Patent Publication Number: US-2016239656-A1

Title: Test for distinguishing between a human and a computer program

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     The present invention relates generally to a test or challenge for distinguishing between a human and a computer program. For example, certain embodiments of the present invention provide a security test for allowing a computer system (e.g. a server) to automatically distinguish between a human user and a computer program (e.g. a “bot”), thereby enabling the computer system to prevent or restrict unauthorised or undesirable activities (e.g. information download or hacking activities) instigated by the computer program. 
     2. Description of the Related Art 
     The ability of a computer system (e.g. a server) to distinguish between a human user and an external computer program is desirable in many situations. For example, some computer programs, referred to as bots, are designed to perform automated tasks, often highly repetitively, over a network (e.g. the Internet). Many bots are created by computer hackers to perform tasks involving unauthorised or undesirable activities. For example, some bots are designed to automatically fetch large volumes of information from a remote web server. This type of activity is often undesirable since it can overload the server, use a large proportion of the available bandwidth, and therefore slow down or prevent other users from accessing information provided by the server. Other bots are designed to perform hacking activities, for example exhaustive password searches in order to gain unauthorised access to user accounts (e.g. email accounts). This type of activity is clearly undesirable from a security point of view. 
     Accordingly, various techniques have been developed for enabling a computer system to automatically distinguish between a human and a computer program. Many of these techniques are based on presenting a test or challenge that is relatively easy for a human to pass, but difficult for an automated computer program to pass. Techniques of this type are sometimes referred to as CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) programs. A computer system may restrict certain activities (e.g. access to data download or the ability to enter a password to log into an account) to human users only by first presenting a CAPTCHA type test, which must be passed before the computer system allows the activity. 
       FIGS. 1 a  and 1 b    illustrate typical CAPTCHA type tests. In these examples, a string of characters are displayed on a screen, and the user is required to correctly enter the displayed characters in a text box using a keyboard in order to pass the test. The effectiveness of these tests depends on the user&#39;s ability to correctly identify the displayed characters, and the inability of an automatic computer program to do the same. In order to achieve this, the displayed characters are typically obfuscated in some way, for example by being distorted and/or overlapped. 
     One problem with existing CAPTCHA type techniques is striking a balance between maintaining acceptable levels of both security and ease of use by a human user. For example, increasing the level of obfuscation applied to the characters reduces the likelihood of an automatic computer program being able to pass the test, and therefore increases security. On the other hand, if the level of obfuscation applied is too high, even a human may find it difficult to correctly identify the characters and pass the test, resulting in user inconvenience. 
     For example, in the test illustrated in  FIG. 1 a   , the level of obfuscation applied to the characters is relatively low. Although this allows a user to easily identify the correct characters, the level of obfuscation may be too low to prevent an automatic computer program from passing the test. On the other hand, in the test illustrated in  FIG. 1 b   , the level of obfuscation applied to the characters is relatively high. Although this level of obfuscation makes it difficult for an automatic computer program to pass the test, a human user may also find it difficult to correctly identify the characters, and may therefore be required to take multiple tests before one is passed. 
     Accordingly, what is desired is a test or challenge for distinguishing between a human and a computer program that maintains acceptable levels of both security and ease of use by a human user. 
     SUMMARY OF THE INVENTION 
     It is an aim of certain exemplary embodiments of the present invention to address, solve and/or mitigate, at least partly, at least one of the problems and/or disadvantages associated with the related art, for example at least one of the problems and/or disadvantages described above. It is an aim of certain exemplary embodiments of the present invention to provide at least one advantage over the related art, for example at least one of the advantages described below. 
     The present invention is defined by the independent claims. Advantageous features are defined by the dependent claims. 
     In accordance with an aspect of the present invention, there is provided a method according to claim  1 ,  34 ,  35  or  43 . 
     In accordance with another aspect of the present invention, there is provided a client device according to claim  32 . 
     In accordance with another aspect of the present invention, there is provided a server according to claim  33 . 
     In accordance with another aspect of the present invention, there is provided a system according to claim  31 . 
     In accordance with another aspect of the present invention, there is provided a computer program comprising instructions arranged, when executed, to implement a method, apparatus and/or system in accordance with any aspect or claim disclosed herein. 
     In accordance with another aspect of the present invention, there is provided a machine-readable storage storing a computer program according to the preceding aspect. 
     Other aspects, advantages, and salient features of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, disclose exemplary embodiments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, and features and advantages of certain exemplary embodiments and aspects of the present invention will be more apparent from the following detailed description, when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1 a    illustrates a first example of a CAPTCHA type test for distinguishing between a human and a computer program; 
         FIG. 1 b    illustrates a second example of a CAPTCHA type test for distinguishing between a human and a computer program; 
         FIG. 2  illustrates a system embodying the present invention; 
         FIG. 3  illustrates an exemplary method for allowing a server to determine whether a request for information and/or a service received from a client device has originated from a human user or a computer program; 
         FIG. 4  illustrates a first exemplary test for distinguishing between a human and a computer program according to an exemplary embodiment of the present invention; 
         FIG. 5  illustrates a second exemplary test for distinguishing between a human and a computer program according to an exemplary embodiment of the present invention; 
         FIG. 6  illustrates an exemplary technique for highlighting selections made by a user; 
         FIG. 7  illustrates an image for a test comprising a graphical symbol “ ”; 
         FIG. 8 a    illustrates a first example of reference coordinates and reference areas for two characters, “A” and “©”; 
         FIG. 8 b    illustrates a second example of reference coordinates and reference areas for two characters, “A” and “©”; 
         FIGS. 9 a - d    illustrate various examples of obfuscation that may be applied to the image used on the test illustrated in  FIG. 5 ; 
         FIGS. 10 a - d    illustrate various examples of rectangular bounding boxes for various certain characters; 
         FIGS. 11 a - b    illustrate various examples of touching points for certain characters; 
         FIGS. 12 a - e    illustrate various examples of character boxes for certain characters; 
         FIGS. 13 a - d    illustrate further examples of character boxes for certain characters; 
         FIGS. 14-17  illustrate an exemplary method for modifying a character box; 
         FIGS. 18 a - c    illustrate an exemplary method for arranging characters in an image; 
         FIGS. 19 a - h    illustrate the various steps in the method of  FIG. 19 ; 
         FIGS. 20 a - b    illustrate examples of an image resulting from the method of  FIG. 18 ; 
         FIG. 21  illustrates an example of a fuzzy area in which the character boxes of two characters overlap; 
         FIGS. 22 a - c    illustrate various examples of a user selection of a character in the image; and 
         FIG. 23  illustrates a case in which the user has selected a point in the fuzzy area of overlap between the bounding boxes of two characters. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The following description of exemplary embodiments of the present invention, with reference to the accompanying drawings, is provided to assist in a comprehensive understanding of the present invention. The description includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope of the present invention, as defined by the claims. 
     The terms, words and phrases used in the following description and claims are not limited to the bibliographical meanings, but, are used to enable a clear and consistent understanding of the present invention. 
     In the description and Figures of this specification, the same or similar features may be designated by the same or similar reference numerals, although they may be illustrated in different drawings. 
     Detailed descriptions of structures, constructions, functions or processes known in the art may be omitted for clarity and conciseness, and to avoid obscuring the subject matter of the present invention. 
     Throughout the description and claims of this specification, the words “comprise”, “include” and “contain” and variations of the words, for example “comprising” and “comprises”, means “including but not limited to”, and is not intended to (and does not) exclude other features, elements, components, integers, steps, processes, operations, characteristics, properties and/or groups thereof. 
     Throughout the description and claims of this specification, the singular forms “a,” “an,” and “the” include plural referents unless the context dictates otherwise. Thus, for example, reference to “an object” includes reference to one or more of such objects. 
     Throughout the description and claims of this specification, language in the general form of “X for Y” (where Y is some action, process, activity, operation or step and X is some means for carrying out that action, process, activity, operation or step) encompasses means X adapted, configured or arranged specifically, but not exclusively, to do Y. 
     Features, elements, components, integers, steps, processes, operations, functions, characteristics, properties and/or groups thereof described in conjunction with a particular aspect, embodiment or example of the present invention are to be understood to be applicable to any other aspect, embodiment or example described herein, unless incompatible therewith. 
     The methods described herein may be implemented in any suitably arranged apparatus or system comprising means for carrying out the method steps. 
       FIG. 2  illustrates a system embodying the present invention. 
     As illustrated in  FIG. 2 , the system  200  comprises a client device  201  and a server  203 . The client device  201  and the server  203  may be connected by a network  205 , for example the Internet or a telecommunications network, allowing signals to be exchanged between the client device  201  and the server  203 . The server  203  may comprise any suitable type of server providing information and/or services which may be accessed over the network  205 . For example, the server  203  may be in the form of a web server providing one or more web pages. The client device  201  may comprise any suitable type of device that may access information and/or services provided by the server  203 . For example, the client device  201  may be in the form of a mobile/portable terminal (e.g. mobile telephone), hand-held device or personal computer (e.g. desktop computer or laptop computer). 
     In the system  200  illustrated in  FIG. 2 , when the client device  201  transmits a request for access to information and/or a service provided by the server  203 , a procedure may be carried out that allows the server  203  to determine whether the request has originated from a human user of the client device  201  or from a computer program (e.g. a bot). 
     An exemplary method  300  is illustrated in  FIG. 3 . In a first step  301 , the client device  201  transmits a request for access to information and/or a service to the server  203  via the network  205 . In a next step  303 , the server  203  generates a test and transmits test information to the client device  201  via the network  205 . In a next step  305 , the client device  201  displays the test based on the received test information and receives input from the user of the client device  201  while the user performs the test. In a next step  307 , the client device  201  transmits test response information, including information based on the user input, to the server  203  via the network  205 . In a next step  309 , the server  203  analyses the test response information received from the client device  201  to determine if the test has been passed. In a next step  311 , if the test response information indicates that the user has passed the test, the server  203  allows the client device  201  to access the information and/or service. 
     In step  305 , the test may require the user to provide multiple individual inputs. In this case, in a variation of steps  305  and  307 , a portion of test response information may be transmitted to the server  203  (e.g. as packet data) each time the user provides an individual input. Alternatively, test response information may be buffered by the client device  201  as the test is conducted, and buffered test response information may be transmitted to the server  203 , for example upon completion of the test. In the case that the server  203  receives test response information in portions as the test is conducted, in a variation of step  309 , the server  203  may analyse portions of test response information as it is received. Alternatively, the server  203  may buffer the received portions of test response information and analyse the buffered test response information, for example upon completion of the test. 
       FIG. 2  illustrates a specific exemplary system embodying the present invention. However, the skilled person will appreciate that the present invention is not limited to this particular arrangement. For example, in alternative embodiments, the client device  201  and the server  203  may communicate without using a network. For example, the client device  201  and server  203  may communicate directly. In another example, the client device  201  may request information from another server, rather than from the server  203 , but the server  203  may determine whether the request has originated from a human or a computer program on the other server&#39;s behalf. 
     In general, the present invention may be implemented in any suitable system comprising a first entity and a second entity, where the first entity performs some activity that another entity wishes to determine whether the activity is a result of a human or a computer program, and where the second entity is used to determine whether the activity is a result of a human or a computer program. 
     In an exemplary embodiment, the client device  201  may comprise: a transmitter for transmitting a request for access to information and/or a service, and for transmitting test response information to the server  203 ; a receiver for receiving test information from the server  203  and for receiving authorisation to access the information and/or service; a display for displaying a test; an input unit for receiving input from the user (e.g. selection of points in an image of the test); a memory for storing various information (e.g. data and software) used and/or generated during operation of the client device  201 ; and a controller for controlling overall operation of the client device  201 . 
     In an exemplary embodiment, the server  203  may comprise: a test generating unit for generating a test; a transmitter for transmitting test information to the client device  201 , and for transmitting a signal indicating whether or not the test has been passed; a receiver for receiving a request to generate a test, and for receiving test response information from the client device  201 ; and a test response analysing unit for analysing the test response information to determine whether or not the test has been passed. 
       FIG. 4  illustrates an exemplary test for distinguishing between a human and a computer program according to an exemplary embodiment of the present invention. For example the test  400  illustrated in  FIG. 4  may be applied in the system  200  illustrated in  FIG. 2  and the method  300  illustrated in  FIG. 3 . 
     In the example of  FIG. 4 , the test  400  comprises a first output in the form of a string  401  of characters (e.g. letters, numbers, and any other suitable types of characters) and/or symbols (e.g. punctuation marks, phonetic symbols, currency symbols, mathematical symbols, icons, graphics, graphical symbols, and any other suitable types of symbols). For example,  FIG. 7  illustrates an image for a test comprising a graphical symbol “ ”. Hereafter, all types of characters and symbols are referred to collectively as “characters” or “graphical entity” for convenience. The test  400  further comprises a second output in the form of an image  403  or “input pad”. The string  401  may be a plaintext string that has relatively little or no obfuscation applied to the characters forming the string  401 . Accordingly, it is easy for a human user (and also a computer program) to correctly identify the characters forming the string  401 . 
     The image  403  comprises an arrangement or configuration of various characters. In particular, the image  403  comprises at least the characters occurring within the string  401 . The image  403  may also comprise one or more additional characters not occurring in the string  401 . In the illustrated example, the arrangement of characters comprises a two-dimensional arrangement of characters. For example, a two-dimensional arrangement may comprise an arrangement in which characters are arranged from left-to-right (or right-to-left) and from top-to-bottom (or bottom-to-top). However, in alternative embodiments, the arrangement of characters may comprise a one-dimensional arrangement of characters. For example, a one-dimensional arrangement may comprise an arrangement in which characters are arranged from left-to-right (or right-to-left), for example in a single row, or alternatively are arranged from top-to-bottom (or bottom-to-top), for example in a single column. Although a one-dimensional arrangement may provide less security than a two-dimensional arrangement, a user may find a one-dimensional arrangement easier or more convenient to use. The image  403  may be any suitable size and/or shape and is not limited to the specific example illustrated in  FIG. 4 . 
     In some embodiments, a user may be given the option to zoom-in and zoom-out of the image. This option may be advantageous in cases where a human user cannot clearly distinguish one or more of the characters in the image  403 . In this case, the user may zoom-in to the image to improve clarity. However, zooming-in would not typically assist a computer program in correctly identifying the characters in the image  403 . 
     The characters forming the image  403  may be arranged in any suitable arrangement or configuration. In the example illustrated in  FIG. 4 , the characters are arranged in a two-dimensional configuration and are arranged roughly in rows. However, in other examples, the characters may be additionally or alternatively arranged roughly in columns, or any other suitable configuration, for example in a spiral pattern, other pattern, randomly, or quasi-randomly in one or two dimensions. 
     At least some of the characters forming the image  403  have at least some level of obfuscation applied to them, for preventing a computer program from being able to correctly identify the characters in the image  403 . Any suitable type of obfuscation may be applied to the characters for this purpose, some example of which will now be described. 
     For example, the obfuscation may be achieved by displaying the characters in a variety of different fonts and/or sizes. 
     The obfuscation may be additionally or alternatively achieved by applying one or more linear or non-linear transformations to a character, or a group thereof. The transformations may comprise, for example, one or more shape-deforming transformations, for example stretching, scaling, tapering, twisting, bending, shearing, warping, and the like. The transformations may additionally or alternatively comprise one or more other types of transformation, for example rotation, reflection, and the like. The skilled person will appreciate that the transformations may additionally or alternatively comprise one or more common or standard transformations. 
     The obfuscation may be additionally or alternatively achieved by applying one or more image processing operations to a character, or a group thereof. The image processing operations may comprise, for example, blurring, shading, patterning, outlining, silhouetting, colouring, and the like. The skilled person will appreciate that the image processing operations may additionally or alternatively comprise one or more common or standard image processing operations. 
     The obfuscation may be additionally or alternatively achieved by overlapping at least some of the characters. For example, a character may be overlapped by N neighbouring characters (where N=1, 2, 3, 4, . . . ). The neighbouring characters of a character may include one or more neighbouring characters in any directions. For example, the neighbouring characters may include any combination of an upper neighbour, a lower neighbour, a left neighbour, a right neighbour, and one or more diagonal neighbours. A character may be overlapped by neighbouring characters in one or two dimensions. 
     The obfuscation may be additionally or alternatively achieved by superimposing another image, pattern, and the like, over the image  403 . For example, a cross-cross pattern of randomly orientated lines may be superimposed over the image  403 . 
       FIGS. 9 a - d    illustrate various examples of obfuscation that may be applied to the image  503  used in the test  500  illustrated in  FIG. 5 . For example,  FIG. 9 a    illustrates an example in which speckling is applied the image.  FIG. 9 b    illustrates an example in which distortion is applied to a middle portion of the image.  FIG. 9 c    illustrates an example in which the edges of the characters are smudged.  FIG. 9 d    illustrates and example in which blurring is applied to the image. 
     In the embodiment illustrated in  FIG. 4 , the image  403  is a static image. However, in alternative embodiments, the image  403  may be a time-varying image, moving image, animated image, and the like. For example, in some embodiments, one or more of the characters may move along any suitable paths, which may be random or non-random. In one example, the characters may float randomly around the image  403 . In another example, the characters may move in straight lines, either bouncing off the edges of the image  403  or disappearing from one side of the image and reappearing in the opposite side of the image. 
     The image  403  may be animated in other ways. For example the size or font of a character, or the transformation or image processing applied to a character, may vary over time. In another example, one or more of the characters may disappear from view for a time (e.g. a random time or predetermined time) and reappear, either in the same position in the image  403  or in a different position. 
     The degree and type of obfuscation applied to the characters forming the image  403  are applied such that a human user is able to correctly identify and locate certain characters in the image  403 , while preventing a computer program from doing the same. The above and/or other methods of obfuscation may be applied in any suitable combination to achieve this goal. 
     The server  203  may generate the test  400  by generating a string  401  comprising a random sequence of characters, and then generating image information (e.g. an image file) defining an image  403 , having a form as described above, comprising the characters occurring within the generated string  401  and optionally one or more additional characters. The server  203  then transmits test information, comprising the generated string  401  and generated image information defining the image  403 , to the client device  201 . The test information allows the client device  201  to reconstruct the test  400  and display the test  400  on a screen of the client device  201  to enable a user to conduct the test  400 . 
     As described further below, the server  203  also stores information allowing the server  203  to determine the position within the image  403  of each character occurring within the string  401 . This allows the server  203  to analyse test response information received back from the client device  201  to determine whether the user of the client device  201  has passed the test. 
     The server  203  may apply any suitable algorithm for generating the string  401 . For example, the string  401  may be generated as a random or quasi-random set of characters. 
     Alternatively, the string  401  may be generated by selecting a word, phrase and/or brand name from a database of words, phrases and/or brand names. The server  203  may apply any suitable algorithm for generating the image  403 . For example, an algorithm may be applied such that the characters forming the image contact and/or overlap in a suitable manner. For example, it is desirable that the characters contact and/or overlap sufficiently to prevent a computer program from correctly identifying the characters, but not so much to prevent a user from doing so. 
     As described above, the client device  201  receives test information from the server  203  and displays the test  400  to the user. For example, the test  400  may be implemented in the form of an applet (e.g. Java applet). In order to conduct the test illustrated in  FIG. 4 , the user identifies each character in the string  401  and selects the corresponding characters in the image  403 . 
     In certain embodiments, the user may be provided with an option of requesting an entirely new test, for example if the user is unable to identify the characters in the image  403  or finds the image  403  confusing. Alternatively, the user may be provided with an option of requesting a new alternative image  403  while the string  401  remains the same. For example,  FIG. 5  illustrates a second example of a test  500  comprising a string  501  that is the same as the string  401  used in the test illustrated in  FIG. 4 , but comprising a different image  503 . 
     In certain embodiments, the user may be required to select the characters in the order that they appear in the string  401 , or in another specified order, in order to pass the test. For example, the characters appearing in the string  401  may be individually and sequentially highlighted in a certain order, and the user may be required to select a character that is currently highlighted. Alternatively, it may be sufficient for the user to select the characters in any order. 
     In certain embodiments, the user may be required to select characters at certain times. For example, an icon or other visual indicator (e.g. a light bulb) displayed to the user may toggle between two states (e.g. on and off). The user may be required to select characters when the visual indicator is in a certain state (e.g. the light bulb is on). 
     The user may select a character in the image  403  using any suitable technique. For example, the user may use an input device, for example a mouse, tracker ball, touch pad, and the like, to move a cursor or pointer over the character and then actuate a button or key to select the character. Alternatively, if the image  403  is displayed on a touch screen, the user may touch the touch screen at the position of the character. 
     In certain embodiments, when the user has made a selection in the image  403 , the selection, or the selected character, may be highlighted in the image  403 , for example as feedback to the user. For example,  FIG. 6  illustrates an exemplary technique for highlighting selections made by a user in an image  603 . As illustrated in  FIG. 6 , the user&#39;s selections are highlighted by displaying a visual indicator  605   a - c  (e.g. a circle in the illustrated example) at the position of each user selection. Optionally, each visual indicator may comprise a number indicating the order in which the selections were made. 
     In certain embodiments, where feedback is provided to the user, the user may be provided with the option to review the selections made and to modify one or more of the selections before submitting the selections for analysis. 
     The client device  201  transmits test response information, comprising information relating to the user&#39;s selections of characters in the image  403 , to the server  203 . For example, the test response information may comprise the coordinates of the user&#39;s individual selections. A portion of test response information may be transmitted to the server each time the user selects a character in the image  403 . Alternatively, the test response information may be buffered by the client device  400  as the test is conducted, and the buffered test response information transmitted to the server  203  following completion of the test  400 . 
     In certain embodiments, the test response information may further comprise information indicating the order of the user&#39;s selections. 
     In certain embodiments, the test response information may further comprise time information indication time points at which the user&#39;s selections were made. The time information may comprise, for example an elapsed time from a predefined reference time (e.g. the time at which animation of the image  403  began). Time information may be required, for example, in embodiments using an image  403  in which the characters move. For example, in order to compare the position of a user&#39;s selection with the position of a character in the image  403  displayed to the user during the test  400 , the server  203  needs to know the positions of the characters in the image  403  at the time the user made the selection. In cases where the characters move, the server uses information indicating the time the user made the selection, together with known motion of the characters in the image  403  to determine the positions of the characters at that time. 
     In cases where the user is allowed to zoom-in and zoom-out of the image  403 , the client device  201  transmits zoom information to the server  203 , either as part of the test response information, or separately. Zooming-in and zooming-out of the image  403  modifies the positions of the characters in the image  403  displayed to the user during the test  400 . The zoom information allows the server  203  to correctly compare the position of a user&#39;s selection with the position of a character in the image  403  that has been zoomed-in or zoomed-out. 
     In order to determine whether the user has passed the test, the server  203  determines whether the user has correctly selected the characters in the image  403  using the test response information received from the client device  401  and the information previously stored when generating the test  400 . For example, the server  203  compares information indicating the coordinates of the user&#39;s selections with information indicating the positions of the characters within the image  403  to determine which characters the user has selected. The server  203  then compares the selected characters with the characters occurring within the string  401 . 
     For example, the information indicating the positions of the characters in the image  403  may comprise reference coordinates and/or a reference area associated with each character in the image  403 . The reference coordinates of a specific character may comprise the position of a centre point of that character in the image  403 . The reference area of a specific character may comprise an area having a certain shape (e.g. rectangle, square or circle) centred on the reference coordinates of that character. Alternatively, the reference area of a specific character may have the same or a similar shape to that character. The reference areas of each character may all have the same fixed size. Alternatively, the reference area of a certain character may have a size proportional to the size of that character. When generating the test  400 , the server  203  stores the reference coordinates and the reference areas of at least those characters occurring within the string  401 . 
       FIG. 8 a    illustrates a first example of first and second reference coordinates  805 ,  807  and first and second reference areas  809 ,  811  for respective first and second characters  801 ,  803 , “A” and “©”. In  FIG. 8 a   , the reference coordinates  805 ,  807  are indicated by crosses and the reference area  809 ,  811  are indicated by dotted boxes. As illustrated in  FIG. 8 a   , in some cases, the reference areas  809 ,  811  of different characters may overlap. In the example of  FIG. 8 a   , the characters  801 ,  803  do not overlap. Also indicated in  FIG. 8 a   , as filled circles  813 ,  815 , are potential selections by a user. 
     In one example, if a selection (e.g. selection  813 ) falls within the reference area of one character only (e.g. reference area  811  of character “@”), then the selection  813  is determined to be a selection of that character (“@”). On the other hand, if a selection (e.g. selection  815 ) falls within the reference areas of two or more characters (e.g. reference areas  809  and  811  of respective characters “A” and “@”), then the selection  815  may be determined to be ambiguous. In this case, to resolve the ambiguity, the character having the closest reference coordinates to the selection  815  may be determined as the selected character (e.g. “A”). 
     In another example, the character having the closest reference coordinates to a selection (e.g. selection  815 ) may be determined directly as the selected character (e.g. “A”), without considering reference areas. 
       FIG. 8 b    illustrates a second example of first and second reference coordinates  825 ,  827  and first and second reference areas  829 ,  831  for respective first and second characters  821 ,  823 , “A” and “©”. In the example illustrated in  FIG. 8 b   , the characters  821 ,  823  overlap, and a selection  833  made by the user falls within the reference areas  829 ,  831  of both characters  821 ,  823  and actually touches both characters  821 ,  821 . The techniques described above in relation to  FIG. 8 a    may be applied equally to the example illustrated in  FIG. 8   b.    
     The skilled person will appreciate that any other suitable technique may be used to determine which character a user has selected, and that the present invention is not limited to the examples described above and illustrated in  FIGS. 8 a    and  8   b.    
     When the user has selected a character in the image  403 , the server  203  determines which character the user has selected by comparing the coordinates of the user&#39;s selection received from the client device  401  with the reference coordinates and the reference areas stored by the server  203 . For example, in certain embodiments, as described above, the character having a reference area into which the coordinates of the user&#39;s selection falls is determined as the character selected by the user. Alternatively (or in the case of ambiguity if a selection falls into two or more reference areas), the character having reference coordinates that are closest to the coordinates of the user&#39;s selection is determined as the character selected by the user. 
     In the case that one or more of the characters move, the reference coordinates and/or reference areas of the moving characters at a particular time may be determined, for example, based on initial reference coordinates and/or reference areas (corresponding to the reference coordinates and/or reference areas at an initial time) together with the known motion of the characters and the known elapsed time since the initial time. 
     When the server  203  has determined which character the user has selected, the server  203  compares the selected character with a corresponding character in the string  401 . The corresponding character refers to a character the user is required to select with the current selection. For example, if the user is required to select characters in the string  201  in a specific order, the corresponding character may be a specific character in the string  201  in that order. If the user is not required to select characters in the string  201  in any particular order, the corresponding character may be any character in the string  201  that has not yet been selected with previous user selections. 
     If the character selected by the user in the image  403  matches the corresponding character in the string  401 , then the server  203  determines that the user has selected the correct character. The above process is repeated for each character in the string  401 , and if the user selects the correct character for each character in the string  401 , then the server  203  determines that the user has passed the test  400 . The server  203  may then transmit a signal to the client device  201  authorizing access by the client device  201  to the information and/or service requested by the client device  201 . 
     In the case that the client device  201  transmits a portion of test response information to the server  203  each time the user selects a character in the image  403 , the server  203  may determine whether the user has correctly selected each character as each portion of test response information is received. Alternatively, the server  203  may buffer the received portions of test response information as they are received from the client device  201  and determine whether the user has correctly selected each character using the buffered information upon completion of the test. 
     Conventional CAPTCHA type tests typically require a user to input characters using a keyboard or keypad. Therefore, either a physical keyboard/keypad must be provided, or a virtual keyboard/keypad must be displayed on a screen. However, many devices, for example a touchscreen-based portable terminal do not typically provide a physical keyboard/keypad. Furthermore, a virtual keyboard/keypad typically occupies a significant portion of the overall screen area of a display, resulting in inconvenience. In contrast, in embodiments of the present invention, the user may conduct a test by directly selecting characters within an image, rather than by typing characters using a physical or virtual keyboard/keypad. This eliminates the need to provide a physical keyboard/keypad or to display a virtual keyboard/keypad, thereby increasing convenience. 
     In addition, since embodiments of the present invention are based on directly selecting characters within an image, rather than by typing characters using a keyboard, this provides an advantage that the test may be easier to perform by a person with dyslexia or other similar condition. 
     Furthermore, by providing a zoom function in certain embodiments of the present invention, the test may be easier to perform by a person with a visual impairment. 
     In the embodiments described above, the first output comprises a string  401 . However, in certain other embodiments, the first output may be provided in any suitable form that indicates a set of characters to a human user. The set of characters may be defined in a particular sequence, or may be unordered. For example, the first output may alternatively be provided in the form of an image, a video or an audio recording. For example, in the case of an audio recording, the user may provide an input (e.g. press a button or select an icon) which causes the playing of an audio recording of a voice that reads out a sequence of one or more characters, or if the sequence of characters is a word or phrase, the voice reads the word or phrase. 
     In certain embodiments of the present invention, the first output may be provided in the form of a logo, brand or advertisement containing a sequence of characters. In this way, the user of the client device  201  is exposed to an advertisement when conducting the test  400 , thereby helping to generally increase the exposure of the logo, brand or advertisement. 
     In other embodiments, the first output may be provided in the form of a sequence of different logos or brands, and the characters forming the image  403  may be replaced with a set of various logos or brands. In this way, multiple brands or logos may be exposed to the user each time a test is conducted. 
     The party wishing to advertise the brands or logos may make a payment to the party managing the server and the test procedure described above, in exchange for increasing exposure to the brand or logo, thereby providing a revenue stream to the party managing the server and test procedure. 
     In addition, any party wishing to use a test or challenge according to the present invention may receive a payment, for example at least a part of the payment made by the party wishing to advertise a brand or logo (e.g. advertising revenue), thereby encouraging the adoption/deployment of embodiments of the present invention. 
     In conventional CAPTCHA-type tests, a displayed “challenge string” is distorted and a user is required to input the characters forming the challenge string into an input text box using a keyboard. In contrast, in certain embodiments of the present invention, a challenge string (e.g. the string  401  illustrated in  FIG. 4 ) may be displayed without any distortion or other type of obfuscation. Furthermore, in certain embodiments of the present invention, rather than using an input text box, an image or “input pad” (e.g. the image  403  illustrated in  FIG. 4 ) is displayed. The user may select points on the input pad (e.g. by “clicking”) to provide input. The input pad comprises one or more characters having at least some obfuscation applied thereto. Accordingly, a computer program cannot easily derive challenge string information from the input pad. 
     In the following, exemplary methods for generating a test for distinguishing between a human and a computer program, and exemplary methods for determining which character has been selected by a user during performance of the test, are described. For example, the test may be in the form of any of the tests described above. 
     As described above, in certain embodiments, the test includes an image comprising a two-dimensional arrangement of various characters. For example, in certain embodiments, the characters may comprise one or more glyphs. Each character may be randomly chosen from a set of characters. In some embodiments, some or all of the characters may have one or more of their characteristics varied. The characteristics may include, for example, one or more of typeface, font size, weight (e.g. bold), slope (e.g. oblique and italic), width and serif. In some embodiments, some or all of the characters may have one or more transformations applied thereto. The transformations may include, for example, one or more of rotation, reflection and a shape-deforming transformation. 
     Once a character has been selected for inclusion in the character array, the characteristics of the character have been determined, and any transformations applied to the character, a “bounding box” of the character may be defined. A bounding box may be defined as an imaginary quadrilateral (e.g. rectangle or square) having the smallest size (e.g. the smallest area) that fully encloses the character. According to this definition, a character will touch the edge of its bounding box at two or more points, which are referred to below as “touching points”.  FIGS. 10 a - d    illustrate various examples of rectangular bounding boxes  1001  for various characters  1003 , “A”, “W”, “T” and “a”.  FIGS. 11 a - b    illustrate examples of touching points  1105  for different orientations of the character “Z”. 
     A bounding box  1001  may be defined such that the sides of the bounding box are aligned with a certain axis, for example the x and y axis of the image comprising the character array. In the case of a square or rectangle, a bounding box  1001  may be defined by the coordinates of two diagonally opposing corners of the bounding box  1001 . For example, the diagonally opposing corners may be the top-left and bottom-right corners (having coordinates (x 1 , y 1 ) and (x 2 , y 2 ), respectively, as illustrated in  FIG. 10 a   ), or the top-right and bottom-left corners (having coordinates (x 2 , y 1 ) and (x 1 , y 2 ), respectively, as illustrated in  FIG. 10 a   ). In this case, the coordinate x 1  is given by the x-coordinate of the point (e.g. pixel) of the character  1003  having the lowest valued x-coordinate. The coordinate x 2  is given by the x-coordinate of the point (e.g. pixel) of the character  1003  having the highest valued x-coordinate. The coordinate y 1  is given by the y-coordinate of the point (e.g. pixel) of the character  1003  having the highest valued y-coordinate. The coordinate y 2  is given by the y-coordinate of the point (e.g. pixel) of the character  1003  having the lowest valued y-coordinate. 
     After a character  1003  has been selected, the characteristics of the character  1003  have been determined, and any transformations applied to the character  1003 , a “character shape” of the character may be defined. A character shape may be defined as a closed shape having minimal perimeter length that completely encloses the character  1003 . The character shape of a character is the shape that an elastic band would form if allowed to contract around the character  1003 . A character shape may be determined by any suitable algorithm. In certain embodiments, a character shape may be approximated by a “character box”, which may be determined in a manner described below. 
     To determine a character box, in a first step, the bounding box  1101  of a character  1103  is determined. In a next step, the touching points  1105   a - d  of the character  1103  (i.e. the points at which the character  1103  touches the bounding box  1101 ) are determined. In a next step, the touching points  1105   a - d  are ordered in a cyclic sequence according to the order in which the touching points  1105   a - d  occur when traversing the perimeter of the bounding box  1101  in a certain direction (e.g. clockwise or ant-clockwise). For example, the touching points  1105   a - d  illustrated in  FIG. 11 a    may be ordered into the sequence { 1105   a ,  1105   b,    1105   c,    1105   d } based on an anti-clockwise traversal of the bounding box  1101  perimeter. In a next step, the character box is defined as a polygon whose edges comprise straight lines formed by connecting consecutive touching points  1105   a - d  in the sequence of touching points  1105   a - d  (including connecting the first and last touching points in the sequence). For example, in the example illustrated in  FIG. 11 a   , the pairs of touching points { 1105   a,    1105   b }, { 1105   b,    1105   c }, { 1105   c,    1105   d } and { 1105   d,    1105   a } are connected by straight lines to form the edges of the character box polygon.  FIGS. 12 a - e    illustrate examples of character boxes  1207  for characters “A”, “T”, “Z”, “m” and “a”. 
     A character shape and a character box  1207  are intended to represent the general shape of a corresponding character  1203 . However, the accuracy with which a character box  1207  determined according to the method described above represents the shape of a corresponding character  1203  may vary. In some cases, a character box  1207  may not represent the shape of a character  1203  sufficiently accurately for some applications, for example in the case of some rotated characters (e.g. some angles for some uppercase letters “C”, “D”, “G”, “Q”, “R”, “U” and “W”).  FIGS. 13 a - d    illustrate character boxes  1307  for characters  1303  “U”, “W”, “C” and “S”. In these examples, it can be seen that a significant portion of each character  1303  falls outside the respective character box  1307 , as indicated by the areas  1309  bounded by dotted lines in  FIGS. 13 a   - d.    
     The size of the area of a character  1303  that falls outside the character&#39;s character box  1307  (referred to below as an “outlying area”  1309 ) may be used to define an accuracy measure for the character box  1307 . For example, the accuracy measure may be based on one or more of the absolute size of the outlying area  1309 , and the size of the outlying area  1309  relative to the total area of the character  1303  (e.g. the size of the outlying area  1309  divided by the total area of the character  1303 ). In some embodiments, a character box  1307  may be regarded as acceptable if the accuracy measure satisfies a certain condition (e.g. the accuracy measure is greater than a threshold value). For example, in some embodiments, based on a certain accuracy measure, the case illustrated in  FIG. 13 a    may be regarded as acceptable, while the cases illustrated in  FIGS. 13 b - d    may be regarded as unacceptable. 
     In cases where the character box  1307  is unacceptable, the character box  1307  may be modified to make the character box  1307  more representative of the shape of the corresponding character  1303 . One exemplary method for modifying the character box  1307  is described in the following with reference to  FIGS. 14-17 . 
     In a first step, the bounding box  1401  of a character  1403  is divided into four equal sized quadrants  1411 , each quadrant  1411  having a width a and height b. Examples of this step are illustrated in  FIGS. 14 a    and  14   b.    
     In a next step, four (e.g. equal sized) squares  1513  (or rectangles) are defined, where the side length of each square  1513  (or the length of the longer side in the case of a rectangle) is less than or equal to the smaller of a and b (i.e. the smaller of the width and height of each quadrant  1411  of the bounding box  1501 ). The squares  1513  are positioned such that each square  1513  is fully enclosed within the bounding box  1501 , and such that a corner of each square  1513  coincides with a respective corner of the bounding box  1501 . Examples of this step are illustrated in  FIGS. 15 a    and  15   b.    
     In a next step, each square  1513  is scanned using a scan-line  1515  that is inclined with respect to the x-axis. The scan-lines  1515   a,    1515   c  for the upper-left and lower-right squares  1513   a,    1513   c  may be inclined by an angle +θ, and the scan-lines  1515   b,    1515   d  for the upper-right and lower-left squares  1513   b,    1513   d  may be inclined by an angle −θ (e.g. θ=45 degrees). The upper-left square  1513   a  is scanned from the upper-left corner to the lower-right corner. The upper-right square  1513   b  is scanned from the upper-right corner to the lower-left corner. The lower-left square  1513   d  is scanned from the lower-left corner to the upper-right corner. The lower-right square  1513   c  is scanned from the lower-right corner to the upper-left corner.  FIG. 15 b    illustrates exemplary scan-lines  1515   a - d . Each square  1513  is scanned until the scan-line  1515  intersects a point of the character  1503  (or possibly a set of points), resulting in four points (one for each square  1513 ). These points (“scan-line points”) and the previously determined touching points  1205  are then combined to form a combined set of points. The modified bounding box  1707  is then defined as a polygon whose edges comprise straight lines formed by sequentially connecting points in the combined set of points (touching points and scan-line points). 
     In the case that the character  1603  is displayed in the form of an array of pixels, the scanning may be achieved by traversing the pixels of a square  1513  in a diagonal zig-zag pattern until arriving at the first pixel forming part of the character  1503 .  FIG. 16  illustrates an exemplary zig-zag pattern for scanning pixel  1513   cs  in the lower-right square. In other embodiments, a zig-zag pattern different from the specific example illustrated in  FIG. 16  may be used, while still generally scanning the squares  1613   a - d  in the same direction (e.g. scanning from the bottom-right corner to the upper-left corner for the bottom-right square  1613   c ). 
       FIGS. 17 a - d    illustrate the modified character boxes  1707  obtained using the method described above for the characters “U”, “W”, “C” and “S”. It can be seen that the modified character boxes  1707  more closely represent the shapes of their respective characters  1703  than the original character boxes  1307  illustrated in  FIGS. 13 a   - d.    
     In the embodiment described above, four squares are used. However, in other embodiments, a different number of squares and/or different shapes may be used. For example, a certain number of squares (or other shapes) may be positioned around a boundary region of the bounding box. Each square (or other shape) may be scanned using a scan line inclined by a suitable amount. The scan-lines may be defined such that each square (or other shape) is scanned in a direction moving from the edge of the bounding box to the interior (e.g. centre) of the bounding box. For example, the inclination of the scan-lines may increase (or decrease), for squares (or other shapes) occurring when traversing the boundary region of the bounding box in a certain direction. For example, in the case that eight squares are positioned around the boundary region of the bounding box, such that three squares are positioned along each side of the bounding box, then the corner squares may use scan-lines as illustrated in  FIG. 15 b   , while the middle squares along each side may use scan-lines inclined either horizontally (for the upper and lower sides) or vertically (for the left and right sides). 
     Next is described a method for generating an image comprising a two-dimensional arrangement of various characters for use in a test. The characters are arranged so that a character connects with one or more of its neighbouring characters. In some embodiments, the connection between neighbouring characters may comprise a certain degree of overlap between one or more characters. However, in the following embodiment, the connection is in the form of touching, but without overlap or with no substantial overlap. In certain embodiments, the characters are arranged so that each character connects with all neighbouring characters in each direction as much as possible. 
     In general, embodiments insert a first character within the image at a certain location, which may be selected randomly or according to a certain pattern. One or more characters may be inserted in this way. To insert a second character in the image, the second character may be initially positioned such that there is no overlap between the second character and a previously inserted character (e.g. the first character). The second character is then slid in a certain direction until the second character touches a previously inserted character (or overlaps a previously inserted character to a desired degree). The direction in which the second character is slid may depend on the particular pattern of characters desired in the final image. The second character may be slid two or more times in different directions order to determine its final position in the image. 
       FIGS. 18 a - c    illustrate one exemplary method for arranging the characters.  FIGS. 19 a - h    illustrate the various steps in the method of  FIG. 19 . 
       FIG. 19 a    illustrates an image into which the characters are to be arranged. In the illustrated example, the image  1901  is provided with a margin  1903  comprising an area that remains empty and a body  1905  comprising an area into which the characters are placed. The margin  1903  may be any suitable size, for example 40 pixels wide. In some embodiments, the margin may be omitted. 
     In the following example, characters are arranged roughly in rows, wherein characters are added sequentially to an existing row, and when a row becomes full, a next row is created, until the image becomes full.  FIG. 18 a    illustrates the part of the method for creating and filling a first row,  FIG. 18 b    illustrates the part of the method for creating a next row, and  FIGS. 18 c  and 18 d    illustrate the part of the method for filling a next row. 
     In a first step  1801 , a character (referred to below as a first character) is placed at a random position within the body to create a first row. For example, as illustrated in  FIG. 18 a   , the character may be placed close to one of the corners of the body. The position of the character within the image may be defined in any suitable way, for example by the central point of the bounding box of the character, or one of the corners of the bounding box. The position of the first character may be denoted by coordinates (x, y), where x and y may be randomly selected. 
     In a next step  1803 , a next character (referred to below as a second character) is initially placed at a position (x max , y+Δ), where x max  denotes a maximum x-coordinate and denotes a random variation in the y-direction. The value Δ, which is generally different for each character, may be generated according to any suitable random distribution, for example a uniform distribution between a minimum value (e.g. −M) and a maximum value (e.g. +M), or a Gaussian distribution having a mean μ (e.g. μ=0) and standard deviation σ. Accordingly, the second character is initially placed at the right-most portion of the image at approximately the same vertical position as the first character but with a random variation in the vertical position. In an alternative embodiments, Δ=0 such that there is no variation in the vertical position of the characters in a row. The second character is then slid leftwards, as indicated by the arrow in  FIG. 19 b   , until the second character touches any previously arranged character (i.e. the first character) at at least one point. The second character may be slid so far as to only touch the first character, with substantially no overlap between the characters. Alternatively, a certain degree of overlap may be allowed between the characters. 
     In a next step  1805 , it is determined whether the second character is lying entirely within the body. If the second character is lying entirely within the body then the second character is regarded as having been successfully added to the current row (as illustrated in  FIG. 19 c   ), and steps  1803  and  1805  are repeated for the next character (as illustrated in  FIG. 19 d   ). On the other hand, if the second character is not lying entirely within the body, for example because there is insufficient space on the right-hand side of the first character, then a similar process is attempted to add the second character to the current row on the left-hand side of the first character, and the method proceeds to step  1807 . 
     In step  1807 , the second character is initially placed at a position (x min , +Δ), where x min  denotes a minimum x-coordinate, and the second character is slid rightwards until the second character touches any previously arranged character (i.e. the first character). In a next step  1809 , it is determined whether the second character is lying entirely within the body. If the second character is lying entirely within the body, then the second character is regarded as having been successfully added to the current row, and steps  1807  and  1809  are repeated for the next character. 
     If the second character is not lying entirely within the body, for example because there is insufficient space on the left-hand side of the first character, this indicates that the current row of characters is full and a next row should be created, in which case, the method proceeds to step  1811 . 
     In step  1811 , a next character (referred to below as a third character) is arranged at a position (x, y max ), where x may be randomly selected and y max  denotes a maximum y-coordinate. The third character is then slid downwards, as indicated by the arrow in  FIG. 19 e   , until the third character touches any previously arranged character (i.e. the characters in the previous row) at at least one point. 
     In a next step  1813 , it is determined whether the third character is lying entirely within the body. If the third character is not lying entirely within the body, this indicates that there is insufficient space for a new row above the previous row. In this case, the method proceeds to step  1815 , wherein creation of a row below the previous row is attempted. 
     In step  1815 , the third character is arranged at a position (x, y min ), where y min  denotes a minimum y-coordinate. The third character is then slid upwards until the third character touches any previously arranged character (i.e. the characters in the previous row) at at least one point. 
     In a next step  1817 , it is determined whether the third character is lying entirely within entirely within the body. If the third character is not lying entirely within the body, this indicates that there is insufficient space for a new row below the previous row. In this case, it is not possible to add any more rows to the image and the method ends. An example of an image resulting from the method of  FIG. 18  is illustrated in  FIG. 20 a   . Another example of an image resulting from the method of claim  18 , in which distortion has been applied to the characters, is illustrated in  FIG. 20   b,    
     If, in either of steps  1813  or  1817 , it is determined that the third character is lying entirely within the body then a new row containing the third character is regarded as having been successfully created, either above the previous row (as illustrated in  FIG. 19 f   ) or below the previous row. The position of the third character may be denoted (x,y). In this case, the method proceeds to either step  1819  (from step  1813 ) or step  1827  (from step  1817 ), wherein characters are added to the new row. 
     In step  1819 , a next character (referred to below as a fourth character) is arranged at a position (x+δ, y max ), where δ denotes a certain displacement in the x-coordinate that is set to be larger than the size of the largest character. As illustrated in  FIG. 19 f   , the fourth character is then slid downwards until it touches a previously arranged character and then slid leftwards until it touches a previously arranged character. 
     In a next step  1821 , it is determined whether the fourth character is lying entirely within the body. If the fourth character is lying entirely within the body, the fourth character is regarded as having been successfully added to the current row, as illustrated in  FIG. 19 g   , and steps  1819  and  1821  are repeated for the next character in the current row. 
     On the other hand, if the fourth character is not lying entirely within the body, this indicates that there is insufficient space for the fourth character on the right-hand side of the current row. In this case, the method proceeds to step  1823  wherein it is attempted to add the fourth character to the left-hand side of the current row. 
     In step  1823 , the fourth character is arranged at a position (x−δ, y max ). The fourth character is then slid downwards until it touches a previously arranged character and then slid rightwards until it touches a previously arranged character. 
     In a next step  1825 , it is determined whether the fourth character is lying entirely within the body. If the fourth character is lying entirely within the body, the fourth character is regarded as having been successfully added to the current row and steps  1823  and  18225  are repeated for the next character in the current row. On the other hand, if the fourth character is not lying entirely within the body, this indicates that the current row of characters is full and a next row should be created, in which case, the method proceeds to step  1811 , wherein creation of a new row above the current row is attempted. 
     Steps  1827  to  1831  illustrated in  FIG. 18 d    are similar to steps  1819  to  1825  illustrated in  FIG. 18 c   , except that the fourth character is slid downwards instead of upwards. In addition, in step  1831 , if the fourth character is not lying entirely within the body, the method proceeds to step  1815 , wherein creation of a new row below the current row is attempted. Accordingly, steps  1827  to  1831  will not be described in detail. 
     In the above example, the characters are arranged roughly in rows. However, in other embodiments, the characters may be arranged differently, for example in columns or inclined rows or columns. Furthermore, in the above example, new characters are first added to the right of an existing row, then to the left, while new rows are first created above existing rows, then below. However, in alternative embodiments, this ordering may be modified. 
     The present invention encompasses many different ways in which characters may be added to the image. For example, in one embodiment in which characters are arranged roughly in a spiral pattern, a first character may be placed at a certain position in the image (e.g. at the centre of the image). A second character may be slid along a spiral pathway emanating from the first character towards the first character until the second character touches the first character. A process of sliding characters towards previously positioned characters along the spiral pathway may be repeated until no more characters can be added. 
     In another example in which characters are arranged randomly, one or more characters may be positioned at random (non-overlapping) positions within the image. Then a new character may be placed at a random initial position on the boundary of the image and then slid into the image in a random direction (e.g. horizontally or vertically selected at random) until the new character touches a previously inserted character (in which case the new character is regarded as having been successfully inserted), or the new character reaches a boundary of the image (in which case the new character is not inserted). A process of sliding new characters in this way may be repeated until no more characters can be added. 
     It will be appreciated that the present invention is not limited to the above examples, and may include any embodiments in which one or more characters are placed at certain positions, and further characters are added by sliding a new characters until the new character touches (or overlaps) a previously inserted character. 
     As described above, when a user performs a test, the user is required to select characters in the image, for example by clicking a point in the image with a mouse. However, in many cases, there may be some ambiguity as to which character the user intended to select. 
     In some embodiments, a user may be regarded as selecting a certain character if the user selects a point (pixel) in the image contained within that character&#39;s bounding box. In other embodiments, a user may be regarded as selecting a certain character if the user selects a point in the image contained within that character&#39;s character box (or character shape). 
     However, in many cases, the bounding boxes, character boxes and/or character shapes of different characters in the image overlap (creating “fuzzy areas”). An example of a fuzzy area  2119  in which the character boxes of two characters “C” and “T” overlap is illustrated in  FIG. 21 . In the case that the user selects a point (pixel) contained within more than one character&#39;s bounding box, character box or character shape (i.e. the user selects a point in a fuzzy area), an ambiguity arises as to which character the user intended to select. 
     In some embodiments, it may be preferable to determine which character a user has selected based on character boxes (or character shapes) rather than bounding boxes. For example, a bounding box does not generally represent the shape of its character very well. consequently, in many cases, a bounding box contains redundant areas (e.g. at its corners) that are outside the character&#39;s outline, which may result in a relatively high number of mistakes or inaccuracies in determining which character a user intended to select. 
     For example,  FIG. 22 a    illustrates a case that a user has selected a point  2217  that may be outside the acceptable boundary of the character “C”, but would be deemed by the system to be a correct selection of “C” since the point is located in the bounding box of “C”.  FIG. 22 b    illustrates a case that a user has selected a point  2217  that lies within the bounding box of “T” but not the bounding box of “C”, even though the selected point is closer to “C” than “T”. Therefore, the system would determine that the user intended to select “T” even though the user may have intended to select “C”.  FIG. 22 c    illustrates a case that a user has selected a point  2217  that lies within the bounding boxes of both “C” and “T”. Therefore, the system would determine that the user intended to select one of “C” and “T”. However, since the selected point lies relatively far from both “C” and “T”, it may not be acceptable for this user selection to represent either “C” or “T”. 
       FIG. 23  illustrates a case in which the user has selected a point  2317  in the fuzzy area  2319  of overlap between the bounding boxes of characters “T” and “A”. Since the user selected a point that lies within the outline of “T” it is likely that the user intended to select “T”. The user may not realise that the selected point lies within a fuzzy area. However, the system cannot resolve the ambiguity based on the bounding boxes alone since the point falls within two bounding boxes. This may lead to incorrect interpretation of the user selection. For example, if the system were to select the character having a boundary box whose centre is closest to the selected point, then the system would select “A” rather than “T”, even though the user selected a point inside the outline of “T”. 
     Accordingly, in certain embodiments, the character box (or character shape), rather than the bounding box, may be used to determine which character the user intended to select. A character box or character shape typically represents the shape of a character more closely than a bounding box, and therefore use of a character box or character shape is more likely to reflect a user&#39;s intended selection than using a bounding box. Using a character box may alleviate many of the problems arising from ambiguous user selection, for example the cases illustrated in  FIGS. 22 and 23 . 
     It will be appreciated that embodiments of the present invention can be realized in the form of hardware, software or a combination of hardware and software. Any such software may be stored in the form of volatile or non-volatile storage, for example a storage device, ROM, whether erasable or rewritable or not, or in the form of memory such as, for example, RAM, memory chips, device or integrated circuits or on an optically or magnetically readable medium such as, for example, a CD, DVD, magnetic disk or magnetic tape or the like. 
     It will be appreciated that the storage devices and storage media are embodiments of machine-readable storage that are suitable for storing a program or programs comprising instructions that, when executed, implement embodiments of the present invention. Accordingly, embodiments provide a program comprising code for implementing apparatus or a method as claimed in any one of the claims of this specification and a machine-readable storage storing such a program. Still further, such programs may be conveyed electronically via any medium such as a communication signal carried over a wired or wireless connection and embodiments suitably encompass the same. 
     While the invention has been shown and described with reference to certain embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the scope of the invention as defined by the appended claims.