Patent Publication Number: US-8976159-B2

Title: Variable size sensing system and method for redefining size of sensing area thereof

Description:
This application claims the priority benefit of Taiwan application serial no. 097150146, filed on Dec. 22, 2008. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to position sensing technology and more specifically to a variable size sensing system and a method for redefining the size of a sensing area thereof. 
     2. Description of Related Art 
     Referring to  FIG. 1 , a conventional sensing system  100  includes a panel  110 , image sensing devices  120  and  130 , and a processing circuit  140 . The panel  110  includes a touch surface  112 , and the shape of the touch surface  112  is a rectangle. The image sensing devices  120  and  130  are located at different corners of the touch surface  112  yet along a same boundary thereof so that the sensing ranges of the image sensing devices  120  and  130  respectively cover the touch surface  112 . The image sensing devices  120  and  130  are both connected to the processing circuit  140 . 
     When a pointer  150  touches the touch surface  112  or is placed in the proximity thereof, the image sensing devices  120  and  130  can respectively sense the pointer  150  along the sensing lines  162  and  164 . Thus, the processing circuit  140  can identify the sensing lines  162  and  164  from the images that the image sensing devices  120  and  130  sense, and calculate the positional coordinates of the pointer  150  according to the two sensing lines so as to complete sensing the position of the pointer  150 . 
     However, since the image sensing devices  120  and  130  are fixed onto or embedded into the panel  110 , the distance between the two image sensing devices is fixed. Once the size of the panel  110  is chosen, the sensing area in which a user can input coordinates is fixed and there is no way to change it. 
     BRIEF SUMMARY 
     The present invention relates to a variable size sensing system, wherein the size of sensing area can be redefined and adjusted. 
     The present invention further relates to a method for redefining the size of the sensing area of the above mentioned variable size sensing system. 
     A preferred embodiment of the present invention provides a variable size sensing system including a first element, a second element, a third element, a fourth element, two image sensing devices and a mark. The first, second, third and fourth elements are consecutively connected and thereby forming a frame. The first element and the third element are capable of increasing their lengths in a predetermined direction so that the size of the frame can be adjusted. The inner edge of the frame defines a sensing area that has a parallelogram shape. Surfaces of the second, the third and the fourth elements facing the sensing area are applied with a reflective material. The two image sensing devices are respectively disposed at the two ends of the first element and staying at two different corners of the sensing area so that the sensing ranges of the image sensing devices cover the sensing area. The mark is disposed on a surface of the third element facing the sensing area and apart from the fourth element by a fixed distance. 
     In another preferred embodiment of the present invention, another variable size sensing system including a first element, a second element, a third element, a fourth element, two image sensing devices and a mark. The first, second, third and fourth elements are consecutively connected and thereby forming a frame. The second element and the fourth element are capable of increasing their lengths in a predetermined direction so that the size of the frame can be adjusted, the predetermined direction being the direction departing from the first element. The inner edge of the frame defines a sensing area that has a parallelogram shape. Surfaces of the second, the third and the fourth elements facing the sensing area are applied with a reflective material. The two image sensing devices are respectively disposed at the two ends of the first element and staying at two different corners of the sensing area so that the sensing ranges of the image sensing devices cover the sensing area. The mark is disposed on a surface of the fourth element facing the sensing area and apart from the first element by a fixed distance. 
     In yet another embodiment of the present invention, a method for redefining the size of a sensing area of a variable size sensing system is provided. The sensing system includes a first element, a second element, a third element and a fourth element, the four elements being consecutively connected and thereby forming a frame. The first element and the third element are capable of increasing their lengths in a predetermined direction so that the size of the frame is adjusted. The inner edge of the frame defines a sensing area that has a parallelogram shape. Surfaces of the second, the third and the fourth elements facing the sensing area are applied with a reflective material. The sensing system further having a mark disposed on a surface of the third element facing the sensing area and apart from the fourth element by a fixed distance. The method includes: changing the sensing system from a first working mode to a second working mode so that the size of the sensing area changes from a predetermined first size to a larger second size; and calculating the change in the length of the third element between the first working mode and the second working mode so as to redefine the size of the sensing area. 
     In still another embodiment of the present invention, another method for redefining the size of a sensing area of a variable size sensing system is provided. The sensing system includes a first element, a second element, a third element and a fourth element, the four elements being consecutively connected and thereby forming a frame. The second element and the fourth element are capable of increasing their lengths in a predetermined direction so that the size of the frame is adjusted, the predetermined direction being the direction departing from the first element. The inner edge of the frame defines a sensing area that has a parallelogram shape. Surfaces of the second, the third and the fourth element facing the sensing area are applied with a reflective material. The sensing system further has a mark disposed on a surface of the fourth element facing the sensing area and apart from the first element by a fixed distance. The method includes changing the sensing system from a first working mode to a second working mode so that the size of the sensing area changes from a predetermined first size to a larger second size; and calculating the change in the length of the fourth element between the first working mode and the second working mode so as to redefine the size of the sensing area. 
     In the embodiments of the present invention, the sensing system includes four elements, a mark and two image sensing devices. The four elements are consecutively connected and thereby forming a frame. Two of the four elements have variable lengths so as to adjust the size of the frame. The inner edge of the frame defines a sensing area of a parallelogram shape. The sensing system has a first and a second working mode. The sensing area has a first size and a second size when the sensing system is in the first working mode and the second working mode respectively, wherein the first size is predetermined and smaller than the second size. The mark is used to mark a fixed length and the image sensing devices are adjusted in position according to the working mode the sensing system is in. When the sensing system is changed from the first working mode to the second working mode, the size of the sensing area changes from the first size to the second size, the mark sensed by the image sensing devices can be utilized to redefine the size of the sensing area in the sensing system. Hence, the size of the sensing area can be redefined according to the adjustment of the working mode of the sensing system so that the size of the area available for a user to input coordinates can be changed according to the specific demand of the user. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the various embodiments disclosed herein will be better understood with respect to the following description and drawings, in which like numbers refer to like parts throughout, and in which: 
         FIG. 1  is a schematic view of a conventional sensing system; 
         FIG. 2A  is a perspective schematic view of a sensing system in accordance with a embodiment of the present invention; 
         FIG. 2B  is a perspective schematic view of one possible design of a light transmitting surface of the sensing system depicted in  FIG. 2A ; 
         FIG. 2C  is a perspective schematic view of another possible design of a light transmitting surface of the sensing system depicted in  FIG. 2A ; 
         FIG. 2D  is a perspective schematic view of yet another possible design of a light transmitting surface of the sensing system depicted in  FIG. 2A ; 
         FIG. 3  is a perspective schematic view of a possible design of a variable length element of the sensing system depicted in  FIG. 2A ; 
         FIG. 4  is a perspective schematic view of another possible design of a variable length element of the sensing system depicted in  FIG. 2A ; 
         FIG. 5  is a top view of the sensing system depicted in  FIG. 2A  illustrating how the sensing system&#39;s size is changed; 
         FIG. 6  illustrates the detection of coordinates of a pointer in the sensing area of the sensing system depicted in  FIG. 2A ; 
         FIG. 7  illustrates how the equation of the sensing line  606  in the sensing system depicted in  FIG. 2A  can be determined; 
         FIG. 8  illustrates how the equation of the sensing line  608  in the sensing system depicted in  FIG. 2A  can be determined; 
         FIG. 9  illustrates the operations that the processing circuit  214  carries out when the sensing system depicted in  FIG. 2A  is in a second working mode and the sensing area thereof has a second size; 
         FIG. 10  illustrates the process of the processing circuit  214  calculating the difference between the length of  FD  and the length of  CD ; 
         FIG. 11  illustrates the operations that the processing circuit  214  carries out when the sensing system depicted in  FIG. 2A  is in a third working mode and the sensing area thereof has a third size; 
         FIG. 12  illustrates the process of the processing circuit  214  calculating the difference between the length of  GA  and the length of  DA ; 
         FIG. 13  illustrates the architecture of an image sensing device suitable for being used with the reflective material in the sensing system depicted in  FIG. 2A ; 
         FIG. 14  illustrates an image sensed by the image sensing device  210  in the sensing system depicted in  FIG. 2A ; 
         FIG. 15  illustrates the main process of a method for redefining the size of a sensing area of a variable size sensing system according to an embodiment of the present invention; 
         FIG. 16  illustrates the main process of a method for redefining the size of a sensing area of a variable size sensing system according to another embodiment of the present invention; 
         FIG. 17  illustrates a way of applying a sensing system according to the embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 2A , an embodiment of the present invention provides a variable size sensing system  200 . The sensing system  200  includes variable length elements  202 ,  204 ,  206  and  208 , image sensing devices  210  and  212 , and a processing circuit  214 . The four variable length elements are consecutively connected into a frame. The inner edge of the frame defines a parallelogram-shaped sensing area. In this embodiment, the sensing area has a rectangular shape. Surfaces of the variable length elements  204 ,  206  and  208  facing the sensing area are applied with a reflective material such as a retro-reflective material. The reflective property of these surfaces is independent of the change in length of the elements  204 ,  206  and  208 . The function of the reflective material will become more clear from discussion which follows later in this description. 
     In this embodiment, the variable length element  202  has a shell structure so that the image sensing devices  210  and  212  and the processing circuit  214  can be disposed therein. In addition, the variable length element  202  has a light transmitting surface  280  so that the image sensing devices  210  and  212  can both sense images coming through the light transmitting surface  280 . The image sensing devices  210  and  212  are respectively disposed on the two ends of the variable length element  202  and located at two different corners of the sensing area so that the sensing ranges of the two image sensing devices respectively cover the sensing area. The two image sensing devices are electrically connected to the processing circuit  214  so that when a pointer  216  enters the sensing area, the processing circuit  214  can calculate the coordinates of the pointer  216  according to the images sensed by the image sensing devices. 
     It should be noted that the light transmitting surface  280  can be designed as a whole transparent plane as depicted in  FIG. 2B . The light transmitting surface  280  can be further designed to allow only infrared light to pass through and function as an infrared pass filter. Alternatively, the light transmitting surface  280  can be designed to have two transparent windows  282  and  284  as illustrated in  FIG. 2C . The transparent windows  282  and  284  must be respectively placed close to the image sensing devices  210  and  212  and must have sufficient area so that the image sensing devices  210  and  212  can respectively sense images coming from the sensing area through the respective transparent windows. In the same way, the transparent windows can be designed to allow only infrared light to pass through. Moreover, the light transmitting surface  280  can be designed to have only one transparent window  286 , as illustrated in  FIG. 2D , as long as the image sensing devices  210  and  212  can sense images coming from the sensing area through the transparent window  286 . Further in the same way, the transparent window  286  can also be designed to allow only infrared light to pass through. 
     Referring to  FIG. 2A , to make the sensing system  200  have communication capability, the variable length element  202  can be designed to further include a communication interface  218 . The communication interface  218  is electrically connected to the processing circuit  214  so as to transmit data output by the processing circuit  214  to a receiver (not shown in  FIG. 2A ) by a communication protocol. The communication interface  218  can be a wired or wireless interface, and can be a USB (Universal Serial Bus) interface as well. 
     The following two examples are given to illustrate how the variable length elements can be structured, although they are not intended to be directly applied to the elements  202 ,  204 ,  206  and  208  or to limit the scope of the present invention. Referring to  FIG. 3 , which illustrates one possible structure of the variable length element, the variable length element  300  includes a shell  302  and another shell  304 , the shell  304  including an element  304 - 1  and an element  304 - 2 . The shell  304  is configured to accommodate the shell  302 . When the element  304 - 2  moves along the direction of the arrow in  FIG. 3 , the shell  302  accommodated in the shell  304  is pulled out by the element  304 - 2 . In other words, the variable length element  300  is a retractable element. Referring to  FIG. 4 , which illustrates another possible structure of the variable length element, the variable length element  400  includes an element  402  and an element  404 . The element  402  has a protrusion  406  and the element  404  has a recess  408 . The element  402  and the element  404  are connected to each other through the protrusion  406  and the recess  408 . In other words, the variable length element  400  is a combined element. 
     Referring to  FIG. 5 , in the four variable length elements, the element  202  and the element  206  are configured to increase their lengths along the direction of the arrow  502 , while the element  204  and the element  208  are configured to increase their lengths along the direction of the arrow  504 , so that the size of the frame composed by the elements  202 ,  204 ,  206  and  208  can be adjusted and the sensing area defined by the frame can be increased. It is noted here that the image sensing device  212  need to move along the direction of the arrow  502  while the element  202  changes its length so as to always stay at the corner of the sensing area after the size of the area is changed and thereby make sure the sensing range of the image sensing device  212  covers the adjusted sensing area. In addition, the image sensing device  212  also needs to stay in electrical connection with the processing circuit  214  after being moved. 
     It is understood that the variable length elements  202  and  206  can also move along a direction opposite to the arrow  502  to increase their lengths while the variable length elements  204  and  208  can also move along a direction of the arrow  504  to increase their lengths, so that a larger sensing area can be defined. It is noted that in this case, the image sensing devices  210  and  212  also need to always stay at the corners of the sensing area after the size thereof is changed. 
     Referring to  FIG. 5 , the sensing area defined by solid lines is the sensing area the sensing system  200  has in a first working mode. The sensing area has a predetermined first size and therefore all the coordinates in the sensing area are predefined. In this working mode, the variable length elements  202 ,  204 ,  206  and  208  all have their original lengths without being elongated. If a pointer is placed in any location in the sensing area, the sensing system  200  can detect the coordinates of the pointer&#39;s position. The method for such detection is illustrated in  FIG. 6 . 
     Referring to  FIG. 6 , the sensing system  200  has image sensing devices  210  and  212 , and a sensing area  602 , which has the above-mentioned first size. The sensing area  602  has consecutive first, second, third and fourth boundaries  AB ,  BC ,  CD  and  DA . A pointer  604  placed in the sensing area  602  is to be detected. The image sensing devices  210  and  212  can sense the pointer  604  respectively along the sensing lines  606  and  608 . Thus, if the equations of the two sensing lines are determined, the coordinates of the cross point of the sensing lines, i.e., the coordinates of the position of the pointer  604  can be calculated. 
       FIG. 7  illustrates how the equation of the sensing line  606  can be determined. In order to determine the equation of the sensing line  606 , the coordinates of the point A and A′ need to be determined first. Because of the size of the sensing area is predetermined, the coordinates of the points A, B, C and D are all known by the system. For the same reason, the vertical coordinate of the point A′ is known, and it is the same as the vertical coordinate of the point D. The horizontal coordinate of the point A′ is yet to be determined. If an imaginary line  610  is drawn between the point B and the point D, the imaginary line  610  will cross the sensing line  606  at a point Z. The triangle composed by  AB ,  BZ  and  ZA  and the triangle composed by  DA′ ,  A′Z  and  ZD  are similar triangles. The proportion of the length of  BZ  to the length of  ZD  is equal to the proportion of the length of  AB  to the length of  DA′ . Hence, if the proportion of the length of  BZ  to the length of  ZD  is determined, the length of  DA′  can be derived. 
     In practice, the imaginary line  610  can be realized by the light reflected by the reflective material on the variable length elements  204  and  206 . The light reflected by the variable length elements  204  and  206  forms a bright line in the image sensed by the image sensing device  210 . The bright line corresponds to the imaginary line  610 . In this bright line, there is a dark mark corresponding to the position of the point Z because of the placement of the pointer  604 . 
     Referring to  FIG. 7 , because the sensing resolution of the image sensing device  210  is known, the proportion of the length of  BZ  to the length of  ZD  can be calculated according to the number of pixels corresponding to  BZ  and the number of pixels corresponding to  ZD  in the bright line in the image sensed by the image sensing device  210 , which corresponds to the imaginary line  610 . This proportion is equal to the proportion of the length of  AB  to the length of  DA′ , and because the length of  AB  is known, the length of  DA′  can be calculated and so can calculate the horizontal coordinate of the point A′. After that, the linear equation of the sensing line  606  can be determined according to the coordinates of the point A and the point A′. 
     In the same way, the linear equation of sensing line  608  can be determined. Referring to  FIG. 8 , the point Z′ is the cross point of the sensing line  608  and an imaginary line  612 . The triangle composed by  AB ,  BZ′  and  Z′A  and the triangle composed by  B′C ,  CZ′  and  Z′B′  are similar triangles. The proportion of the length of  Z′A  to the length of  CZ′  is equal to the proportion of the length of  AB  to the length of  B′ C . Hence, if the proportion of the length of  Z′A  to the length of  CZ′  is determined, the length of  B′C  can be derived. 
     In practice, the imaginary line  612  can be realized by the light reflected by the reflective material on the variable length elements  206  and  208 . The light reflected by the variable length elements  206  and  208  forms a bright line in the image sensed by the image sensing device  212 . The bright line corresponds to the imaginary line  612 . In this bright line, there is a dark mark corresponding to the position of the point Z′ because of the placement of the pointer  604 . 
     Referring to  FIG. 8 , because the sensing resolution of the image sensing device  212  is known, the proportion of the length of  CZ′  to the length of  Z′A  can be calculated according to the number of pixels corresponding to  CZ′  and the number of pixels corresponding to  Z′A  in the bright line in the image sensed by the image sensing device  212 , which corresponds to the imaginary line  612 . This proportion is equal to the proportion of the length of  B′C  to the length of  AB , and because the length of  AB  is known, the length of  B′C  can be calculated and so can calculate the horizontal coordinate of the point B′. After that, the linear equation of the sensing line  608  can be determined according to the coordinates of the point B and the point B′. According to the linear equations of sensing lines  606  and  608 , the coordinates of the cross point of the two sensing lines can be calculated. 
     Referring to  FIG. 5 , the dashed lines illustrate an object working mode of the sensing system  200 . The sensing area in this working mode is enlarged to an object size comparing to the first working mode. Transforming from the first working mode to the object working mode, the sensing system  200  undergoes two stages. In the first stage, the variable length elements  202  and  206  are elongated along the direction of the arrow  502 . In the second stage, the variable length elements  204  and  208  are elongated along the direction of the arrow  504 . 
     Because the sensing area of the sensing system  200  in the object working mode is not predetermined, most of the coordinates in the sensing area in this mode are undefined. Thus the processing circuit  214  need to determine, for the object sensing area comparing to the sensing area of the first size, by how much it is elongated in the horizontal direction and by how much it is elongated in the vertical direction, so as to redefine the size of the sensing area  602  and the coordinates in the sensing area  602  and thereby determine a pointer&#39;s position in the object sensing area. 
       FIG. 9  illustrates the operations that the processing circuit  214  carries out after the above-mentioned first stage is completed before the second stage is initiated. Referring to  FIG. 9 , the sensing system  200  is in a second working mode, in which the sensing area  602  has a second size and consecutive fifth, sixth, seventh and eighth boundaries  AE ,  EF ,  FD  and  DA . It is noted that the first, second, third and fourth boundaries  AB ,  BC ,  CD  and  DA  defining the sensing area in the first working mode are also shown in  FIG. 9  for comparison. As shown in  FIG. 9 ,  AE ,  FD  and  DA  respectively partially overlap  AB ,  CD  and  DA , and the lengths of  AE  and  FD  are respectively greater than the lengths of  AB  and  CD . 
     Referring to  FIG. 9 , the difference between the length of  FD  and the length of  CD , i.e., the length of  CF  is the elongated amount of the sensing area  602  in the horizontal direction when changing from the first working mode to the second working mode. To determine the length of  CF  correctly, a first mark in made on a surface of the variable length element  206  facing the sensing area  602  for marking the length of  CD . The first mark is illustrated by the arrow  902  in  FIG. 9 . By utilizing the first mark  902  sensed by the image sensing devices  210  and  212 , the processing circuit  214  can calculate the length of  CF .  FIG. 10  illustrates the process of such calculation. 
     Referring to  FIG. 9  and  FIG. 10 , to calculate the length of  CF , from the images of the sensing area  602  taken by the image sensing devices  210  and  212 , the processing circuit  214  identifies the cross point of  AE  and  DA  and regards it as the point A, identifies the cross point of  AE  and  EF  and regards it as the point E, and identifies the cross point of  EF  and  FD  and regards it as the point F, as shown in the step S 1002 . In addition, the image sensing device  212  can sense the first mark  902  along the path of the sensing line  904 , which is a line connecting the point E and the first mark  902 . 
     If an imaginary line  906  connecting the point A and the point F is provided, the sensing line  904  will cross the imaginary line  906  at a point V, and the triangle composed by  AE ,  EV  and  VA  and the triangle composed by  CF ,  FV  and  VC  are similar triangles. The processing circuit  214  can calculate difference in the length of  FD  and the length of  CD  according to the proportional relationship between the corresponding sides of the similar triangles, as shown in the step S 1004 . More specifically, for the two similar triangles, 
     
       
         
           
             
               
                 
                   
                     
                       
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     In practice, the imaginary line  906  can be realized by the light reflected by the reflective material on the variable length elements  206  and  208 . The light reflected by the variable length elements  206  and  208  forms a bright line in the image sensed by the image sensing device  212 . The bright line corresponds to the imaginary line  906 . In this bright line, there is a dark mark corresponding to the position of the point V because of the placement of the first mark  902 . Because the sensing resolution of the image sensing device  212  is known, the proportion of the length of  FV  to the length of  VA  can be calculated according to the number of pixels corresponding to  FV  and the number of pixels corresponding to  VA  in the bright line in the image sensed by the image sensing device  212 , which corresponds to the imaginary line  906 . In addition, because the length of  AB  is known, the processing circuit  214  can calculate the length of  CF , i.e., the difference between the length of  FD  and the length of  CD  from the above equation (2). 
       FIG. 11  illustrates the operations of the processing circuit  214  carries out after the above-mentioned second stage is completed. Referring to  FIG. 11 , the sensing system  200  is in the object working mode, in which the sensing area  602  has a third size and consecutive ninth, tenth, eleventh, twelfth boundaries  AE ,  EH ,  HG  and  GA . It is noted the fifth, sixth, seventh, and eighth boundaries  AE ,  EF ,  FD  and  DA  defining the sensing area in the second working mode are also shown in  FIG. 11  for comparison. As shown in  FIG. 11 ,  AE ,  EH  and  GA  respectively partially overlap  AE ,  EF  and  DA , and the lengths of  EH  and  GA  are respectively greater than the lengths of  EF  and  DA . 
     Referring to  FIG. 11 , the difference between the length of  GA  and the length of  DA , i.e., the length of  DG  is the elongated amount of the sensing area  602  in the vertical direction when changing from the second working mode to the object working mode. To determine the length of  DG  correctly, a second mark in made on a surface of the variable length element  208  facing the sensing area  602  for marking the length of  DA . The second mark is illustrated by the arrow  908  in  FIG. 11 . By utilizing the second mark  908  sensed by the image sensing devices  210  and  212 , the processing circuit  214  can calculate the length of  DG .  FIG. 12  illustrates the process of such calculation. 
     Referring to  FIG. 11  and  FIG. 12 , to calculate the length of  DG , from the images of the sensing area  602  taken by the image sensing devices  210  and  212 , the processing circuit  214  identifies the cross point of  AE  and  GA  and regards it as the point A, identifies the cross point of  AE  and  EH  and regards it as the point E, and identifies the cross point of  EH  and  HG  and regards it as the point H, as shown in the step S 1202 . In addition, the image sensing device  212  can sense the second mark  908  along the path of the sensing line  910 , which is a line connecting the point E and the second mark  908 . 
     If an imaginary line  912  connecting the point A and the point H is provided, the sensing line  910  will cross the imaginary line  912  at a point V′, and the triangle composed by  AV′ ,  V′D  and  DA  and the triangle composed by  V′E ,  EH  and  HV′  are similar triangles. The processing circuit  214  can calculate difference in the length of  GA  and the length of  DA  according to the proportional relationship between the corresponding sides of the similar triangles, as shown in the step S 1204 . More specifically, for the two similar triangles, 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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                   4 
                   ) 
                 
               
             
           
         
       
     
     In practice, the imaginary line  912  can be realized by the light reflected by the reflective material on the variable length elements  206  and  208 . The light reflected by the variable length elements  206  and  208  forms a bright line in the image sensed by the image sensing device  212 . The bright line corresponds to the imaginary line  912 . In this bright line, there is a dark mark corresponding to the position of the point V′ because of the placement of the second mark  908 . Because the sensing resolution of the image sensing device  212  is known, the proportion of the length of  AV′  to the length of  HV′  can be calculated according to the number of pixels corresponding to  AV′  and the number of pixels corresponding to  HV′  in the bright line in the image sensed by the image sensing device  212 , which corresponds to the imaginary line  912 . In addition, because the lengths of  DA  and  EF  are known, the processing circuit  214  can calculate the length of  DG , i.e., the difference between the length of  GA  and the length of  DA  from the above equation (4). After the lengths of  CF  and  DG  are both calculated, the processing circuit  214  can redefine the coordinates in the sensing area  602  according to the redefined size of the sensing area  602 . Thereby, after being increased to the object size, the sensing system  200  can detect the coordinates of a pointer placed in the sensing area  602  in the same way as described by  FIG. 6-FIG .  8 . 
     In this embodiment, after the sensing system  200  increases its size, the size of the sensing area  602  thereof is redefined. Thus the area available for inputting coordinates in the system  200  can be changed by demand. Referring to  FIG. 5 , it is understood that the sensing system  200  in the first working mode can also be operated to increase the lengths of the variable length elements  204  and  208  in the direction of the arrow  504  and then be operated to increase the lengths of the variable length elements  202  and  206  in the direction of the arrow  502 . In this case, the operations of the processing circuit  214  should change accordingly. 
     Further, the sensing system  200  in the first working mode can be operated to only increase the lengths of the variable length elements  202  and  206  in the direction of the arrow  502 , as long as after that the sensing area  602  is redefined according to the first mark  902 . Similarly, the sensing system  200  in the first working mode can be operated to only increase the lengths of the variable length elements  204  and  208  in the direction of the arrow  504 , as long as after that the sensing area  602  is redefined according to the second mark  908 . In addition, the variable length elements  204  and  208  as shown in  FIG. 2A  may be replaced by regular fixed length elements, or the variable length elements  202  and  206  as shown in  FIG. 2A  may be replaced by regular fixed length elements. 
     Referring to  FIG. 13 , an image sensing device  1300  suitable for being used with the above-mentioned reflective material is provided. The image sensing device  1300  includes an infrared lighting device  1302 , an infrared pass optical filtering device  1304  and a photosensor  1306 . The photosensor  1306  receives images through the infrared pass optical filtering device  1304  and is electrically connected to the processing circuit  214 . In addition, the infrared lighting device  1302  can be realized by an infrared light emitting diode (IR LED). The infrared pass optical filtering device  1304  can be realized by an infrared pass filter. 
     If the image sensing device  210  in  FIG. 2A  has the same architecture as the image sensing device  1300 , and the sensing system  200  has a size increase, the image sensed by the image sensing device  210  is illustrated in  FIG. 14 . Referring to  FIG. 14 , in the image sensing window  1400  of the image sensing device  210 , a bright zone  1402  is formed on the image by the light reflected from the reflective material on the variable length elements. The bright zone corresponds to the main sensing area. The dark mark  1404  is formed because of the pointer  216 . The mark  1406  is the sensed first mark. Thus, when the image sensing device  210  senses images of the sensing area, the reflective material is configured to generate a background for the pointer  216  to make the position of the pointer  216  clear. It is understood the pointer used in this embodiment is not limited to a certain shape, but it should be able to be sensed by the image sensing device and can be differentiated from the reflective material. 
     Although in the above embodiment, the first mark  902  is used to mark the length of the variable length element  206  when the sensing area has a first size, it is understood as long as the first mark  902  is disposed on a surface of the variable length element  206  facing the sensing area  602 , and apart from the variable length element  208  by a fixed distance, then the image sensing devices in the system can still sense the first mark  902  and using the first mark  902  to calculate the difference in length of the variable length element  206  between the first working mode and the second working mode so that the size of the sensing area  602  can be redefined. Similarly, as long as the second mark  908  is disposed on a surface of the variable length element  208  facing the sensing area  602 , and apart from the variable length element  202  by a fixed distance, then the image sensing devices in the system can still sense the second mark  908  and using the second mark  908  to calculate the difference in length of the variable length element  208  between the first working mode and the second working mode so that the size of the sensing area  602  can be redefined. 
     From the above embodiments, two methods for redefining the size of the sensing area can be deduced. Referring to  FIG. 15 , one of such methods suitable for a variable size sensing system is provided. The sensing system includes a first element, a second element, a third element and a fourth element, the four elements being consecutively connected and thereby forming a frame. The first element and the third element are capable of increasing their lengths in a predetermined direction so that the size of the frame is adjusted. The inner edge of the frame defines a sensing area. The sensing area has a parallelogram shape. Reflective material is applied to surfaces of the second, the third and the fourth elements facing the sensing area. The sensing system further has a mark disposed on a surface of the third element facing the sensing area and apart from the fourth element by a fixed distance. In the method includes: changing the sensing system from a first working mode to a second working mode so that the size of the sensing area changes from a predetermined first size to a larger second size (step S 1502 ); calculating the change in the length of the third element between the first working mode and the second working mode so as to redefine the size of the sensing area (S 1504 ). 
     Referring to  FIG. 16 , another method for redefining the size of a sensing area of a variable size sensing system is provided. The sensing system includes a first element, a second element, a third element and a fourth element, the four elements being consecutively connected and thereby forming a frame. The second element and the fourth element are capable of increasing their lengths in a predetermined direction so that the size of the frame is adjusted. The predetermined direction is the direction departing from the first element. The inner edge of the frame defines a sensing area. The sensing area has a shape of a parallelogram. Reflective material is applied to surfaces of the second, the third and the fourth elements facing the sensing area. The sensing system further has a mark disposed on a surface of the fourth element facing the sensing area and apart from the first element by a fixed distance. The method includes changing the sensing system from a first working mode to a second working mode so that the size of the sensing area changes from a predetermined first size to a larger second size (step S 1502 ); and calculating the change in the length of the fourth element between the first working mode and the second working mode so as to redefine the size of the sensing area (S 1504 ). 
       FIG. 17  illustrates a way of applying the sensing system provided by the embodiments of the present invention. Referring to  FIG. 17 , a user may attach a variable size sensing system  1702  provided by the present invention to a regular computer screen  1704  and use a finger or other pointers to input coordinates, which makes the computer screen function as a touch screen. If the communication interface that the sensing system  1702  has is a wired interface, then the sensing system  1702  can transmit the positional information to the computer host  1706  by wired communication. If the communication interface that the sensing system  1702  has is a wireless interface, then the sensing system  1702  can transmit the positional information to the computer host  1706  by wireless communication. 
     In summary, the sensing system provided by the embodiments of the present invention includes four elements, a mark and two image sensing devices. The four elements are consecutively connected and thereby forming a frame. Two of the four elements have variable lengths so as to adjust the size of the frame. The inner edge of the frame defines a sensing area of a parallelogram shape. The sensing system has a first and a second working mode. The sensing area has a first size and a second size when the sensing system is in the first working mode and the second working mode respectively, wherein the first size is predetermined and smaller than the second size. The mark is used to mark a fixed length and the image sensing devices are adjusted in position according to the working mode the sensing system is in. When the sensing system is changed from the first working mode to the second working mode, the size of the sensing area changes from the first size to the second size, the mark sensed by the image sensing device can be utilized to redefine the size of the sensing area in the sensing system. Hence, the size of the sensing area can be redefined according to the adjustment of the working mode of the sensing system so that the size of the area available for a user to input coordinates can be changed according to the specific demand of the user. 
     The above description is given by way of example, and not limitation. Given the above disclosure, one skilled in the art could devise variations that are within the scope and spirit of the invention disclosed herein, including configurations ways of the recessed portions and materials and/or designs of the attaching structures. Further, the various features of the embodiments disclosed herein can be used alone, or in varying combinations with each other and are not intended to be limited to the specific combination described herein. Thus, the scope of the claims is not to be limited by the illustrated embodiments.