Embedded interaction code recognition

Abstract

In accordance with embodiments of the invention, embedded interaction code (EIC) symbols are recognized. EIC dots are generated based on effective EIC symbols, which have been generated by processing an image containing the EIC symbols, by obtaining graylevels of selected positions of the EIC-symbols. Rotated EIC dots are generated based on the EIC dots by determining which grid cells correspond to the EIC symbols and by determining which direction is a correct orientation of the EIC symbols. A homography matrix is updated with orientation information based on the EIC dots. EIC bits are extracted from the rotated EIC dots based on graylevels of selected positions of the rotated EIC dots.

Description

TECHNICAL FIELD

Embodiments of the invention relate to image processing and more particularly relate to embedded interaction code recognition.

BACKGROUND

Computer users are accustomed to using a mouse and keyboard as a way of interacting with a personal computer. While personal computers provide a number of advantages over written documents, most users continue to perform certain functions using printed paper. Some of these functions include reading and annotating written documents. In the case of annotations, the printed document assumes a greater significance because of the annotations made on it by the user. One of the difficulties, however, with having a printed document with annotations is the need to have the annotations subsequently entered back into the electronic form of the document. This requires the original user or another user to wade through the annotations and enter them into a personal computer. In some cases, a user will scan in the annotations and the original text, thereby creating a new document. These multiple steps make the interaction between the printed document and the electronic version of the document difficult to handle on a repeated basis. Further, scanned-in images are frequently non-modifiable. There may be no way to separate the annotations from the original text. This makes using the annotations difficult. Accordingly, an improved way of handling annotations would be desirable.

One technique for capturing handwritten information is by using an image capturing pen whose location may be determined during writing. One image capturing pen that provides this capability is the Anoto pen by Anoto Inc. This pen functions by using a camera to capture an image of paper encoded with a predefined pattern. An example of the image pattern is shown in FIG. 11. This pattern is used by the Anoto pen to determine a location of the pen on a piece of paper (or other positionally encoded medium).

Improved techniques for recognizing embedded interaction code (EIC) information, based on images of EIC documents, would be desirable.

SUMMARY

In accordance with embodiments of the invention, embedded interaction code (EIC) symbols are recognized. EIC dots are generated based on effective EIC symbols, which have been generated by processing an image containing the EIC symbols, by obtaining graylevels of selected positions of the EIC-symbols. Rotated EIC dots are generated based on the EIC dots by determining which grid cells correspond to the EIC symbols and by determining which direction is a correct orientation of the EIC symbols. A homography matrix is updated with orientation information based on the EIC dots. EIC bits are extracted from the rotated EIC dots based on graylevels of selected positions of the rotated EIC dots

These and other aspects of the present invention will become known through the following drawings and associated description.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing summary of the invention, as well as the following detailed description of preferred embodiments, is better understood when read in conjunction with the accompanying drawings, which are included by way of example, and not by way of limitation with regard to the claimed invention.

FIG. 1 shows a general description of a computer that may be used in conjunction with embodiments of the present invention.

FIGS. 2A and 2B show an image capture system and corresponding captured image in accordance with embodiments of the present invention.

FIGS. 3A through 3F show various sequences and folding techniques in accordance with embodiments of the present invention.

FIGS. 4A through 4E show various encoding systems in accordance with embodiments of the present invention.

FIGS. 5A through 5D show four possible resultant corners associated with the encoding system according to FIGS. 4A and 4B.

FIG. 6 shows rotation of a captured image portion in accordance with embodiments of the present invention.

FIG. 7 shows various angles of rotation used in conjunction with the coding system of FIGS. 4A through 4E.

FIG. 8 shows a process for determining the location of a captured array in accordance with embodiments of the present invention.

FIG. 9 shows a method for determining the location of a captured image in accordance with embodiments of the present invention.

FIG. 10 shows another method for determining the location of captured image in accordance with embodiments of the present invention.

FIG. 11 shows a representation of encoding space in a document according to prior art.

FIG. 12 shows a flow diagram for decoding extracted bits from a captured image in accordance with embodiments of the present invention.

FIG. 13 shows an example of a camera captured image of an EIC document in accordance with embodiments of the invention.

FIG. 14 shows symbol, grid, and image coordinate systems in accordance with embodiments of the invention.

FIG. 15 shows symbol, grid, and image coordinate systems of a different image than the image shown in FIG. 14 in accordance with embodiments of the invention.

FIG. 16 shows a flow diagram of a system for performing EIC symbol recognition in accordance with embodiments of the invention.

FIG. 17 shows effective EIC symbols in accordance with embodiments of the invention.

FIG. 18 shows EIC bits in accordance with embodiments of the invention.

FIG. 19 shows a shifted coordinate system in which grid intersections in an image have non-negative coordinates in accordance with embodiments of the invention.

FIG. 20 shows positions on each edge of an EIC symbol in accordance with embodiments of the invention.

FIG. 21 shows an example of an EIC symbol in accordance with embodiments of the invention.

FIG. 22 shows a pixel index in accordance with embodiments of the invention.

FIG. 23 shows pixels for bilinear sampling in accordance with embodiments of the invention.

FIG. 24 shows position naming on each edge in accordance with embodiments of the invention.

FIG. 25 shows grid cells and various symbol orientations in accordance with embodiments of the invention.

FIG. 26 shows EIC symbol rotation in accordance with embodiments of the invention.

FIG. 27 shows EIC symbol offset in accordance with embodiments of the invention.

FIG. 28 shows coordinate systems of symbol, grid, and image when Q=0 in accordance with embodiments of the invention.

FIG. 29 shows coordinate systems of symbol, grid, and image when Q=1 in accordance with embodiments of the invention.

FIG. 30 shows coordinate systems of symbol, grid, and image when Q=2 in accordance with embodiments of the invention.

FIG. 31 shows coordinate systems of symbol, grid, and image when Q=3 in accordance with embodiments of the invention.

FIG. 32 shows assignment of bit values in accordance with embodiments of the invention.

DETAILED DESCRIPTION

Aspects of the present invention relate to determining the location of a captured image in relation to a larger image. The location determination method and system described herein may be used in combination with a multi-function pen.

The following is separated by subheadings for the benefit of the reader. The subheadings include: terms, general-purpose computer, image capturing pen, encoding of array, decoding, error correction, location determination, and embedded interaction code recognition.

Terms

Pen—any writing implement that may or may not include the ability to store ink. In some examples, a stylus with no ink capability may be used as a pen in accordance with embodiments of the present invention.

Camera—an image capture system that captures an image from paper or any other medium.

General Purpose Computer

FIG. 1 is a functional block diagram of an example of a conventional general-purpose digital computing environment that can be used to implement various aspects of the present invention. In FIG. 1, a computer 100 includes a processing unit 110, a system memory 120, and a system bus 130 that couples various system components including the system memory to the processing unit 110. The system bus 130 may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. The system memory 120 includes read only memory (ROM) 140 and random access memory (RAM) 150.

A basic input/output system 160 (BIOS), containing the basic routines that help to transfer information between elements within the computer 100, such as during start-up, is stored in the ROM 140. The computer 100 also includes a hard disk drive 170 for reading from and writing to a hard disk (not shown), a magnetic disk drive 180 for reading from or writing to a removable magnetic disk 190, and an optical disk drive 191 for reading from or writing to a removable optical disk 192 such as a CD ROM or other optical media. The hard disk drive 170, magnetic disk drive 180, and optical disk drive 191 are connected to the system bus 130 by a hard disk drive interface 192, a magnetic disk drive interface 193, and an optical disk drive interface 194, respectively. The drives and their associated computer-readable media provide nonvolatile storage of computer readable instructions, data structures, program modules and other data for the personal computer 100. It will be appreciated by those skilled in the art that other types of computer readable media that can store data that is accessible by a computer, such as magnetic cassettes, flash memory cards, digital video disks, Bernoulli cartridges, random access memories (RAMs), read only memories (ROMs), and the like, may also be used in the example operating environment.

A number of program modules can be stored on the hard disk drive 170, magnetic disk 190, optical disk 192, ROM 140 or RAM 150, including an operating system 195, one or more application programs 196, other program modules 197, and program data 198. A user can enter commands and information into the computer 100 through input devices such as a keyboard 101 and pointing device 102. Other input devices (not shown) may include a microphone, joystick, game pad, satellite dish, scanner or the like. These and other input devices are often connected to the processing unit 110 through a serial port interface 106 that is coupled to the system bus, but may be connected by other interfaces, such as a parallel port, game port or a universal serial bus (USB). Further still, these devices may be coupled directly to the system bus 130 via an appropriate interface (not shown). A monitor 107 or other type of display device is also connected to the system bus 130 via an interface, such as a video adapter 108. In addition to the monitor, personal computers typically include other peripheral output devices (not shown), such as speakers and printers. In a preferred embodiment, a pen digitizer 165 and accompanying pen or stylus 166 are provided in order to digitally capture freehand input. Although a direct connection between the pen digitizer 165 and the serial port is shown, in practice, the pen digitizer 165 may be coupled to the processing unit 110 directly, via a parallel port or other interface and the system bus 130 as known in the art. Furthermore, although the digitizer 165 is shown apart from the monitor 107, it is preferred that the usable input area of the digitizer 165 be co-extensive with the display area of the monitor 107. Further still, the digitizer 165 may be integrated in the monitor 107, or may exist as a separate device overlaying or otherwise appended to the monitor 107.

The computer 100 can operate in a networked environment using logical connections to one or more remote computers, such as a remote computer 109. The remote computer 109 can be a server, a router, a network PC, a peer device or other common network node, and typically includes many or all of the elements described above relative to the computer 100, although only a memory storage device 111 has been illustrated in FIG. 1. The logical connections depicted in FIG. 1 include a local area network (LAN) 112 and a wide area network (WAN) 113. Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets and the Internet.

When used in a LAN networking environment, the computer 100 is connected to the local network 112 through a network interface or adapter 114. When used in a WAN networking environment, the personal computer 100 typically includes a modem 115 or other means for establishing a communications over the wide area network 113, such as the Internet. The modem 115, which may be internal or external, is connected to the system bus 130 via the serial port interface 106. In a networked environment, program modules depicted relative to the personal computer 100, or portions thereof, may be stored in the remote memory storage device.

It will be appreciated that the network connections shown are illustrative and other techniques for establishing a communications link between the computers can be used. The existence of any of various well-known protocols such as TCP/IP, Ethernet, FTP, HTTP, Bluetooth, IEEE 802.11x and the like is presumed, and the system can be operated in a client-server configuration to permit a user to retrieve web pages from a web-based server. Any of various conventional web browsers can be used to display and manipulate data on web pages.

Image Capturing Pen

Aspects of the present invention include placing an encoded data stream in a displayed form that represents the encoded data stream. (For example, as will be discussed with FIG. 4B, the encoded data stream is used to create a graphical pattern.) The displayed form may be printed paper (or other physical medium) or may be a display projecting the encoded data stream in conjunction with another image or set of images. For example, the encoded data stream may be represented as a physical graphical image on the paper or a graphical image overlying the displayed image (e.g., representing the text of a document) or may be a physical (non-modifiable) graphical image on a display screen (so any image portion captured by a pen is locatable on the display screen).

This determination of the location of a captured image may be used to determine the location of a user's interaction with the paper, medium, or display screen. In some aspects of the present invention, the pen may be an ink pen writing on paper. In other aspects, the pen may be a stylus with the user writing on the surface of a computer display. Any interaction may be provided back to the system with knowledge of the encoded image on the document or supporting the document displayed on the computer screen. By repeatedly capturing images with a camera in the pen or stylus as the pen or stylus traverses a document, the system can track movement of the stylus being controlled by the user. The displayed or printed image may be a watermark associated with the blank or content-rich paper or may be a watermark associated with a displayed image or a fixed coding overlying a screen or built into a screen.

FIGS. 2A and 2B show an illustrative example of pen 201 with a camera 203. Pen 201 includes a tip 202 that may or may not include an ink reservoir. Camera 203 captures an image 204 from surface 207. Pen 201 may further include additional sensors and/or processors as represented in broken box 206. These sensors and/or processors 206 may also include the ability to transmit information to another pen 201 and/or a personal computer (for example, via Bluetooth or other wireless protocols).

FIG. 2B represents an image as viewed by camera 203. In one illustrative example, the field of view of camera 203 (i.e., the resolution of the image sensor of the camera) is 32×32 pixels (where N=32). In the embodiment, a captured image (32 pixels by 32 pixels) corresponds to an area of approximately 5 mm by 5 mm of the surface plane captured by camera 203. Accordingly, FIG. 2B shows a field of view of 32 pixels long by 32 pixels wide. The size of N is adjustable, such that a larger N corresponds to a higher image resolution. Also, while the field of view of the camera 203 is shown as a square for illustrative purposes here, the field of view may include other shapes as is known in the art.

The images captured by camera 203 may be defined as a sequence of image frames {I_i}, where I_i is captured by the pen 201 at sampling time t_i. The sampling rate may be large or small, depending on system configuration and performance requirement. The size of the captured image frame may be large or small, depending on system configuration and performance requirement.

The image captured by camera 203 may be used directly by the processing system or may undergo pre-filtering. This pre-filtering may occur in pen 201 or may occur outside of pen 201 (for example, in a personal computer).

The image size of FIG. 2B is 32×32 pixels. If each encoding unit size is 3×3 pixels, then the number of captured encoded units would be approximately 100 units. If the encoding unit size is 5×5 pixels, then the number of captured encoded units is approximately 36.

FIG. 2A also shows the image plane 209 on which an image 210 of the pattern from location 204 is formed. Light received from the pattern on the object plane 207 is focused by lens 208. Lens 208 may be a single lens or a multi-part lens system, but is represented here as a single lens for simplicity. Image capturing sensor 211 captures the image 210.

The image sensor 211 may be large enough to capture the image 210. Alternatively, the image sensor 211 may be large enough to capture an image of the pen tip 202 at location 212. For reference, the image at location 212 is referred to as the virtual pen tip. It is noted that the virtual pen tip location with respect to image sensor 211 is fixed because of the constant relationship between the pen tip, the lens 208, and the image sensor 211.

The following transformation F_S→P transforms position coordinates in the image captured by camera to position coordinates in the real image on the paper:L_paper=F_SΔP(L_Sensor).

During writing, the pen tip and the paper are on the same plane. Accordingly, the transformation from the virtual pen tip to the real pen tip is also F_S→P:L_pentip=F_S→P(L_{virtual-pentip}).

The transformation F_S→P may be estimated as an affine transform, which approximates F_S→P as:

$F_{S->P}^{\prime }=\left[\begin{array}{ccc}\frac{\sin {\theta }_y}{s_x}& \frac{\cos {\theta }_y}{s_x}& 0\\ \frac{-\sin {\theta }_x}{s_y}& \frac{-\cos {\theta }_x}{s_y}& 0\\ 0& 0& 1\end{array}\right],$ in which θ_x, θ_y, s_x, and s_y are the rotation and scale of two orientations of the pattern captured at location 204. Further, one can refine F′_S→P by matching the captured image with the corresponding real image on paper. “Refine” means to get a more precise estimation of the transformation F_S→P by a type of optimization algorithm referred to as a recursive method. The recursive method treats the matrix F′_S→P as the initial value. The refined estimation describes the transformation between S and P more precisely.

Next, one can determine the location of virtual pen tip by calibration.

One places the pen tip 202 on a fixed location L_pentip on paper. Next, one tilts the pen, allowing the camera 203 to capture a series of images with different pen poses. For each image captured, one may obtain the transformation F_S→P. From this transformation, one can obtain the location of the virtual pen tip L_{virtual-pentip}:L_{virtual-pentip}=F_S→P(L_pentip),where L_pentip is initialized as (0, 0) andF_S→P=(F_S→P)⁻¹.

By averaging the L_{virtual-pentip} obtained from each image, a location of the virtual pen tip L_{virtual-pentip} may be determined. With L_{virtual-pentip}, one can get a more accurate estimation of L_pentip. After several times of iteration, an accurate location of virtual pen tip L_{virtual-pentip} may be determined.

The location of the virtual pen tip L_{virtual-pentip} is now known. One can also obtain the transformation F_S→P from the images captured. Finally, one can use this information to determine the location of the real pen tip L_pentip:L_pentip=F_S→P(L_{virtual-pentip}).Encoding of Array

A two-dimensional array may be constructed by folding a one-dimensional sequence. Any portion of the two-dimensional array containing a large enough number of bits may be used to determine its location in the complete two-dimensional array. However, it may be necessary to determine the location from a captured image or a few captured images. So as to minimize the possibility of a captured image portion being associated with two or more locations in the two-dimensional array, a non-repeating sequence may be used to create the array. One property of a created sequence is that the sequence does not repeat over a particular length (or window size). The following describes the creation of the one-dimensional sequence then the folding of the sequence into an array.

Sequence Construction

A sequence of numbers may be used as the starting point of the encoding system. For example, a sequence (also referred to as an m-sequence) may be represented as a q-element set in field F_q. Here, q=pⁿ, where n≧1 and p is a prime number. The sequence or m-sequence may be generated by a variety of different techniques including, but not limited to, polynomial division. Using polynomial division, the sequence may be defined as follows:

$\frac{R_l\left(x\right)}{P_n\left(x\right)},$

where P_n(x) is a primitive polynomial of degree n in field F_q[x] (having qⁿ elements). R_l(x) is a nonzero polynomial of degree l (where l<n) in field F_q[x]. The sequence may be created using an iterative procedure with two steps: first, dividing the two polynomials (resulting in an element of field F_q) and, second, multiplying the remainder by x. The computation stops when the output begins to repeat. This process may be implemented using a linear feedback shift register as set forth in an article by Douglas W. Clark and Lih-Jyh Weng, “Maximal and Near-Maximal Shift Register Sequences: Efficient Event Counters and Easy Discrete Logarithms,” IEEE Transactions on Computers 43.5 (May 1994, pp 560-568). In this environment, a relationship is established between cyclical shifting of the sequence and polynomial R_l(x): changing R_l(x) only cyclically shifts the sequence and every cyclical shifting corresponds to a polynomial R_l(x). One of the properties of the resulting sequence is that, the sequence has a period of qⁿ−1 and within a period, over a width (or length) n, any portion exists once and only once in the sequence. This is called the “window property”. Period qⁿ−1 is also referred to as the length of the sequence and n as the order of the sequence. In our implementation, q is chosen as 2.

The process described above is but one of a variety of processes that may be used to create a sequence with the window property.

Array Construction

The array (or m-array) that may be used to create the image (of which a portion may be captured by the camera) is an extension of the one-dimensional sequence or m-sequence. Let A be an array of period (m₁, m₂), namely A(k+m₁,l)=A(k,l+m₂)=A(k,l). When an n₁×n₂ window shifts through a period of A, all the nonzero n₁×n₂ matrices over F_q appear once and only once. This property is also referred to as a “window property” in that each window is unique. A widow may then be expressed as an array of period (m₁, m₂) (with m₁ and m₂ being the horizontal and vertical number of bits present in the array) and order (n₁, n₂).

A binary array (or m-array) may be constructed by folding the sequence. One approach is to obtain a sequence then fold it to a size of m₁×m₂ where the length of the array is L=m₁×m₂=2ⁿ−1. Alternatively, one may start with a predetermined size of the space that one wants to cover (for example, one sheet of paper, 30 sheets of paper or the size of a computer monitor), determine the area (m₁×m₂), then use the size to let L≧m₁×m₂, where L=2ⁿ−1.

A variety of different folding techniques may be used. For example, FIGS. 3A through 3C show three different sequences. Each of these may be folded into the array shown as FIG. 3D. The three different folding methods are shown as the overlay in FIG. 3D and as the raster paths in FIGS. 3E and 3F. We adopt the folding method shown in FIG. 3D.

To create the folding method as shown in FIG. 3D, one creates a sequence {a_i} of length L and order n. Next, an array {b_kl} of size m₁×m₂, where gcd(m₁, m₂)=1 and L=m₁×m₂, is created from the sequence {a_i} by letting each bit of the array be calculated as shown by equation 1:b_kl=a_i, where k=i mod(m₁), l=i mod(m₂), i=0, . . . ,L−1.  (1)

This folding approach may be alternatively expressed as laying the sequence on the diagonal of the array, then continuing from the opposite edge when an edge is reached.

FIG. 4A shows sample encoding techniques that may be used to encode the array of FIG. 3D. It is appreciated that other encoding techniques may be used. For example, an alternative coding technique is shown in FIG. 11.

Referring to FIG. 4A, a first bit 401 (for example, “1”) is represented by a column of dark ink. A second bit 402 (for example, “0”) is represented by a row of dark ink. It is appreciated that any color ink may be used to represent the various bits. The only requirement in the color of the ink chosen is that it provides a significant contrast with the background of the medium to be differentiable by an image capture system. The bits in FIG. 4A are represented by a 3×3 matrix of cells. The size of the matrix may be modified to be any size as based on the size and resolution of an image capture system. Alternative representation of bits 0 and 1 are shown in FIGS. 4C-4E. It is appreciated that the representation of a one or a zero for the sample encodings of FIGS. 4A-4E may be switched without effect. FIG. 4C shows bit representations occupying two rows or columns in an interleaved arrangement. FIG. 4D shows an alternative arrangement of the pixels in rows and columns in a dashed form. Finally FIG. 4E shows pixel representations in columns and rows in an irregular spacing format (e.g., two dark dots followed by a blank dot).

Referring back to FIG. 4A, if a bit is represented by a 3×3 matrix and an imaging system detects a dark row and two white rows in the 3×3 region, then a zero is detected (or one). If an image is detected with a dark column and two white columns, then a one is detected (or a zero).

Here, more than one pixel or dot is used to represent a bit. Using a single pixel (or bit) to represent a bit is fragile. Dust, creases in paper, non-planar surfaces, and the like create difficulties in reading single bit representations of data units. However, it is appreciated that different approaches may be used to graphically represent the array on a surface. Some approaches are shown in FIGS. 4C through 4E. It is appreciated that other approaches may be used as well. One approach is set forth in FIG. 11 using only space-shifted dots.

A bit stream is used to create the graphical pattern 403 of FIG. 4B. Graphical pattern 403 includes 12 rows and 18 columns. The rows and columns are formed by a bit stream that is converted into a graphical representation using bit representations 401 and 402. FIG. 4B may be viewed as having the following bit representation:

$\left[\begin{array}{ccccccccc}0& 1& 0& 1& 0& 1& 1& 1& 0\\ 1& 1& 0& 1& 1& 0& 0& 1& 0\\ 0& 0& 1& 0& 1& 0& 0& 1& 1\\ 1& 0& 1& 1& 0& 1& 1& 0& 0\end{array}\right]\hspace{1em}$ Decoding

When a person writes with the pen of FIG. 2A or moves the pen close to the encoded pattern, the camera captures an image. For example, pen 201 may utilize a pressure sensor as pen 201 is pressed against paper and pen 201 traverses a document on the paper. The image is then processed to determine the orientation of the captured image with respect to the complete representation of the encoded image and extract the bits that make up the captured image.

For the determination of the orientation of the captured image relative to the whole encoded area, one may notice that not all the four conceivable corners shown in FIG. 5A-5D can present in the graphical pattern 403. In fact, with the correct orientation, the type of corner shown in FIG. 5A cannot exist in the graphical pattern 403. Therefore, the orientation in which the type of corner shown in FIG. 5A is missing is the right orientation.

Continuing to FIG. 6, the image captured by a camera 601 may be analyzed and its orientation determined so as to be interpretable as to the position actually represented by the image 601. First, image 601 is reviewed to determine the angle θ needed to rotate the image so that the pixels are horizontally and vertically aligned. It is noted that alternative grid alignments are possible including a rotation of the underlying grid to a non-horizontal and vertical arrangement (for example, 45 degrees). Using a non-horizontal and vertical arrangement may provide the probable benefit of eliminating visual distractions from the user, as users may tend to notice horizontal and vertical patterns before others. For purposes of simplicity, the orientation of the grid (horizontal and vertical and any other rotation of the underlying grid) is referred to collectively as the predefined grid orientation.

Next, image 601 is analyzed to determine which corner is missing. The rotation amount o needed to rotate image 601 to an image ready for decoding 603 is shown as o=(θ plus a rotation amount {defined by which corner missing}). The rotation amount is shown by the equation in FIG. 7. Referring back to FIG. 6, angle θ is first determined by the layout of the pixels to arrive at a horizontal and vertical (or other predefined grid orientation) arrangement of the pixels and the image is rotated as shown in 602. An analysis is then conducted to determine the missing corner and the image 602 rotated to the image 603 to set up the image for decoding. Here, the image is rotated 90 degrees counterclockwise so that image 603 has the correct orientation and can be used for decoding.

It is appreciated that the rotation angle θ may be applied before or after rotation of the image 601 to account for the missing corner. It is also appreciated that by considering noise in the captured image, all four types of corners may be present. We may count the number of corners of each type and choose the type that has the least number as the corner type that is missing.

Finally, the code in image 603 is read out and correlated with the original bit stream used to create image 403. The correlation may be performed in a number of ways. For example, it may be performed by a recursive approach in which a recovered bit stream is compared against all other bit stream fragments within the original bit stream. Second, a statistical analysis may be performed between the recovered bit stream and the original bit stream, for example, by using a Hamming distance between the two bit streams. It is appreciated that a variety of approaches may be used to determine the location of the recovered bit stream within the original bit stream.

As will be discussed, EIC pattern analysis obtains recovered bits from image 603. Once one has the recovered bits, one needs to locate the captured image within the original array (for example, the one shown in FIG. 4B). The process of determining the location of a segment of bits within the entire array is complicated by a number of items. First, the actual bits to be captured may be obscured (for example, the camera may capture an image with handwriting that obscures the original code). Second, dust, creases, reflections, and the like may also create errors in the captured image. These errors make the localization process more difficult. In this regard, the image capture system may need to function with non-sequential bits extracted from the image. The following represents a method for operating with non-sequential bits from the image.

Let the sequence (or m-sequence) I correspond to the power series I(x)=1/P_n(x), where n is the order of the m-sequence, and the captured image contains K bits of I b=(b₀ b₁ b₂ . . . b_K-1)^t, where K≧n and the superscript t represents a transpose of the matrix or vector. The location s of the K bits is just the number of cyclic shifts of I so that b₀ is shifted to the beginning of the sequence. Then this shifted sequence R corresponds to the power series x^s/P_n(x), or R=T^s (I), where T is the cyclic shift operator. We find this s indirectly. The polynomials modulo P_n(x) form a field. It is guaranteed that x^s≡r₀+r₁x+ . . . r_n-1xⁿ⁻¹mod(P_n(x)). Therefore, we may find (r₀,r₁, . . . , r_n-1) and then solve for s.

The relationship x^s≡r₀+r₁x+ . . . r_n-1xⁿ⁻¹mod(P_n(x)) implies that R=r₀+r₁T(I)+ . . . +r_n-1Tⁿ⁻¹(I). Written in a binary linear equation, it becomes:R=r^tA  (2)where r=(r₀ r₁ r₂ . . . r_n-1)^t, and A=(I T(I) . . . Tⁿ⁻¹(I)^t which consists of the cyclic shifts of I from 0-shift to (n−1)-shift. Now only sparse K bits are available in R to solve r. Let the index differences between b_i and b₀ in R be k_i, i=1,2, . . . , k−1, then the 1^st and (k_i+1)-th elements of R, i=1,2, . . . ,k−1, are exactly b₀, b₁, . . . , b_k−1. By selecting the 1^st and (k_i+1)-th columns of A, i=1,2, . . . ,k−1, the following binary linear equation is formed:b^t=r^tM  (3)

where M is an n×K sub-matrix of A.

If b is error-free, the solution of r may be expressed as:r^t={tilde over (b)}^t{tilde over (M)}⁻¹  (4)

where {tilde over (M)} is any non-degenerate n×n sub-matrix of M and {tilde over (b)} is the corresponding sub-vector of b.

With known r, we may use the Pohlig-Hellman-Silver algorithm as noted by Douglas W. Clark and Lih-Jyh Weng, “Maximal and Near-Maximal Shift Register Sequences: Efficient Event Counters and Easy Discrete Logorithms,” IEEE Transactions on Computers 43.5 (May 1994, pp 560-568) to find s so that x^s≡r₀+r₁x+ . . . r_n-1xⁿ⁻¹mod(P_n(x)).

As matrix A (with the size of n by L, where L=2ⁿ−1) may be huge, we should avoid storing the entire matrix A. In fact, as we have seen in the above process, given extracted bits with index difference k_i, only the first and (k_i+1)-th columns of A are relevant to the computation. Such choices of k_i is quite limited, given the size of the captured image. Thus, only those columns that may be involved in computation need to saved. The total number of such columns is much smaller than L (where L=2ⁿ−1 is the length of the m-sequence).

Error Correction

If errors exist in b, then the solution of r becomes more complex. Traditional methods of decoding with error correction may not readily apply, because the matrix M associated with the captured bits may change from one captured image to another.

We adopt a stochastic approach. Assuming that the number of error bits in b, n_e, is relatively small compared to K, then the probability of choosing correct n bits from the K bits of b and the corresponding sub-matrix {tilde over (M)} of M being non-degenerate is high.

When the n bits chosen are all correct, the Hamming distance between b^t and r^tM, or the number of error bits associated with r, should be minimal, where r is computed via equation (4). Repeating the process for several times, it is likely that the correct r that results in the minimal error bits can be identified.

If there is only one r that is associated with the minimum number of error bits, then it is regarded as the correct solution. Otherwise, if there is more than one r that is associated with the minimum number of error bits, the probability that n_e exceeds the error correcting ability of the code generated by M is high and the decoding process fails. The system then may move on to process the next captured image. In another implementation, information about previous locations of the pen can be taken into consideration. That is, for each captured image, a destination area where the pen may be expected next can be identified. For example, if the user has not lifted the pen between two image captures by the camera, the location of the pen as determined by the second image capture should not be too far away from the first location. Each r that is associated with the minimum number of error bits can then be checked to see if the location s computed from r satisfies the local constraint, i.e., whether the location is within the destination area specified.

If the location s satisfies the local constraint, the X, Y positions of the extracted bits in the array are returned. If not, the decoding process fails.

FIG. 8 depicts a process that may be used to determine a location in a sequence (or m-sequence) of a captured image. First, in step 801, a data stream relating to a captured image is received. In step 802, corresponding columns are extracted from A and a matrix M is constructed.

In step 803, n independent column vectors are randomly selected from the matrix M and vector r is determined by solving equation (4). This process is performed Q times (for example, 100 times) in step 804. The determination of the number of loop times is discussed in the section Loop Times Calculation.

In step 805, r is sorted according to its associated number of error bits. The sorting can be done using a variety of sorting algorithms as known in the art. For example, a selection sorting algorithm may be used. The selection sorting algorithm is beneficial when the number Q is not large. However, if Q becomes large, other sorting algorithms (for example, a merge sort) that handle larger numbers of items more efficiently may be used.

The system then determines in step 806 whether error correction was performed successfully, by checking whether multiple r's are associated with the minimum number of error bits. If yes, an error is returned in step 809, indicating the decoding process failed. If not, the position s of the extracted bits in the sequence (or m-sequence) is calculated in step 807, for example, by using the Pohig-Hellman-Silver algorithm.

Next, the (X, Y) position in the array is calculated as: x=s mod m₁ and y=s mod m₂ and the results are returned in step 808.

Location Determination

FIG. 9 shows a process for determining the location of a pen tip. The input is an image captured by a camera and the output may be a position coordinates of the pen tip. Also, the output may include (or not) other information such as a rotation angle of the captured image.

In step 901, an image is received from a camera. Next, the received image may be optionally preprocessed in step 902 (as shown by the broken outline of step 902) to adjust the contrast between the light and dark pixels and the like.

Next, in step 903, the image is analyzed to determine the bit stream within it.

Next, in step 904, n bits are randomly selected from the bit stream for multiple times and the location of the received bit stream within the original sequence (or m-sequence) is determined.

Finally, once the location of the captured image is determined in step 904, the location of the pen tip may be determined in step 905.

FIG. 10 gives more details about 903 and 904 and shows the approach to extract the bit stream within a captured image. First, an image is received from the camera in step 1001. The image then may optionally undergo image preprocessing in step 1002 (as shown by the broken outline of step 1002). The pattern is extracted in step 1003. Here, pixels on the various lines may be extracted to find the orientation of the pattern and the angle θ.

Next, the received image is analyzed in step 1004 to determine the underlying grid lines. If grid lines are found in step 1005, then the code is extracted from the pattern in step 1006. The code is then decoded in step 1007 and the location of the pen tip is determined in step 1008. If no grid lines were found in step 1005, then an error is returned in step 1009.

Outline of Enhanced Decoding and Error Correction Algorithm

With an embodiment of the invention as shown in FIG. 12, given extracted bits 1201 from a captured image (corresponding to a captured array) and the destination area, a variation of an m-array decoding and error correction process decodes the X, Y position. FIG. 12 shows a flow diagram of process 1200 of this enhanced approach. Process 1200 comprises two components 1251 and 1253.

• Decode Once. Component 1251 includes three parts.
  • random bit selection: randomly selects a subset of the extracted bits 1201 (step 1203)
  • decode the subset (step 1205)
  • determine X, Y position with local constraint (step 1209)Decoding with Smart Bit Selection. Component 1253 includes four parts.
  • smart bit selection: selects another subset of the extracted bits (step 1217)
  • decode the subset (step 1219)
  • adjust the number of iterations (loop times) of step 1217 and step 1219 (step 1221)
  • determine X, Y position with local constraint (step 1225)

The embodiment of the invention utilizes a discreet strategy to select bits, adjusts the number of loop iterations, and determines the X, Y position (location coordinates) in accordance with a local constraint, which is provided to process 1200. With both components 1251 and 1253, steps 1205 and 1219 (“Decode Once”) utilize equation (4) to compute r.

Let {circumflex over (b)} be decoded bits, that is:{circumflex over (b)}^t=r^tM  (5)The difference between b and {circumflex over (b)} are the error bits associated with r.

FIG. 12 shows a flow diagram of process 1200 for decoding extracted bits 1201 from a captured image in accordance with embodiments of the present invention. Process 1200 comprises components 1251 and 1253. Component 1251 obtains extracted bits 1201 (comprising K bits) associated with a captured image (corresponding to a captured array). In step 1203, n bits (where n is the order of the m-array) are randomly selected from extracted bits 1201. In step 1205, process 1200 decodes once and calculates r. In step 1207, process 1200 determines if error bits are detected for b. If step 1207 determines that there are no error bits, X,Y coordinates of the position of the captured array are determined in step 1209. With step 1211, if the X,Y coordinates satisfy the local constraint, i.e., coordinates that are within the destination area, process 1200 provides the X, Y position (such as to another process or user interface) in step 1213. Otherwise, step 1215 provides a failure indication.

If step 1207 detects error bits in b, component 1253 is executed in order to decode with error bits. Step 1217 selects another set of n bits (which differ by at least one bit from the n bits selected in step 1203) from extracted bits 1201. Steps 1221 and 1223 determine the number of iterations (loop times) that are necessary for decoding the extracted bits. Step 1225 determines the position of the captured array by testing which candidates obtained in step 1219 satisfy the local constraint. Steps 1217-1225 will be discussed in more details.

Smart Bit Selection

Step 1203 randomly selects n bits from extracted bits 1201 (having K bits), and solves for r₁. Using equation (5), decoded bits can be calculated. Let I₁={kε{1,2, . . . , K}|b_k={circumflex over (b)}_k}, Ī₁={kε{1,2, . . . , K}|b_k≠{circumflex over (b)}_k}, where {circumflex over (b)}_k is the k^th bit of {circumflex over (b)}, B₁={b_k|kεI₁} and B₁={b_k|kεĪ₁}, that is, B₁ are bits that the decoded results are the same as the original bits, and B₁ are bits that the decoded results are different from the original bits, I₁ and Ī₁ are the corresponding indices of these bits. It is appreciated that the same r₁ will be obtained when any n independent bits are selected from B₁. Therefore, if the next n bits are not carefully chosen, it is possible that the selected bits are a subset of B₁, thus resulting in the same r₁ being obtained.

In order to avoid such a situation, step 1217 selects the next n bits according to the following procedure:

• • 1. Choose at least one bit from B₁ 1303 and the rest of the bits randomly from B₁ 1301 and B₁ 1303, as shown in FIG. 13 corresponding to bit arrangement 1351. Process 1200 then solves r₂ and finds B₂ 1305, 1309 and B₂ 1307, 1311 by computing {circumflex over (b)}₂^t=r₂^tM₂.
  • 2. Repeat step 1. When selecting the next n bits, for every B_i(i=1, 2, 3 . . . , x−1, where x is the current loop number), there is at least one bit selected from B_i. The iteration terminates when no such subset of bits can be selected or when the loop times are reached.Loop Times Calculation

With the error correction component 1253, the number of required iterations (loop times) is adjusted after each loop. The loop times is determined by the expected error rate. The expected error rate p_e in which not all the selected n bits are correct is:

$\begin{array}{cc}{p}_e={\left(1-\frac{C_{K-n_e}^n}{C_K^n}\right)}^{\mathrm{lt}}\approx -e^{-{\mathrm{lt}\left(\frac{K-n}{K}\right)}^{n_e}}& \left(6\right)\end{array}$ where lt represents the loop times and is initialized by a constant, K is the number of extracted bits from the captured array, n_e represents the minimum number of error bits incurred during the iteration of process 1200, n is the order of the m-array, and C_Kⁿ is the number of combinations in which n bits are selected from K bits.

In the embodiment, we want p_e to be less than e⁻⁵=0.0067. In combination with (6), we have:

$\begin{array}{cc}{\mathrm{lt}}_i=\min \left({\mathrm{lt}}_{i-1},\frac{5}{{\left(\frac{K-n}{K}\right)}^{n_e}}+1\right)& \left(7\right)\end{array}$ Adjusting the loop times may significantly reduce the number of iterations of process 1253 that are required for error correction.Determine X, Y Position with Local Constraint

In steps 1209 and 1225, the decoded position should be within the destination area. The destination area is an input to the algorithm, and it may be of various sizes and places or simply the whole m-array depending on different applications. Usually it can be predicted by the application. For example, if the previous position is determined, considering the writing speed, the destination area of the current pen tip should be close to the previous position. However, if the pen is lifted, then its next position can be anywhere. Therefore, in this case, the destination area should be the whole m-array. The correct X, Y position is determined by the following steps.

In step 1224 process 1200 selects r_i whose corresponding number of error bits is less than:

$\begin{array}{cc}{N}_e=\frac{{\log }_{10}\left(\frac{3}{\mathrm{lt}}\right)}{{\log }_{10}\left(\frac{K-n}{K}\right)\times {\log }_{10}\left(\frac{10}{\mathrm{lr}}\right)}& \left(8\right)\end{array}$ where lt is the actual loop times and lr represents the Local Constraint Rate calculated by:

$\begin{array}{cc}\mathrm{lr}=\frac{\mathrm{area}\mathrm{of}\mathrm{the}\mathrm{destination}\mathrm{area}}{L}& \left(9\right)\end{array}$ where L is the length of the m-array.

Step 1224 sorts r_i in ascending order of the number of error bits. Steps 1225, 1211 and 1212 then finds the first r_i in which the corresponding X, Y position is within the destination area. Steps 1225, 1211 and 1212 finally returns the X, Y position as the result (through step 1213), or an indication that the decoding procedure failed (through step 1215).

Illustrative Example of Enhanced Decoding and Error Correction Process

An illustrative example demonstrates process 1200 as performed by components 1251 and 1253. Suppose n=3, K=5, I=(I₀ I₁ . . . I₆)^t is the m-sequence of order n=3. Then

$\begin{array}{cc}A=\left(\begin{array}{ccccccc}{I}_0& I_1& I_2& I_3& I_4& I_5& I_6\\ I_6& I_0& I_1& I_2& I_3& I_4& I_5\\ I_5& I_6& I_0& I_1& I_2& I_3& I_4\end{array}\right)& \left(10\right)\end{array}$ Also suppose that the extracted bits b=(b₀ b₁ b₂ b₃ b₄)^t, where K=5, are actually the s^th, (s+1)^th, (s+3)^th, (s+4)^th, and (s+6)^th bits of the m-sequence (these numbers are actually modulus of the m-array length L=2ⁿ−1=2³−1=7). Therefore

$\begin{array}{cc}M=\left(\begin{array}{ccccc}{I}_0& I_1& I_3& I_4& I_6\\ I_6& I_0& I_2& I_3& I_5\\ I_5& I_6& I_1& I_2& I_4\end{array}\right)& \left(11\right)\end{array}$ which consists of the 0^th, 1^st, 3^rd, 4^th, and 6^th columns of A. The number s, which uniquely determines the X, Y position of b₀ in the m-array, can be computed after solving r=(r₀ r₁ r₂)^t that are expected to fulfill b^t=r^tM. Due to possible error bits in b, b^t=r^tM may not be completely fulfilled.

Process 1200 utilizes the following procedure. Randomly select n=3 bits, say {tilde over (b)}₁^t=(b₀ b₁ b₂), from b. Solving for r₁:{tilde over (b)}₁^t=r₁^t{tilde over (M)}₁  (12)where {tilde over (M)}₁ consists of the 0th, 1st, and 2nd columns of M. (Note that {tilde over (M)}₁ is an n×n matrix and r₁^t is a 1×n vector so that {tilde over (b)}₁^t is a 1×n vector of selected bits.)

Next, decoded bits are computed:{circumflex over (b)}₁^t=r₁^tM  (13)where M is an n×K matrix and r₁^t is a 1×n vector so that {circumflex over (b)}₁^t, is a 1×K vector. If {circumflex over (b)}₁ is identical to b, i.e., no error bits are detected, then step 1209 determines the X, Y position and step 1211 determines whether the decoded position is inside the destination area. If so, the decoding is successful, and step 1213 is performed. Otherwise, the decoding fails as indicated by step 1215. If {circumflex over (b)}₁ is different from b, then error bits in b are detected and component 1253 is performed. Step 1217 determines the set B₁, say {b₀ b₁ b₂ b₃}, where the decoded bits are the same as the original bits. Thus, B₁={b₄} (corresponding to bit arrangement 1351 in FIG. 13). Loop times (lt) is initialized to a constant, e.g., 100, which may be variable depending on the application. Note that the number of error bits corresponding to r₁ is equal to 1. Then step 1221 updates the loop time (lt) according to equation (7), lt₁=min(lt,13)=13.

Step 1217 next chooses another n=3 bits from b. If the bits all belong to B₁, say {b₀ b₂ b₃}, then step 1219 will determine r₁ again. In order to avoid such repetition, step 1217 may select, for example, one bit {b₄} from B₁, and the remaining two bits {b₀ b₁} from B₁.

The selected three bits form {tilde over (b)}₂^t=(b₀ b₁ b₄). Step 1219 solves for r₂:{tilde over (b)}₂^t=r₂^t{tilde over (M)}₂  (14)where {tilde over (M)}₂ consists of the 0^th, 1^st, and 4^th columns of M.

Step 1219 computes {circumflex over (b)}₂^t=r₂^tM. Find the set B₂, e.g., {b₀ b₁ b₄}, such that {circumflex over (b)}₂ and b are the same. Then B₂={b₂ b₃} (corresponding to bit arrangement 1353 in FIG. 13). Step 1221 updates the loop times (lt) according to equation (7). Note that the number of error bits associated with r₂ is equal to 2. Substituting into (7), lt₂=min(lt₁, 32)=13.

Because another iteration needs to be performed, step 1217 chooses another n=3 bits from b. The selected bits shall not all belong to either B₁ or B₂. So step 1217 may select, for example, one bit {b₄} from B₁, one bit {b₂} from B₂, and the remaining one bit {b₀}.

The solution of r, bit selection, and loop times adjustment continues until we cannot select any new n=3 bits such that they do not all belong to any previous B_i's, or the maximum loop times lt is reached.

Suppose that process 1200 calculates five r_i(i=1,2,3,4,5), with the number of error bits corresponding to 1, 2, 4, 3, 2, respectively. (Actually, for this example, the number of error bits cannot exceed 2, but the illustrative example shows a larger number of error bits to illustrate the algorithm.) Step 1224 selects r_i's, for example, r₁,r₂,r₄,r₅, whose corresponding numbers of error bits are less than N_e shown in (8).

Step 1224 sorts the selected vectors r₁,r₂,r₄,r₅ in ascending order of their error bit numbers: r₁,r₂, r₅, r₄. From the sorted candidate list, steps 1225, 1211 and 1212 find the first vector r, for example, r₅, whose corresponding position is within the destination area. Step 1213 then outputs the corresponding position. If none of the positions is within the destination area, the decoding process fails as indicated by step 1215.

Embedded Interaction Code Recognition

Introduction to Embedded Interaction Code Recognition

As previously mentioned, to determine the location of a digital pen during interaction with one or more surfaces, images are captured by the digital pen. FIG. 13 shows an example image. Images first undergo pre-processing, and then features of effective EIC pattern in the image are analyzed to obtain grid lines in image. Once the grid lines are determined, black dots on the grid lines are identified. Positions of the black dots help to determine which grid cells correspond to EIC symbols and which direction is the correct orientation of EIC symbols.

The grid cells formed by grid lines may or may not correspond to EIC symbols. As can be seen in FIG. 14, grid cells within the squares, which are formed by the horizontal and vertical dashed lines in FIG. 14, correspond to EIC symbols 1400, whereas grid cells in between rows of symbols do not correspond to EIC symbols. In FIG. 14, the grid cell 1402 is not an EIC symbol. For these reasons, a determination is made as to which grid cells in image correspond to EIC symbols.

Correct orientation of EIC symbols is also determined. EIC symbols captured in image may be rotated due to pen rotation. When EIC symbols are at the correct orientation (i.e. oriented the same as EIC symbols in EIC symbol array), the segment of EIC symbols captured in image can be matched against EIC symbol array, i.e. bits extracted from EIC symbols can be matched against the m-array.

Once we know which grid cells correspond to EIC symbols and the correct orientation of the symbols, the EIC symbols captured in an image are recognized. We then consider a large enough section 1404 of EIC symbol array that encompasses the grid lines and corresponding EIC symbols of the image.

In FIG. 14, X, Y is the coordinate system (referenced generally as 1412 in FIG. 14) of the image, with the image center as the origin, and pixels as the unit of measure. The X, Y coordinate system 1412 is determined in relation to the image, i.e. facing the image, X is left to right and Y is top to bottom.

H′, V′ is the coordinate system (referenced generally as 1410 in FIG. 14) of the grid, with the top (relative to image) intersection point of the farthest grid lines in image, C_H′V′, as the origin, and grid cells as the unit of measure. The H′, V′ coordinate system 1410 is determined in relation to the image. The rotation angle from X to H′ is smaller than that from X to V′, and intersections of grid lines in image have non-negative coordinates in the H′, V′ coordinate system 1410.

What is depicted inside the image in FIG. 14 should not be thought of as what a real image may look like. Grid lines are typically not seen in image. But if we assume a perspective transform from paper to image, effective EIC pattern in image may appear to lie on grid lines that are a perspective transform of the grid lines in EIC symbol array (i.e., the diagonal lines in FIG. 14). Therefore, we can draw grid lines in image and the H′, V′ coordinate system 1410 based on a perspective transform of the grid lines in EIC symbol array 1406.

X′, Y′ is the coordinate system (referenced generally as 1408 in FIG. 14) of the section 1404 of EIC symbol array encompassing the grid lines and corresponding EIC symbols of the image, with the top-left corner of the section, C_X′Y′, as the origin, and EIC symbols as the unit of measure. X′, Y′ is in the direction of EIC symbol array, and the origin is at the top-left corner of a symbol.

FIG. 15 shows the three coordinate systems of another image (which may be thought of as taken after the pen is moved and rotated). The coordinate systems of X, Y 1412 and H′, V′ 1410 stay in relation to the image. The coordinate system of X′, Y′ 1408 is in the direction of EIC symbol array 1406. Therefore, the rotation from H′, V′ to X′, Y′ now is

$-\frac{\pi }{4}$ whereas it was

$\frac{3\pi }{4},$ in FIG. 15.

Given a particular EIC symbol design, and the identified correct orientation of EIC symbols in an image, a transformation from the section 1404 of EIC symbol array (that encompasses the grid lines and corresponding EIC symbols of the image) to grid, i.e. from X′, Y′ to H′, V′, can be obtained. For example, with EIC symbol 8-a-16 (FIG. 26A), the scale from the unit of measure in H′, V′ to that of X′, Y′ is √{square root over (2)}, and the rotation from H′, V′ to X′, Y′ may be

$-\frac{\pi }{4},\frac{\pi }{4},\frac{3\pi }{4},\mathrm{or}\frac{5\pi }{4},$ depending on the correct orientation of EIC symbols in image (FIGS. 14 and 15 show two of these situations). We refer to the homography matrix describing the transformation from X′, Y′ to H′, V′ as H_{Symbol→Grid}.

From a previous step, a homography matrix describing the perspective transform from grid to image, i.e. from H′, V′ to X, Y, H_Grid→Image, is known. Herein we assume digital pen is used on a plane (such as a paper plane where EIC pattern is printed on) and the spatial transformation from the plane to image (also assumed a plane) is a perspective transform. That is, effective EIC pattern in image should lie on grid lines that are a perspective transform of the grid lines in EIC symbol array. The perspective transform is first assumed to be an affine transform, i.e. evenly spaced parallel lines are kept evenly spaced and parallel, but perpendicular lines may not be perpendicular anymore. Rotation, scale and translation of the affine transform are estimated from analyzing effective EIC pattern in image. The perspective transform is then obtained by fitting effective EIC pattern to affine transformed grid lines. A homography matrix H_Grid→Image that describes the perspective transform from grid lines in EIC symbol array to image is obtained.

Thus, a homography matrix, H_{Symbol →Image}, describing the transformation from X′, Y′ to X, Y can be obtained as:H_{Symbol→Image}=H_Grid→Image·H_{Symbol→Grid}

The homography matrix H_{Symbol→Image} specifies the transformation of points in the section 1404 of EIC symbol array encompassing the image to a point in the image coordinate system 1412. The homography matrix H_{Symbol→Image}⁻¹, specifies the transformation of each point in the image coordinate system 1412 to a point in the section 1404 of EIC symbol array encompassing the image.

From recognized EIC symbols in the section of EIC symbol array encompassing the image, EIC bits are extracted. For each m-array, a stream of bits is extracted. Any bit can be chosen as the bit whose position in m-array is decoded.

For convenience, we choose the top-left corner of the section 1404 of EIC symbol array encompassing the image, C_X′Y′, as the position to decode. In the bit stream starting from C_X′Y′, some of the bits are known (bits extracted from recognized symbols), and some are unknown (bits that can't be extracted or EIC symbols are not captured in image). As long as the number of extracted bits is more than the order of the m-array, decoding can be done.

We call this process EIC symbol recognition. FIG. 16 shows a flow diagram of a system for performing EIC symbol recognition in accordance with embodiments of the invention.

From EIC pattern analysis, H_Grid→Image is obtained, with which grid lines in image are obtained. Grid cells thus obtained are effective EIC symbols. Given effective EIC symbols, the next step is to recognize the symbols. The goal of EIC symbol recognition is to obtain bits encoded in EIC symbols and obtain a homography matrix H_{Symbol→Image}, which describes the transformation from the section of EIC symbol array encompassing the image to image. Input of EIC symbol recognition is homography matrix obtained from EIC pattern analysis H_Grid→Image, normalized image, and document content mask. Example input to EIC symbol recognition is shown in FIG. 17. Output of EIC symbol recognition is extracted bits (and confidence values of the bits) and homography matrix H_{Symbol→Image}. FIG. 18 shows example recognized EIC bits and corresponding confidence values for the recognized EIC bits.

The EIC symbol recognition system shown in FIG. 16 includes an EIC-dot-detection module 1604, an EIC-symbol-orientation-determination module 1612, and an EIC-bit-extraction module 1616, each of which is described in more detail below.

EIC Dot Detection

The EIC-dot-detection module 1604 detects black dots on each edge. First, we move the origin of H, V to get the H′, V′ coordinate system. By moving the origin of H, V, all grid intersections in the image have non-negative coordinates. We call the new coordinate system H′, V′, as shown in FIG. 19.

Suppose C′ has coordinates (h′,v′) in H, V coordinate system. After moving, its coordinates are (0, 0).

Suppose the homography matrix obtained from EIC pattern analysis is:

$H=\left[\begin{array}{ccc}{h}_{11}& h_{12}& h_{13}\\ h_{21}& h_{22}& h_{23}\\ h_{31}& h_{32}& h_{33}\end{array}\right]$

The homography matrix that transforms a point in the H′, V′ coordinate system to a point in the X, Y coordinate system is:

$H=\left[\begin{array}{ccc}{h}_{11}& h_{12}& h_{13}+h_{11}·h\text{'}+h_{12}·v^{\prime }\\ h_{21}& h_{22}& h_{23}+h_{21}·h\text{'}+h_{22}·v^{\prime }\\ h_{31}& h_{32}& h_{33}+h_{31}·h\text{'}+h_{32}·v^{\prime }\end{array}\right]$

This homography matrix is referred to herein as the final H_Grid→Image.

With homography matrix H_Grid→Image, all the grid lines in image are obtained (by transforming the grid lines in EIC symbol array using the homography matrix) and form the H′, V′ coordinate system, as shown in FIG. 20.

These grid lines are referred to as H lines and V lines. Grid cells are indexed by the H′, V′ coordinates of the top corner of the cell. Edges of the cells are identified as either on the H lines or on the V lines. For example, in FIG. 20, the cell (i, j) has two edges: edge h, i, j and edge v, i, j.

Next, graylevels are obtained of selected positions on each edge. For EIC symbol 8-a-16, for example, there are 5 EIC dot positions on each edge, as shown in FIG. 21. The EIC symbol in FIG. 21 occupies all of the rows and columns of grid spaces shown in FIG. 21 except for the bottom row and the right-most column. That row and that column belong to adjacent EIC symbols. Accordingly, while black dots 2102-1 and 21024 belong to the EIC symbol shown in FIG. 21, black dots 2102-2 and 2102-3 are not part of that EIC symbol. There are EIC data dot positions and EIC orientation dot positions. Data dots 2106-1 through 2106-16 may be black or white for representing bits of information. Of the 4 data dot positions on each edge, there may be only one black dot. Orientation dots 2104-1 through 2104-4 are always white to facilitate properly orienting camera-captured EIC-symbol images.

Graylevels of the 5 positions on each edge, as shown in FIG. 20, are obtained. First, coordinates of the positions in the H′, V′ coordinate system are obtained. Suppose the total number of H lines is N_h+1, and the total number of V lines is N_v+1. For each position s on each edge (i, j) on the H line, where s=1, 2, . . . , 5, i=0, 1, . . . , N_h−1, j=0, 1, . . . , N_v, the H′, V′ coordinates are:

${\left(i+\frac{s+1}{8},j,1\right)}^t.$ For each position s on each edge (i, j) on the V line, where s=1, 2, . . . , 5, i=0, 1, . . . N_h, j=0, 1, . . . , N_v−1, the H′, V′ coordinates are:

${\left(i,j+\frac{s+1}{8},1\right)}^t.$

Next, with the homography matrix H_Grid→Image, coordinates of the positions in the X, Y coordinate system are obtained. For each position s on each edge (i, j) on the H line, where s=1, 2, . . . , 5, i=0, 1, . . . , N_h−1, j=0, 1, . . . , N_v, the X, Y coordinates are: (x_s^h,i,j, y_x^h,i,j, 1)^t=H_Grid→Image

${\left(x_s^{h,i,j},y_s^{h,i,j},1\right)}^t=H_{\mathrm{grid}\to \mathrm{Image}}·{\left(i+\frac{s+1}{8},j,1\right)}^t.$ For each position s on each edge (i, j) on the V line, where s=1, 2, . . . , 5, i=0, 1, . . . , N_h, j=0, 1, . . . , N_v−1, the X, Y coordinates are: (x_s^v,i,j,y_s^v,i,j,1)^t=H_Grid→Image

${\left(x_s^{v,i,j},y_s^{v,i,j},1\right)}^t=H_{\mathrm{grid}\to \mathrm{Image}}·{\left(i,j+\frac{s+1}{8},1\right)}^t$

Graylevels of the positions are calculated using bilinear sampling of the pixels surrounding the positions. For each position s on edge (i, j) on the H line, where s=1, 2, . . . , 5, i=0, 1, . . . , N_h−1, j=0, 1, . . . , N_v, get the index of the first pixel for bilinear sampling: x₁≈int(x_s^h,i,j+63.5), y₁=int(y_s^h,i,j+49.5).

If

• • 0≦x₁≦126
  • 0≦y₁≦98
  • Document Content Mask (x₁,y₁)=0
  • Document Content Mask (x₁+1,y₁)=0
  • Document Content Mask (x₁,y₁+1)=0
  • Document Content Mask (x₁+1,y₁+1)=0

then,

• • The position is valid.
  • η_x=decimal(x_s^h,i,j+63.5)
  • η_y=decimal(y_s^h,i,j+49.5)G_s^h,i,j=(1−η_y)·[(1−η_x)·G_(x1,y1)+η_x·G_x1+1,y1)]+η_y·[(1−η_x)·G_(x1,y1+1)+η_x·G_(x1+1,y1+1)]

else,

• • The position is not valid.
  • G_s^h,i,j=null.

The function decimal(x) returns the decimal fraction part of x, where x≧0. For example, decimal(1.8)=0.8. (x₁,y₁), (x₁+1,y₁), (x₁,y₁+1) and (x₁+1,y₁+1) are indexes of the pixels used for bilinear sampling, defined by the coordinate system shown in FIG. 22. FIG. 23 shows an illustration of the pixels used for bilinear sampling.

Similarly, for each position s on edge (i, j) on the V line, where s=1, 2, . . . , 5, i=0, 1, . . . , N_h, j=0, 1, . . . , N_v−1, get the index of the first pixel for bilinear sampling:x₁=int(x_s^v,i,j+63.5)y₁=int(y_s^v,i,j+49.5)

If

• • 0≦x₁≦126
  • 0≦y₁≦98
  • Document Content Mask (x₁,y₁)=0
  • Document Content Mask (x₁+1,y₁)=0
  • Document Content Mask (x₁, y₁+1)=0
  • Document Content Mask (x₁+1,y₁+1)=0

then,

• • The position is valid.
  • η_x=decimal(x_s^v,i,j+63.5)
  • η_y=decimal(y_s^v,i,j+49.5)G_s^v,i,j=(1−η_y)·[(1−η_x)·G_(x1,y1)+η_x·G_x1+1,y1)]+η_y·[(1−η_x)·G_(x1,y1+1)+η_x·G_(x1+1,y1+1)]

else,

• • The position is not valid.G_s^v,i,j=null

Next, black dots are detected.

Based on the relative graylevels of the positions, black dots are determined. First, the five positions on each edge are named as follows (see FIG. 24):he_s^i,j|s=1, 2, . . . , 5 when the edge is on an H line and mod(i+j,2)=0;ho_s^i,j|s=1, 2, . . . , 5 when the edge is on an H line and mod(i+j,2)=1;ve_s^i,j|s=1, 2, . . . , 5 when the edge is on a V line and mod(i+j,2)=0;vo_s^i,j|s=1, 2, . . . , 5 when the edge is on a V line and mod(i+j,2)=1.

For each edge, let the count of valid positions be VD^k,i,j, where k=h, v. If there are at least two valid positions on an edge, i.e. VD^k,i,j≧2, let

$u_1=\underset{1\le s\le 5}{\mathrm{Arg}\mathrm{Min}}{G}_s^{k,i,j}$ and

$u_2=\underset{1\le s\le 5,s\ne u_1}{\mathrm{Arg}\mathrm{Min}}{G}_s^{k,i,j}$ i.e., u₁ is the darkest position and u₂ is the second darkest position. If the graylevel difference between the darkest and the second darkest position is large enough, i.e. exceeds a threshold (e.g., T₀=20), the darkest position is considered a black dot.

For each edge (i, j) on the H line, where i=0, 1, . . . , N_h−1, j=0, 1, . . . , N_v and mod(i+j,2)=0,

• • If (G_u2^h,i,j−G_u1^h,i,j)>T₀, then,
    • he_u1^i,j=1, where 1≦u₁≦5
    • he_s^i,j=0, where s=1, 2, . . . , 5 and s≠u₁
    • D_s^h,i,j=he_s^i,jDiff^h,i,j=G_u2^h,i,j−G_u1^h,i,j
  • else,
    • he_s^i,j=0, where s=1, 2, . . . , 5
    • D_s^h,i,j=null
    • Diff^h,i,j=null

For each edge (i, j) on the H line, where i=0, 1, . . . , N_h−1, j=0, 1, . . . , N_v and mod(i+j,2)=1,

• • If (G_u2^h,i,j−G_u1^h,i,j)>T₀, then,
    • ho_u1^i,j=1, where 1≦u₁≦5
    • ho_s^i,j=0, where s=1, 2, . . . , 5 and s≠u₁
    • D_s^h,i,j=ho_s^i,j
    • Diff^h,i,j=G_u2^h,i,j−G_u1^h,i,j
  • else,
    • ho_s^i,j=0, where s=1, 2, . . . , 5
    • D_s^h,i,j=null
    • Diff^h,i,j=null

For each edge (i, j) on the V line, where i=0, 1, . . . , N_h, j=0, 1, . . . , N_v−1 and mod(i+j,2)=0,

• • If (G_u2^v,i,j−G_u1^v,i,j)>T₀, then,
    • ve_u1^i,j=1, where 1≦u₁≦5
    • ve_s^i,j=0, where s=1, 2, . . . , 5 and s≠u₁
    • D_s^v,i,j=ve_s^i,j
    • Diff^v,i,j=G_u2^v,i,j−G_u1^v,i,j
  • else,
    • ve_s^i,j=0, where s=1, 2, . . . , 5
    • D_s^v,i,j=null
    • Diff^v,i,j=null

For each edge (i, j) on the V line, where i=0, 1, . . . , N_h, j=0, 1, . . . , N_v−1 and mod(i+j,2)=1,

• • If (G_u2^v,i,j−G_u1^v,i,j)>T₀, then,
    • vo_u1^i,j=1, where 1≦u₁≦5
    • vo_s^i,j=0, where s=1, 2, . . . , 5 and s≠u₁
    • D_s^v,i,j=vo_s^i,j Diff^v,i,j=G_u2^v,i,j−G_u1^v,i,j
  • else,
    • vo_s^i,j=0, where s=1, 2, . . . , 5
    • D_s^v,i,j=null
    • Diff^v,i,j=null

By now, substantially all of the black dots are detected. he_s^i,j, ho_s^i,j, ve_s^i,j and vo_s^i,j will be used to determine which grid cells correspond to EIC symbols and the correct orientation of the symbols. D_s^h,i,j and D_s^v,i,j will be used for bit extraction.

EIC Symbol Orientation Determination

Now that the black dots are detected, the EIC-symbol-orientation-determination module 1612, which accepts EIC dots 1610 as input, determines which grid cells correspond to EIC symbols and which direction is the correct orientation of the symbols, as illustrated in FIG. 25.

Recall that the orientation dot positions are designed to help to determine the correct orientation of a symbol. When EIC symbols are rotated, the location of the orientation dot positions are different, as illustrated in FIGS. 26A-D. FIG. 26A shows the symbol shown in FIG. 21. FIG. 26B shows the symbol rotated 90 degrees clockwise. FIG. 26C shows the symbol rotated 180 degrees clockwise. FIG. 26D shows the symbol rotated 270 degrees clockwise.

Since there should be no black dots at orientation dot positions, the total number of detected black dots at orientation dot positions assuming no rotation, rotated 90 degrees clockwise, rotated 180 degrees clockwise, and rotated 270 degrees clockwise, can be obtained. The assumption (of a correct orientation) is accepted if the total count under the assumption is the smallest.

Therefore, the EIC-symbol-orientation-determination module first obtains the total number of black dots at orientation dot positions under different assumptions about which grid cells correspond to EIC symbols and the correct orientation of the symbols. Then, based on the smallest count, which grid cells correspond to EIC symbols and the correct orientation of the symbols are determined.

The section of EIC symbol array encompassing the image, i.e. the X′, Y′ coordinate system discussed above in connection with FIG. 15, is then determined. A homography matrix H_{Symbol→Grid}, which describes the transformation from the section of EIC symbol array encompassing the image to grid, i.e. from the X′, Y′ coordinate system, to the H′, V′ coordinate system, can be obtained.

Finally, given H_{Symbol→Grid} and H_Grid→Image obtained from EIC pattern analysis, a homography matrix H_{Symbol→Image}, which describes the transformation from the section of EIC symbol array encompassing the image to image, i.e. from the X′, Y′ coordinate system 1408 to the X, Y coordinate system 1412, is obtained.

The total number of black dots at orientation dot positions is determined as follows.

Let

$Q_0=\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{he}}_5^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{ho}}_5^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{ve}}_5^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{vo}}_1^{i,j}$ $Q_1=\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{he}}_1^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{ho}}_5^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{ve}}_5^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{vo}}_5^{i,j}$ $Q_2=\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{he}}_1^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{ho}}_1^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{ve}}_5^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{vo}}_1^{i,j}$ $Q_3=\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{he}}_1^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{ho}}_5^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{ve}}_1^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{vo}}_1^{i,j}$ $Q_4=\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{he}}_5^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{ho}}_5^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{ve}}_1^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{vo}}_5^{i,j}$ $Q_5=\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{he}}_5^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{ho}}_1^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{ve}}_5^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{vo}}_5^{i,j}$ $Q_6=\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{he}}_1^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{ho}}_1^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{ve}}_1^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{vo}}_5^{i,j}$ $Q_7=\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{he}}_5^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{ho}}_1^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{ve}}_1^{i,j}+\sum _{i=0}^{N_h}\sum _{j=0}^{N_v}{\mathrm{vo}}_1^{i,j}$

Here Q_i, where i=1, 2, . . . , 7, represent the total number of detected black dots at orientation dot positions, given different assumptions about which grid cells correspond to EIC symbols and the correct orientation of the symbols.

Q₀ is the total number of detected black dots at orientation dot positions if grid cell (i, j) is a symbol and (i, j) is the top corner of the symbol (assuming mod(i+j,2)=0, see FIG. 24). Q₁ is the total number of detected black dots at orientation dot positions if grid cell (i, j) is a symbol, and (i+1, j) is the top corner of the symbol. Q₂ is the total number of detected black dots at orientation dot positions if grid cell (i, j) is a symbol, and (i+1, j+1) is the top corner of the symbol. Q₃ is the total number of detected black dots at orientation dot positions if grid cell (i, j) is a symbol, and (i, j+1) is the top corner of the symbol.

Q₄ is the total number of detected black dots at orientation dot positions if grid cell (i+1, j) is a symbol, and (i+1, j) is the top corner of the symbol. Q₅ is the total number of detected black dots at orientation dot positions if grid cell. (i+1, j) is a symbol, and (i+2, j) is the top corner of the symbol. Q₆ is the total number of detected black dots at orientation dot positions if grid cell (i+1, j) is a symbol, and (i+2, j+1) is the top corner of the symbol. Q₇ is the total number of detected black dots at orientation dot positions if grid cell (i+1, j) is a symbol, and (i+1, j+1) is the top corner of the symbol.

Next, determinations are made with respect to which grid cells correspond to EIC symbols and what the correct orientation is for the symbols.

$\mathrm{Let}j=\underset{0\le i\le 7}{\mathrm{Arg}\mathrm{Min}}\left(Q_i\right).$

Let O=int(j/4). O represents which grid cells correspond to EIC symbols. If O=0, grid cell (0, 0) is a symbol. If O=1, grid cell (1, 0) is a symbol. See FIG. 27. Here, we call O an offset.

Let Q=mod(j,4). Q represents the correct orientation of the symbols. EIC symbols in image are rotated

$Q·\frac{\pi }{2}$ clockwise.

Next, the homography matrix, which transforms symbol to image, is obtained.

Now that we know which grid cells correspond to EIC symbols and the correct orientation of the symbols, the section of EIC symbol array encompassing the image, i.e. the X′, Y′ coordinate system 1408, can be determined. And the homography matrix H_{Symbol→Grid}, which describes the transformation from X′, Y′ 1408 to H′, V′ 1410, is obtained.

First, we introduce the H″, V″ coordinate system. H″, V″ is H′, V′ rotated, with the origin moved to the corner of the grid lines that correspond to the top corner of a symbol.

When Q=0, the top corner of the H′, V′ grid lines corresponds to the top corner of a symbol. H″, V″ is the same as H′, V′. X′, Y′ is the section of EIC symbol array encompassing the image. See FIG. 28, which assumes O=1 and which shows symbol, grid, and image coordinate systems when Q=0.

When Q=1, the far right corner of the H′, V′ grid lines corresponds to the top corner of a symbol. H″, V″ is H′, V′ rotated 90 degrees clockwise, with the origin moved to the far right corner of the H′, V′ grid lines. X′, Y′ is the section of EIC symbol array encompassing the image. See FIG. 29, which shows symbol, grid, and image coordinate systems when Q=1.

When Q=2, the bottom corner of the H′, V′ grid lines corresponds to the top corner of a symbol. H″, V″ is H′, V′ rotated 180 degrees clockwise, with the origin moved to the bottom corner of the H′, V′ grid lines. X′, Y′ is the section of EIC symbol array encompassing the image. See FIG. 30, which shows symbol, grid, and image coordinate systems when Q=2.

When Q=3, the far left corner of the H′, V′ grids corresponds to the top corner of a symbol. H″, V″ is H′, V′ rotated 270 degrees clockwise, with the origin moved to the far left corner of the H′, V′ grid lines. X′, Y′ is the section of EIC symbol array encompassing the image. See FIG. 31, which shows symbol, grid, and image coordinate systems when Q=3.

Let the rotation angle from H′, V′ to H″, V″ be θ_Q:

${\theta }_Q=Q·\frac{\pi }{2},i.e.{\theta }_Q\in \left\{0,\frac{\pi }{2},\pi ,\frac{3\pi }{2}\right\}.$

Let θ_s be the angle from H′, V′ to X′, Y′:

${\theta }_s=Q·\frac{\pi }{2}-\frac{\pi }{4},i.e.{\theta }_s\in \left\{-\frac{\pi }{4},\frac{\pi }{4},\frac{3\pi }{4},\frac{5\pi }{4}\right\}.$

Let the origin of the H″, V″ coordinate system, C_H″V″, have the coordinates (h′_CH″V″,v′_CH″V″) in H′, V′ coordinates. We then have,

$h_{C_{HV}}^{\prime }=\mathrm{int}\left(\frac{\mathrm{mod}\left(Q+1,4\right)}{2}\right)·N_h,v_{C_{H^{\mathrm{\prime \prime }}{V}^{\mathrm{\prime \prime }}}}^{\prime }=\mathrm{int}\left(\frac{\mathrm{mod}\left(Q,4\right)}{2}\right)·N_v.$

Let the transform from H″, V″ to H′, V′ be ΔH_Q, i.e.

$\left[\begin{array}{c}{h}^{\prime }\\ v^{\prime }\\ 1\end{array}\right]=\Delta H_Q·\left[\begin{array}{c}{h}^{\mathrm{\prime \prime }}\\ v^{\mathrm{\prime \prime }}\\ 1\end{array}\right].$

We will have,

$\Delta H_Q=\left(\begin{array}{ccc}\cos {\theta }_Q& -\sin {\theta }_Q& h_{C_{H^{\mathrm{\prime \prime }}{V}^{\mathrm{\prime \prime }}}}^{\prime }\\ \sin {\theta }_Q& \cos {\theta }_Q& v_{C_{H^{\mathrm{\prime \prime }}{V}^{\mathrm{\prime \prime }}}}^{\prime }\\ 0& 0& 1\end{array}\right).$

Now, ΔH₀ is obtained. ΔH₀ is the transform from X′, Y′ to H″, V″, i.e.

$\left[\begin{array}{c}{h}^{\mathrm{\prime \prime }}\\ v^{\mathrm{\prime \prime }}\\ 1\end{array}\right]=\Delta H_0·\left[\begin{array}{c}{x}^{\prime }\\ y^{\prime }\\ 1\end{array}\right].$

Let O₀ be the offset in H″, V″ coordinate system. We will have,

$O_0=\{\begin{array}{c}O,\mathrm{if}Q=0\\ \mathrm{mod}\left(N_h+O+1,2\right),\mathrm{if}Q=1\\ \mathrm{mod}\left(N_h+N_v+O,2\right),\mathrm{if}Q=2\\ \mathrm{mod}\left(N_v+O+1,2\right),\mathrm{if}Q=3\end{array}.$

Let N_h⁰+1 and N_v⁰+1 be the total number of H and V lines in H″, V″ coordinate system. We will have,

$N_h^0=\{\begin{array}{c}{N}_h,\mathrm{if}Q=0\\ N_v,\mathrm{if}Q=1\\ N_h,\mathrm{if}Q=2\\ N_v,\mathrm{if}Q=3\end{array},N_v^0=\{\begin{array}{c}{N}_v,\mathrm{if}Q=0\\ N_h,\mathrm{if}Q=1\\ N_v,\mathrm{if}Q=2\\ N_h,\mathrm{if}Q=3\end{array}.$

Let the origin of the X′, Y′ coordinate system, C_X′Y′, have the coordinates (h″_CX′Y′,v″_CX′Y′) in the H″, V″ coordinate system:

$h_{C_{X^{\prime }{Y}^{\prime }}}^{\mathrm{\prime \prime }}=-\mathrm{int}\left(\frac{N_v^0+O_0}{2}\right)-\frac{1}{2},v_{C_{X^{\prime }{Y}^{\prime }}}^{\mathrm{\prime \prime }}=\mathrm{int}\left(\frac{N_v^0+O_0}{2}\right)+\frac{1}{2}-O_0.$

Since the rotation from H″, V″ to X′, Y′ is −π/4, and the scale is √{square root over (2)} from the unit of measure in H″, V″ to X′, Y′, we will have,

$\Delta H_0=\left(\begin{array}{ccc}\sqrt{2}\cos -\frac{\pi }{4}& -\sqrt{2}\sin -\frac{\pi }{4}& h_{C_{X^{\prime }{Y}^{\prime }}}^{\mathrm{\prime \prime }}\\ \sqrt{2}\cos -\frac{\pi }{4}& \sqrt{2}\cos -\frac{\pi }{4}& v_{C_{X^{\prime }{Y}^{\prime }}}^{\mathrm{\prime \prime }}\\ 0& 0& 1\end{array}\right)=\left(\begin{array}{ccc}1& 1& h_{C_{X^{\prime }{Y}^{\prime }}}^{\mathrm{\prime \prime }}\\ -1& 1& v_{C_{X^{\prime }{Y}^{\prime }}}^{\mathrm{\prime \prime }}\\ 0& 0& 1\end{array}\right).$

Therefore, the transform from X′, Y′ to H′, V′ is:H_{Symbol→Grid}=ΔH_Z·ΔH₀.

From EIC pattern analysis, H_Grid→Image is obtained, i.e.

$\left[\begin{array}{c}x\\ y\\ 1\end{array}\right]=H_{\mathrm{Grid}\to \mathrm{Image}}·\left[\begin{array}{c}{h}^{\prime }\\ v^{\prime }\\ 1\end{array}\right].$

Therefore, a transform from the coordinate system of the section of EIC symbol array encompassing the image (X′, Y′ coordinate system) to the coordinate system of the image (the X, Y coordinate system), H_{Symbol→Image} can be obtained:

$\left[\begin{array}{c}x\\ y\\ 1\end{array}\right]=H_{\mathrm{Grid}\to \mathrm{Image}}·\left[\begin{array}{c}{h}^{\prime }\\ v^{\prime }\\ 1\end{array}\right]=H_{\mathrm{Grid}\to \mathrm{Image}}·H_{\mathrm{Symbol}\to \mathrm{Grid}}·\left[\begin{array}{c}{x}^{\prime }\\ y^{\prime }\\ 1\end{array}\right],$ i.e.,H_{Symbol→Image}=H_Grid→Image·H_{Symbol→Grid}.

An output of this step is H_{Symbol→Image}, i.e. the updated homography matrix with orientation information 1622 in FIG. 16. As explained above, we can estimate the orientation and offset of actual EIC symbols. Rotated EIC Dots 1614 and updated homography matrix with orientation information 1622 are the two aspects of the estimation. Based on the homography matrix without orientation information obtained in previous step and orientation & offset estimation, 1622 is obtained. Based on the orientation & offset estimation, we can rotate the EIC dots and associate them with EIC symbols, thus based on this result we can recognize the information embedded in each EIC symbol respectively.

Rotated EIC Dots 1614 (i.e., D₀ and Diff₀) are also output of 1612 in FIG. 16. First, we obtain positions of black dots on each edge in H″, V″ coordinate system, based on the positions of black dots in H′, V′ coordinate system. We also obtain the graylevel difference of the darkest and the second darkest positions on each edge. Note that the edges are now named based on coordinates of intersection points in H″, V″ coordinate system.

For each position s on edge (i, j) on the H line in H″, V″ coordinate system, where s=1, 2, . . . , 5, i=0, 1, . . . , N_h⁰−1, j=0, 1, . . . , N_v⁰,

$D_{0,s}^{h,i,j}=\{\begin{array}{c}{D}_s^{h,i,j},\mathrm{if}Q=0\\ D_s^{v,N_v^0-j,i},\mathrm{if}Q=1\\ D_{6-s}^{h,N_h^0-i-1,N_v^0-j},\mathrm{if}Q=2\\ D_{6-s}^{v,j,N_h^0-i-1},\mathrm{if}Q=3\end{array},{\mathrm{Diff}}_0^{h,i,j}=\{\begin{array}{c}{\mathrm{Diff}}^{h,i,j},\mathrm{if}Q=0\\ {\mathrm{Diff}}^{v,N_v^0-j,i},\mathrm{if}Q=1\\ {\mathrm{Diff}}^{h,N_h^0-i-1,N_v^0-j},\mathrm{if}Q=2\\ {\mathrm{Diff}}^{v,j,N_h^0-i-1},\mathrm{if}Q=3\end{array}.$

For each position s on edge (i, j) on the V line in H″, V″ coordinate system, where s=1, 2, . . . , 5, i=0, 1, . . . , N_h⁰, j=0, 1, . . . , N_v⁰−1,

$D_{0,s}^{v,i,j}=\{\begin{array}{c}{D}_s^{v,i,j},\mathrm{if}Q=0\\ D_{6-s}^{h,N_v^0-j-1,i},\mathrm{if}Q=1\\ D_{6-s}^{v,N_h^0-i,N_v^0-j-1},\mathrm{if}Q=2\\ D_s^{h,j,N_h^0-i},\mathrm{if}Q=3\end{array},{\mathrm{Diff}}_0^{v,i,j}=\{\begin{array}{c}{\mathrm{Diff}}^{v,i,j},\mathrm{if}Q=0\\ {\mathrm{Diff}}^{h,N_v^0-j-1,i},\mathrm{if}Q=1\\ {\mathrm{Diff}}^{v,N_h^0-i,N_v^0-j-1},\mathrm{if}Q=2\\ {\mathrm{Diff}}^{h,j,N_h^0-i},\mathrm{if}Q=3\end{array}.$

Recall that 2 bits are encoded on each edge of an EIC symbol. Let B_l^h,i,j and B_l^v,i,j be the two bits, where l=0, 1.

EIC Bit Extraction

Now that it is known which grid cells correspond to EIC symbols and the correct orientation of the symbols, bits can be extracted based on the positions of black dots on each edge of a symbol. The EIC-bit-extraction module 1616 takes as input the rotated EIC dots 1614 and produces EIC bits 1620.

Bit extraction is done in H″, V″ coordinate system, i.e. EIC symbols are oriented at the correct orientation.

For each edge, if there is a black dot detected, and all 5 positions on the edge are valid, bits are extracted. Otherwise, bits are not extracted.

For each edge (i, j) on the H line in H″, V″ coordinate system, where i=0, 1, . . . , N_h⁰−1, j=0, 1, . . . , N_v⁰,

If there exists w and D_0,w^h,i,j=1, where wε{1,2,3,4}, and VD^h,i,j=5, then,

$B_0^{h,i,j}=\mathrm{int}\left(\frac{\mathrm{mod}\left(w,4\right)}{2}\right),B_1^{h,i,j}=\mathrm{int}\left(\frac{w-1}{2}\right),$

else,B₀^h,i,j=B₁^h,i,j=null.

Similarly, for each edge (i, j) on the V line in H″, V″ coordinate system, where i=0, 1, . . . , N_h⁰, j=0, 1, . . . , N_v⁰−1, let q=mod(i+j+O₀,2),

If there exists w and D_0,w+q^v,i,j=1, where wε{1,2,3,4}, and VD^v,i,j=5, then,

$B_0^{v,i,j}=\mathrm{int}\left(\frac{\mathrm{mod}\left(w,4\right)}{2}\right),B_1^{v,i,j}=\mathrm{int}\left(\frac{w-1}{2}\right),$

else,B₀^v,i,j=B_l^v,i,j=null.

The bits extracted are B₁^h,i,j B₀^h,i,j, i.e. if the 1^st position on the edge is a black dot, the bits are 00; if the 2^nd position on the edge is a black dot, the bits are 01; if the 3^rd position on the edge is a black dot, the bits are 11; if the 4^th position on the edge is a black dot, the bits are 10. Note that 00, 01, 11, 10 is a Gray code, which ensures that the number of error bits is at most 1 if the position of black dot is incorrect. See FIG. 20, which assumes H″, V″ is the same as H′, V′ for an illustration.

Recall that a total of 8 bits are encoded in an EIC symbol. Each bit is a bit from an m-array (one dimension). Bits are now obtained from each dimension.

Let B_b^m,n be the bit of dimension b, where b=0, 1, . . . , 7, encoded in EIC symbol (m, n), where (m, n) are the coordinates of the symbol in X′, Y′ coordinate system. Let C_b^m,n be the confidence of bit B_b^m,n (FIG. 32).

Note that B_b^m,n is a matrix in which substantially all the bits encoded in all the EIC symbols in the section of EIC symbol array encompassing the image, are stored. Each element (m, n) in matrix B_b^m,n corresponds to a square (formed by the horizontal and vertical dashed lines in FIG. 32) whose top-left corner has the coordinates of (m, n) in X′, Y′ coordinate system.

For EIC symbols not captured in image, values of the corresponding elements in B_b^m,n will be null. Even if EIC symbols are captured in image, if we are unable to extract the bits encoded in the symbols, values of the corresponding elements in B_b^m,n will also be null. Only when bits are extracted, the corresponding elements in B_b^m,n will have the value of the bits.

We now store all the extracted bits in B_b^m,n, and their confidence values in C_b^m,n.

For each dimension b, where b=0, 1, . . . , 7, initialize B_b^m,n and C_b^m,n as:B_b^m,n=null,C_b^m,n=null.

For each bit l on edge (i, j) on H line, where i=0,1, . . . , N_h⁰−1, j=0,1, . . . , N_v⁰, l=0, 1, find the corresponding b, m and n, and assign values to B_b^m,n and C_b^m,n:

$b=2+l+2·\mathrm{mod}\left(i+j+O_0,2\right),m=\mathrm{int}\left(\mathrm{int}\left(\frac{N_v^0+O_0}{2}\right)+\frac{i-j+O_0}{2}\right),n=\mathrm{int}\left(\frac{i+j+O_0}{2}\right),B_b^{m,n}=B_l^{h,i,j},C_b^{m,n}={\mathrm{Diff}}_0^{h,i,j}.$

For each bit l on edge (i, j) on V line, where i=0,1, . . . , N_h⁰, j=0,1, . . . , N_v⁰−1, l=0, 1, find the corresponding b, m and n, and assign values to B_b^m,n and C_b^m,n:

$b=l+6·\mathrm{mod}\left(i+j+O_0,2\right),m=\mathrm{int}\left(\mathrm{int}\left(\frac{N_v^0+O_0}{2}\right)+\frac{i-j+O_0}{2}\right)-\mathrm{mod}\left(i+j+O_0,2\right),n=\mathrm{int}\left(\frac{i+j+O_0}{2}\right),B_b^{m,n}=B_l^{v,i,j},C_b^{m,n}={\mathrm{Diff}}_0^{v,i,j}.$

We now normalize the confidence values. Let C_max=max(C_b^m,n), where B_b^m,n≠null. The normalized confidence values are:

$C_b^{m,n}=\mathrm{int}\left(100·\frac{C_b^{m,n}}{C_{m\mathrm{ax}}}\right).$

This completes EIC symbol recognition in accordance with embodiments of the invention. Output of EIC symbol recognition is homography matrix H_{Symbol→Image}, which is shown as homography matrix 1624 in FIG. 16, matrix B_b^m,n containing the extracted bits, and matrix C_b^m,n containing the confidence values of the bits. Matrix B_b^m,n and matrix C_b^m,n are shown as EIC bits 1620 in FIG. 16.

As can be appreciated by one skilled in the art, a computer system with an associated computer-readable medium containing instructions for controlling the computer system can be utilized to implement the exemplary embodiments that are disclosed herein. The computer system may include at least one computer such as a microprocessor, digital signal processor, and associated peripheral electronic circuitry.

Although the invention has been defined using the appended claims, these claims are illustrative in that the invention is intended to include the elements and steps described herein in any combination or sub combination. Accordingly, there are any number of alternative combinations for defining the invention, which incorporate one or more elements from the specification, including the description, claims, and drawings, in various combinations or sub combinations. It will be apparent to those skilled in the relevant technology, in light of the present specification, that alternate combinations of aspects of the invention, either alone or in combination with one or more elements or steps defined herein, may be utilized as modifications or alterations of the invention or as part of the invention. It is intended that the written description of the invention contained herein covers all such modifications and alterations.

Claims

1. A method in a computer system for identifying a location of an embedded interaction code (EIC) symbol on a surface, the method comprising using a computer to carry out the steps of: capturing an image of the surface containing a portion of an EIC pattern having a plurality of grid cells formed by a plurality of grid lines, each grid line having a plurality of data dot positions and orientation dot positions;for each of a plurality of grid cells within the captured image, for each grid line portion associated with the grid cell, determining a graylevel for each of a plurality of data dot positions, anddetermining positions of black data dots by comparing the determined graylevels of the data dot positions;for each of a plurality of orientations of the grid cells, determining the number of black data dots at orientation dot positions;determining a correct orientation for the EIC pattern and which grid cells correspond to EIC symbols based on which rotated position of the grid cells contains the fewest number of black dots at orientation dot positions;orienting the EIC pattern based on the determined correct orientation for the EIC pattern;identifying an EIC symbol in the EIC pattern with the corrected orientation, the EIC symbol having an associated grid cell with edges; andextracting bits from at least one edge of the grid cell associated with the identified EIC symbol based on the position of a black data dot on the at least one edgewherein the extracted bits are used to determine the position of the identified EIC symbol relative to the surface.

2. The method of claim 1, wherein the extracted bits are selected based on whether a predetermined number of neighboring dots of the black dot on the at least one edge are valid.

3. The method of claim 1, further comprising: updating a homography matrix based on the determined correct orientation for the EIC pattern, the homography matrix for transforming from the coordinate system of the EIC pattern to the coordinate system of the image,wherein the homography matrix is used in the process of identifying grid lines.

4. The method of claim 1, further comprising: producing confidence values that correspond to the extracted bits.

5. The method of claim 1 wherein determining positions of black data dots includes: identifying the darkest data dot position based on the determined graylevels;identifying the second darkest data dot position based on the determined graylevels; andwhen the difference between the graylevels of the darkest data dot position and the second darkest data dot position exceeds a threshold, designating the darkest data dot position as a black dot.

6. The method of claim 1 wherein each grid line portion associated with each grid cell includes four data dot positions and one orientation dot position.

7. The method of claim 6, further comprising: when a first data dot position is black, the extracted bits are 00,when a second data dot position is black, the extracted bits are 01,when a third data dot position is black, the extracted bits are 11, andwhen a fourth data dot position is black, the extracted bits are 10.

8. A computer-readable storage medium containing instructions for executing a method for identifying a location of an embedded interaction code (EIC) symbol on a surface, the method comprising: capturing an image of the surface containing a portion of an EIC symbol array having a plurality of grid cells formed by a plurality of grid lines, each grid line having a plurality of data dot positions and orientation dot positions;for each of a plurality of grid cells within the captured image, determining a graylevel for each of a plurality of data dot positions located on each grid line portion associated with the grid cell, anddetermining positions of black data dots by comparing the determined graylevels of the data dot positions;for each of a plurality of orientations of the grid cells, determining the number of black data dots at orientation dot positions;determining a correct orientation for the EIC symbol array and which grid cells correspond to EIC symbols based on which rotated position of the grid cells contains the fewest number of black dots at orientation dot positions;orienting the EIC symbol array based on the determined correct orientation for the EIC symbol array;identifying an EIC symbol in the EIC pattern with the corrected orientation, the EIC symbol having an associated grid cell;extracting bits from at least one edge of the grid cell associated with the identified EIC symbol based on the position of a black data dot on the at least one edge; anddetermining the position of the identified EIC symbol relative to the surface based at least in part on the extracted bits.

9. The computer-readable storage medium of claim 8, wherein the extracted bits are selected based on whether a predetermined number of neighboring dots of the black dot on the at least one edge are valid.

10. The computer-readable storage medium of claim 8, further comprising: updating a homography matrix based on the determined correct orientation for the EIC symbol array, the homography matrix for transforming from the coordinate system of the EIC symbol array to the coordinate system of the image,wherein the homography matrix is used in the process of identifying grid lines.

11. The computer-readable storage medium of claim 8, further comprising: producing confidence values that correspond to the extracted bits.

12. The computer-readable storage medium of claim 8 wherein determining positions of black data dots includes: identifying the darkest data dot position based on the determined graylevels;identifying the second, darkest data dot position based on the determined graylevels; andwhen the difference between the graylevels of the darkest data dot position and the second darkest data dot position exceeds a threshold, designating the darkest data dot position as a black dot.

13. The computer-readable storage medium of claim 8 wherein each grid line portion associated with each grid cell includes four data dot positions and one orientation dot position.

14. The computer-readable storage medium of claim 13, further comprising: when a first data dot position is black, the extracted bits are 00,when a second data dot position is black, the extracted bits are 01,when a third data dot position is black, the extracted bits are 11, andwhen a fourth data dot position is black, the extracted bits are 10.

15. A system for identifying a location of an embedded interaction code (EIC) symbol on a surface, the system comprising: a component that captures an image of the surface containing a portion of an EIC symbol array having a plurality of grid cells formed by a plurality of grid lines, each grid line having a plurality of data dot positions and orientation dot positions;a component that, for each of a plurality of grid cells within the captured image, determines a graylevel for each of a plurality of data dot positions located on each grid line portion associated with the grid cell, anddetermines positions of black data dots by comparing the determined graylevels of the data dot positions;a component that, for each of a plurality of orientations of the grid cells, determines the number of black data dots at orientation dot positions;a component that determines a correct orientation for the EIC symbol array and which grid cells correspond to EIC symbols based on which rotated position of the grid cells contains the fewest number of black dots at orientation dot positions;a component that orients the EIC symbol array based on the determined correct orientation for the EIC symbol array;a component that identifies an EIC symbol in the EIC pattern with the corrected orientation, the EIC symbol having an associated grid cell;a component that extracts bits from at least one edge of the grid cell associated with the identified EIC symbol based on the position of a black data dot on the at least one edge; anda component that determines the position of the identified EIC symbol relative to the surface based at least in part on the extracted bits.

16. The system of claim 15, wherein the extracted bits are selected based on whether a predetermined number of neighboring dots of the black dot on the at least one edge are valid.

17. The system of claim 15, further comprising: a component that updates a homography matrix based on the determined correct orientation for the EIC symbol array, the homography matrix for transforming from the coordinate system of the EIC symbol array to the coordinate system of the image,wherein the homography matrix is used in the process of identifying grid lines.

18. The system of claim 15, further comprising: producing confidence values that correspond to the extracted bits.

19. The system of claim 15 wherein the component that determines positions of black data dots includes: a component that identifies the darkest position based on the determined graylevels;a component that identifies the second darkest position based on the determined graylevels; anda component that, when the difference between the graylevels of the darkest position and the second darkest position exceeds a threshold, designates the darkest position as a black dot.

20. The system of claim 15 wherein each grid line portion associated with each grid cell includes four data dot positions and one orientation dot position and wherein the orientation dot position is always white.