CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims the benefit of the earlier filing date of U.S. Provisional Patent Application Ser. No. 61/498,805 filed on Jun. 20, 2011, the entire contents of which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Disclosure 
         [0003]    The present disclosure relates to an electronic terminal including a display unit, an input correction method performed in the electronic terminal, and a program for achieving the input correction method. 
         [0004]    2. Description of Related Art 
         [0005]    Various processing operations of electronically reading a picture or a character handwritten on paper or the like to obtain image data or character data have been performed. 
         [0006]    In this case, a method of preparing an input pen designed for handwriting detection as a writing implement and causing a receiver near the input pen to detect handwriting obtained by the movement of the input pen on paper is actually used. 
         [0007]    For example, Japanese Unexamined Patent Application Publication No. 2006-309474 describes an example in which an input pen is configured to output an infrared signal and an ultrasonic signal from a leading end thereof at the same time and a receiver disposed near the input pen receives the infrared signal and the ultrasonic signal and detects handwriting. In this case, the receiver performs processing for detecting the position of the leading end of the pen on the basis of the difference between a time at which the infrared signal is received and a time at which the ultrasonic signal is received. By repeatedly performing this position detection processing at short intervals, the change in the position of the leading end of the pen is detected and handwriting is detected. 
         [0008]    Any handwriting detection receiver having a function of receiving an infrared signal and an ultrasonic signal may be used. For example, when a radiotelephone terminal called smartphone has a function of receiving an infrared signal and an ultrasonic signal, the radiotelephone terminal can have a handwriting detection function. 
       SUMMARY 
       [0009]    In the above-described detection of handwriting input by an input pen which is performed using an infrared signal and an ultrasonic signal, it is necessary to reduce the change in positional relationship between paper on which writing is performed and a receiver. That is, in processing for detecting handwriting input by an input pen using an infrared signal and an ultrasonic signal, when a receiver is moved, a relative position to be detected is changed and a handwriting detection condition is disturbed. This is caused because relative positions of the input pen and the receiver are detected in the handwriting detection processing. 
         [0010]    When the handwriting detection condition is disturbed by the change in the position of the receiver, it is difficult to accurately detect handwriting. Detected handwriting differs from a picture or a character actually written on paper with an input pen. 
         [0011]    It is an object of the present disclosure to appropriately detect handwriting even when the relative positions of a writing implement for transmitting a signal and a receiver for receiving the signal transmitted from the writing implement at a position near the writing implement are changed. 
         [0012]    According to a first embodiment, the disclosure is directed to an information processing apparatus that includes a first sensor that receives a first type of signal; second and third sensors that receive a second type of signal; a control unit that calculates a position of a source from which a signal of the first type and a signal of the second type are transmitted based on a first time at which the signal of the first type is received at the first sensor, a second time at which the signal of the second type is received at the second sensor and a third time at which the signal of the second type is received at the third sensor; and a display that displays an image corresponding to a path obtained based on changes in position of the source calculated by the control unit. 
         [0013]    According to another exemplary embodiment, the disclosure is directed to a method performed by an information processing apparatus. The method including receiving a first type of signal transmitted from a source at a first sensor at a first time; receiving a second type of signal transmitted from a source at a second sensor at a second time; receiving the second type of signal transmitted from the source at a third sensor at a third time; calculating a position of the source based on the first time, the second time, and the third time; and displaying an image corresponding to a path obtained based on changes in position of the source calculated by the control unit. 
         [0014]    According to another exemplary embodiment, the disclosure is directed to a non-transitory computer-readable medium including computer program instructions, which when executed by an information processing apparatus, causes the information processing apparatus to perform a method. The method including receiving a first type of signal transmitted from a source at a first sensor at a first time; receiving a second type of signal transmitted from a source at a second sensor at a second time; receiving the second type of signal transmitted from the source at a third sensor at a third time; calculating a position of the source based on the first time, the second time, and the third time; and displaying an image corresponding to a path obtained based on changes in position of the source calculated by the control unit. 
         [0015]    According to the present disclosure, even when an apparatus for receiving a signal of a first type and a signal of a second type is moved, it is possible to continuously perform correct handwriting display without disturbing the detected position of a writing implement since correction processing is performed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]      FIG. 1  is a perspective view illustrating an example of a system according to an embodiment of the present disclosure. 
           [0017]      FIG. 2  is a block diagram illustrating an exemplary configuration of an electronic terminal according to an embodiment of the present disclosure. 
           [0018]      FIG. 3  is a block diagram illustrating an exemplary configuration of a writing implement according to an embodiment of the present disclosure. 
           [0019]      FIG. 4  is a diagram describing an example of a usage pattern according to an embodiment of the present disclosure. 
           [0020]      FIG. 5  is a flowchart illustrating an example of a correction process according to an embodiment of the present disclosure. 
           [0021]      FIG. 6  is a diagram describing a display example of touch points at the time of correction according to an embodiment of the present disclosure. 
           [0022]      FIG. 7  includes diagrams describing an exemplary case at the time of start of a correction process according to an embodiment of the present disclosure. 
           [0023]      FIG. 8  includes diagrams describing an exemplary case at the time of a correction process according to an embodiment of the present disclosure. 
           [0024]      FIG. 9  is a flowchart illustrating an example of a distance measurement process according to an embodiment of the present disclosure. 
           [0025]      FIG. 10  is a diagram describing a display example of a distance according to an embodiment of the present disclosure. 
           [0026]      FIG. 11  is a perspective view illustrating an exemplary configuration of an electronic terminal according to another embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    An exemplary embodiment of the present disclosure will be described in the following order. 
       1. Exemplary Entire Configuration of System (FIG. 1) 
     2. Exemplary Configuration of Electronic Terminal (FIG. 2) 
     3. Exemplary Configuration of Writing Implement (FIG. 3) 
     4. Example of Usage Pattern (FIG. 4) 
     5. Description of Correction Process (FIGS. 5 to 8) 
     6. Description of Distance Display Process (FIGS. 9 and 10) 
     7. Modification (FIG. 11) 
     1. Exemplary Entire Configuration of System 
       [0028]      FIG. 1  is a diagram illustrating an example of a system according to an embodiment of the present disclosure (hereinafter referred to as “this example”). 
         [0029]    In this example, as illustrated in  FIG. 1 , an electronic terminal  100  and an input pen  200  are prepared. The electronic terminal  100  is a radiotelephone terminal called smartphone whose functionality can be increased by installing various pieces of application software into the radiotelephone terminal. On the surface of a casing  101  of the electronic terminal  100 , a display unit  102  that is a relatively large display panel is disposed. As the display unit  102 , a display device such as a liquid crystal display panel or an organic electroluminescence (EL) display panel is used. 
         [0030]    On the surface of the electronic terminal  100 , operation keys included in an operation unit  119 , a call speaker  121 , and a microphone  122  are further disposed. 
         [0031]    On a side surface  101   a  of the casing  101  of the electronic terminal  100 , three sensors  111 ,  112 , and  113  are aligned in a substantially linear array. That is, at the substantially center of the side surface  101   a , the infrared sensor  111  for receiving an infrared signal is disposed. The first ultrasonic sensor  112  is disposed at a position apart from the infrared sensor  111  by a predetermined distance on the left side of the infrared sensor  111 . The second ultrasonic sensor  113  is disposed at a position apart from the infrared sensor  111  by a predetermined distance on the right side of the infrared sensor  111 . The distance between the infrared sensor  111  and each of the ultrasonic sensors  112  and  113  is, for example, several centimeters. 
         [0032]    The input pen  200  is a writing implement such as a ballpoint pen used for writing on paper near the electronic terminal  100 . A pen tip switch  202  is disposed at a leading end  201  of the input pen  200 . When the leading end of the pen is brought into contact with paper or the like, the pen tip switch  202  is turned on. Inside the leading end  201  of the input pen  200 , an infrared transmission unit  211  and an ultrasonic oscillation unit  212  are disposed. During the pen tip switch  202  is in an ON state, the infrared transmission unit  211  periodically transmits an infrared signal and the ultrasonic oscillation unit  212  periodically oscillates an ultrasonic signal. The transmission of an infrared signal and the oscillation of an ultrasonic signal are performed at the same time. 
         [0033]    There is a mode in which the measurement of the position of the leading end of the pen is continued by continuously transmitting an infrared signal and an ultrasonic signal regardless of whether the pen tip switch is in the ON or OFF state. As a method of notifying a body of information about the ON or OFF state of the pen tip switch, a method of changing the frequencies of an infrared signal and an ultrasonic signal may be used. 
         [0034]    As illustrated in  FIG. 1 , an infrared signal IR 1  transmitted from the input pen  200  is detected by the infrared sensor  111  in the electronic terminal  100 . An ultrasonic signal SW 1  oscillated by the input pen  200  is detected by the first ultrasonic sensor  112  and the second ultrasonic sensor  113  in the electronic terminal  100 . 
         [0035]    The infrared signal IR 1  and the ultrasonic signal SW 1  are output from the input pen  200  at the same time. However, since speeds at which these signals are transmitted in space differ from each other, the ultrasonic signal SW 1  is detected by the two ultrasonic sensors  112  and  113  after the infrared signal IR 1  has been detected by the infrared sensor  111 . On the basis of the detection time difference between the infrared signal IR 1  and the ultrasonic signal SW 1 , the distance between the input pen  200  and the side surface  101   a  of the electronic terminal  100  is determined. 
         [0036]    In accordance with the position of the input pen  200 , the time at which the ultrasonic signal SW 1  is detected by the first ultrasonic sensor  112  and the time at which the ultrasonic signal SW 1  is detected by the second ultrasonic sensor  113  are changed. When the ultrasonic signal SW 1  is detected by the two ultrasonic sensors  112  and  113  at the same time, the two ultrasonic sensors  112  and  113  are at the same distance from the input pen  200 . On the other hand, when the ultrasonic signal SW 1  is detected by the two ultrasonic sensors  112  and  113  at different times, there is a difference in the distance from the input pen  200  between the two ultrasonic sensors  112  and  113  which is changed in accordance with the time difference. 
         [0037]    Accordingly, the position of the leading end of the input pen  200  as viewed from the electronic terminal  100  is calculated on the basis of the difference between a time at which the infrared signal IR 1  is detected and a time at which the ultrasonic signal SW 1  is detected and the difference between times at which the ultrasonic signal SW 1  is detected by the two ultrasonic sensors  112  and  113 . 
         [0038]    By detecting the infrared signal IR 1  and the ultrasonic signal SW 1  output from the leading end  201  of the input pen  200  at periodic intervals (e.g. several tens of times per second), the position of the leading end of the pen is detected at short intervals. By connecting the positions that have been detected at short intervals and generating a continuous line, handwriting recorded on paper with the input pen  200  is detected. The detected handwriting is displayed on the display unit  102  and is stored in the electronic terminal  100  as image data. 
       2. Exemplary Configuration of Electronic Terminal 
       [0039]    Next, an exemplary configuration of the electronic terminal  100  will be described with reference to a block diagram in  FIG. 2 . 
         [0040]    The electronic terminal  100  includes a communication processing unit  115  for performing radio processing as a radiotelephone terminal. An antenna  116  is connected to the communication processing unit  115 , and the communication processing unit  115  performs radio communication with a radiotelephone base station via the antenna  116 . The communication processing unit  115  performs speech communication and data communication under the control of the control unit  117 . For example, at the time of speech communication, transmission data and receiving data are transmitted between the communication processing unit  115  and the speech processing unit  120 , and communication is performed using the speaker  121  and the microphone  122  connected to the speech processing unit  120 . At the time of data communication, a memory  118  stores data received by the communication processing unit  115 , and an image and text are displayed on the display unit  102  on the basis of display data. Transmission data stored in the memory  118  is wirelessly transmitted from the communication processing unit  115 . 
         [0041]    As illustrated in  FIG. 1 , the electronic terminal  100  includes the infrared sensor  111 , the first ultrasonic sensor  112 , and the second ultrasonic sensor  113 , and supplies data detected by each of these sensors to the control unit  117  via an internal bus line. The infrared sensor  111  is a sensor functioning as a receiving unit for receiving an infrared signal transmitted from the input pen  200 . The first ultrasonic sensor  112  and the second ultrasonic sensor  113  are sensors functioning as receiving units for receiving an ultrasonic signal transmitted from the input pen  200 . 
         [0042]    The control unit  117  performs processing for calculating the position of the input pen  200  on the basis of pieces of data of detection performed by the sensors  111 ,  112 , and  113 . When the control unit  117  calculates the position of the input pen  200 , data of handwriting obtained by the change in the calculated position is stored in the memory  118  and is displayed on the display unit  102 . This storage processing and this display processing are performed by causing the control unit  117  to execute a program (software) stored in the memory  118 . 
         [0043]    The infrared sensor  111  and the ultrasonic sensors  112  and  113  may be used in an application other than the detection of the position of the input pen  200 . For example, using the infrared sensor  111 , the electronic terminal  100  may perform data communication with another terminal near the electronic terminal  100 . 
         [0044]    The electronic terminal  100  further includes an acceleration sensor  114  that is an acceleration detection unit for detecting acceleration applied to the electronic terminal  100 , and supplies data of detection performed by the acceleration sensor  114  to the control unit  117 . The control unit  117  detects the movement of the electronic terminal  100  on the basis of the supplied data of acceleration detection. 
         [0045]    The operation unit  119  included in the electronic terminal  100  sets an operation mode performed by the control unit  117 . For example, the operation unit  119  can set a mode in which storage and display of handwriting input with the input pen  200  are performed. Furthermore, the operation unit  119  can set a reset mode in which a terminal position is reset. This reset mode will be described later. 
         [0046]    A display panel with a touch panel may be used as the display unit  102  included in the electronic terminal  100 , and the operation unit  119  may output an operation instruction when a touch operation performed on the display panel is detected. 
       3. Exemplary Configuration of Writing Implement 
       [0047]    Next, the configuration of the input pen  200  that is a writing implement with which a user performs writing by hand will be described with reference to  FIG. 3 . 
         [0048]    The input pen  200  includes the pen tip switch  202  that is turned on when a writing operation is performed on paper or the like. When the control unit  213  detects that the pen tip switch  202  has been turned on, the transmission of an infrared signal from the infrared transmission unit  211  and the oscillation of an ultrasonic signal from the ultrasonic oscillation unit  212  are periodically performed in conjunction with each other. When the pen tip switch  202  is turned off, the control unit  213  terminates the transmission of an infrared signal from the infrared transmission unit  211  and the oscillation of an ultrasonic signal from the ultrasonic oscillation unit  212 . 
         [0049]    Although not illustrated in  FIG. 3 , the input pen  200  further has a function of a typical writing implement such as a ballpoint pen. 
       4. Example of Usage Pattern 
       [0050]      FIG. 4  illustrates an example of a usage pattern of the input pen  200  and the electronic terminal  100  when a handwriting input is performed with the input pen  200  and the electronic terminal  100 . 
         [0051]    In the example illustrated in  FIG. 4 , a user draws a certain picture (figure) on a notebook  300  with the input pen  200  held by a hand H. 
         [0052]    On the upper side of the notebook  300 , the electronic terminal  100  is disposed. At that time, the electronic terminal  100  is placed so that the side surface  101   a  on which the sensors  111 ,  112 , and  113  are disposed faces the notebook  300 . 
         [0053]    When a handwriting display mode is set in the electronic terminal  100  in the state illustrated in  FIG. 4 , the electronic terminal  100  detects handwriting input with the input pen  200 . Accordingly, the electronic terminal  100  detects handwriting of a picture  301  that a user has handwritten on the notebook  300  with the input pen  200 , and an image  102   a  that is the same as the picture  301  is displayed on the display unit  102 . In the example illustrated in  FIG. 4 , the image  102   a  displayed on the display unit  102  is a reduced image of the picture  301  that has been handwritten. 
         [0054]    When the electronic terminal  100  according to this example is moved while performing handwriting detection illustrated in  FIG. 4 , the electronic terminal  100  performs processing for correcting the position thereof relative to the input pen. When the control unit  117  determines that acceleration larger than a predetermined threshold value has been detected on the basis of data of detection performed by the acceleration sensor  114  ( FIG. 2 ) included in the electronic terminal  100 , a program for position correction processing is started. When the operation unit  119  included in the electronic terminal  100  sets the reset mode, the program for position correction processing can also be started. 
       5. Description of Correction Process 
       [0055]      FIG. 5  is a flowchart illustrating a position correction process performed under the control of the control unit  117 . 
         [0056]    First, the control unit  117  performs processing for calculating the position of the input pen  200  in the handwriting display mode. In a state in which the storage and display of handwriting input at the detected position are performed (step S 11 ), acceleration detection processing is performed on the basis of the output of the acceleration sensor  114  and it is determined whether acceleration larger than a threshold value has been detected (step S 12 ). When acceleration larger than the threshold value is not been detected, it is determined whether an operation of setting a reset mode has been performed (step S 13 ). When the operation of setting the reset mode is not performed, the process returns to step S 11  in which the storage and display of handwriting is continuously performed. It is assumed that the storage and display of handwriting in step S 11  is performed in an exemplary state illustrated in  FIG. 4  in which writing is performed on the notebook  300  with the input pen  200 . 
         [0057]    When it is determined in step S 12  that acceleration larger than the threshold value has been detected and it is determined in step S 13  that the operation of setting the reset mode has been performed, the process proceeds to step S 14  in which the detection and storage of writing information are terminated and the reset mode is started. 
         [0058]    When the reset mode is started, the control unit  117  determines a first singular point (hereinafter referred to as a point A) on a current writing image displayed in step S 11  and displays an image on which the point A is identifiable on the display unit  102  (step S 15 ). At that time, for example, it is desired that a mark indicating the point A and a character or sign encouraging a user to touch the point A with the input pen  200  be displayed on the image. 
         [0059]    When a user sees the displayed image, the user checks the point A on the displayed image, searches the picture  301  on the notebook  300  for the point A, and touches the point A with the leading end of the input pen  200 . 
         [0060]    The control unit  117  included in the electronic terminal  100  waits until an infrared signal and an ultrasonic signal transmitted from the input pen  200  are detected. When an infrared signal and an ultrasonic signal transmitted from the input pen  200  are detected, the process proceeds (step S 16 ). 
         [0061]    Next, the control unit  117  determines a second singular point (hereinafter referred to as a point B) on a currently displayed writing image and displays an image on which the point B is identifiable on the display unit  102  (step S 17 ). In this case, it is also desired that a character or sign encouraging a user to touch the point B with the input pen  200  be displayed on the image. 
         [0062]    When a user sees the displayed image, the user checks the point B on the displayed image, searches the picture  301  on the notebook  300  for the point B, and touches the point B with the leading end of the input pen  200 . 
         [0063]    The control unit  117  included in the electronic terminal  100  waits until an infrared signal and an ultrasonic signal transmitted from the input pen  200  are detected. When an infrared signal and an ultrasonic signal transmitted from the input pen  200  are detected, the process proceeds (step S 18 ). 
         [0064]    Next, the position of the leading end of the input pen  200  detected in step S 16  is registered as the point A in the displayed image, the position of the leading end of the input pen  200  detected in step S 18  is registered as the point B in the displayed image, and processing for correcting a detected position is performed (step S 19 ). 
         [0065]    When it is determined that the picture  301  on the notebook  300  has been rotated as viewed from the electronic terminal  100  in the position correction in step S 19 , the image displayed on the display unit  102  is rotated by the rotation angle of the picture  301  (step S 20 ). Thus, even after correction, the horizontal and vertical directions of the picture  301  on the notebook  300  and the horizontal and vertical directions of the image  102   a  displayed on the display unit  102  are made to conform to each other. 
         [0066]    After the rotation of the image has been performed in step S 20 , the reset mode ends and the process returns to step S 11  in which handwriting input with the input pen  200  is detected and the storage and display of the detected handwriting are performed in the handwriting display mode. In the handwriting display mode after the reset mode, position detection reflecting the position correction in step S 19  is performed. 
         [0067]    Next, an example of processing for setting the point A in step S 15  in the flowchart illustrated in  FIG. 5  and an example of processing for setting the point B in step S 17  in the flowchart illustrated in  FIG. 5  will be described with reference to  FIG. 6 . 
         [0068]      FIG. 6  is a diagram illustrating an image  102   b  that is input by a pen and is displayed on the display unit  102 . On the image  102   b  in  FIG. 6 , characters are displayed. 
         [0069]    As illustrated in  FIG. 6 , on the displayed image, an X direction (horizontal direction) and a Y direction (vertical direction) are set. At that time, a substantially center of the displayed image, that is, an intersection of an X directional axis and a Y directional axis illustrated in  FIG. 6 , is set as an origin at which an evaluation value is 0. The farther from the origin in the X and Y directions, the larger the evaluation value. On the left side of the origin in the X direction and the lower side of the origin in the Y direction, the evaluation value becomes negative. The X directional axis and the Y directional axis are illustrated in  FIG. 6  to describe the evaluation value, and are not actually displayed on an image on the display unit  102 . 
         [0070]    The control unit  117  checks a value obtained by adding an evaluation value in the X direction and an evaluation value in the Y direction at each pixel on a displayed line corresponding to handwriting on the image  102   b , sets a point at which the maximum value is obtained as a point Pa that is the point A, and sets a point at which the minimum value is obtained as a point Pb that is the point B. Accordingly, in the example in  FIG. 6 , the upper right end and the lower left end of handwriting in the image are the points A and B, respectively. 
         [0071]    In the example in  FIG. 6 , in order to make the points A and B identifiable, the character of A in a circle and the character of B in a circle are displayed at corresponding positions. 
         [0072]    The evaluation value determination processing illustrated in  FIG. 6  is merely an example, and another processing for determining the points A and B may be performed. For example, an evaluation value in only one of the X direction and the Y direction may be used, and a point at which the maximum evaluation value is obtained and a point at which the minimum evaluation value is obtained may be set as the points A and B, respectively. 
         [0073]    For example, when the evaluation value in the Y direction is used, the upper end and the lower end of handwriting are the points A and B, respectively. When the evaluation value in the X direction is used, the right end and the left end of handwriting are the points A and B, respectively. 
         [0074]    These evaluation values may not be used, and randomly selected two points on a line corresponding to handwriting on an image may be set as the points A and B. In this case, it is desired that the points A and B be apart from each other by a certain distance. 
         [0075]    Alternatively, when there is a single (independent) point or the intersection of a plurality of lines in handwriting on an image, the independent point or the intersection may be selected as the point A or B. By selecting the independent point or the intersection, a user can easily check the position of the point and points an input pen at the point. For example, a point on the upper side of “i” or the intersection of a vertical line and a horizontal line of “t” may be set as the point A or B. 
         [0076]    Exemplary cases in which correction processing in the reset mode described with reference to the flowchart in  FIG. 5  is actually performed will be described with reference to  FIGS. 7 and 8 . 
         [0077]      FIG. 7A  illustrates an exemplary case in which the electronic terminal  100  is placed on the far side of the notebook  300 , an image  102   c  that is the same as a picture  302  drawn on the notebook  300  with the input pen  200  is displayed on the display unit  102 , and the displayed image is stored. 
         [0078]    It is assumed that a hand H of a user holding the input pen  200  is brought into contact with the electronic terminal  100  and the electronic terminal  100  is obliquely moved as illustrated in, for example,  FIG. 7B  in a state the display and storage of handwriting illustrated in  FIG. 7A  are performed. At that time, the movement of the electronic terminal  100  is detected on the basis of data of detection performed by the acceleration sensor  114  in the electronic terminal  100  and the reset mode is set. 
         [0079]    When the electronic terminal  100  is moved slightly or very slowly, the reset mode may not be set on the basis of data of acceleration detection. In such a case, the reset mode may be compulsorily set by a user&#39;s operation. 
         [0080]      FIG. 8  is a diagram illustrating an exemplary case after the reset mode has been set. 
         [0081]    When the reset mode is set, the point Pa (the point A) is displayed on the image  102   c  on the display unit  102  as illustrated in  FIG. 8A . A user sees the position of the point Pa (the point A), determines the position of the point Pa (the point A) on the picture  302  on the notebook  300 , and touches a determined point  302   a  with the leading end of the input pen  200 . 
         [0082]    After the point A has been touched, the point Pb (the point B) is displayed on the image  102   c  on the display unit  102  as illustrated in  FIG. 8B . The user sees the position of the point Pb (the point B), determines the position of the point Pb (the point B) on the picture  302  on the notebook  300 , and touches a determined point  302   b  with the leading end of the input pen  200 . 
         [0083]    When the touches of the points A and B illustrated in  FIGS. 8A and 8B  have been completed, a corrected position is calculated in the electronic terminal  100 . After the corrected position has been calculated and correction has been performed, the handwriting of the picture  302  handwritten on the notebook  300  with the input pen  200  is continuously detected by the electronic terminal  100  and newly input handwriting is added to a displayed image  102   d  as illustrated in  FIG. 8C . 
         [0084]    By performing the image rotation processing in step S 20  in the flowchart illustrated in  FIG. 5 , the picture  301  on the notebook  300  and the displayed image  102   d  remain parallel even if the electronic terminal  100  is obliquely moved as illustrated in  FIG. 8C . The image rotation processing in step S 20  may not be performed. 
         [0085]    On the image  102   c  including the point A in  FIG. 8A  and the image  102   c  including the point B in the  FIG. 8B , the portions of these images surrounding points A and B may be enlarged to make the selection of these points clearer for the user. 
         [0086]    As described previously, when the electronic terminal  100  according to this example is moved by some impact during detection of the position thereof relative to the input pen  200  and storage and display of a handwriting image written with the input pen  200 , the correction mode is automatically set. Accordingly, since correction processing is automatically started, detection of the relative position can be continuously performed under the same condition. A handwriting detection condition is not therefore disturbed by the movement of the electronic terminal  100 . 
         [0087]    In the processing for determining two points to be touched by a user at the time of the correction processing, as illustrated in, for example, the example in  FIG. 6 , it is possible to automatically select points easily recognizable to the user using evaluation values in the horizontal and vertical directions which are obtained on a line corresponding to handwriting. 
         [0088]    Furthermore, as illustrated in  FIG. 8C , it is possible to keep a picture or a character string being written with the input pen  200  and an image displayed on the electronic terminal  100  parallel to each other by rotating the displayed image on the basis of a state of correction of the relative position. 
       6. Description of Distance Display Process 
       [0089]    Next, a process for measuring the distance between certain two points and displaying the distance using the electronic terminal  100  according to this example and the input pen  200  in a measurement mode will be described. 
         [0090]    The process in the measurement mode is performed in accordance with a procedure illustrated in a flowchart in  FIG. 9  under the control of the control unit  117 . 
         [0091]    First, the control unit  117  determines whether the measurement mode has been set by the operation unit  119  (step S 21 ). When the measurement mode is not set, the control unit  117  waits until the measurement mode is set. When it is determined that the measurement mode has been set, it is determined whether a first point that is a measurement starting point has been touched by the input pen  200  (step S 22 ). When it is determined that the touch of the first point has been detected, the detected point is set as a starting point (step S 23 ). 
         [0092]    Subsequently, it is determined whether a second point that is a measurement end point has been touched by the input pen  200  (step S 24 ). When it is determined that the touch of the second point has been detected, the detected point is set as an end point (step S 25 ). The distance between the starting point and the end point is determined, and the determined distance is displayed on the display unit  102  (step S 26 ). 
         [0093]      FIG. 10  is a diagram illustrating an example of a processing state in the measurement mode. 
         [0094]    For example, as illustrated in  FIG. 10 , it is assumed that a specific point  311  in a figure written on the notebook  300  is set as a starting point and another specific point  312  is set as an end point. At that time, the electronic terminal  100  detects the positions of these points and calculates the distance between the positions. 
         [0095]    In the example illustrated in  FIG. 10 , a scale image  102   f  is displayed on the display unit  102  in the electronic terminal  100  and the scale image  102   f  indicates the calculated distance. 
         [0096]    Thus, it is possible to measure the distance between certain two points and display the measured distance using a position detection function of the electronic terminal  100 . The electronic terminal  100  functions as an electronic measuring device. A distance is presented in a scale in the example in  FIG. 10 , but may be numerically represented. 
       7. Modification 
       [0097]    In the example illustrated in  FIG. 1 , the casing  101  of the electronic terminal  100  contains the sensors  111 ,  112 , and  113 . A casing containing the sensors  111 ,  112 , and  113  and the body of the electronic terminal  100  may be separately formed. 
         [0098]    For example, as illustrated in  FIG. 11 , in an electronic terminal  100 ′, an adaptor connection unit  131  is formed on the side surface of a casing and a receiving adaptor  400  is connected to the adaptor connection unit  131 . The electronic terminal  100 ′ is the same as the electronic terminal  100  illustrated in  FIG. 2  except that the infrared sensor  111 , the first ultrasonic sensor  112 , and the second ultrasonic sensor  113  are not disposed and the adaptor connection unit  131  is disposed. 
         [0099]    An infrared sensor  401 , a first ultrasonic sensor  402 , and a second ultrasonic sensor  403  are disposed in the receiving adaptor  400  illustrated in  FIG. 11 . The detection outputs of the sensors  401 ,  402 , and  403  are supplied to the electronic terminal  100 ′. For example, power for the sensors  401 ,  402 , and  403  in the receiving adaptor  400  are supplied from the electronic terminal  100 ′ via the adaptor connection unit  131 . 
         [0100]    In the electronic terminal  100 ′, the position of a leading end of an input pen is calculated on the basis of pieces of detection data acquired by the sensors  401 ,  402 , and  403  in the receiving adaptor  400  and correction processing is started when acceleration detected in the electronic terminal  100 ′ exceeds a threshold value. 
         [0101]    Using the configuration illustrated in  FIG. 11 , this example can be applied to an electronic terminal including no infrared sensor and no ultrasonic sensor and versatility can be increased. 
         [0102]    The position calculation processing may be performed in the receiving adaptor when each sensor performs detection, and data of the calculated position may be transmitted to an electronic terminal. In this case, when an acceleration sensor in the electronic terminal detects that acceleration has exceeded a threshold value, the electronic terminal may transmit an instruction for setting a reset mode to the receiving adaptor and processing in the reset mode may be performed in the receiving adaptor. 
         [0103]    In the above-described examples, a radiotelephone terminal is used as an electronic terminal. A configuration and processing according to the present disclosure may be applied to various electronic terminals such as an electronic terminal having a function of reproducing music or video for portable devices and an electronic terminal having a function of playing a video game. Alternatively, a handwriting input apparatus having only a function of detecting handwriting using a writing implement such as an input pen may be used. 
         [0104]    Each sensor may be disposed in a general-purpose information processing apparatus such as a personal computer apparatus, the output of the sensor may be processed in the information processing apparatus, and similar processing may be performed by executing a program (software) for correction processing installed in the general-purpose information processing apparatus. 
         [0105]    When the radiotelephone terminal illustrated in  FIG. 2  is employed, a program using detection data acquired by each sensor may be externally supplied to the radiotelephone terminal and processing according to the present disclosure may be performed in accordance with the program. 
         [0106]    In the above-described description, in order to detect the position of an input pen, an infrared signal and an ultrasonic signal are transmitted from the input pen. Other types of signals may be used. That is, two types of signals having different transmission speeds may be used in combination. For example, an infrared signal and a radio signal in the range of several GHz to several hundred MHz may be used in combination. Alternatively, instead of an infrared signal, a visible light signal may be used. 
         [0107]    In the correction process illustrated in the flowchart in  FIG. 5 , the reset mode is started when acceleration is applied to a terminal or a user&#39;s operation is performed. Only one of these conditions may be used as the reset mode start condition. For example, only when acceleration is applied to a terminal, the reset mode may be started. Alternatively, only when a user&#39;s operation is performed, the reset mode may be started. 
         [0108]    The input pen  200  illustrated in  FIGS. 1 and 3  is merely an example. Various writing implements can be used under the condition that they have a function of transmitting two types of signals to a terminal.
An information processing apparatus that includes a first sensor that receives a first type of signal; second and third sensors that receive a second type of signal; a control unit that calculates a position of a source from which a signal of the first type and a signal of the second type are transmitted based on a first time at which the signal of the first type is received at the first sensor, a second time at which the signal of the second type is received at the second sensor and a third time at which the signal of the second type is received at the third sensor; and a display that displays an image corresponding to a path obtained based on changes in position of the source calculated by the control unit.