Abstract:
An interactive input system comprises at least one imaging device having a field of view looking into a region of interest, a bezel at least partially surrounding the region of interest and having a surface in the field of view of the at least one imaging device, a first radiation source emitting radiation into the region of interact that is generally matched to the characteristics of the bezel so that the radiation emitted by the first radiation source is reflected by the bezel surface generally towards the at least one imaging device and a second radiation source emitting radiation into the region of interest that is generally unmatched to the characteristics of the bezel so that the radiation emitted by the second radiation source is not reflected by the bezel surface.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an interactive input system and to a bezel therefor. 
       BACKGROUND OF THE INVENTION 
       [0002]    Interactive input systems that allow users to inject input (e.g. digital ink, mouse events etc.) into an application program using an active pointer (eg. a pointer that emits light, sound or other signal), a passive pointer (eg. a finger, cylinder or other object) or other suitable input device such as for example, a mouse or trackball, are well known. These interactive input systems include but are not limited to: touch systems comprising touch panels employing analog resistive or machine vision technology to register pointer input such as those disclosed in U.S. Pat. Nos. 5,448,263; 6,141,000; 6,337,681; 6,747,636; 6,803,906; 7,232,986; 7,236,162; and 7,274,356 and in U.S. Patent Application Publication No. 2004/0179001 assigned to SMART Technologies ULC of Calgary, Alberta, Canada, assignee of the subject application, the contents of which are incorporated by reference; touch systems comprising touch panels employing electromagnetic, capacitive, acoustic or other technologies to register pointer input; tablet personal computers (PCs); laptop PCs; personal digital assistants (PDAs); and other similar devices. 
         [0003]    Above-incorporated U.S. Pat. No. 6,803,906 to Morrison et al. discloses a touch system that employs machine vision to detect pointer interaction with a touch surface on which a computer-generated image is presented. A rectangular bezel or frame surrounds the touch surface and supports digital cameras at its corners. The digital cameras have overlapping fields of view that encompass and look generally across the touch surface. The digital cameras acquire images looking generally across the touch surface from different vantages and generate image data. Image data acquired by the digital cameras is processed by on-board digital signal processors to determine if a pointer exists in the captured image data. When it is determined that a pointer exists in the captured image data, the digital signal processors convey pointer characteristic data to a master controller, which in turn processes the pointer characteristic data to determine the location of the pointer in (x,y) coordinates relative to the touch surface using triangulation. The pointer coordinates are conveyed to a computer executing one or more application programs. The computer uses the pointer coordinates to update the computer-generated image that is presented on the touch surface. Pointer contacts on the touch surface can therefore be recorded as writing or drawing or used to control execution of application programs executed by the computer. 
         [0004]    U.S. Patent Application Publication No. 2004/0179001 to Morrison et al. discloses a touch system and method that differentiates between passive pointers used to contact a touch surface so that pointer position data generated in response to a pointer contact with the touch surface can be processed in accordance with the type of pointer used to contact the touch surface. The touch system comprises a touch surface to be contacted by a passive pointer and at least one imaging device having a field of view looking generally along the touch surface. At least one processor communicates with the at least one imaging device and analyzes images acquired by the at least one imaging device to determine the type of pointer used to contact the touch surface and the location on the touch surface where pointer contact is made. The determined type of pointer and the location on the touch surface where the pointer contact is made are used by a computer to control execution of an application program executed by the computer. 
         [0005]    In order to determine the type of pointer used to contact the touch surface, in one embodiment a curve of growth method is employed to differentiate between different pointers. During this method, a horizontal intensity profile (HIP) is formed by calculating a sum along each row of pixels in each acquired image thereby to produce a one-dimensional profile having a number of points equal to the row dimension of the acquired image. A curve of growth is then generated from the HIP by forming the cumulative sum from the HIP. 
         [0006]    U.S. Pat. No. 7,202,860 to Ogawa discloses a camera-based coordinate input device allowing coordinate input using a pointer or finger. The coordinate input device comprises a pair of cameras positioned in the upper left and upper right corners of a display screen. The field of view of each camera extends to a diagonally opposite corner of the display screen in parallel with the display screen. Infrared emitting diodes are arranged close to the imaging lens of each camera and illuminate the surrounding area of the display screen. An outline frame is provided on three sides of the display screen. A narrow-width retro-reflection tape is arranged near the display screen on the outline frame. A non-reflective reflective black tape is attached to the outline frame along and in contact with the retro-reflection tape. The retro-reflection tape reflects the light from the infrared emitting diodes allowing the reflected light to be picked up as a strong white signal. When a user&#39;s finger is placed proximate to the display screen, the finger appears as a shadow over the bright image of the retro-reflection tape. 
         [0007]    The video signals from the two cameras are fed to a control circuit, which detects the border between the white image of the retro-reflection tape and the outline frame. A horizontal line of pixels from the white image close to the border is selected. The horizontal line of pixels contains information related to a location where the user&#39;s finger is in contact with the display screen. The control circuit determines the coordinates of the touch position, and the coordinate value is then sent to a computer. 
         [0008]    When a pen having a retro-reflective tip touches the display screen, the light reflected therefrom is strong enough to be registered as a white signal. The resulting image is not discriminated from the image of the retro-reflection tape. However, the resulting image is easily discriminated from the image of the black tape. In this case, a line of pixels from the black image close to the border of the outline frame is selected. Since the signal of the line of pixels contains information relating to the location where the pen is in contact with the display screen. The control circuit determines the coordinate value of the touch position of the pen and the coordinate value is then sent to the computer. 
         [0009]    In the Ogawa coordinate input device, resolution issues can arise if a finger that is illuminated by ambient light and/or by other source light is brought into proximity of the cameras as the finger may appear as bright as or brighter than the retro-reflection tape in images captured by the cameras. In such cases, separating the pointer from the retro-reflection tape in the captured images can provide to be difficult. As will be appreciated, improvements in interactive input systems are sought. 
         [0010]    It is therefore an object of the present invention at least to provide a novel interactive input system and a novel bezel therefor. 
       SUMMARY OF THE INVENTION 
       [0011]    Accordingly, in one aspect there is provided an interactive input system comprising at least one imaging device having a field of view looking into a region of interest; a bezel at least partially surrounding the region of interest and having a surface in the field of view of the at least one imaging device; a first radiation source emitting radiation into the region of interest that is generally matched to the characteristics of the bezel so that the radiation emitted by the first radiation source is reflected by the bezel surface generally towards the at least one imaging device; and a second radiation source emitting radiation into the region of interest that is generally unmatched to the characteristics of the bezel so that the radiation emitted by the second radiation source is not reflected by the bezel surface. 
         [0012]    In one embodiment, the interactive input system further comprises a first filter associated with the first radiation source through which radiation emitted by the first radiation source passes and a second filter on the bezel that is matched to the first filter. A third filter is associated with the second radiation source through which radiation emitted by the second radiation source passes. The third filter in unmatched to the first and second filters. Each of the first and second radiation sources comprises a light source. In one embodiment, each light source comprises one or more light emitting diodes. The first and second filters may take the form of polarizing filters having the same axis of polarization. In this case, the third filter is a polarizing filter having an axis of polarization generally orthogonal to the axes of polarization of the first and second filters. 
         [0013]    In one embodiment, the interactive input system further comprises processes structure communicating with the at least one imaging device and processing image data output thereby. The processing structure compares image data acquired by the at least one imaging device when the first radiation source is on and the second radiation source is off, with image data acquired by the at least one imaging device when the first radiation source is off and the second radiation source is on. A switching circuit connects alternately the first and second radiation sources to a power source. 
         [0014]    According to another aspect there is provided a bezel for an interactive touch surface comprising a reflective surface oriented to reflect radiation toward at least one imaging device and a filter overlying the reflective surface and matched to intermittent radiation emitted across said touch surface. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Embodiments will now be described more fully with reference to the accompanying drawings in which: 
           [0016]      FIG. 1  is a perspective view of an interactive input system; 
           [0017]      FIG. 2  is a front elevation view of the interactive input system of  FIG. 1 ; 
           [0018]      FIG. 3  is a block diagram of an imaging assembly forming part of the interactive input system of  FIG. 1 ; 
           [0019]      FIG. 4A  is a perspective view of an image sensor and radiation sources forming part of the imaging assembly of  FIG. 3 ; 
           [0020]      FIG. 4B  is a cross-sectional view of  FIG. 4A  taken along line  4 - 4 ; 
           [0021]      FIG. 5  is a front elevational view of a portion of a bezel segment forming part of the interactive input system of  FIG. 1 ; 
           [0022]      FIG. 6  is a block diagram of a digital signal processor forming part of the interactive input system of  FIG. 1 ; 
           [0023]      FIGS. 7A and 7B  are image frames captured by the imaging assembly of  FIG. 3  in the absence of a pointer; 
           [0024]      FIG. 7C  is a difference image frame generated from the image frames of  FIGS. 7A and 7B ; 
           [0025]      FIG. 7D  shows a plot of normalized intensity values I(x) calculated for pixel columns of the difference image frame of  FIG. 7C ; 
           [0026]      FIGS. 8A and 8B  are image frames captured by the imaging assembly of  FIG. 3  when a stylus is positioned adjacent to a bezel segment; 
           [0027]      FIG. 8C  is a difference image frame generated from the image frames of  FIGS. 8A and 8B ; 
           [0028]      FIG. 8D  shows a plot of normalized intensity values I(x) calculated for pixel columns of the difference image frame of  FIG. 8C ; 
           [0029]      FIGS. 9A and 9B  are image frames captured by the imaging assembly of  FIG. 3  when a stylus is positioned proximate an image sensor; 
           [0030]      FIG. 9C  is a difference image frame generated from the image frames of  FIGS. 9A and 9B ; 
           [0031]      FIG. 9D  shows a plot of normalized intensity values I(x) calculated for pixel columns of the difference image frame of  FIG. 9C ; 
           [0032]      FIG. 10  is a side elevational view of a pen tool used in conjunction with the interactive input system of  FIG. 1 ; and 
           [0033]      FIGS. 11A and 11B  show illumination of a passive pointer and the bezel by radiation emitted by the radiation sources of the imaging assembly of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0034]    Turning now to  FIGS. 1 and 2 , an interactive input system that allows a user to inject input such as digital ink, mouse events etc. into an application program is shown and is generally identified by reference numeral  20 . In this embodiment, interactive input system  20  comprises an assembly  22  that engages a display unit (not shown) such as for example, a plasma television, a liquid crystal display (LCD) device, a flat panel display device, a cathode ray tube display or monitor etc. and surrounds the display surface  24  of the display unit. The assembly  22  employs machine vision to detect pointers brought into a region of interest in proximity with the display surface  24  and communicates with a digital signal processor (DSP) unit  26  via communication lines  28 . The communication lines  28  may be embodied in a serial bus, a parallel bus, a universal serial bus (USB), an Ethernet connection or other suitable wired connection. The DSP unit  26  in turn communicates with a computer  30  executing one or more application programs via a USB cable  32 . Alternatively, the DSP unit  26  may communicate with the computer  30  over another wired connection such as for example, a parallel bus, an RS-232 connection, an Ethernet connection etc. or may communicate with the computer  30  over a wireless connection using a suitable wireless protocol such as for example Bluetooth, WiFi, ZigBee, ANT, IEEE 802.15.4, Z-Wave etc. Computer  30  processes the output of the assembly  22  received via the DSP unit  26  and adjusts image data that is output to the display unit so that the image presented on the display surface  24  reflects pointer activity. In this manner, the assembly  22 , DSP unit  26  and computer  30  allow pointer activity proximate to the display surface  24  to be recorded as writing or drawing or used to control execution of one or more application programs executed by the computer  30 . 
         [0035]    Assembly  22  comprises a frame assembly that is mechanically attached to the display unit and surrounds the display surface  24 . Frame assembly comprises a bezel having three bezel segments  40 ,  42  and  44 , four corner pieces  46  and a tool tray segment  48 . Bezel segments  40  and  42  extend along opposite side edges of the display surface  24  while bezel segment  44  extends along the top edge of the display surface  24 . The tool tray segment  48  extends along the bottom edge of the display surface  24  and supports one or more active pen tools P. The corner pieces  46  adjacent the top left and top right corners of the display surface  24  couple the bezel segments  40  and  42  to the bezel segment  44 . The corner pieces  46  adjacent the bottom left and bottom right corners of the display surface  24  couple the bezel segments  40  and  42  to the tool tray segment  48 . In this embodiment, the corner pieces  46  adjacent the bottom left and bottom right corners of the display surface  24  accommodate imaging assemblies  60  that look generally across the entire display surface  24  from different vantages. The bezel segments  40 ,  42  and  44  are oriented so that their inwardly facing surfaces are seen by the imaging assemblies  60 . 
         [0036]    Turning now to  FIGS. 3 and 4 , one of the imaging assemblies  60  is better illustrated. As can be seen, the imaging assembly  60  comprises an image sensor  70  such as that manufactured by Micron Technology, Inc. of Boise, Id. under model No. MT9V022 fitted with an 880 nm lens of the type manufactured by Boowon Optical Co. Ltd. of Korea under model No. BW25B. The lens has an IR-pass/visible light blocking filter thereon  70   a  and provides the image sensor  70  with a 98 degree field of view so that the entire display surface  24  is seen by the image sensor  70 . The image sensor  70  is connected to a connector  72  that receives one of the communication lines  28  via an I 2 C serial bus. The image sensor  70  is also connected to an electrically erasable programmable read only memory (EEPROM)  74  that stores image sensor calibration parameters as well as to a clock (CLK) receiver  76 , a serializer  78  and a current control module  80 . The clock receiver  76  and the serializer  78  are also connected to the connector  72 . Current control module  80  is also connected to infrared (IR) light sources  82   a  and  82   b  as well as to a power supply  84  and the connector  72 . In this embodiment, each IR light source comprises one or more IR light emitting diodes (LEDs). A filter  90  is provided over the IR light source  82   a  and a filter  92  is provided over IR light source  82   b . In this embodiment, the filters  90  and  92  are polarizing filters, with each polarizing filter having a single axis of polarization and with the axis of polarization of filter  90  being generally orthogonal to the axis of polarization of filter  92 . The configuration of the LEDs of each IR light source  82  is selected so that the bezel segments  40 ,  42  and  44  are generally evenly illuminated over their entire lengths. Further specifics concerning the IR light sources  82  are described in U.S. patent application Ser. No. 12/118,552 to Hansen et al. entitled “Interactive Input System And Illumination Assembly Therefor” filed on May 9, 2008 and assigned to SMART Technologies ULC of Calgary, Alberta, the content of which is incorporated herein by reference. Of course, those of skill in the art will appreciate that other types of suitable radiation sources to provide illumination to the region of interest may be used. 
         [0037]    The clock receiver  76  and serializer  78  employ low voltage, differential signaling (LVDS) to enable high speed communications with the DSP unit  26  over inexpensive cabling. The clock receiver  76  receives timing information from the DSP unit  26  and provides clock signals to the image sensor  70  that determines the rate at which the image sensor  70  captures and outputs image frames. Each image frame output by the image sensor  70  is serialized by the serializer  78  and output to the DSP unit  26  via the connector  72  and communication lines  28 . 
         [0038]      FIG. 5  shows a portion of the inwardly facing surface  100  of one of the bezel segments  40 ,  42  and  44 . As can be seen, the inwardly facing surface  100  of each bezel segment comprises a single horizontal strip or band  102  of retro-reflective material. To take best advantage of the properties of the retro-reflective material, the bezel segments  40 ,  42  and  44  are oriented so that their inwardly facing surfaces extend in a plane generally normal to that of the display surface  24 . A filter (not shown) is also provided on each bezel segment and overlies the retro-reflective band  102 . The axis of polarization of the filter over the retro-reflective band  102  of each bezel segment is matched to filter  90  of radiation source  82   a . In this manner, IR light emitted by the IR light source  82   a  that passes through filter  90 , passes through the filter over the retro-reflective band  102  of each bezel segment and is reflected by the retro-reflective band  102 . IR light emitted by IR light source  82   b  that passes through filter  92  is blocked by the filter over the retro-reflective band  102  of each bezel segment as a result of the IR light being polarized along an axis orthogonal to the axis of polarization of the filter on the bezel segments  40 ,  42  and  44 . 
         [0039]    Turning now to  FIG. 6 , the DSP unit  26  is better illustrated. As can be seen, DSP unit  26  comprises a controller  120  such as for example, a microprocessor, microcontroller, DSP etc. having a video port VP connected to connectors  122  and  124  via deserializers  126 . The controller  120  is also connected to each connector  122 ,  124  via an I 2 C serial bus switch  128 . I 2 C serial bus switch  128  is connected to clocks  130  and  132 , each clock of which is connected to a respective one of the connectors  122 ,  124 . The controller  120  communicates with an external antenna  136  via a wireless receiver  138 , a USB connector  140  that receives USB cable  32  and memory  142  including volatile and non-volatile memory. The clocks  130  and  132  and deserializers  126  similarly employ low voltage, differential signaling (LVDS). 
         [0040]    The interactive input system  20  is able to detect passive pointers such as for example, a user&#39;s finger, a cylinder or other suitable object as well as active pen tools P as shown in  FIG. 10  that are brought into proximity with the display surface  24  and within the fields of view of the imaging assemblies  60 . For ease of discussion, the operation of the interactive input system  20 , when a passive pointer is brought into proximity with the display surface  24 , will be described. 
         [0041]    During operation, the controller  120  conditions the clocks  130  and  132  to output clock signals that are conveyed to the imaging assemblies  60  via the communication lines  28 . The clock receiver  76  of each imaging assembly  60  uses the clock signals to set the frame rate of the associated image sensor  70 . In this embodiment, the controller  120  generates clock signals so that the frame rate of each image sensor  70  is twice the desired image frame output rate. The controller  120  also signals the current control module  80  of each imaging assembly  60  over the I 2 C serial bus. In response, each current control module  80  initially connects only the IR light source  82   a  to the power supply  84  and then disconnects the IR light source  82   a  from the power supply  84  and connects IR light source  82   b  to the power supply  84 . The timing of the on/off IR light source switching is controlled so that for each pair of subsequent image frames captured by each image sensor  70 , one image frame is captured when the IR light source  82   a  is on and one image frame is captured when the IR light source  82   b  is on. 
         [0042]    When the IR light sources  82   a  are on, each LED of the IR light sources  82   a  floods the region of interest over the display surface  24  with infrared illumination that has been polarized by the filters  90 . As the filters  90  are matched to the filters on the bezel segments  40 ,  42  and  44 , the infrared illumination passes through the filters on the bezel segments and impinges on the retro-reflective bands  102 . Infrared illumination that impinges on the retro-reflective bands  102  is returned to the imaging assemblies  60 . As a result, in the absence of a pointer P, each imaging assembly  60  sees a bright band  160  having a substantially even intensity over its length and possibly ambient light from sources such as the sun, light bulbs, projectors as represented by the white circle  144  above the bright band  160  and/or reflections of ambient light from sources such as the sun, light bulbs, projectors as represented by the white circle  146  below the bright band  160  as shown in  FIG. 7A . When a pointer is brought into proximity with the display surface  24  and is sufficiently distant from the IR light sources  82   a , the pointer occludes infrared illumination reflected by the retro-reflective bands  102 . As a result, each imaging assembly sees a dark region  166  that interrupts the bright band  160  in captured image frames as shown in  FIG. 8A . When a pointer P is brought into proximity with the display surface  24  and is sufficiently proximate to an IR light source  82   a , the pointer reflects infrared illumination that is returned to the imaging assemblies  60 . As a result, the pointer appears as a bright region  168  that crosses the bright band  160  in captured frames as shown in  FIG. 9A . 
         [0043]    When the IR light sources  82   b  are on, each LED of the IR light sources  82   b  floods the region of interest over the display surface  24  with infrared illumination that has been polarized by the filters  92 . As the filters  92  are orthogonal (i.e. unmatched) to the filters over the retro-reflective bands  102  of the bezel segments  40 ,  42  and  44 , the infrared illumination is unable to pass through the filters on the bezel segments. As a result, in the absence of a pointer P, the image sensor  70  of each imaging assembly  60  sees darkness and possibly the ambient light and reflections of ambient light as represented by the white circles  144  and  146  as shown in  FIG. 7B . When a pointer is brought into proximity with the display surface  24  and is sufficiently distant from the IR light sources  82   b , the pointer reflects very little infrared illumination that is returned to the image sensors  70  of the imaging assemblies  60 . As a result, the pointer appears a dark region  170  that blends into the dark background in captured image frames as shown in  FIG. 8B . When a pointer is brought into proximity with the display surface  24  and is sufficiently proximate to an IR light source  82   b , the pointer reflects infrared radiation that is returned to the image sensors  70  of the imaging assemblies  60 . As a result, the pointer appears as bright region  172  against the dark background in captured image frames as shown in  FIG. 9B . 
         [0044]    As mentioned above, each image frame output by the image sensor  70  of each imaging assembly  60  is conveyed to the DSP unit  26 . When the DSP unit  26  receives image frames from the imaging assemblies  60 , the controller  120  processes the image frames to detect the existence of a pointer therein and if a pointer exists, to determine the position of the pointer relative to the display surface  24  using triangulation. To reduce the effects unwanted light may have on pointer discrimination, the controller  120  measures the difference in the intensity of light within the image frames to detect the existence of a pointer. There are generally three sources of unwanted light, namely ambient light, light from the display unit and infrared illumination that is emitted by the IR light sources  82  and scattered off of objects proximate to the imaging assemblies  60 . As will be appreciated, if a pointer is close to an imaging assembly  60 , infrared illumination emitted by the associated IR light source  82   a  may illuminate the pointer directly resulting in the pointer being as bright as or brighter than the retro-reflective bands  102  in captured image frames. As a result, the pointer will not appear in the image frames as a dark region interrupting the bright band  160  but rather will appear as a bright region  168  that extends across the bright band  160  as shown in  FIG. 9A . 
         [0045]    The controller  120  processes successive image frames output by the image sensor  70  of each imaging assembly  60  in pairs with one image frame captured with IR light source  82   a  on and the other image frame captured with IR light source  82   b  on. When the first image frame of a pair is received, the controller  120  stores the image frame in a buffer. When the successive image frame of the pair is received, the controller  120  similarly stores the image frame in a buffer. With the successive image frames available, the controller  120  subtracts the two image frames to form a difference image frame. Provided the frame rates of the image sensors  70  are high enough, ambient light levels and display unit light levels in successive image frames will typically not change significantly and as a result, ambient light and display unit light are substantially cancelled out and do not appear in the difference image frame. The end result is a high contrast image of the pointer and the retro-reflective band  102 . Once the difference image frame has been generated, the controller  120  examines the intensity of the difference image frame for values that represent the bezel and the pointer. When no pointer is in proximity with the display surface  24 , the intensity values are high and uninterrupted. When a pointer is in proximity with the display surface  24 , some of the intensity values fall below a threshold value allowing the existence of the pointer in the difference image frame to be readily determined. In order to generate the intensity values for each difference image frame, the controller  120  calculates a vertical intensity profile (VIP retro ) for each pixel column of the difference image frame. 
         [0046]      FIG. 7C  shows a difference image frame generated from the image frames of  FIGS. 7A and 7B  and  FIG. 7D  shows a plot of the normalized intensity values I(x) calculated for the pixel columns of the difference image frame of  FIG. 7C . As will be appreciated, in this difference image frame no pointer exists and thus, the intensity values I(x) remain high and uninterrupted for all of the pixel columns of the difference image frame.  FIG. 8C  shows a difference image frame generated from the image frames of  FIGS. 8A and 8B  and  FIG. 8D  shows a plot of the normalized intensity values I(x) calculated for the pixel columns of the difference image frame of  FIG. 8C . As can be seen, the I(x) curves drop to low values at a region corresponding to the location of the pointer in the difference image frame.  FIG. 9C  shows a difference image frame generated from the image frames of  FIGS. 9A and 9B  and  FIG. 9D  shows a plot of the normalized intensity values I(x) calculated for the pixel columns of the difference image frame of  FIG. 9C . As can be seen, the I(x) curves also drop to low values at a region corresponding to the location of the pointer in the difference image frame. 
         [0047]    Once the intensity values I(x) for the pixel columns of each difference image frame have been determined, the resultant I(x) curve for each difference image frame is examined to determine if the I(x) curve falls below a threshold value signifying the existence of a pointer and if so, to detect left and right edges in the I(x) curve that represent opposite sides of a pointer. In particular, in order to locate left and right edges in each difference image frame, the first derivative of the I(x) curve is computed to form a gradient curve ∇I(x). If the I(x) curve drops below the threshold value signifying the existence of a pointer, the resultant gradient curve ∇I(x) will include a region bounded by a negative peak and a positive peak representing the edges formed by the dip in the I(x) curve. In order to detect the peaks and hence the boundaries of the region, the gradient curve ∇I(x) is subjected to an edge detector. 
         [0048]    In particular, a threshold T is first applied to the gradient curve ∇I(x) so that, for each position x, if the absolute value of the gradient curve ∇I(x) is less than the threshold, that value of the gradient curve ∇I(x) is set to zero as expressed by: 
         [0000]      ∇ I ( x )=0, if |∇ I ( x )|&lt; T  
 
         [0049]    Following the thresholding procedure, the thresholded gradient curve ∇I(x) contains a negative spike and a positive spike corresponding to the left edge and the right edge representing the opposite sides of the pointer, and is zero elsewhere. The left and right edges, respectively, are then detected from the two non-zero spikes of the thresholded gradient curve ∇I(x). To calculate the left edge, the centroid distance CD left  is calculated from the left spike of the thresholded gradient curve ∇I(x) starting from the pixel column X left  according to: 
         [0000]    
       
         
           
             
               CD 
               left 
             
             = 
             
               
                 
                   ∑ 
                   i 
                 
                  
                 
                   
                     ( 
                     
                       
                         x 
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                         left 
                       
                     
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         [0000]    where x i  is the pixel column number of the i-th pixel column in the left spike of the gradient curve ∇I(x), i is iterated from 1 to the width of the left spike of the thresholded gradient curve ∇I(x) and X left  is the pixel column associated with a value along the gradient curve ∇I(x) whose value differs from zero (0) by a threshold value determined empirically based on system noise. The left edge in the thresholded gradient curve ∇I(x) is then determined to be equal to X left +CD left . 
         [0050]    To calculate the right edge, the centroid distance CD right  is calculated from the right spike of the thresholded gradient curve ∇I(x) starting from the pixel column X right  according to: 
         [0000]    
       
         
           
             
               CD 
               right 
             
             = 
             
               
                 
                   ∑ 
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                  
                 
                   
                     ( 
                     
                       
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                         right 
                       
                     
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                       ) 
                     
                   
                 
               
             
           
         
       
     
         [0000]    where x i  is the pixel column number of the j-th pixel column in the right spike of the thresholded gradient curve ∇I(x), j is iterated from 1 to the width of the right spike of the thresholded gradient curve ∇I(x) and X right  is the pixel column associated with a value along the gradient curve ∇I(x) whose value differs from zero (0) by a threshold value determined empirically based on system noise. The right edge in the thresholded gradient curve is then determined to be equal to X right +CD right . 
         [0051]    Once the left and right edges of the thresholded gradient curve ∇I(x) are calculated, the midpoint between the identified left and right edges is then calculated thereby to determine the location of the pointer in the difference image frame. 
         [0052]    After the location of the pointer in each difference frame has been determined, the controller  120  uses the pointer positions in the difference image frames to calculate the position of the pointer in (x,y) coordinates relative to the display surface  24  using well known triangulation such as that described in above-incorporated U.S. Pat. No. 6,803,906 to Morrison et al. The calculated pointer coordinate is then conveyed by the controller  120  to the computer  30  via the USB cable  32 . The computer  30  in turn processes the received pointer coordinate and updates the image output provided to the display unit, if required, so that the image presented on the display surface  24  reflects the pointer activity. In this manner, pointer interaction with the display surface  24  can be recorded as writing or drawing or used to control execution of one or more application programs running on the computer  30 . 
         [0053]    If desired, as the image frames captured when the IR light sources  82   b  are on, include image data relating only to the pointer and not the bezel segments  40  to  44 , these image frames can be separately analyzed to extract additional information concerning the pointer. For example, these image frames can be analyzed to verify display surface pointer contact and/or to recognize surface features of the pointer to determine the pointer type or in the case of multi-touch scenarios to disambiguate multiple pointers in contact with the display surface  24 . 
         [0054]    When the active pointer P is brought into proximity with the display surface  24 , the IR light sources remain off so that the imaging assemblies see the pointer P as a bright region interrupting a dark band. 
         [0055]    To reduce the amount of data to be processed, only the area of the image frames occupied by the bezel segments need be processed. A bezel finding procedure similar to that described in U.S. patent application Ser. No. 12/118,545 to Hansen et al. entitled “Interactive Input System and Bezel Therefor” filed on May 9, 2008 and assigned to SMART Technologies ULC of Calgary, Alberta, the content of which is incorporated herein by reference, may be employed to locate the bezel segments in captured image frames. Of course, those of skill in the art will appreciate that other suitable techniques may be employed to locate the bezel segments in captured image frames. 
         [0056]    Although the use of polarizing filters associated with the IR light sources  82   a  and  82   b  and bezel segments  40 ,  42  and  44  has been described, those of skill in the art will appreciate that other types of filters can be used so that radiation emitted by the IR light sources  82   a  is reflected by the retro-reflective bands  102  and radiation emitted by the IR light sources  82   b  is blocked by the filter over the retro-reflective band of each bezel segment. For example, if a non-colored pointer (i.e. a white or grey pointer) that reflects radiation emitted by IR light sources  82   a  and  82   b  is used, different colored filters can be used with the IR light sources with the filters over the bezel segments being the same color as one of the filters associated with the light sources. 
         [0057]    In an alternative embodiment, the IR light sources  82   a  and  82   b  are selected to emit radiation at different wavelengths in the visible or non-visible spectrum. For example, the IR light sources  82   a  may emit radiation at a wavelength of 850 nm and the IR light sources  82   b  may emit radiation at a wavelength of 880 nm. An IR filter is provided on the bezel segments  40 ,  42  and  44  that blocks the emitted radiation at wavelength 850 nm and that allows the emitted radiation at wavelength 880 nm to pass. An IR filter on the lens of each image sensor is matched to the emitted radiation at both wavelengths. 
         [0058]    If desired, the IR light sources  82  can be further modulated as described in U.S. patent application Ser. No. 12/118,521 to McReynolds et al. entitled “Interactive Input System with Controlled Lighting” filed on May 9, 2008 and assigned to SMART Technologies ULC of Calgary, Alberta, the content of which is incorporated by reference. In this manner, image frames for each imaging assembly based only on the contribution of illumination from its associated IR light source can be generated. The modulated signals output by the pen tool P can also be modulated. 
         [0059]    Although specific embodiments have been described above with reference to the figures, those of skill in the art will appreciate that other alternatives are available. For example, in the above embodiment, the DSP unit  26  is shown as comprising an antenna  136  and a wireless receiver  138  to receive the modulated signals output by the pen tool P. Alternatively, each imaging assembly  60  can be provided with an antenna and a wireless receiver to receive the modulated signals output by the pen tool P. In this case, modulated signals received by the imaging assemblies are sent to the DSP unit  26  together with the image frames. The pen tool P may also be tethered to the assembly  22  or DSP unit  26  allowing the signals output by the pen tool P to be conveyed to one or more of the imaging assemblies  60  or the DSP unit  26  or imaging assembly(s) over a wired connection. 
         [0060]    In the above embodiments, each bezel segment  40 ,  42  and  44  is shown as comprising a single retro-reflective band. Those of skill in the art will appreciate that alternatives are available. For example, rather than using a retro-reflective band, a band formed of highly reflective material such as a micro-mirror array may be used. Alternatively, each bezel segment may comprise two or more retro-reflective bands and two or more filters covering the retro-reflective bands. 
         [0061]    If desired the tilt of each bezel segment can be adjusted to control the amount of light reflected by the display surface itself and subsequently toward the image sensors  70  of the imaging assemblies  60 . 
         [0062]    Although the frame assembly is described as being attached to the display unit, those of skill in the art will appreciate that the frame assembly may take other configurations. For example, the frame assembly may be integral with the bezel  38 . If desired, the assembly  22  may comprise its own panel to overlie the display surface  24 . In this case it is preferred that the panel be formed of substantially transparent material so that the image presented on the display surface  24  is clearly visible through the panel. The assembly can of course be used with a front or rear projection device and surround a substrate on which the computer-generated image is projected. 
         [0063]    Although the imaging assemblies are described as being accommodated by the corner pieces adjacent the bottom corners of the display surface, those of skill in the art will appreciate that the imaging assemblies may be placed at different locations relative to the display surface. Also, the tool tray segment is not required and may be replaced with a bezel segment. 
         [0064]    Those of skill in the art will appreciate that although the operation of the interactive input system  20  has been described with reference to a single pointer or pen tool P being positioned in proximity with the display surface  24 , the interactive input system  20  is capable of detecting the existence of multiple pointers/pen tools that are proximate to the touch surface as each pointer appears in the image frames captured by the image sensors. 
         [0065]    Although preferred embodiments have been described, those of skill in the art will appreciate that variations and modifications may be made with departing from the spirit and scope thereof as defined by the appended claims.