Patent Publication Number: US-2015070331-A1

Title: Digital pen

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is based on and claims priority of Japanese Patent Application No. 2013-185675 filed on Sep. 6, 2013. The entire disclosure of the above-identified application, including the specification, drawings and claims is incorporated herein by reference in its entirety. 
     FIELD 
     One or more exemplary embodiments relate to a digital pen for making a drawing in a planar drawing area. 
     BACKGROUND 
     Recently, instead of conventional pens or pencils, a digital pen capable of storing, on a personal computer, motion trajectory information representing characters or figures drawn by moving the pen tip on a paper is becoming popular. 
     Such a digital pen requires positional information of the pen tip to obtain the motion trajectory information. In order to obtain the positional information of the pen tip, absolute positional information representing absolute positional information of the pen tip on the paper is needed. In order to obtain the absolute positional information, the pen tip must be on the paper. 
     Patent Literature (PTL) 1 discloses that a white board provided with a specific pattern for position detection thereon can be used to determine where information is written and what is written. 
     CITATION LIST 
     Patent Literature 
     
         
         [PTL 1] Japanese Patent No. 4920153 
       
    
     SUMMARY 
     Technical Problem 
     In the PTL 1, however, a specific pattern for position detection is required to identify the position of a pen tip, which leads to a lack of versatility. 
     There is a technique of: when a pen tip is in contact with a paper disposed on a desk, capturing an image of the paper and the desk surface with a camera from above; dividing the captured image into a light portion in the vicinity of the pen tip (a white and near-white portion) and a dark portion away from the pen tip (a color away from white); when there is a predetermined difference between the light portion and the dark portion, deciding that the pen tip exists on the paper. 
     Unfortunately, when a part of the light portion in the image is, for example, a “high-luminance desk surface portion in the vicinity of the pen tip”, the part of the light portion is also regarded as a part of the image of the paper. In other words, although the part of the light portion does not belong to the image of the paper, the “high-luminance desk surface portion in the vicinity of the pen tip” is incorrectly determined as the paper because there is a difference between the average luminance of the high-luminance desk surface portion approximately equal to the average luminance of the paper and the average luminance of the dark portion. 
     Moreover, when the part of the light portion in the image is the “high-luminance desk surface portion in the vicinity of the pen tip”, the luminance value of the “high-luminance desk surface portion in the vicinity of the pen tip” is used to calculate the average luminance of the light portion. As a result, the calculated average is different from the average obtained when the light portion in the image includes only the paper. When there are multiple “high-luminance desk surface portions in the vicinity of the pen tip”, the difference between the averages increases as more unnecessary luminance values are used to calculate the average. 
     One non-limiting and exemplary embodiment has been conceived to solve the aforementioned problem, and provides a digital pen capable of accurately determining a drawing area even when a non-drawing area having a relatively high average luminance exists in the vicinity of the digital pen. 
     Solution to Problem 
     In general aspect, the techniques disclosed here feature a digital pen for making a drawing in a drawing area having a planar configuration, the digital pen including: an image obtainment unit configured to obtain an image of an area including the digital pen and the drawing area; an area division unit configured to divide the image obtained by the image obtainment unit into: a first area that is displayed in a color of a first reference point and a color close to the color of the first reference point; and a second area that is displayed in a color of a second reference point and a color close to the color of the second reference point, the second reference point being farther away from the digital pen than the first reference point; an average calculation unit configured to calculate a first average and a second average, the first average being an average luminance in the first area, the second average being an average luminance in the second area; a storage unit for storing the first average; a control unit configured to repeat a first average determination process a predetermined number of times while changing the first reference point and the second reference point, the first average determination process being a process in which: whether or not a current first average exceeds a current second average is decided; and when the current first average exceeds the current second average and the first average stored in the storage unit, the first average stored in the storage unit is updated with the current first average; and a drawing area determination unit configured to determine, as the drawing area, the first area corresponding to the first average stored in the storage unit after the first average determination process is repeated the predetermined number of times by the control unit. 
     With this, the first average determination process is performed a predetermined number of times, and then the first area corresponding to the first average stored in the storage unit is determined as the drawing area. 
     In this way, even when an area where the luminance is relatively high but lower than that of the drawing area, such as a non-drawing area, is included in the first area, the average luminance of the first area is lower than that of the first area including only the drawing area because the luminance of the non-drawing area is lower than that of a paper. 
     As a result, when the first area includes the area where the luminance is relatively high but lower than that of the drawing area, it is possible to prevent the first area from being incorrectly determined to be a paper. Thus, the accuracy of identifying the drawing area is improved. 
     For example, the first average stored in the storage may be greater than or equal to a predetermined initial reference value. 
     Furthermore, for example, the first reference point may be an inner end of a line segment, and the second reference point may be an outer end of the line segment, the line segment being one of a plurality of line segments radially arranged around a predetermined point in the image. 
     With this, the first reference point is an inner end of each of the line segments radially arranged around a predetermined point in the image, and the second reference point is an outer end of each line segment. 
     Accordingly, compared with a random setting of the first reference point and the second reference point in the image, the even arrangement of the first reference point and the second reference point can be achieved in a wide range. 
     Furthermore, for example, the first reference point and the second reference point may be each a predetermined point on the line segment. 
     With this, the first reference point and the second reference point are also placed on each of the radially-arranged line segments, and thus more first reference points and more second reference points can be reserved as the number of line segments increase. 
     Accordingly, the number of first averages to be used for update when the first average exceeds the second average increases as more reference points are reserved. As a result, the accuracy of determining the first area as the drawing area is further improved. 
     Furthermore, for example, the drawing area determination unit may determine, as the drawing area, a rectangle including all points on a boundary between the first area corresponding to the first average stored in the storage unit and the second area after the first average determination process is repeated the predetermined number of times. 
     With this, multiple points are drawn on the boundary between the first area determined as the drawing area and the second area, and a rectangle including all the points is obtained as an image representing the drawing area. Accordingly, it is possible to obtain information representing the drawing area through easy software processing. 
     Furthermore, for example, the digital pen may further include an estimation unit for estimating a position of the digital pen based on the drawing area determined by the drawing area determination unit, in which the storage unit further stores positional information indicating the position estimated by the estimation unit. 
     With this, the digital pen estimates the position based on the drawing area, and thus it is possible to estimate the position of the digital pen without requiring the specific pattern for position detection. 
     Furthermore, for example, the digital pen may further include a positional information output unit for outputting the positional information to a communication device used in combination with the digital pen. 
     With this, the foregoing digital pen can provide the positional information to the communication device, and thus the communication device can obtain the motion trajectory of the digital pen from the provided positional information. Accordingly, characters or figures drawn in the drawing area with the digital pen can be stored on the communication device in a paperless manner. 
     It is to be noted that one or more exemplary embodiments disclosed herein may be implemented as not only a digital pen but also: a method including, as steps, processing units included in the digital pen; a program for causing a computer to execute the steps; a computer readable recording medium, such as a CD-ROM, storing the program; and information, data, or signal indicating the program. The program, information, data, and signal may be distributed via a communication network such as an Internet. 
     Advantageous Effects 
     According to one or more exemplary embodiments, it is possible to accurately determine a drawing area even when a non-drawing area having a relatively high average luminance exists in the vicinity of a digital pen. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings that illustrate a specific embodiment of the present invention. 
         FIG. 1  illustrates a side view showing an exemplary outline of a digital pen according to one exemplary embodiment 1. 
         FIG. 2  illustrates a block diagram showing an exemplary functional configuration of the digital pen. 
         FIG. 3  illustrates an example of an image captured by a camera module. 
         FIG. 4  illustrates an example of a corrected image obtained by correcting the captured image in an image correction unit. 
         FIG. 5  illustrates a flowchart showing an exemplary basic operation of the digital pen according to the embodiment 1. 
         FIG. 6  illustrates an example of a divided image divided by an area division unit. 
         FIG. 7  illustrates a black-and-white image obtained by binarizing the divided image. 
         FIG. 8  illustrates a diagram for describing an operation of a digital pen according to one exemplary embodiment 2. 
         FIG. 9  also illustrates a diagram for describing the operation of the digital pen according to the embodiment 2. 
         FIG. 10  illustrates a diagram for describing an operation of a digital pen according to one exemplary embodiment 3. 
         FIG. 11  illustrates a diagram for describing an operation of a digital pen according to one exemplary embodiment 4. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, certain exemplary embodiments are described in detail with reference to the accompanying Drawings. Each of the exemplary embodiments described below shows a general or specific example. The structural elements, the arrangement and connection of the structural elements etc. shown in the following exemplary embodiments are mere examples, and therefore do not limit the scope of the appended Claims and their equivalents. Therefore, among the structural elements in the following exemplary embodiments, structural elements not recited in any one of the independent claims are described as arbitrary structural elements. 
     Embodiment 1 
     &lt;Structure of Digital Pen&gt; 
       FIG. 1  illustrates a side view showing an exemplary outline of a digital pen according to one exemplary embodiment 1. It is to be noted that, besides a digital pen  1 ,  FIG. 1  also shows a paper  103  disposed on the desk surface  104 S of a desk  104  as a planar drawing area in which a drawing is made with the digital pen  1 . In  FIG. 1 , the x-axis is along a width direction of the paper  103 , the y-axis is along a length direction of the paper  103 , and the z-axis is along a height direction of the digital pen  1 . 
     The digital pen  1  includes a pen tip assembly  100  and a main body  101 . The pen tip assembly  100  includes a pen tip  100 A in contact with the paper surface  1035  which is the surface of the paper  103 . Ink supplied from an ink cartridge in the main body  101  may exit from the pen tip  100 A. Alternatively, the structure of the pen tip assembly  100  may be the same as that of a tip portion of a pencil. 
     The main body  101  includes a lower main body portion  101 A and an upper main body portion  101 B which are separated by a camera module  10 . The lower main body portion  101 A is used when a user grips it with his/her hand to draw characters or figures on the paper surface  103 S. Accordingly, the lower main body portion  101 A may include the above-mentioned ink cartridge. 
     An inner head portion of the upper main body portion  101 B is provided with a hemisphere mirror  102  so that the mirror surface  102 S faces to the paper  103 . The mirror surface  102 S of the hemisphere mirror  102  reflects the digital pen  1 , the paper surface  103 S of the paper  103  disposed on the desk surface  1045  of the desk  104 , and the desk surface  104 S. 
     The camera module  10  is provided with an image-capturing surface  105  facing to the hemisphere mirror  102 . The camera module  10  captures a mirror image on the mirror surface  102 S of the hemisphere mirror  102  to obtain an image IM 1  of the mirror image. 
       FIG. 2  illustrates a block diagram showing an exemplary functional configuration of the digital pen  1 .  FIG. 3  illustrates an example of an image IM 1  captured by a camera module  10 .  FIG. 4  illustrates an example of a corrected image IM 2  obtained by correcting the captured image IM 1  in an image correction unit  11 . 
     It is to be noted that, in  FIG. 3  and  FIG. 4 , the first area AR 1  represented as a white area is an area that is displayed in a color of a first reference point (for example, white) and a color close to the color of the first reference point (for example, a color within a predetermined range from white), while the second area AR 2  represented as a hatched area is an area that is displayed in a color of a second reference point which is farther away from the digital pen  1  than the first reference point (for example, green) and a color close to the color of the second reference point (for example, a color within a predetermined range from green). 
     The digital pen  1  include the camera module  10 , the image correction unit  11 , an area division unit  12 , a average calculation unit  13 , an estimation unit  14 , a storage unit  15 , a control unit  16 , a drawing area determination unit  17 , and a positional information output unit  18 . 
     As described above, the camera module  10  captures the image IM 1  of the mirror image on the mirror surface  102 S of the hemisphere mirror  102 . In other words, the camera module  10  obtains the image IM 1  of an area including the digital pen  1  and the paper  103  (drawing area). The image IM 1  obtained in this manner is shown in  FIG. 3 . 
     The image correction unit  11  performs predetermined correction on the image IM 1  captured by the camera module  10 . For example, the image IM 1  captured by the camera module  10  is distorted as shown in  FIG. 3 , and thus the image correction unit  11  corrects the distortion to facilitate the color-based division of the image. Specifically, as shown in  FIG. 3 , the image IM 1  includes portions unnecessary to decide whether or not the digital pen  1  faces to the paper surface  103 S (for example, the rim  102 C of the hemisphere mirror  102 , and a ceiling  105 ), and thus the image correction unit  11  removes the unnecessary portions and transform the resulting image into a substantially rectangular image. In this way, the corrected image IM 2  as shown in  FIG. 4  is generated. 
     The area division unit  12  divides the corrected image IM 2  generated by making the correction in the image correction unit  11  into: the first area AR 1  of the color of the first reference point and the color close to the color of the first reference point; and the second are AR 2  of the color of the second reference point and the color close to the color of the second reference point. In other words, the corrected image IM 2  is divided into: the first area that is displayed in the color of the first reference point and the color close to the color of the first reference point; and the second area that is displayed in the color of the second reference point and the color close to the color of the second reference point. For example, the area division unit  12  makes the division using the well-known WATERSHED method. 
     With the WATERSHED method, the luminance of each of the pixels in the corrected image IM 2  is regarded as altitude information or depth information of a map. The same luminances are connected with a contour line or a depth contour line, and a dividing ridge is detected when water is filled from the first seed point serving as the first reference point P 1  and the second seed point serving as the second reference point P 2 . Thus, the area division unit  12  divides the corrected image IM 2  into the first area AR 1  and the second area AR 2 . 
     The average calculation unit  13  calculates respective average luminances in the first area AR 1  and the second area AR 2  obtained by dividing the corrected image IM 2  in the area division unit  12 . In other words, the average calculation unit  13  calculates a first average being the average luminance in the first area AR 1  and a second average being the average luminance in the second area AR 2 . 
     The estimation unit  14  estimates the position of the digital pen  1  based on an area determined as the paper  103  (drawing area) by the drawing area determination unit  17  to be described below. Specifically, the estimation unit  14  takes advantage of the digital pen  1  located at the center in the corrected image IM 2  to obtain the center coordinate of the corrected image IM 2  with respect to the area determined as the paper  103 . Thus, the position of the digital pen  1  relative to the paper  103  is estimated. Furthermore, the estimation unit  14  may include a gyro sensor. The position of the digital pen  1  may be estimated using the angular velocity about the x-axis and the angular velocity about the y-axis (see  FIG. 1 ) detected by the gyro sensor. 
     The storage unit  15  stores a control program for activating the digital pen  1 . Furthermore, the storage unit  15  stores an initial reference value to be used in a first average determination process to be described below. The storage unit  15  also stores the first average (an average luminance in the first area AR 1 ) obtained by the first average determination process. In addition, the storage unit  15  stores positional information indicating the position of the digital pen  1  estimated by the estimation unit  14 . 
     The control unit  16  totally controls the digital pen  1 . The control unit  16  includes CPU (Central Processing Unit), RAM (Random Access Memory), ROM (Read Only Memory), and others, and the CPU executes a predetermined control program to achieve the functionality. The control unit  16  repeats a first average determination process a predetermined number of times while changing the first reference point P 1  and the second reference point P 2 , the first average determination process being a process in which: whether or not a current first average exceeds a current second average is decided; and when the current first average exceeds the current second average and the first average stored in the storage unit  15 , the first average stored in the storage unit  15  is updated with the current first average. The control unit  16  has a counter  16 A used in the first average determination process. 
     The drawing area determination unit  17  determines, as the paper  103  (drawing area), the first area corresponding to the first average stored in the storage unit  15  after the first average determination process is repeated the predetermined number of times by the control unit  16 . 
     When the digital pen  1  connected to a communication device such as a personal computer receives a specific instruction, the positional information output unit  18  outputs the positional information stored in the storage unit  15  to the communication device. 
     &lt;Operation of Digital Pen&gt; 
       FIG. 5  illustrates a flowchart showing an exemplary basic operation of the digital pen  1  according to the embodiment 1.  FIG. 6  illustrates an example of a divided image IM 3  divided by the area division unit  12 .  FIG. 7  illustrates a black-and-white image IM 4  obtained by binarizing the divided image IM 3  such that the light portion is represented by white and the dark portion is represented by black to clarify the shading of the divided image IM 3  shown in  FIG. 6 . 
     Like  FIG. 3  and  FIG. 4 , in  FIG. 6 , the first area AR 1  represented as a white area is an area that is displayed in the color of the first reference point (for example, white) and a color close to the color of the first reference point (for example, a color within a predetermined range from white), while the second area AR 2  represented as a hatched area is an area that is displayed in the color of the second reference point which is farther away from the digital pen  1  than the first reference point (for example, green) and a color close to the color of the second reference point (for example, a color within a predetermined range from green). 
     The control unit  16  obtains the image IM 1  shown in  FIG. 3  with the camera module  10  (Step S 10 ). The image IM 1  includes an image of the digital pen  1 , an image of the first area AR 1  that is assumed as the paper surface  103 S, and an image of the second area AR 2  that is assumed as the desk surface  104 S. As described above, the image IM 1  also includes an image of the rim  102 C of the hemisphere mirror  102  and an image of the ceiling  105 . 
     Next, the control unit  16  causes the image correction unit  11  to make the foregoing correction to convert the image IM 1  into an image appropriate to the area division (Step S 11 ). In other words, the image correction unit  11  corrects the distortion of the image IM 1 , and removes the image of the rim  102 C of the hemisphere mirror  102  and the image of the ceiling  105 . In this way, the corrected image IM 2  shown in  FIG. 4  can be obtained. 
     Next, as shown in  FIG. 4 , the control unit  16  places, in the corrected image IM 2 , the first reference point P 1  in the vicinity of the digital pen  1  and the second reference point P 2  in a position farther away from the digital pen  1  than the first reference point P 1  (Step S 12 ). 
     Next, the control unit  16  causes the area division unit  12  to divide the corrected image IM 2  into: the first area AR 1  whose color is the color of the first reference point P 1  and a color within a predetermined range from the color of the first reference point P 1 ; and the second area AR 2  whose color is the color of the second reference point P 2  and a color within a predetermined range from the color of the second reference point P 2  (Step S 13 ). 
     In this way, as shown in  FIG. 6 , the corrected image IM 2  is divided into: the first area AR 1  whose color is the color of the first reference point P 1  (in this case, white) and a color within a predetermined range from the color of the first reference point P 1 ; and the second area AR 2  whose color is the color of the second reference point P 2  (in this case, green) and a color within a predetermined range from the color of the second reference point P 2 . 
     Next, the control unit  16  causes the average calculation unit  13  to calculate the respective average luminances in the first area AR 1  and the second area AR 2  (Step S 14 ). In other words, the first average being the average luminance in the first area AR 1  and the second average being the average luminance in the second area AR 2  are calculated by the average calculation unit  13 . 
     Next, when a count of the counter  16 A is “0” (YES in Step S 15 ), the control unit  16  decides whether or not the average luminance in the first area AR 1  (referred to as the first average) exceeds the average luminance in the second area AR 2  (referred to as the second average) (Step S 16 ). 
     If the first average exceeds the second average (YES in Step S 16 ), then the control unit  16  stores the first average in the storage unit  15  (Step S 17 ). The control unit  16  also stores the first area AR 1  corresponding to the stored first average, in the storage unit  15  as a paper candidate. Next, the control unit  16  increments the count by 1 (Step S 18 ). 
     In contrast, if the first average is lower than or equal to the second average (NO in Step S 16 ), then the control unit  16  increments the count by 1 without storing the first average in the storage unit  15  (Step S 18 ). 
     It is to be noted that the processing when the count of the counter  16 A is “0” as described above (Step S 16  to Step S 18 ) is processing for obtaining the foregoing “initial reference value” (hereinafter, referred to as a “initial reference value determination process”). Accordingly, instead of obtaining the initial reference value in the initial reference value determination process, it is possible to pre-store the initial reference value in the storage unit  15 . The initial reference value is at least a luminance to be determined as the paper  103  (drawing area). For example, it is higher than or equal to a predetermined luminance. 
     The control unit  16  performs Step S 12  to Step S 14  again, and decides whether or not the count is α (where α is an integer greater than “0”) (Step S 19 ). 
     If the count is not α (NO in Step S 19 ), then the control unit  16  determines the first area AR 1  as the paper  103  candidate when the first average exceeds the second average (YES in Step S 20 ) and the first average exceeds a first average stored in the storage unit  15  (YES in Step S 21 ) (Step S 22 ). 
     Next, the control unit  16  updates the first average stored in the storage unit  15  (Step S 24 ). The control unit  16  also updates the first area AR 1  stored in the storage unit  15  as the paper candidate. 
     In other words, the control unit  16  performs the following first average determination process on the first area AR 1  and the second area AR 2  obtained by dividing, in the division process (Step S 13 ), the corrected image based on the first reference point P 1  and the second reference point P 2  placed in the reference point setting process (Step S 12 ). The first average determination process is a process in which a current first average is decided whether or not to exceed a current second average, and a first average stored in the storage unit  15  is updated with the current first average when (i) the current first average exceeds the current second average and (ii) the current first average exceeds the first average stored in the storage unit  15 . 
     Here, the first average stored in the storage  15  is greater than or equal to a predetermined initial reference value. In other words, a value stored in the storage unit  15  prior to the first average determination process is the “initial reference value” being the first average obtained when the count of the counter  16 A is “0”. In the first average determination process, the initial reference value stored in the storage unit  15  is updated with a first average when the first average exceeds the initial reference value. Accordingly, after the first average determination process, the first average stored in the storage unit  15  is greater than or equal to the “initial reference value”. 
     In contrast, if the count is not a (NO in Step S 19 ), then when the first average is lower than or equal to the second average (NO in Step S 20 ) or when the first average is lower than or equal to a first average stored in the storage unit  15  (NO in Step S 21 ), the first area AR 1  is not determined as the paper candidate (Step S 23 ). In other words, the first area AR 1  is determined to be a desk surface  104 S candidate. 
     Next, the control unit  16  further increments the count by 1 (Step S 25 ). 
     The control unit  16  repeats a set of the foregoing steps, i.e. Step S 12 , S 13 , S 14 , S 15 , S 19 , S 20 , S 21 , S 22 , S 23 , S 24 , and S 25 , until the count reaches α. 
     Even when the count has reached α (YES in Step S 19 ), the control unit  16  decides whether or not the first average exceeds the second average (Step S 26 ). If the first average exceeds the second average (YES in Step S 26 ), then the control unit  16  decides whether or not the first average is higher than or equal to a first average stored in the storage unit  15  (Step S 27 ). 
     If the first average is higher than or equal to the first average stored in the storage unit  15  (YES in Step S 27 ), then the control unit  16  determines, as the paper  103 , the first area AR 1  of when the count is α (Step S 28 ). 
     In contrast, if the count is α (YES in Step S 19 ), then when the first average is lower than or equal to the second average (NO in Step S 26 ) or when the first average is lower than the first average stored in the storage unit  15  (NO in Step S 27 ), the control unit  16  determines, as the paper  103 , the paper candidate stored in the storage unit  15  (Step S 29 ). Specifically, the control unit  16  causes the drawing area determination unit  17  to determine, as the paper  103  (drawing area), the first area corresponding the first average stored in the storage unit  15 . That means the drawing area determination unit  17  determines, as the paper  103  (drawing area), the first area corresponding the first average stored in the storage unit  15 . In other words, the first area AR 1  of when the count is a is determined to be an area other than the paper  103 , i.e. the desk surface  104 S. 
     Like the case of the count of less than α, even when the count is α (YES in Step S 19 ), the first average stored in the storage unit  15  may be updated with a current first average when (i) the current first average exceeds a current second average and (ii) the current first average exceeds the first average stored in the storage unit  15 . 
     In other words, the control unit  16  repeat a first average determination process α times (a predetermined number of times) while changing the first reference point and the second reference point, the first average determination process being a process in which: whether or not a current first average exceeds a current second average is decided; and when the current first average exceeds the current second average and the first average stored in the storage unit  15 , the first average stored in the storage unit  15  is updated with the current first average. The drawing area determination unit  17  determines, as the paper  103  (drawing area), the first area corresponding to the first average stored in the storage unit  15  after the first average determination process is repeated α times (the predetermined number of times) by the control unit  16 . 
     Next, the control unit  16  resets the counter  16 A (Step S 30 ). 
     &lt;Advantages&gt; 
     As described above, the embodiment takes advantage of the luminance of the paper surface  103 S higher than that of the desk surface  104 S to identify the paper  103  (drawing area) in the following manner. 
     When the digital pen  1  faces to the paper surface  103 S, i.e. when the pen tip of the digital pen  1  is on the paper  103 , the first average in the first area AR 1  close to the digital pen  1  exceeds the second average in the second area AR 2  away from the digital pen  1 . In such a case, the first area AR 1  is determined as the paper candidate, and the first average is stored in the storage unit  15 . 
     In contrast, when the digital pen  1  does not face to the paper surface  103 S, i.e. when the pen tip of the digital pen  1  is not on the paper  103 , the first average does not exceeds the second average. In such a case, the first area AR 1  is not determined as the paper candidate, and the first average is not stored in the storage unit  15 . 
     The control unit  16  repeats the first average determination process a predetermined number of times to obtain the highest first average being the highest average luminance, stored in the storage unit  15 . Then, the drawing area determination unit  17  determines, as the paper surface  103 S, the first area AR 1  corresponding the first average stored in the storage unit  15 . 
     In this way, even when an area where the luminance is relatively high but lower than that of the paper  103 , such as a non-paper area (for example, the desk  104 ), is included in the first area AR 1 , the average luminance of the first area AR 1  is lower than that of the first area AR 1  including only the paper  103  because the luminance of the non-paper area is lower than that of the paper  103 . 
     As a result, when the area where the luminance is relatively high but lower than that of the paper  103  (drawing area) is included in the first area AR 1 , it is possible to prevent the area from being incorrectly determined to be a part of the paper  103 . Thus, the accuracy of identifying the paper  103  (drawing area) is improved. 
     Furthermore, in the embodiment, the digital pen  1  further includes the estimation unit  14  for estimating the position of the digital pen  1  based on the paper  103  (drawing area) determined by the drawing area determination unit  17 , and the storage unit  15  further stores the positional information indicating the position estimated by the estimation unit  14 . 
     With this, the digital pen  1  estimates its position based on the paper  103  (drawing area), and thus the position of the digital pen  1  can be estimated without requiring a specific pattern for position detection. 
     Furthermore, in the embodiment, the digital pen  1  further includes the positional information output unit  18  for outputting the positional information to a communication device used in combination with the digital pen. 
     With this, the digital pen  1  can provide the positional information to the communication device, and thus the communication device can obtain the motion trajectory of the digital pen  1  from the provided positional information. Accordingly, characters or figures drawn on the paper  103  (in the drawing area) with the digital pen  1  can be stored on the communication device in a paperless manner. 
     Embodiment 2 
       FIG. 8  and  FIG. 9  each illustrate a diagram for describing an operation of a digital pen according to one exemplary embodiment 2. It is to be noted that the basic structure of the digital pen according to the embodiment 2 is the same as that of the digital pen according to the embodiment 1, and thus the illustration and the description are omitted. 
     &lt;Operation of Digital Pen&gt; 
     The image correction unit  11  generates a black-and-white image by binarizing the divided image IM 3  divided into the first area AR 1  determined as the paper  103  by the control unit  16  and the second area AR 2  determined as an area other than the paper  103 , and rotates the first area AR 1  until the long side becomes horizontal. 
     The control unit  16  places multiple points P 3  on the boundary between the first area AR 1  converted into white and the second area AR 2  converted into black. Then the black-and-white image IM 5  shown in  FIG. 8  is generated. The black-and-white image IM 5  shown in  FIG. 8  differs from the divided image IM 3  in that the digital pen  1  is substantially located at the center of the paper  103 . 
     As shown in  FIG. 9 , the drawing area determination unit  17  determines, as the paper  103  (drawing area), a rectangle  106  including all the points P 3 . Specifically, the drawing area determination unit  17  determines, as the paper  103  (drawing area), the rectangle  106  including all the points P 3  on the boundary between the first area AR 1  corresponding to the first average stored in the storage unit  15  and the second area AR 2  after the first average determination process is repeated the predetermined number of times by the control unit  16 . Here, the “rectangle including all the points” means, for example, the rectangle circumscribing the polygon whose vertices are the points. 
     &lt;Advantages&gt; 
     With this, as described above, multiple points P 3  are drawn on the boundary between the first area AR 1  determined as the paper  103  and the second area AR 2 , and the rectangle  106  including all the points P 3  is obtained as an image of the paper  103  (drawing area). Accordingly, it is possible to obtain information representing the paper  103  through easy software processing. 
     Embodiment 3 
       FIG. 10  illustrates a diagram for describing an operation of a digital pen according to one exemplary embodiment 3. It is to be noted that the basic structure of the digital pen according to the embodiment 3 is the same as that of the digital pen according to the embodiment 1, and thus the illustration and the description are omitted. 
     &lt;Operation of Digital Pen&gt; 
     The control unit  16  does not randomly place the first reference point P 1  and the second reference point P 2  in the corrected image IM 2 . As shown in  FIG. 10 , the first reference points P 10  to P 17  are respectively placed on one ends of line segments L 1  to L 8  radially arranged around a predetermined position (in this case, the position of the digital pen  1 ), and the second reference points P 20  to P 27  are respectively placed on the other ends. Specifically, the first reference points P 10  to P 17  are each an inner end of one of the line segments L 1  to L 8  radially arranged around a predetermined point in the image, and the second reference points P 20  to P 27  are each an outer end of the line segment. 
     &lt;Advantages&gt; 
     With this, a set of the first reference points P 10  to P 17  and a set of the second reference points P 20  to P 27  each can be circularly arranged around the position of the digital pen  1 . Accordingly, compared with a random setting of the first reference point P 1  and the second reference point P 2  in the corrected image IM 2 , the even arrangement of the first reference points P 10  to P 17  and the second reference points P 20  to P 27  can be achieved in a wide range. As a result, it is possible to more accurately determine the first area AR 1  as the paper  103 . 
     Embodiment 4 
       FIG. 11  illustrates a diagram for describing an operation of a digital pen according to one exemplary embodiment 4. It is to be noted that the basic structure of the digital pen according to the embodiment 4 is the same as that of the digital pen according to the embodiment 1, and thus the illustration and the description are omitted. 
     &lt;Operation of Digital Pen&gt; 
     The control unit  16  places the first reference point P 18  and the second reference point P 28  not only on the ends of each of the line segments L 1  to L 8  (in this case, the line segment L 1 ), but also along each line segment. 
     &lt;Advantages&gt; 
     With this, the first reference point P 18  and the second reference point P 28  are also placed on each of the radially-arranged line segments L 1  to L 8  (in this case, the line segment L 1 ), and thus more first reference points P 18  and more second reference points P 28  can be reserved as the number of line segments increase. 
     Accordingly, more possible first areas AR 1  to be determined as the paper  103  and more possible second areas AR 2  to be determined as the area other than the paper  103  can be used to decide whether or not each of the possible first areas AR 1  is the paper, and thus the accuracy of identifying the paper is further improved. 
     The herein disclosed subject matter is to be considered descriptive and illustrative only, and the appended Claims are of a scope intended to cover and encompass not only the particular embodiments disclosed, but also equivalent structures, method, and/or uses. 
     For example, in the foregoing description, the digital pen has been for making a drawing on a paper, but a target on which a drawing is made with the digital pen is not limited to the paper. A planar drawing area such as a white board is possible. 
     Furthermore, in the foregoing description, the image obtainment unit has included a mirror such as the hemisphere mirror  102  and the camera module  10  for capturing a mirror image on the mirror, but need not include the mirror. In other words, the image obtainment unit may include a camera module provided with an image-capturing surface facing to the paper, and obtain an image captured by such a camera module. 
     Furthermore, in the foregoing description, the digital pen has included the image correction unit  11 , but it is optional. In other words, the area division unit may divide not the corrected image but an image captured by the camera module into the first area and the second area. 
     Furthermore, in the foregoing description, the digital pen has included the positional information output unit  18 , but it is optional. In other wards, the positional information stored in the storage unit  15  may be provided to an external memory (for example, a USB memory) connected to the digital pen. 
     Furthermore, in the foregoing description, the digital pen has included the estimation unit  14 , but it is optional. In other words, the digital pen may only identify the paper, and need not estimate the position of the digital pen. 
     For example, a part of structural elements included in the foregoing digital pen may be implemented, specifically, as a microprocessor, a ROM, a RAM, and so on. A computer program is stored in the RAM or a hard disk drive. Each of the structural elements achieves its function through the microprocessor&#39;s operation according to the computer program. Here, the computer program is configured by combining plural instruction codes indicating instructions for the computer in order to achieve predetermined functions. 
     Furthermore, a part of the structural elements included in the foregoing digital pen may be configured from a single System-LSI (Large-Scale Integration). The System-LSI is a super-multi-function LSI manufactured by integrating constituent units on one chip, and is specifically a computer system configured by including a microprocessor, a ROM, a RAM, and so on. A computer program is stored in the RAM. The System-LSI achieves its function through the microprocessor&#39;s operation according to the computer program. 
     Furthermore, a part of the structural elements included in the foregoing digital pen may be configured as an IC card which can be attached and detached from the respective apparatuses or as a stand-alone module. The IC card or the module is a computer system configured from a microprocessor, a ROM, a RAM, and so on. The IC card or the module may also include the aforementioned super-multi-function LSI. The IC card or the module achieves its function through the microprocessor&#39;s operation according to the computer program. The IC card or the module may also be implemented to be tamper-resistant. 
     INDUSTRIAL APPLICABILITY 
     One or more exemplary embodiments disclosed herein are applicable to input devices of user controlled apparatuses such as a STB (Set Top Box), a gaming system, and an audio visual apparatus.