Abstract:
A camera-based touch system includes at least one pair of cameras having overlapping fields of view and a touch surface encompassed within the overlapping fields of view across which a pointer is moved. The cameras of the at least one pair acquire images at intervals asynchronously. In order to estimate the position of the pointer relative to the touch surface from image data acquired by the at least one pair of cameras, the images are synthetically synchronized. During this process, for each camera in the pair, each acquired image is processed to determine the position of the pointer therein and the position of the pointer is recorded together with a timestamp representing the time elapsed between a reference point common to the cameras and the time the image was acquired. Successive pairs of recorded positions are interpolated to generate interpolated positions and the interpolated positions are recorded together with synchronization times representing times the images would have been acquired had the cameras been synchronized. Interpolated positions generated by the cameras having equivalent associated synchronization times are determined and these interpolated positions are triangulated to estimate the position of the pointer relative to the touch surface.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to camera-based touch systems and in particular to synchronization of camera images in a camera-based touch system to enhance position determination of fast moving objects.  
         BACKGROUND OF THE INVENTION  
         [0002]    Camera-based touch systems that use cameras to acquire images of a touch surface and process the image data to determine the position of a pointer relative to the touch surface are known. For example, International PCT Application No. WO 02/03316 to Smart Technologies Inc. et al discloses a camera-based touch system including a passive touch surface and a plurality of cameras associated with the touch surface. The cameras have overlapping fields of view encompassing the touch surface. The cameras acquire images of the touch surface from different locations and generate image data. A processor receives and processes the image data generated by the cameras to determine the location of a pointer captured in the images relative to the touch surface using triangulation.  
           [0003]    In order to triangulate the position of the pointer accurately, especially in situations where the pointer is moving quickly across the touch surface, it is necessary to synchronize the cameras. This is due to the fact that if the cameras are not synchronized, each camera will capture an image of the pointer at a different time and therefore, will see the pointer at a different position on the touch surface. This of course makes the results of triangulation unpredictable and inaccurate.  
           [0004]    It is therefore an object of the present invention to provide a novel system and method for synchronizing camera images in a camera-based touch system to enhance position determination of fast moving objects.  
         SUMMARY OF THE INVENTION  
         [0005]    According to one aspect of the present invention there is provided in a camera-based touch system including at least one pair of cameras having overlapping fields of view and a touch surface encompassed within said overlapping fields of view across which a pointer is moved, wherein the cameras of said at least one pair acquire images at intervals asynchronously, a method of synchronizing image data acquired by said at least one pair of cameras comprising the step of:  
           [0006]    for each camera in said pair:  
           [0007]    processing each acquired image to determine the position of said pointer therein and recording the position together with a timestamp representing the time elapsed between a reference point common to said cameras and the time the image was acquired; and  
           [0008]    interpolating between pairs of recorded positions to generate interpolated positions and recording each interpolated position together with a synchronization time representing a time each image would have been acquired had said cameras been synchronized.  
           [0009]    Preferably, the interpolating is performed between each successive pair of recorded positions. The reference point is preferably, a signal sent to each of the cameras simultaneously. A timer associated with each camera is initiated in response to the signal and the value of the timer is read when each image is acquired thereby to determine the timestamp.  
           [0010]    According to another aspect of the present invention there is provided in a camera-based touch system including at least one pair of cameras having overlapping fields of view and a touch surface encompassed within said overlapping fields of view across which a pointer is moved, wherein the cameras of said at least one pair acquire images at intervals asynchronously, a method of estimating the position of said pointer relative to said touch surface from image data acquired by said at least one pair of cameras, said method comprising the step of:  
           [0011]    for each camera in said pair:  
           [0012]    processing each acquired image to determine the position of said pointer therein and recording the position together with a timestamp representing the time elapsed between a reference point common to said cameras and the time the image was acquired; and  
           [0013]    interpolating between successive pairs of recorded positions to generate interpolated positions and recording said interpolated positions together with synchronization times representing times the images would have been acquired had said cameras been synchronized; and  
           [0014]    determining interpolated positions generated by said cameras having equivalent associated synchronization times and triangulating the interpolated positions to estimate the position of the said pointer relative to said touch surface.  
           [0015]    In accordance with yet another aspect of the present invention there is provided a camera-based touch system comprising:  
           [0016]    at least one pair of cameras associated with a touch surface and having overlapping fields of view encompassing said touch surface, said at least one pair of cameras acquiring images of said touch surface from different locations and generating image data;  
           [0017]    a processor receiving and processing the image data generated by said at least one pair of cameras to determine the location of an object relative to the touch surface by triangulation when the object is captured in images acquired by the at least one pair of cameras; and  
           [0018]    a synchronization mechanism to synchronize image data generated by said at least one pair of cameras.  
           [0019]    In accordance with still yet another aspect of the present invention there is provided a method of determining the position of a pointer relative to a touch surface comprising the steps of:  
           [0020]    acquiring synchronized image data of said touch surface from different locations using cameras having overlapping fields of view; and  
           [0021]    processing the image data to yield pointer position data; and  
           [0022]    triangulating the pointer position data to determine the position of said pointer relative to said touch surface.  
           [0023]    The present invention provides advantages in that since the position of the pointer is derived from synchronized camera image data, the pointer position relative to the touch surface can be accurately determined using triangulation. In the case of asynchronously captured images, the pointer position data derived from images is adjusted to approximate pointer position data that would have been derived from the images had the images been synchronously captured. In this manner, the position of the pointer can be accurately determined using triangulation notwithstanding the asynchronous image acquisition.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    Embodiments of the present invention will now be described more fully with reference to the accompanying drawings in which:  
         [0025]    [0025]FIG. 1 is a schematic diagram of a camera-based touch system in accordance with the present invention;  
         [0026]    [0026]FIG. 2 is an isometric view of a touch screen forming part of the touch system of FIG. 1;  
         [0027]    [0027]FIG. 3 is an isometric view of a corner portion of the touch screen of FIG. 2;  
         [0028]    [0028]FIG. 4 is a schematic diagram of a digital camera forming part of the touch screen of FIG. 2;  
         [0029]    [0029]FIG. 5 is a schematic diagram of a master controller forming part of the touch system of FIG. 1;  
         [0030]    [0030]FIG. 6 shows triangulation geometry used to calculate a pointer contact position on the touch surface of the touch screen;  
         [0031]    [0031]FIG. 7 is an isometric view of a portion of the touch screen showing how a pair of cameras sees the position of a pointer when the pointer is moved quickly across the touch surface during asynchronous image acquisition; and  
         [0032]    [0032]FIG. 8 is a flowchart showing the steps performed by each camera during x-position adjustment to synthesize camera synchronization. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0033]    Turning now to FIG. 1, a camera-based touch system such as that described in International PCT No. WO 02/03316 filed on Jul. 5, 2001, assigned to the assignee of the present invention, the contents of which are incorporated herein by reference, is shown and is generally identified by reference numeral  50 . As can be seen, touch system  50  includes a touch screen  52  coupled to a digital signal processor (DSP) based master controller  54 . Master controller  54  is also coupled to a computer  56 . Computer  56  executes one or more application programs and provides display output that is presented on the touch screen  52  via a projector  58 . The touch screen  52 , master controller  54 , computer  56  and projector  58  form a closed-loop so that user contacts with the touch screen  52  can be recorded as writing or drawing or used to control execution of application programs executed by the computer  56 .  
         [0034]    FIGS.  2  to  4  better illustrate the touch screen  52 . Touch screen  52  includes a touch surface  60  bordered by a rectangular frame  62 . Touch surface  60  is in the form of a rectangular planar sheet of passive material. DSP-based CMOS digital cameras  63   0  to  63   3  are positioned adjacent each corner of the touch screen  52 . Each digital camera  63   N  is mounted on a frame assembly  64 . Each frame assembly  64  includes an angled support plate  66  on which the digital camera  63   N  is mounted. Supporting frame elements  70  and  72  are mounted on the plate  66  by way of posts  74  and secure the plate  66  to the frame  62 .  
         [0035]    Each digital camera  63   N  includes a two-dimensional CMOS image sensor  80  having an associated lens assembly, a first-in-first-out (FIFO) buffer  82  coupled to the image sensor  80  by a data bus and a digital signal processor (DSP)  84  coupled to the FIFO  82  by a data bus and to the image sensor  80  by a control bus. A boot EPROM  86  and a power supply subsystem  88  are also included.  
         [0036]    In the present embodiment, the CMOS camera image sensor  80  is a National LM9617 image sensor configured for a 640×20 pixel subarray that can be operated to capture image frames at rates in excess of 200 frames per second. Arbitrary pixel rows of the image sensor  80  can be selected. Since the pixel rows can be arbitrarily selected, the pixel subarray can be exposed for a greater duration for a given digital camera frame rate providing for good operation in darker rooms in addition to well lit rooms. The FIFO buffer  82  is manufactured by Cypress under part number CY7C4211V and the DSP  84  is manufactured by Analog Devices under part number ADSP2185M.  
         [0037]    The DSP  84  receives and processes image frames from the image sensor  80  to determine the x-positions of a pointer within the image frames. In addition, the DSP  84  provides control information to the image sensor  80  via the control bus. The control information allows the DSP  84  to control parameters of the image sensor  80  such as exposure, gain, array configuration, reset and initialization. The DSP  84  also provides clock signals to the image sensor  80  to control the frame rate of the image sensor  80 .  
         [0038]    The angle of the plate  66  and the optics of the digital cameras  63   N  are selected so that the field of view (FOV) of each digital camera  63   N  is slightly beyond 90°. In this way, the entire touch surface  60  is within the field of view of each digital camera  63   N  with the field of view of each digital camera  63   N  extending slightly beyond a designated peripheral edge of the touch surface  60  as shown in FIG. 6.  
         [0039]    Master controller  54  is best illustrated in FIG. 5 and includes a DSP  90 , a boot EPROM  92 , a serial line driver  94  and a power supply subsystem  95 . The DSP  90  communicates with the DSPs  84  of the digital cameras  63   0  to  63   3 , over a data bus via a serial port  96  and communicates with the computer  56  over a data bus via a serial port  98  and the serial line driver  94 . In this embodiment, the DSP  90  is manufactured by Analog Devices under part number ADSP2185M. The serial line driver  94  is manufactured by Analog Devices under part number ADM222.  
         [0040]    The master controller  54  and each digital camera  63   N  follow a communication protocol that enables bi-directional communications via a common serial cable similar to a universal serial bus (USB). The transmission bandwidth is divided into thirty-two (32) 16-bit channels. Of the thirty-two channels, six (6) channels are assigned to each of the DSPs  84  in the digital cameras  63   0  to  63   3  and to the DSP  90  in the master controller  54  and the remaining two (2) channels are unused. The master controller  54  monitors the twenty-four (24) channels assigned to the DSPs  84 . The DSPs  84  monitor the six (6) channels assigned to the DSP  90  of the master controller  54 . Communications between the master controller  54  and the digital cameras  63   0  to  63   3  are performed as background processes in response to interrupts.  
         [0041]    The operation of the touch system  50  will now be described. Initially, a camera offset angle calibration routine is performed to determine the offset angle δ of each digital camera  63   N  (see FIG. 6). Details of the camera offset angle calibration are described in Applicants&#39; co-pending U.S. application Ser. No. 09,870,698 entitled “Calibrating Camera Offsets to Facilitate Object Position Determination Using Triangulation” filed on Jun. 1, 2001, the content of which is incorporated herein by reference.  
         [0042]    With the touch system  50  calibrated, each digital camera  63   N  acquires image frames of the touch surface  60  within the field of view of its image sensor  80  at a desired frame rate and processes each acquired image frame to determine if a pointer is in the acquired image frame. During this operation, the DSP  84  reads each image frame from the FIFO buffer  82  and processes the image frame.  
         [0043]    If a pointer is in the acquired image frame, the image frame is further processed by the DSP  84  to determine the x-position of the pointer. The z-position of the pointer is also determined so that a determination can be made as to whether the pointer is contacting or hovering above the touch surface  60 . The x-position data generated by the DSP  84  is then adjusted for camera synchronization purposes, as will be described. Pointer information packets (PIPs) including the pointer position information, status and/or diagnostic information are then generated by the DSP  84  and the PIPs are queued for transmission to the master controller  54 . The digital cameras  63   0  to  63   3  also receive and respond to command PIPs generated by the master controller  54 .  
         [0044]    The master controller  54  polls the digital cameras  63   0  to  63   3  for PIPs in the queues. In this particular embodiment, the master controller  54  polls the digital cameras at a rate exceeding the image sensor frame rates. Upon receipt of PIPs from the digital cameras  63   N , the master controller  54  examines the PIPs to determine if the PIPs include pointer location data. If the PIPs include pointer location data, the master controller  54  triangulates the pointer location data in the PIPS to determine the position of the pointer relative to the touch surface  60  in Cartesian rectangular coordinates. The master controller  54  in turn transmits calculated pointer position data, status and/or diagnostic information to the computer  56 . In this manner, the pointer position data transmitted to the computer  56  can be recorded as writing or drawing or can be used to control execution of application programs executed by the computer  56 . The computer  56  also updates the display output conveyed to the projector  58  so that information presented on the touch surface  60  reflects the pointer activity.  
         [0045]    The master controller  54  also receives commands from the computer  56  and responds accordingly as well as generates and conveys command PIPs to the digital cameras  63   N . Specifics of the manner in which the cameras  63   N  determine the pointer x and z positions from the image frame data and create PIPs is described in International PCT Application No. WO 02/03316 and therefore, will not be described herein.  
         [0046]    When a pointer is stationary on the touch surface  60  or when the pointer is moving slowly across the touch surface  60 , the triangulated positions of the pointer relative to the touch surface  60  over time are accurate. However, when the pointer moves quickly across the touch surface  60 , a pair of digital cameras  63   N  capturing images of the pointer will see the pointer at different positions on the touch surface  60  if the digital cameras  63   N  are capturing images at different times. FIG. 7 illustrates the above scenario. In this example, camera  63 , captures images of the pointer slightly ahead of digital camera  63   0 . Therefore, as line L is drawn across the touch surface  60 , the pointer x-position returned by each digital camera  63   N , each time that digital camera acquires an image, is different. As a result, triangulating the x-positions returned by the digital cameras, results in inaccuracies.  
         [0047]    Accordingly, to deal with the above problem, in one embodiment of the present invention the camera-based touch system  50  performs synthetic camera synchronization to maintain triangulation accuracy notwithstanding the fact that the digital cameras  63   N  acquire images asynchronously. In particular, during synthetic camera synchronization the DSPs  84  in the digital cameras  63   N  adjust the x-position data derived from captured image frames to approximate x-position data that would have been derived from the image frames had the image frames been synchronously captured by the digital cameras  63   N . Specifics concerning synthetic camera synchronization will now be described with particular reference to FIG. 8.  
         [0048]    When the DSP  84  in a digital camera  63   N  receives an EOF signal from its associated image sensor  80  (step  150 ), signifying that a new image frame is ready to be read from the FIFO buffer  82 , the DSP  84  examines the status of the digital camera  63   N  to determine if the digital camera has stalled (step  152 ). If the camera has not stalled, the value of the DSP internal timer (“TimeStamp”) is read and the image frame is processed to determine the x-position of the pointer in the image frame (step  154 ). The TimeStamp and the pointer x-position form an entry that is used to update a camera history table maintained by the DSP  84  (step  156 ).  
         [0049]    At step  152 , if the digital camera  63   N  has stalled (i.e. image processing for the prior image frame has not been completed by the DSP  84 ), to avoid losing the TimeStamp read from the DSP internal timer, the x-position is estimated by extrapolating the x-positions of the previous two entries in the camera history table (step  158 ). The extrapolated x-position and the TimeStamp form an entry that is used to update the camara history table (step  156 ). This procedure is considered as error recovery, due to the fact that problems can arise if stalled image frames become frequent or consecutive.  
         [0050]    Once the camera history table has been updated, the DSP  84  updates a synchronization table maintained by the DSP  84  using the entries in the camera history table (step  162 ).  
         [0051]    In the present embodiment, the camera history table includes six entries to ensure that triangulation can be performed even if the digital cameras  63   N  become out of phase by four image frames. During updating of the camera history table at step  156 , whenever the DSP  84  generates a new x-position and reads the TimeStamp from the DSP internal timer in response to an EOF signal, the DSP  84  rolls the camera history table back by one position. In this manner, the oldest entry in the camera history table at position [0] is discarded and the new entry is placed in the camera history table at position [5].  
         [0052]    Table 1 below shows camera history tables maintained by the DSPs  84  of digital cameras  63   0  and  63   1 . As can be seen, each entry in each of the camera history table include TimeStamp and the associated x-position.  
                                                                                                   Camera 63 0     Camera 63 1                      TimeStamp   x-position   TimeStamp   x-position                            200   10   3700   10           700   20   400   20           1200   30   900   30           1700   40   1400   40           2200   50   1900   50           2700   60   2400   60                      
 
         [0053]    Table 2 below shows the synchronization tables that are maintained by the DSP&#39;s  84  of digital cameras  63   0  and  63   1 , based on the camera history tables of Table 1.  
                                                         Interpolated   Interpolated       Sync Time   x-position X s     x-position X s         T s     Camera 63 0     Camera 63 1                                  500   16   22       1000   26   32       1500   36   42       2000   46   52       2500   56   —       3000   —   —       3500   —   —       4000   —   16                  
 
         [0054]    As can be seen, each entry in the synchronization tables includes a synchronization time T S  and an interpolated x-position X S . The interpolated x-positions X S  are determined as follows. Assuming that the velocity of the pointer is constant as the pointer travels between two (2) points, the interpolated x-positions are calculated using the equation:  
           X   S =(( X   1   −X   0 )/( T   1   −T   0 ))*( T   S   −T   0 )+ X   0    (1)  
         [0055]    where:  
         [0056]    X 0  and X 1  are successive x-position entries in the camera history tables;  
         [0057]    T 1  and T 0  are successive TimeStamps corresponding to the x-position entries X 1  and X 0 ; and  
         [0058]    T S  is a given synchronization time, where T 0 ≦T S  ≦T 1 .  
         [0059]    For example, using the first two entries in the camera history table maintained by digital camera  63   0  shown in Table 1 and a synchronization time T S  equal to 500, equation (1) yields:  
           X   S =((20−10)/(700−200))*(500−200)+10=16  
         [0060]    In order to interpolate the x-position data accurately, the DSP internal timers need to be calibrated against a common reference. This is due to the fact that the DSP internal timers, although similar, are not identical. As a result, if the DSP internal timers are not reset, a phase error will be introduced. Moreover, the maximum TimeStamp allowable for 16-bit integer math is 32767. If the TimeStamp is permitted to exceed this maximum limit, problems arise. In the present embodiment, a signal embedded in the command PIPs generated by the master controller  54 , that are sent to each of the digital cameras  63   N  simultaneously, is used by the digital cameras  63   N  to reset the DSP internal timers.  
         [0061]    Since the DSP internal timers are simultaneously reset, the TimeStamps constitute baseline data that exhibits the following known attributes:  
         [0062]    i) each TimeStamp is greater than its predecessor (T 1 &gt;T 0 ) except following a DSP internal timer reset; and  
         [0063]    ii) the elapsed time between successive TimeStamps is constant (C=T 1 −T 0 ) since the camera frame rates are constant.  
         [0064]    As mentioned above, the attribute T 1 &gt;T 0  does not hold true following a DSP internal timer reset. In this case when using equation (1) to calculate the interpolated x-position X S  following a DSP internal timer reset, the term (T 1 −T 0 ) in equation (1) yields an incorrect and unpredictable value. Thus, when the attribute T 1 &gt;T 0  does not hold true, the occurrence of a DSP internal timer reset can be recognized by the DSP  84  allowing TimeStamp T 1  to be corrected prior to performing the interpolation calculation. Since the elapsed time between successive TimeStamps is a known constant C, following a DSP internal timer reset, the TimeStamp T 1  is adjusted by assigning the TimeStamp T 1  a replacement value equal to C+T 0 .  
         [0065]    The synchronization times T S  used by each digital camera  63   N  during the interpolation calculations are the same and the interval between successive synchronization times T S  is constant. As a result, although the TimeStamps in the camera history tables fluctuate, the synchronization times T S  in the synchronization tables do not. Since the interval between successive synchronization times T S  is also a known constant, following a DSP internal timer reset, the synchronization time T S  can also be corrected so that the term (T S −T 0 ) in equation (1) yields a meaningful result during the interpolation calculation.  
         [0066]    The interval between successive synchronization times T S  should be at least the same as the elapsed time constant C for camera-based touch systems that include only include one pair of cameras. For example, if the elapsed time constant C between T 1  and T 0  is equal to 500, the interval between synchronization times T S  should also be equal to 500. As the number of cameras in the camera-based touch system increases, the interval between synchronization times T S  should be greater than the elapsed time constant C. This results in an increase in the number of non-redundant interpolated points that are generated by the digital cameras  63   N  and hence, an increase in touch system resolution.  
         [0067]    When the digital cameras  63   N  are polled by the master controller  54 , the digital cameras  63   N  package the interpolated x-positions X S  in the synchronization tables into PIPs and convey the PIPs to the master controller  54  (step  164 ). During polling, the master controller  54  sends a poll number to each digital camera  63   N  The poll number signifies the synchronization time T S  for which an interpolated x-position X S  is desired. If the synchronization table maintained by the DSP  84  of the digital camera  63   N  includes an interpolated x-position X S  for the specified synchronization time T S , the interpolated x-position X S  is packaged into a PIP and the PIP is conveyed to the master controller  54 .  
         [0068]    Upon receipt of the PIPs, the master controller  54  uses interpolated x-positions X S  in the PIPs received from pairs of digital cameras having equivilent synchronization times to triangulate the position of the pointer. In the example of Table 2, synchronization time 2000 is the most recent synchronization time at which digital cameras  63   0  and  63   1  will return interpolated x-positions X S  to the master controller  54  if polled for these interpolated x-positions. As a result, these interpolated x-positions X S  can used by the master controller  54  to triangulate the pointer position. Specifics of the triangulation methodology are described in International PCT Application No. WO 02/03316 and therefore, will not be described herein.  
         [0069]    If desired, the camera history tables and the synchronization tables can be maintained by the DSP  90  of the master controller  54  or by the computer  56 . In this case, the camera frame rates must be constant and known. Using the DSPs  84  to maintain the camera history tables and the synchronization tables allows the camera frame rates to be variable.  
         [0070]    As an alternate solution to achieve digital camera synchronization, the DSP  84  in each of the digital cameras can be provided with synchronization logic that is responsive to a programmable high-speed signal generator that generates the horizontal and vertical synchronization signals for the camera image sensor  80 . The high-speed signal generators are programmed to ensure that each of the digital cameras  63   N  captures an image of the touch surface  60  at the same time so that the images captured by the digital cameras are synchronized. Although this results in synchronized camera images, it is a more costly solution than that of the first embodiment due to the fact that the synchronization logic and programmable high-speed signal generators take up real estate on the digital camera boards and therefore, increase costs.  
         [0071]    Although the equation (1) interpolates within the interval T 0  to T 1 , those of skill in the art will appreciate that it is possible to interpolate outside of the interval T 0  to T 1  although the assumption that the velocity of the pointer is constant during the interpolation interval becomes less true.  
         [0072]    Although preferred embodiments of the present invention have been described, those of skill in the art will appreciate that variations and modifications may be made without departing from the spirit and scope thereof as defined by the appended claims.