shows the macrodot layout of the triangular tag of Figure 16;

Figure 18 shows a logical layout of a two-fold rotationally symmetric linear tag, with two interleaved 2³ -ary

(7,/c) codewords; Figure 19 shows the macrodot layout of the linear tag of Figure 18;

Figure 20 shows an alternative macrodot layout based on the layout of the linear tag of Figure 18, with a purely one-dimensional structure analogous to the structure of a conventional barcode;

Figure 21 shows a logical layout of an alternative hexagonal tag, with one local 2⁴ -ary (12, k) codeword, interleaved with eighteen 3-symbol fragments of eighteen distributed 2⁴ -ary (9, k) codewords; Figure 22 shows the logical layout of the hexagonal tag of Figure 21, re-arranged to show the distributed 3- symbol fragments which contribute to the same codewords;

Figure 23 shows a continuous tiling of three hexagonal tag types each with the structure of the tag of Figure

21;

Figure 24 shows a layout of an alternative square tag using the symbol of Figure 2, with eight interleaved 2⁴ - ary (6, k) codewords;

Figure 25 shows a layout of an alternative hexagonal tag using the symbol of Figure 1, with twelve interleaved 2⁴ -ary (4,k) codewords; and

Figure 26 shows a tag image processing and decoding process flow. DESCRIPTION OF THE PREFERRRED AND OTHER EMBODIMENTS

In the preferred embodiment, the invention is configured to work with the netpage networked computer system, an overview of which follows. It will be appreciated that not every implementation will necessarily embody all or even most of the specific details and extensions discussed in relation to the basic netpage system.

In its preferred form, the netpage system relies on the production of, and human interaction with, netpages. These are pages of text, graphics and images printed on ordinary paper, but which work like interactive web pages. Information is encoded on each page using ink which is substantially invisible to the unaided human eye. The ink, however, and thereby the coded data, can be sensed by an optically imaging pen and transmitted to the netpage system.

In the preferred form, active buttons and hyperlinks on each page can be clicked with a sensing pen to request information from the network or to signal preferences to a network server. In one embodiment, text written by hand on a netpage is automatically recognized and converted to computer text in the netpage system, allowing forms to be filled in. In other embodiments, signatures recorded on a netpage are automatically verified, allowing e-commerce transactions to be securely authorized.

A detailed description of several forms of the netpage system, as well as a netpage printer and a netpage sensing device in the form of a stylus and pen is described in the present applicants' co-pending PCT application WO 00/72230 entitled "Sensing Device, filed 24 May 2000; and co-pending US application USSN 09/721,893 entitled "Sensing Device", filed 25 November 2000.

1.1 NETPAGES Netpages are the foundation on which a netpage network is built. They provide a paper-based user interface to published information and interactive services.

A netpage consists of a printed page (or other surface region) invisibly tagged with references to an online description of the page. The online page description is maintained persistently by a netpage page server. The page description describes the visible layout and content of the page, including text, graphics and images. It also describes the input elements on the page, including buttons, hyperlinks, and input fields. A netpage allows markings made with a netpage pen on its surface to be simultaneously captured and processed by the netpage system.

Multiple netpages can share the same page description. However, to allow input through otherwise identical pages to be distinguished, each netpage is assigned a unique page identifier. This page ID has sufficient precision to distinguish between a very large number of netpages. Each reference to the page description is encoded in a printed tag. The tag identifies the unique page on which it appears, through the page ID, and thereby indirectly identifies the page description. The tag also identifies its own position on the page. Characteristics of the tags are described in more detail below.

Tags are printed in infrared-absoφtive ink on any substrate which is infrared-reflective, such as ordinary paper. Near-infrared wavelengths are invisible to the human eye but are easily sensed by a solid-state image sensor with an appropriate filter.

A tag is sensed by an area image sensor in the netpage pen, and the decoded tag data is transmitted to the netpage system via the nearest netpage relay, such as a suitably- enabled personal computer, PDA, mobile phone or netpage printer. The pen is wireless and communicates with the netpage relay via a short-range radio link. The pen may also incoφorate the relay functionality. Tags are sufficiently small and densely arranged that the pen can reliably image at least one tag even on a single click on the page. It is important that the pen recognize the page ID and position on every interaction with the page, since the interaction is stateless.

As only single tags may be sensed with one interaction, tags are preferably error-correctably encoded to make them partially tolerant to surface damage.

The netpage page server maintains a unique page instance for each printed netpage, allowing it to maintain a distinct set of user-supplied values for input fields in the page description for each printed netpage.

1.2 CODEDDATAON SURFACES USINGNETPAGETAGS

Various netpage coding schemes and patterns are described in the present applicants' co-pending PCT application WO 00/72249 entitled "Identity-Coded Surface with Reference Points", filed 24 May 2000; co- pending PCT application WO 02/84473 entitled "Cyclic Position Codes", filed 11 October 2001; co-pending US application USSN 10/309,358 entitled "Rotationally Symmetric Tags", filed 4 December 2002; and Australian Provisional Application 2002952259 entitled "Methods and Apparatus (NPT019)", filed 25 October 2002.

1.2.1 Tag Data Content

In a preferred form, each tag identifies the region in which it appears, and the location of that tag within the region. A tag may also contain flags which relate to the region as a whole or to the tag. One or more flag bits may, for example, signal a tag sensing device to provide feedback indicative of a function associated with the immediate area of the tag, without the sensing device having to refer to a description of the region. A netpage pen may, for example, illuminate an "active area" LED when in the zone of a hyperlink. The tags preferably tile the entire page, and are sufficiently small and densely arranged that the pen can reliably image at least one tag even on a single click on the page. It is important that the pen recognize the page ID and position on every interaction with the page, since the interaction is stateless.

In a preferred embodiment, the region to which a tag refers coincides with an entire page, and the region ID encoded in the tag is therefore synonymous with the page ID of the page on which the tag appears. In other embodiments, the region to which a tag refers can be an arbitrary subregion of a page or other surface. For example, it can coincide with the zone of an interactive element, in which case the region ID can directly identify the interactive element.

Each tag typically contains 16 bits of tag ID, at least 90 bits of region ID, and a number of flag bits. Assuming a maximum tag density of 64 per square inch, a 16-bit tag ID supports a region size of up to 1024 square inches. Larger regions can be mapped continuously without increasing the tag ID precision simply by using abutting regions and maps. The distinction between a region ID and a tag ID is mostly one of convenience. For most puφoses the concatenation of the two can be considered as a globally unique tag ID. Conversely, it may also be convenient to introduce structure into the tag ID, for example to define the x and y coordinates of the tag. A 90-bit region ID allows 2⁹⁰ (~10²⁷ or a thousand trillion trillion) different regions to be uniquely identified. Tags may also contain type information, and a region may be tagged with a mixture of tag types. For example, a region may be tagged with one set of tags encoding x coordinates and another set, interleaved with the first, encoding y coordinates. It will be appreciated the region tag ID and tag ID precision may be more or less than just described depending on the environment in which the system will be used.

1.2.2 Tag Data Encoding Figure 11 shows one embodiment of a netpage tag in the form of a square tag 726 with four perspective targets 17. It is similar in appearance to tags described by Bennett et al. in US Patent 5051746. The tag 726 represents sixty 4-bit Reed-Solomon symbols 747 (see the description of Figure 1, Figure 2 and Figure 3 below for a discussion of symbols), for a total of 240 bits. The tag represents each "one" bit by the presence of a mark 748, referred to as a macrodot, and each "zero" bit by the absence of the corresponding macrodot. Figure 13 shows a square tiling 728 of nine tags, containing all "one" bits for illustrative puφoses. It will be noted that the perspective targets are designed to be shared between adjacent tags.

In general, each tag includes a codeword encoded four times within the tag using the 4-bit symbols. Using a (15, 7) Reed-Solomon code, 112 bits of tag data are redundantly encoded to produce 240 encoded bits, corresponding to 60-bits, or 15 4-bit symbols, per codeword.

In one example, each 4-bit symbol is represented in a spatially coherent way in the tag, with the symbols of the four codewords being interleaved spatially within the tag. This is shown by the example in Figure 11, where each symbol is labeled with the number of its codeword (1-4) and the position of the symbol within the codeword(A-0).

This ensures that a burst error (an error affecting multiple spatially adjacent bits) damages a minimum number of symbols overall and a minimum number of symbols in any one codeword, thus maximizing the likelihood that the burst error can be fully corrected.

This arrangement of using a (15, 7) Reed-Solomon code, with 112 bits of tag data redundantly encoded to produce 240 encoded bits, and spatial interleaving of the codewords within the tag allows up to 4 symbol errors to be corrected per codeword, i.e. it is tolerant of a symbol error rate of up to 27% per codeword.

Any suitable error-correcting code can be used in place of a (15, 7) Reed-Solomon code, for example: a Reed-Solomon code with more or less redundancy, with the same or different symbol and codeword sizes; another block code; or a different kind of code, such as a convolutional code (see, for example, Stephen B. Wicker, Error Control Systems for Digital Communication and Storage, Prentice-Hall 1995, the contents of which a herein incoφorated by reference thereto).

In order to support "single-click" interaction with a tagged region via a sensing device, the sensing device must be able to see at least one entire tag in its field of view no matter where in the region or at what orientation it is positioned. The required diameter of the field of view of the sensing device is therefore a function of the size and spacing of the tags.

The data-bearing macrodots 748 of the tag are designed to not overlap their neighbors, so that groups of lags cannot produce structures that resemble targets. This also saves ink. The perspective targets allow detection of the tag, so further targets are not required.

Although the tag may contain an orientation feature to allow disambiguation of the four possible orientations of the tag relative to the sensor, the present invention is concerned with embedding orientation and related transform data in the tag data. For example, the four codewords can be arranged so that each tag orientation (in a rotational sense) contains one codeword placed at that orientation, as shown in Figure 11.

Tag decoding then consists of decoding one codeword at each rotational orientation. Each codeword can either contain a single bit indicating whether it is the first codeword, or two bits indicating which codeword it is. The latter approach has the advantage that if, say, the data content of only one codeword is required, then at most two codewords need to be decoded to obtain the desired data. This may be the case if the region ID is not expected to change within a stroke and is thus only decoded at the start of a stroke. Within a stroke only the codeword containing the tag ID is then desired. Furthermore, since the rotation of the sensing device changes slowly and predictably within a stroke, only one codeword typically needs to be decoded per frame. It is possible to dispense with perspective targets altogether and instead rely on the data representation being self-registering. In this case each bit value (or multi-bit value) is typically represented by an explicit glyph, i.e. no bit value is represented by the absence of a glyph. This ensures that the data grid is well-populated, and thus allows the grid to be reliably identified and its perspective distortion detected and subsequently corrected during data sampling.

To allow tag boundaries to be detected, each tag data must contain a marker pattern, and these must be redundantly encoded to allow reliable detection. The overhead of such marker patterns is similar to the overhead of explicit perspective targets. Such scheme uses dots positioned at various points relative to grid vertices to represent different glyphs and hence different multi-bit values, as discussed in the present applicants' co-pending PCT application WO 02/084473 entitled "Cyclic Position Codes", filed 11 October 2001.

The arrangement 728 of Figure 13 shows that the square tag 726 can be used to fully tile or tessellate, i.e. without gaps or overlap, a plane of arbitrary size.

Although in preferred embodiments the tagging schemes described herein encode a single data bit using the presence or absence of a single undifferentiated macrodot, they can also use sets of differentiated glyphs to represent single-bit or multi-bit values, such as the sets of glyphs illustrated in the present applicants' co- pending PCT application WO 02/084473 entitled "Cyclic Position Codes", filed 11 October 2001.

Macrodot Packing Schemes

Figure 1 shows a triangular macrodot packing 700 with a four-bit symbol unit 702 outlined. The area of the symbol unit is given by A_ τ = 2 -^J 3s² = 3.5s², where s the spacing of adjacent macrodots. Figure 2 shows a square macrodot packing 704 with a four-bit symbol unit 706 outlined. The area of the symbol unit is given by ^ωwr = 4s². Figure 3 shows a hexagonal macrodot packing 708 with a four-bit symbol unit 710 outlined. The area of the symbol unit is given by Aumr ~ ^ - 3s² = 5.2s². Of these packing schemes, the triangular packing scheme gives the greatest macrodot density for a particular macrodot spacing s.

In preferred embodiments, s has a value between 100 μm and 200 μm.

Tag Designs

Figure 4 shows a one-sixth segment 712 of a hexagonal tag, with the segment containing a maximum of 11 four-bit symbols with the triangular macrodot packing shown in Figure 1. The target 17 is shared with adjacent segments. Each tag segment can, by way of example, support a codeword of an (11, k) Reed- Solomon code, i.e. a punctured (15, k) code, with the ability to detect u = 1 \-k symbol errors, or correct t =

|_(11 — k) 12j symbol errors. For example, ifk= 7 then u = 4 and t=2. (EQ 1) gives the area of the corresponding hexagonal tag. (EQ 2) gives the radius of the required field of view. (EQ 3) gives the area of the required field of view. (EQ 4) gives the recovered bit data density for the field of view.

AT_AG = (10s) x (5 -/^" 3s) x 6 = 300 «J^~ 3s² = 520s² (EQ 1)

R_FOy = (10sx 2) + 2s = 22s (EQ 2)

K_Fov = R.²FO_V = 1521s² (EQ 3)

D_F0V = (6 x 11 x 4)/A_Fov = 0.17/s² (EQ 4)

Figure 5 shows a one-sixth segment 716 of another hexagonal tag, with the segment containing a maximum of 17 four-bit symbols with the triangular macrodot packing shown in Figure 1. Each tag segment can, by way of example, support a codeword of a (17, k) Reed-Solomon code, i.e. an augmented (15, k) code, with the ability to detect u = Yl-k symbol errors, or correct t = [(17 - k)/2] symbol errors. For example, if k = 7 then u = 10 and = 5.

(EQ 5) gives the area of the corresponding hexagonal lag. (EQ 6) gives the radius of the required field of view. (EQ 7) gives the area of the required field of view. (EQ 8) gives the recovered bit data density for the field of view.

AT_AG = (12s) x (6 J^~ 3s) x 6 = 432 3s² = 748s (EQ 5)

RF_OV = (12s x 2) + 2s = 26s (EQ 6)

D_F0V = (6 x 17 x 4)/A_F0V = 0.191s¹ (EQ 8)

Figure 6 shows a one-quarter segment 718 of a square tag, with the segment containing a maximum of 15 four-bit symbols with the square macrodot packing shown in Figure 2. Each tag segment can, by way of example, support a codeword of a (15, k) Reed-Solomon code, with the ability to detect u = 15 - k symbol errors, or correct f = |_( 15 ^— k) / 2j symbol errors. For example, if k = 7 then u = 8 and t = A.

(EQ 9) gives the area of the corresponding square tag. (EQ 10) gives the radius of the required field of view. (EQ 11) gives the area of the required field of view. (EQ 12) gives the recovered bit data density for the field of view.

ATAG = (8s) x (8 x 4 = 256s² (EQ 9)

RFO_V = ((8s x 2) + 2s) x τ[^~ 2 = 25.5s (EQ 10) π ² _FOV = 2036s² (EQ ll) D_Fov = (4 x 15 x 4)/A_F0V ≡ 0.12/s^{2 '} (EQ 12)

Figure 7 shows a one-sixth segment 720 of an hexagonal tag, with the segment containing a maximum of 14 four-bit symbols with the hexagonal macrodot packing shown in Figure 3. Each tag segment can, by way of example, support a codeword of a (14, k) Reed-Solomon code, i.e. a punctured (15, k) code, with the ability to detect u = 14 - k symbol errors, or correct t = |_( 14 — ) / 2] symbol errors. For example, if k = 6 then and u = 8 and t = 4.

(EQ 13) gives the area of the hexagonal tag. (EQ 14) gives the radius of the required field of view. (EQ 15) gives the area of the required field of view. (EQ 16) gives the recovered bit data density for the field of view.

AT_AG = (8 s[^~ 3s) x (12s) x 6 = 576 -/^~ 3s² = 998s² (EQ 13)

RFOV = (8 _Λ[~ 3S x 2) + 2s S 29.7s (EQ 14)

^^■R² _Fov ≡ 2774s² (EQ 15) D_F0V = (6 x 14 x 4)/A_FOV ≡ 0.12/s² (EQ 16)

Of the tag designs considered, hexagonal tag designs using the tag segments shown in Figure 4 and Figure 5 yield fields of view with the highest recovered bit data densities. Square and hexagonal tag designs using the tag segments shown respectively in Figure 6 and Figure 7 yield fields of view with lower recovered bit data densities.

Hexagonal Tag Design

Figure 8 shows a logical layout of a hexagonal tag 722 using the tag segment 712 of Figure 4, with six interleaved 2⁴ -ary (11, k) codewords. Figure 9 shows the macrodot layout of the hexagonal tag 722 of Figure 8. Figure 10 shows an arrangement 724 of seven abutting tags 722 of the design of Figure 9, with shared targets 17. The arrangement 724 shows that the hexagonal tag 722 can be used to tessellate a plane of arbitrary size.

Alternative Square Tag Design Figure 14 shows a logical layout of another square tag 730, with four interleaved 2³ -ary (7, k) codewords. Each 3-bit symbol 732 is represented by an L-shaped arrangement of three macrodots. Figure 15 shows the macrodot layout of the square tag 730 of Figure 14.

Triangular Tag Design Figure 16 shows a logical layout of a triangular tag 734, with three interleaved 2³ -ary (7, k) codewords. Each 3-bit symbol 736 is represented by a triangular arrangement of three macrodots. Figure 17 shows the macrodot layout of the triangular tag 734 of Figure 16. As with the square and hexagonal tags described above, the triangular tag 734 can be used to tessellate a plane of arbitrary size.

Linear Tag Design Figure 18 shows a logical layout of a two-fold rotationally symmetric linear tag 738, with two interleaved 2³ -ary (7, k) codewords. Each 3-bit symbol 740 is represented by a linear arrangement of three macrodots. Figure 19 shows the macrodot layout of the linear tag 738 of Figure 18.

Figure 20 shows an alternative macrodot layout based on the layout of the linear tag of Figure 18, with a purely one-dimensional structure analogous to the structure of a conventional barcode. Although the layout is based on that of the linear tag of Figure 18, the target structures 17 are linear rather than circular. This embodiment is particularly suitable for use with prior art barcode scanners (with suitable software changes) adapted for linear scanning rather than area scanning.

Distributed Hexagonal Tag Design

Figure 21 shows a logical layout of a hexagonal tag 750 using the tag segment of Figure 22, with one local 2⁴ -ary (12, k) codeword interleaved with eighteen 3-symbol fragments of eighteen distributed 2⁴ -ary (9, k) codewords. Figure 9 shows, as before, the macrodot layout of the hexagonal tag 750 of Figure 21, and Figure 10 shows an arrangement of seven abutting tags of the design of Figure 9, with shared targets.

In the layout of Figure 21, the twelve 4-bit symbols of the local codeword are labeled Gl through G12, and are shown with a dashed outline. Each symbol of the eighteen fragments of the eighteen distributed codewords is labeled with an initial prefix of A through F, indicating which of six nominal codewords the symbol belongs to, a subsequent prefix of S through U, indicating which 3-symbol part of the codeword the symbol belongs to, and a suffix of 1 through 3, indicating which of the three possible symbols the symbol is.

Figure 23 shows a continuous tiling 752 of three hexagonal tag types labeled P, Q and R, each with the structure of tag 750. There are six distributed codewords (A through F) associated with each tag type (P, Q and R). The distributed tagging scheme is predicated on the data content of each respective distributed codeword AP, AQ, AR, etc. through FP, FQ and FR being constant throughout the tiling. Only the G codewords can contain unique data.

Figure 23 shows lattice vectors 754 associated with the physical tag layout, and corresponding (i.e. iso- directional rather than minimal) lattice vectors 755 associated with the PQR tag layout. The ratio of the length of the PQR lattice vectors 755 to the length of the corresponding physical lattice vectors 754 is three, consistent with there being three distinct tag types P, Q and R in the PQR tag layout. Figure 23 also shows the coverage 756 and 757 of a minimal imaging field of view in two arbitrary positions with respect to the tiling. The field of view is minimal with respect to the tiling in the sense that it the smallest field of view which is guaranteed to include at least one complete tag. Tag 750 is structured so that the minimal field of view allows the recovery of the local codeword G of at least one tag, and the entire set of distributed codewords AP through FR via fragments of tags of type P, Q and R included in the field of view. Furthermore, the continuous tiling of tag 750 ensures that there is a codeword available with a known layout for each possible rotational and translational combination (of which there are eighteen). Each distributed codeword includes data which identifies the rotation of the codeword in relation to the tiling, thus allowing the rotation of the tiling with respect to the field of view to be determined from decoded data rather than from other structures, and the local codeword to be decoded at the correct orientation.

Once the field of view is imaged, decoding proceeds as follows. A codeword is sampled at an arbitrary rotation (of the possible six rotations with respect to the six targets). This consists of sampling three S symbols from the current tag, and sampling, at the same rotation, T and U symbols from adjacent tags, modulo the size and alignment of the field of view with respect to the tiling. The result is a single distributed 2⁴ -ary (9, k) codeword. Decoding the codeword yields, among other data, the codeword orientation. Once the orientation is known, the local G codeword can be sampled and decoded at the correct orientation. Decoding the G codeword yields, among other data, the type of the corresponding tag (i.e. P, Q or R). This in turn allows the data content of the various distributed codewords to be correctly inteφreted, once they are sampled and decoded. It also allows the distributed codewords to be selectively sampled and decoded. For example, codewords known to contain data which is assumed not to change during a stroke, such as the region ID, may be ignored.

Figure 22 shows the logical layout of the hexagonal tag 750 of Figure 21, re-arranged to show the distributed 3-symbol fragments which contribute to the same codewords. For example, if the central tag shown in Figure 22 were a P-type tag, then the six distributed codewords shown in the figure would be the AP, BP, CP, DP, EP and FP codewords. Figure 22 also shows the local G codeword of the tag. Clearly, given the distributed and repeating nature of the distributed codewords, different fragments from the ones shown in the figure can be used to build the corresponding codewords.

Although it is more space-efficient to record the tag type (P, Q or R) in the G codeword, the tag type corresponding to each (or to selected) distributed codewords can also be recorded in the distributed codewords themselves. In this case there is a fixed relationship between a codeword's alignment and the tag to which its tag type refers. For example, a codeword's tag type may always refer to the tag containing the S fragment of the codeword.

Note that with a larger field of view, a particular codeword need not be constant throughout the tiling, but may instead be allowed to be constant only in a certain direction. For example, a codeword used to encode a coordinate can be constant in the direction where the coordinate is constant, but can vary in (semi- )orthogonal directions (with respect to the rotational symmetry of the tiling). This has the advantage that semi-variable information can be encoded in distributed codewords rather than being restricted to local codewords.

Mirror-Symmetric Tag Designs

Figure 24 shows a layout of a square tag 760 using the symbol of Figure 2, with eight interleaved 2⁴ -ary (6, k) codewords. Like the square tag of Figure 11 discussed earlier, the tag of Figure 24 contains four codewords, each arranged according to the same layout but each rotated according to a different one of the four possible rotations of the tag with respect to its own four-fold rotational symmetry. As discussed earlier, this allows a codeword to be sampled and decoded independently of the actual rotation of the tag, which allows the rotation of the tag to be encoded in the codeword(s).

The tag of Figure 24 contains four additional codewords, laid out in the mirror image of the original four codewords with respect to the tag's two axes of reflection 762 and 764. This allows a codeword to be sampled and decoded independently of the actual rotation and reflection of the tag, which allows the rotation and reflection of the tag to be encoded in the codeword(s). This is useful if the tag may be imaged in reflection, e.g. reflected in a mirror or through the back of a transparent substrate.

Each codeword may explicitly indicate its rotation and reflection via a three-bit value, or implicitly via a one- bit value indicating whether the codeword corresponds to a pre-determined rotation and reflection or not.

In Figure 24, each original symbol has a prefix (1-4) indicating which codeword it belongs to, and a suffix (A-F) indicating its position in the codeword. Each reflected symbol is shown with a bar above its label, as well as with a dashed outline.

Figure 25 shows a layout of hexagonal tag 770 using the symbol of Figure 1, with twelve interleaved 2⁴ -ary (4, k) codewords. Like the hexagonal tag of Figure 8 discussed earlier, the tag of Figure 25 contains six codewords, each arranged according to the same layout but each rotated according to a different one of the six possible rotations of the tag with respect to its own six-fold rotational symmetry.

The tag of Figure 25 contains six additional codewords, laid out in the mirror image of the original six codewords with respect to the tag's three axes of reflection 772, 774 and 776. As discussed above, this allows a codeword to be sampled and decoded independently of the actual rotation and reflection of the tag, which allows the rotation and reflection of the tag to be encoded in the codeword(s).

Each codeword may explicitly indicate its rotation and reflection via a four-bit value, or implicitly via a one- bit value indicating whether the codeword corresponds to a pre-determined rotation and reflection or not. In Figure 25, each original symbol has a prefix (1-6) indicating which codeword it belongs to, and a suffix (A-D) indicating its position in the codeword. Each reflected symbol is shown with a bar above its label, as well as with a dashed outline.

Note that the tags of Figure 24 and Figure 25 are not ideally sized for the number of symbols each codeword contains, since the data content of each codeword is low once allowance is made for useful redundancy. However, both serve to illustrate interleaved rotation-symmetric and mirror-symmetric tag designs, and the design principles obviously extend to tags of larger size and/or to tags incoφorating different error-detecting and error-correcting schemes.

1.2.3 Tag Image Processing and Decoding

Figure 26 shows a tag image processing and decoding process flow. A raw image 802 of the tag pattern is acquired (at 800), for example via an image sensor such as a CCD image sensor, CMOS image sensor, or a scanning laser and photodiode image sensor. The raw image is then typically enhanced (at 804) to produce an enhanced image 806 with improved contrast and more uniform pixel intensities. Image enhancement may include global or local range expansion, equalization, and the like.

The enhanced image 806 is then typically filtered (at 808) to produce a filtered image 810. Image filtering may consist of low-pass filtering, with the low-pass filter kernel size tuned to obscure macrodots but to preserve targets. The filtering step 808 may include additional filtering (such as edge detection) to enhance target features.

The filtered image §10 is then processed to locate target features (at 812), yielding a set of target points. This may consist of a search for target features whose spatial inter-relationship is consistent with the known geometry of a tag. Candidate targets may be identified directly from maxima in the filtered image 810, or may the subject of further characterization and matching, such as via their (binary or grayscale) shape moments (typically computed from pixels in the enhanced image 806 based on local maxima in the filtered image 810), as described in US patent application serial number 09/575,154.

The search typically starts from the center of the field of view. The target points 814 found by the search step 812 indirectly identify the location of the tag in the three-dimensional space occupied by the image sensor and its associated optics. Since the target points 814 are derived from the (binary or grayscale) centroids of the targets, they are typically defined to sub-pixel precision.

It may be useful to determine the actual 3D transform of the tag (at 816), and, by extension, the 3D transform (or pose) 818 of the sensing device relative to the tag. This may be done analytically, as described in US patent application serial number 09/575,154, or using a maximum likelihood estimator (such as least squares adjustment) to fit parameter values to the 3D transform given the observed perspective-distorted target points

(as described in P.R. Wolf and B.A. Dewitt, Elements of Photogrammetry with Applications in GIS, 3rd Edition, McGraw Hill, February 2000, the contents of which are herein incoφorated by reference thereto). The 3D transform includes the 3D translation of the tag, the 3D orientation (rotation) of the tag, and the focal length and viewport scale of the sensing device, thus giving eight parameters to be fitted, or six parameters if the focal length and viewport scale are known (e.g. by design or from a calibration step). Each target point yields a pair of observation equations, relating an observed coordinate to a known coordinate. If eight parameters are being fitted, then five or more target points are needed to provide sufficient redundancy to allow maximum likelihood estimation. If six parameters are being fitted, then four or more target points are needed. If the tag design contains more targets than are rninimally required to allow maximum likelihood estimation, then the tag can be recognized and decoded even if up to that many of its targets are damaged beyond recognition.

To allow macrodot values to be sampled accurately, the perspective transform of the tag must be inferred. Four of the target points are taken to be the perspective-distorted corners of a rectangle of known size in tag space, and the eight-degree-of-freedom perspective transform 822 is inferred (at 820), based on solving the well-understood equations relating the four tag-space and image-space point pairs (see Heckbert, P., Fundamentals of Texture Mapping and Image Waφing, Masters Thesis, Dept. of EECS, U. of California at Berkeley, Technical Report No. UCB/CSD 89/516, June 1989, the contents of which are herein incoφorated by reference thereto). The perspective transform may alternatively be derived from the 3D transform 818, if available.

The inferred tag-space to image-space perspective transform 822 is used to project (at 824) each known data bit position in tag space into image space where the real-valued position is used to bi-linearly (or higher- order) inteφolate (at §24) the four (or more) relevant adjacent pixels in the enhanced input image 806. The resultant macrodot value is compared with a suitable threshold to determine whether it represents a zero bit or a one bit.

Once the bits of one or more complete codeword have been sampled, the codewords are decoded (at 828) to obtain the desired data 830 encoded in the tag. Redundancy in the codeword may be used to detect errors in the sampled data, or to correct errors in the sampled data.

As discussed in US patent application serial number 09/575,154, the obtained tag data 830 may directly or indirectly identify the surface region containing the tag^* and the position of the tag within the region. An accurate position of the sensing device relative to the surface region can therefore be derived from the tag data 830 and the 3D transform 818 of the sensing device relative to the tag.

1.2.4 Tag Map

Decoding a tag results in a region ID, a tag ID, and a tag-relative pen transform. Before the tag ID and the tag-relative pen location can be translated into an absolute location within the tagged region, the location of the tag within the region must be known. This is given by a tag map, a function which maps each tag ID in a tagged region to a corresponding location.

A tag map reflects the scheme used to tile the surface region with tags, and this can vary according to surface type. When multiple tagged regions share the same tiling scheme and the same tag numbering scheme, they can also share the same tag map.

The tag map for a region must be retrievable via the region JD. Thus, given a region ID, a tag ID and a pen transform, the tag map can be retrieved, the tag ID can be translated into an absolute tag location within the region, and the tag-relative pen location can be added to the tag location to yield an absolute pen location within the region.

The tag ID may have a structure which assists translation through the tag map. It may, for example, encode Cartesian coordinates or polar coordinates, depending on the surface type on which it appears. The tag ID structure is dictated by and known to the tag map, and tag IDs associated with different tag maps may therefore have different structures. For example, the tag ID may simply encode a pair of x and y coordinates of the tag, in which case the tag map may simply consist of record of the coordinate precision. If the coordinate precision is fixed, then the tag map can be implicit.

1.2.5 Tagging Schemes

Two distinct surface coding schemes are of interest, both of which use the tag structure described earlier in this section. The preferred coding scheme uses "location-indicating" tags as already discussed. An alternative coding scheme uses object-indicating tags.

A location-indicating tag contains a tag ID which, when translated through the tag map associated with the tagged region, yields a unique tag location within the region. The tag-relative location of the pen is added to this tag location to yield the location of the pen within the region. This in turn is used to determine the location of the pen relative to a user interface element in the page description associated with the region.

Not only is the user interface element itself identified, but a location relative to the user interface element is identified. Location-indicating tags therefore trivially support the capture of an absolute pen path in the zone of a particular user interface element.

An object-indicating tag contains a tag ID which directly identifies a user interface element in the page description associated with the region. All the tags in the zone of the user interface element identify the user interface element, making them all identical and therefore indistinguishable.

Object-indicating tags do not, therefore, support the capture of an absolute pen path. They do, however, support the capture of a relative pen path. So long as the position sampling frequency exceeds twice the encountered tag frequency, the displacement from one sampled pen position to the next within a stroke can be unambiguously determined.

With either tagging scheme, the tags function in cooperation with associated visual elements on the netpage as user interactive elements in that a user can interact with the printed page using an appropriate sensing device in order for tag data to be read by the sensing device and for an appropriate response to be generated in the netpage system.

The invention may also be said to broadly consist in the parts, elements and features referred to or indicated herein, individually or collectively, in any or all combinations of two or more of the parts, elements or features, and wherein specific integers are mentioned herein which have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incoφorated herein as if individually set forth.

Although the preferred embodiment has been described in detail, it should be understood that various changes, substitutions, and alterations can be made by one of ordinary skill in the art without departing from the scope of the present invention.

Claims

The claims: 1) Coded data for disposal on or in a surface, the coded data being arranged in accordance with at least one layout having n-fold rotational symmetry about a center of rotation, where n is at least two, and having mirror symmetry, the layout including: (k) n identical first sub-layouts rotated 1/n revolutions apart about the center of rotation, and, (1) n identical second sub-layouts rotated 1/n revolutions apart about the center of rotation, each second sub-layout being a reflection of a corresponding one of the first sub-layouts, at least one sub-layout including: (i) rotation-indicating data that distinguishes that sub-layout from at least one other sub- layout; and, (ii) reflection-indicating data that distinguishes the reflection of that sub-layout from the reflection of a corresponding reflected sub-layout. 2) The coded data of claim 1 , each second sub-layout being a reflection of a corresponding first sub-layout about a symmetry axis, the symmetry axis passing through the center of rotation. 3) The coded data of claim 1 or claim 2, the rotation-indicating data of each sub-layout being adapted to distinguish the rotation of the sub-layout from the rotation of each other sub-layout. 4) The coded data of any one of the claims 1 to 3, the reflection-indicating data being adapted to distinguish the reflection of a respective sub-layout from the reflection of each other sub-layout within the layout. 5) The coded data of any one of the claims 1 to 4, the coded data being redundantly encoded. 6) The coded data of claim 5, the coded data being redundantly encoded using Reed-Solomon encoding. 7) The coded data of any one of the claims 1 to 6, the coded data including at least one codeword associated with each layout, the codeword being encoded in each sub-layout. 8) The coded data of any one of the claims 1 to 7, the coded data including a plurality of layouts, each layout having at least one codeword that is different to at least one codeword of at least one other layout. 9) The coded data of any one of the claims 1 to 8, the coded data including a plurality of layouts, each layout having at least one codeword that is identical to at least one codeword of at least one other layout. 10) The coded data of any one of the claims 1 to 9, each sub-layout having at least one codeword that is different to at least one codeword of at least one other sub-layout. 11) The coded data of any one of the claims 1 to 10, each sub-layout having at least one codeword that is identical to at least one codeword of at least one other sub-layout. 12) The coded data of claim 1, each sub-layout having a codeword formed from a number of data elements, each sub-layout defining the positions of the data elements. 13) The coded data of claim 12, the sub-layouts being arranged such that each data element has a unique position. 14) The coded data of claim 12 or claim 13, the positions of the data elements of respective sub-layouts being interleaved. 15) The coded data of any one of the claims 1 to 14, the layout having at least one of the following shapes: (a) linear; (b) square; (c) rectangular; (d) triangular; or (e) hexagonal. 16) The coded data of any one of the claims 1 to 15, each layout including at least one target feature. 17) The coded data of claim 16, the at least one target feature being used to determine at least one of the location of the layout and rotation of the layout. 18) The coded data of claim 16 or claim 17, the coded data being adapted to be sensed using a sensing device, the target feature being configured to enable perspective correction of the coded data as sensed by the sensing device. 19) The coded data of any one of the claims 16 to 18, each layout including at least four target features. 20) The coded data of any one of the claims 16 to 19, the coded data including a plurality of layouts, at least some target features being common to at least two layouts. 21) The coded data of any one of the claims 1 to 20, the coded data including a plurality of layouts arranged over the surface in accordance with a super-layout. 22) The coded data of claim 21, the super-layout being arranged such that there is a predetermined spacing between adjacent layouts in the super-layout. 23) A surface bearing machine-readable coded data, the coded data being arranged in accordance with at least one layout having n-fold rotational symmetry about a center of rotation, where n is at least two, and having mirror symmetry, the layout including: (a) n identical first sub-layouts rotated 1/n revolutions apart about the center of rotation, and, (b) n identical second sub-layouts rotated 1/n revolutions apart about the center of rotation, each second sub-layout being a reflection of a corresponding one of the first sub-layouts, at least one sub-layout including: (i) rotation-indicating data that distinguishes that sub-layout from at least one other sub- layout; and, (ii) reflection-indicating data that distinguishes the reflection of that sub-layout from the ' reflection of a corresponding reflected sub-layout. 24) A surface according to claim 23, the coded data being substantially invisible to the unaided human eye. 25) A surface according to claim 23 or claim 24, the surface further including additional visible markings. 26) A surface according to any one of the claims 23 to 25, the surface being configured as an interface surface for enabling user interaction with a computer. 27) A surface according to any one of the claims 23 to 26, the surface including at least one region and the coded data representing an identity of the at least one region. 28) A method of generating an interface surface, the method including the steps of: (a) receiving user data in a printer; (b) generating machine-readable coded data incoφorating the user data, the coded data being arranged in accordance with at least one layout having n-fold rotational symmetry about a center of rotation, where n is at least two, and having mirror symmetry, the layout including: (i) n identical first sub-layouts rotated 1/n revolutions apart about the center of rotation, and, (ii) n identical second sub-layouts rotated 1/n revolutions apart about the center of rotation, each second sub-layout being a reflection of a corresponding one of the first sub-layouts, at least one sub-layout including: (1) rotation-indicating data that distinguishes that sub-layout from at least one other sub- layout; and, (2) reflection-indicating data that distinguishes the reflection of that sub-layout from the reflection of a corresponding reflected sub-layout. (c) printing the coded data onto a surface. 29) A method according to claim 28, the coded data being substantially invisible to the unaided human eye. 30) A method according to claim 28 or claim 29, the surface further including additional visible markings, the method including printing visible markings on the surface substantially simultaneously with the coded data. 31) A method of using a sensing device to read machine-readable coded data, the coded data being arranged in accordance with at least one layout having n-fold rotational symmetry about a center of rotation, where n is at least two, and having mirror symmetry, the layout including: (i) n identical first sub-layouts rotated 1/n revolutions apart about the center of rotation, and, (ii) n identical second sub-layouts rotated 1/n revolutions apart about the center of rotation, each second sub-layout being a reflection of a corresponding one of the first sub-layouts, at least one sub-layout including:

(b) sensing at least one layout using the sensing device;

(c) decoding the coded data of at least one of the sub-layouts of the sensed layout, thereby determining at least the rotation-indicating data of that sub-layout; and

(d) using the rotation-indicating data to inteφret the meaning of at least some of the coded data.

32) A method according to claim 31, each layout including at least one target feature representing at least a position and a rotation of the layout, the method of reading the coded data including:

(a) imaging the surface to generate an image thereof;

(b) processing the image to locate one or more target features; and (c) on the basis of the located target features, determining at least one of the position and the rotation of at least one of the sub-layouts.

33) A method according to claim 31 or claim 32, each layout including a codeword, the codeword being encoded in each sub-layout, the method including decoding the codeword from at least one of the sub- layouts.

34) A method according to any one of the claims 31 to 33, at least one sub-layout including type-indicating data, the method including:

(a) determining at least the type-indicating data of the at least one sub-layou and, (b) using the type-indicating data to inteφret the meaning of at least some of the coded data.

35) Coded data for disposal on or in a surface, the coded data being arranged in accordance with at least one layout having n-fold rotational symmetry about a center of rotation, where n is at least two, the layout including n identical sub-layouts rotated 1/n revolutions apart about the center of rotation, each sub- layout encoding a codeword formed from a number of data elements, the sub-layout defining the position of the data elements, and including rotation-indicating data that distinguishes that sub-layout from at least one other sub-layout.

36) The coded data of claim 35, the sub-layouts being arranged such that each data element has a unique position.

37) The coded data of claim 35 or claim 36, the positions of the data elements of respective sub-layouts being interleaved.

38) The coded data of any one of the claims 35 to 37, the coded data including a plurality of layouts, each layout having at least one codeword that is different to at least one codeword of at least one other layout.

39) The coded data of any one of the claims 35 to 38, the coded data including a plurality of layouts, each layout having at least one codeword that is identical to at least one codeword of at least one other layout.

40) The coded data of any one of the claims 35 to 39, each sub-layout having at least one codeword that is different to at least one codeword of at least one other sub-layout.

41) The coded data of any one of the claims 35 to 40, each sub-layout having at least one codeword that is identical to at least one codeword of at least one other sub-layout.

42) The coded data of any one of the claims 35 to 1, the coded data being coded data according to any one of the claims 1 to 22.

43) A surface bearing machine-readable coded data, the coded data being arranged in accordance with at least one layout having n-fold rotational symmetry about a center of rotation, where n is at least two, the layout including n identical sub-layouts rotated 1/n revolutions apart about the center of rotation, each sub-layout encoding a codeword formed from a number of data elements, the sub-layout defining the position of the data elements, and including rotation-indicating data that distinguishes that sub-layout from at least one other sub-layout.

44) A surface according to claim 43, the coded data being substantially invisible to the unaided human eye.

45) A surface according to claim 43 or claim 44, the surface further including additional visible markings.

46) A surface according to any one of the claims 43 to 45, the surface being configured as an interface surface for enabling user interaction with a computer. 47) A surface according to claim 46, the surface including at least one region and the coded data representing an identity of the at least one region.

48) A surface according to any one of the claims 43 to 48, the surface being a surface according to any one of the claims 23 to 27.

49) A method of generating an interface surface, the method including the steps of:

(a) receiving user data in a printer;

(b) generating machine-readable coded data incoφorating the user data, the coded data being arranged in accordance with at least one layout having n-fold rotational symmetry about a center of rotation, where n is at least two, the layout including n identical sub-layouts rotated 1/n revolutions apart about the center of rotation, each sub-layout encoding a codeword formed from a number of data elements, the sub-layout defining the position of the data elements, and including rotation-indicating data that distinguishes that sub-layout from at least one other sub-layout. (c) printing the coded data onto a surface.

50) A method according to claim 49, the coded data being substantially invisible to the unaided human eye.

51) A method according to claim 49 or claim 50, the surface further including additional visible markings, the method including printing visible markings on the surface substantially simultaneously with the coded data.

52) A method according to any one of the claims 49 to 51, the method being a method according to any one of the claims 28 to 30.

53) A method of using a sensing device to read machine-readable coded data, the coded data being arranged in accordance with at least one layout having n-fold rotational symmetry about a center of rotation, where n is at least two, the layout including n identical sub-layouts rotated 1/n revolutions apart about the center of rotation, each sub-layout encoding a codeword formed from a number of data elements, the sub-layout defining the position of the data elements, and including rotation-indicating data that distinguishes that sub-layout from at least one other sub-layout.

(a) sensing at least one layout using the sensing device;

(b) decoding the coded data of at least one of the sub-layouts of the sensed layout, thereby determining at least the rotation-indicating data of that sub-layout; and (c) using the rotation-indicating data to inteφret the meaning of at least some of the coded data.

54) A method according to claim 53, each layout including at least one target feature representing at least a position and a rotation of the layout, the method of reading the coded data including:

(a) imaging the surface to generate an image thereof; (b) processing the image to locate one or more target features; and

(c) on the basis of the located target features, deternήning at least one of the position and the rotation of at least one of the sub-layouts.

55) A method according to claim 53 or claim 54, each layout including a codeword, the codeword being encoded in each sub-layout, the method including decoding the codeword from at least one of the sub- layouts.

56) A method according to any one of the claims 53 to 55, at least one sub-layout including type-indicating data, the method including:

(a) determining at least the type-indicating data of the at least one sub-layout; and,

(b) using the type-indicating data to inteφret the meaning of at least some of the coded data.

57) A method according to any one of the claims 53 to 56, the method being a method according to any one of the claims 31 to 34.