Abstract:
A method of inhibiting a subject&#39;s eyes from being exposed to projected light when the subject is positioned in front of a background on which an image is displayed comprises capturing an image of the background including the displayed image, processing the captured image to detect the existence of the subject and to locate generally the subject and masking image data used by the projector to project the image corresponding to a region that encompasses at least the subject&#39;s eyes, the image data masking compensating for differing camera and projector viewpoints.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 11/420,146 to Morrison et al. filed on May 24, 2006, the content of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to projection systems and in particular to a method and apparatus for inhibiting a subject&#39;s eyes from being exposed to projected light when the subject is positioned in front of a background on which a projected image is displayed. 
       BACKGROUND OF THE INVENTION 
       [0003]    During presentations, use of a projector to project an image onto a projection screen so that the image can be seen by attendees is common. In many instances, presenters interact with the displayed images using a pointer to highlight and emphasize displayed information. During such interactions, presenters must often move between the projector and the projection screen and into the projected image. If the presenter moves into such a position and turns to face the audience, the projected light can be blinding. Subjecting a presenter&#39;s eyes to light of this nature is of course undesired. 
         [0004]    Techniques to avoid exposing a presenter&#39;s eyes to projected light have been considered. For example, U.S. Pat. No. 6,361,173 to Vlahos et al. discloses a method and apparatus for inhibiting projection of selected areas of a projected image. The apparatus includes a video projector for projecting a desired video image onto a projection screen. An infrared source close to the video projector uniformly floods the projection screen with non-visible infrared radiation. An infrared sensitive camera, close to the video projector, observes the projection screen and sees only the uniform infrared illumination flooding the projection screen. Upon entry of a subject into the projected video image, the infrared radiation reflected from the subject will not match that of the projection screen and thus, the subject area can be identified. All pixels of the projected video image, in the area occupied by the subject, are inhibited before reaching the video projector. This allows the subject to look directly at an audience without being blinded by light projected by the video projector. 
         [0005]    U.S. Pat. No. 6,860,604 to Vlahos discloses a method and apparatus for inhibiting the projection of a shadow of a presenter onto a projection screen. Left and right rectilinear corrections of an image are simultaneously projected onto the projection screen from each of two positions that are off-axis on opposite sides of the centerline of the projection screen such that both projections register as a single image. A center of mass of the presenter&#39;s silhouette is generated. A vertical join line is located on the projection screen directly behind the presenter&#39;s center of mass. The portion of the projected image from the left projector that extends to the right of the join line is inhibited. The portion of the projected image from the right projector that extends to the left of the join line is also inhibited thereby providing the full projected image on the projection screen from the left and right image segments, without either segment projecting the image onto the presenter and without casting the presenter&#39;s shadow onto the projection screen. 
         [0006]    U.S. Pat. No. 6,945,653 to Kobori et al. discloses an anti-glare device for a projector designed to minimize stimulus to the eyes of a presenter. A displayed image captured by a camera is compared with an input image signal to generate a difference image and thus, detect the location of the presenter. The input image signal is then masked so that pixels corresponding to the area at which the presenter is located are set to black. The resulting projected image signal is then projected onto the projection screen. 
         [0007]    Although the above-references disclose techniques to inhibit projected light from being directed into the eyes of a presenter, the systems disclosed in these references suffer disadvantages. For example, although the systems disclosed in U.S. Pat. No. 6,361,173 to Vlahos et al. and U.S. Pat. No. 6,860,604 to Vlahos avoid light projected by the projector from being directed into the eyes of a presenter, the systems still expose the presenter&#39;s eyes to infrared light. Also, each of the above-described systems determines a tight-mask defining the location of the presenter and thus, requires high-resolution cameras. 
         [0008]    As will be appreciated, there exists a need for a projection system designed to protect a presenter&#39;s eyes from virtually any form of projected light, which is easy to manufacture and cost effective. It is therefore an object of the present invention to provide a novel method and apparatus for inhibiting a subject&#39;s eyes from being exposed to projected light when the subject is positioned in front of a background on which a projected image is displayed. 
       SUMMARY OF THE INVENTION 
       [0009]    Accordingly, in one aspect there is provided in a projection system comprising a projector to project an image for display on a background and at least one camera to capture an image of said background, said at least one camera having a viewpoint differing from the viewpoint of said projector, a method of inhibiting a subject&#39;s eyes from being exposed to projected light when the subject is positioned in front of the background, said method comprising: 
         [0010]    capturing the image of the background including the displayed image; 
         [0011]    processing the captured image to detect the existence of a subject therein and to locate generally the subject; and 
         [0012]    masking image data used by the projector to project the image corresponding to a region that encompasses at least the subject&#39;s eyes, said image data masking compensating for said differing camera and projector viewpoints. 
         [0013]    In one embodiment, during the processing, the subject as seen by the at least one camera is converted to a size as would be seen by the camera if positioned at the projector. The subject as seen by the at least one camera is also scaled to compensate for different camera and projector resolutions. 
         [0014]    According to another aspect there is provided in a projection system comprising a projector to project an image for display on a background and at least two laterally spaced cameras to capture an image of said background, a method of inhibiting a subject&#39;s eyes from being exposed to projected light when the subject is positioned in front of the background, said method comprising: 
         [0015]    successively capturing sets of images of the background including the displayed image; 
         [0016]    processing each set of captured images to detect the existence of the subject and to locate generally the subject&#39;s head; and 
         [0017]    modifying image data used by the projector to project the image taking into account differing viewpoints of said cameras and projector, so that the displayed image comprises an area of darkness that at least encompasses the subject&#39;s eyes and that does not extend appreciably beyond the subject&#39;s head. 
         [0018]    According to yet another aspect there is provided a projection system comprising: 
         [0019]    a projector receiving image data and projecting an image onto a background; 
         [0020]    at least one camera capturing an image of the background including the projected image, said at least one camera being spaced from said projector and having a viewpoint differing from that of said projector; and 
         [0021]    processing structure processing the captured image to locate a subject in the captured image and to map the subject to the viewpoint of said projector, said processing structure masking image data used by the projector to project the image corresponding to a region that encompasses at least the subject&#39;s eyes. 
         [0022]    According to still yet another aspect there is provided in a projection system comprising a projector to project an image for display on a background and at least two spaced cameras to capture images of said background, a method of determining the distance a subject in the captured image is from the background, said method comprising: 
         [0023]    comparing the images captured by the cameras to determine image disparity; and 
         [0024]    using the image disparity and the geometry of the projection system to calculate the background to subject distance. 
         [0025]    In one embodiment, during the comparing subsets of the captured images are compared. During comparing a subset of one image is selected and scanned across the other image to locate a matching subset in the other image. 
         [0026]    According to still yet another aspect there is provided in a projection system comprising a projector to project an image for display on a background and at least one camera to capture an image of said background, a method of setting up said projection system comprising: 
         [0027]    determining the orthogonal distance between the background and the projector and determining the width of said background; 
         [0028]    assuming a maximum distance that a subject will stand in front of said background; 
         [0029]    using the orthogonal distance, the width and the maximum distance to calculate a maximum camera to projector spacing; and 
         [0030]    positioning said at least one camera relative to and within said maximum camera to projector spacing of said projector. 
         [0031]    The method and apparatus provide advantages in that captured images are analyzed to detect the existence of a subject so that the image projected onto the background is modified to inhibit projected light from being directed at the subject&#39;s eyes. Thus, neither visible light nor non-visible light is directed at the subject&#39;s eyes when the subject is positioned in front of the background on which the projected image is displayed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    Embodiments will now be described more fully with reference to the accompanying drawings in which: 
           [0033]      FIG. 1  is a schematic side view of a projection system including an apparatus for inhibiting a subject&#39;s eyes from being exposed to projected light; 
           [0034]      FIG. 2  is a top plan view of the projection system of  FIG. 1  illustrating a pair of low resolution cameras and a projector positioned therebetween; 
           [0035]      FIG. 3  is a front view of a projection screen on which a projected image is displayed; 
           [0036]      FIG. 4  is a flowchart showing the steps performed during processing of captured images to detect the presence of a presenter and to mask an output video data stream; 
           [0037]      FIG. 5  is a front view showing a presenter positioned in front of the projection screen; 
           [0038]      FIG. 6  is a flowchart showing the steps performed during processing of captured images to generate a disparity image; 
           [0039]      FIGS. 7   a  and  7   b  are flowcharts showing the steps performed during processing of captured images to detect the center of the presenter&#39;s head; 
           [0040]      FIG. 8  is a flowchart showing the steps performed during processing of captured images to track the center of the presenter&#39;s head; 
           [0041]      FIG. 9  shows the horizontal geometry of the projection system of  FIG. 1  with respect to the left low resolution camera; 
           [0042]      FIGS. 10   a  to  10   c  are tables showing the maximum horizontal camera to projector spacing for different projection system configurations; 
           [0043]      FIG. 11  shows the horizontal geometry with respect to the left low resolution camera for a different projection system configuration; 
           [0044]      FIG. 12  shows a portion of the horizontal geometry of  FIG. 11 ; 
           [0045]      FIG. 13  shows template matching between right and left captured images to calculate stereo disparity; and 
           [0046]      FIG. 14  shows the stereo disparity geometry. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0047]    Turning now to  FIGS. 1 to 3 , a projection system is shown and is generally identified by reference numeral  10 . In this embodiment, projection system  10  is similar to that disclosed in above-incorporated U.S. patent application Ser. No. 11/420,146 to Morrison et al. As can be seen, projection system  10  includes a projector  12  receiving an input video data stream from a processing device such as for example, a personal computer (PC)  14 . Projector  12  in response to the input video data stream projects an image I onto a projection screen  16  as shown in  FIG. 3  and by the dotted lines  12   a  in  FIGS. 1 and 2 . Projection screen  16  in this embodiment is an interactive touch board or panel of the type manufactured by SMART Technologies, Inc. of Calgary, Alberta, Canada and sold under model No. SB 680. 
         [0048]    A pair of low resolution video cameras  18  is positioned adjacent the projector  12 . The cameras  18  are horizontally spaced, with each camera being fixed in position on either side of the projector  12 . Each camera  18  has a field of view (FOV) encompassing the projection screen  16  as shown in  FIG. 3  and by the dotted lines  18   a  in  FIG. 1 . In this manner, the cameras  18  capture images including the entire projection screen  16  and the image I displayed thereon as well as any objects, such as a presenter P, positioned in front of the projection screen  16 . The cameras  18  are coupled to the personal computer  14  via universal serial bus (USB) connections  20 . 
         [0049]    To avoid a presenter P from being blinded by light projected by the projector  12  when the presenter is positioned in front of the projection screen  16 , the projection system  10  makes use of image analysis to mask the input video data stream provided to the projector  12  so that pixels corresponding to a region generally encompassing the presenter&#39;s head are set to black or near black (i.e. dimmed). In this manner, the presenter P is able to stand in front of the projection screen  16  and look back at the projector  12  without being blinded by projected light. 
         [0050]    During set up of the projection system  10 , it is desired to determine the maximum spacing between the projector  12  and the cameras  18  so that the projection system  10  can be installed in a manner that ensures desired operation. Referring now to  FIG. 9 , the horizontal geometry of the projection system  10  is shown assuming that the optical axis of the projector  12  is orthogonal to the plane of the projection screen  16 . As can be seen, the width of the projection screen  16  is designated by reference character W. The horizontal distance between the left camera  18  and the projector  12  is designated by reference character dCP. The distance between the projection screen  16  and the left camera  18  is designated by reference character dSB. The distance between the projection screen  16  and the presenter P is designated by reference character d. The distance between the presenter P and the projector  12  can therefore be expressed as dSB-d. As will be appreciated by those of skill in the art, although not illustrated the vertical geometry of the projection system  10  is very similar to that shown in  FIG. 9  with two exceptions. When examining the vertical geometry of the projection system  10 , the projection screen width W is replaced with the projection screen height H and the horizontal camera to projector spacing dCP is replaced with the vertical camera to projector spacing dCP. 
         [0051]    Based on the above, the extreme viewing angle θ of the left camera  18  at which the entire planar surface of the projection screen  16  as well as a presenter P in front of the projection screen can still be seen, can be expressed as: 
         [0000]    
       
         
           
             
               
                 
                   
                     tan 
                      
                     
                       ( 
                       θ 
                       ) 
                     
                   
                   = 
                   
                     
                       d 
                       / 
                       
                         ( 
                         
                           W 
                           / 
                           2 
                         
                         ) 
                       
                     
                     = 
                     
                       
                         
                           d 
                           SB 
                         
                         - 
                         d 
                       
                       
                         d 
                         CP 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    Equation (1) can be rewritten as: 
         [0000]    
       
         
           
             
               
                 
                   
                     d 
                     CP 
                   
                   = 
                   
                     
                       W 
                        
                       
                         ( 
                         
                           
                             
                               d 
                               SB 
                             
                             d 
                           
                           - 
                           1 
                         
                         ) 
                       
                     
                     2 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0052]    From Equation (2), it can be seen that the maximum camera to projector spacing dCP increases linearly with the distance between the camera  18  and the projection screen  16 . The maximum camera to projector spacing dCP however decreases as the distance d between the presenter P and the projection screen  16  increases. Using the minimum and maximum projection screen to camera distances dSB and the minimum and maximum projection screen to presenter distances d, from Equation (2), the maximum camera to projector spacing can be calculated for different projection system configurations allowing the cameras  18  and projector  12  to be quickly and effectively oriented while ensuring that the presenter P in front of the projection screen  16  remains within the fields of view of the cameras  18  so that the position of the presenter P can be properly tracked. 
         [0053]    For example, assume that the projection screen  16  has a diagonal length equal to 1.1956 m (77 inches) and an aspect ratio of 4:3 and that the minimum and maximum projection screen to camera distances dSB are 1.524 m (5 feet) and 4.267 m (14 feet) respectively. Also assume that the presenter P will typically stand at distances d in front of the projection screen  16  in the range of from about 0.31 m to 0.61 m (1 to 2 feet). Based on the above assumptions and solving for Equation (2),  FIG. 10   a  is a table showing the maximum horizontal camera to projector spacing dCP assuming the projection screen to presenter distance d is equal to 0.31 m at each of the minimum and maximum projection screen to camera distances dSB as well as the maximum camera to projector spacing dCP assuming the projection screen to presenter distance d is equal to 0.61 m at each of the minimum and maximum projection screen to camera distances dSB.  FIG. 10   b  is a table showing the maximum vertical camera to projector spacing dCP assuming the projection screen to presenter distance d is equal to 0.31 m at each of the minimum and maximum projection screen to camera distances dSB as well as the maximum camera to projector spacing dCP assuming the projection screen to presenter distance d is equal to 0.61 m at each of the minimum and maximum projection screen to camera distances dSB. As will be appreciated from the above, at a minimum projection screen to camera distance dSB equal to 5 feet, the maximum camera to projector spacing dCP is about 3 to 4 feet. 
         [0054]      FIG. 10   c  is a table showing the maximum horizontal and vertical camera to projector spacing dCP assuming the projection screen to presenter distance d is equal to 0.61 m and that the width and height of the projection screen  16  have been decreased by 5% at each of the minimum and maximum projection screen to camera distances dSB. In this case, the width of the projection screen  16  is equal to 1.1485 m and the height of the projection screen  16  is equal to 1.115 m. As can be seen, as the size of the projection screen  16  decreases, the camera to projector spacing dCP decreases. 
         [0055]    Prior to general use of the projection system  10  once the cameras  18  and projector  12  have been properly oriented with respect to the projection screen  16 , the projection system  10  is calibrated in the manner described in above-incorporated U.S. patent application Ser. No. 11/420,146 to Morrison et al. Accordingly, specifics of the calibration will not be discussed further herein. Calibration of the projection system  10  matches the views of the cameras  18  through an image transformation with respect to the surface of the projection screen  16  such that image superposition creates one image of the viewed scene. Foreground objects such as the presenter P are separated from the plane of the projection screen  16  due to the effect of planar parallax and show up as two images of finite separation related to the stereo camera positions. The calibration is fully automated without requiring intervention from the presenter or requiring projecting of special images that are visible during calibration. 
         [0056]    Once calibrated, in use the personal computer  14  outputs a video data stream that is received by the projector  12 . In response to the input video data stream, the projector  12  in turn projects an image I onto the projection screen  16 . The cameras  18 , which see the projection screen  16 , the displayed image I and the presenter P, if the presenter exists in their fields of view, capture images and convey the captured images to the personal computer  14  over the USB connections  20 . Thus, the computer  14  receives pairs of images from the cameras  18 . 
         [0057]    Upon receipt of each pair of images from the cameras  18 , the computer  14  processes the images to detect whether a presenter is positioned in front of the projection screen  16  and if so, to determine the location of the presenter&#39;s head so that the video data stream input to the projector  12  can be masked to inhibit the presenter from being blinded by projected light. In particular as shown in  FIG. 4 , when the computer  14  receives the images from the cameras  18  (step  100 ), the computer  14  applies a homography transformation matrix calculated during calibration to the right camera image to transform the right camera image to the coordinate system of the left camera image (step  102 ). With the right and left camera images in the same coordinate system, a statistical cross-correlation between the images is performed to generate a disparity image (step  104 ). The disparity image is then examined to determine if a presenter exists in the disparity image (step  106 ). If the existence of a presenter is not detected, the camera images are discarded and the process reverts back to step  100  to await receipt of the next pair of camera images. 
         [0058]    If the existence of a presenter is detected at step  106 , the disparity image is further processed to detect the center of the presenter&#39;s head (step  108 ). With the center of the presenter&#39;s head detected, the presenter&#39;s head is tracked (step  110 ) and the average center position and velocity of the presenter&#39;s head over successive image pairs is used to mask the video data stream so that pixels corresponding to a circular region encompassing the presenter&#39;s head are set to black or near black (step  112 ). As a result, when the video data stream is output to the projector  12 , projected light is inhibited from being directed at the presenter&#39;s eyes as a circular area of darkness  24  encompasses the presenter&#39;s head as shown in  FIG. 5 . As the cameras  18  continually capture images of the projection screen  16  and displayed image I, movement of the presenter P is tracked allowing the area of darkness  24  to be moved to follow the presenter. 
         [0059]    At step  104 , during generation of the disparity image, images A, B and C are initially calculated (see step  150  in  FIG. 6 ) according to: 
         [0000]    
       
      
       A=I 
       LC 
       ×I 
       LC  
      
     
         [0000]    
       
      
       B=I 
       TRC 
       ×I 
       TRC  
      
     
         [0000]    
       
      
       C=I 
       LC 
       ×I 
       TRC  
      
     
         [0000]    where: 
         [0060]    I LC  is the image captured by the left camera; and 
         [0061]    I TRC  is the transformed image captured by the right camera. 
         [0062]    A box filter of the appropriate kernel size is then applied to each image A, B and C (step  152 ). The statistical cross-correlation is then calculated to generate the disparity image P (step  154 ) according to: 
         [0000]        P=A /sqrt( B×C ) 
         [0063]    The equation used to generate the disparity image is subject to a divide by zero (or close to zero) exception during tracking of dark objects in the captured images. Hardware and/or software interrupts are therefore employed to replace such divide by zero exceptions with numbers that are in a range that will yield a reasonable result. Overexposing the cameras  18  such that dark objects do not appear as absolute black and have some minimal level of illumination can also be performed to avoid the divide by zero exceptions. 
         [0064]    Alternatively, the disparity image may be found by filtering a difference image based on the captured left and transformed right captured images or by generating a Gaussian weighted difference image based on the captured left and transformed right captured images. As will be appreciated, in these instances, the complexity of calculating a square root and performing a divide operation are avoided. 
         [0065]    During step  108 , in order to detect the location of the presenter&#39;s head, the disparity image is initially preprocessed. During preprocessing as shown in  FIG. 7   a , the disparity image is firstly converted to greyscale so that the pixel values are in the range of 0 to 255 (step  200 ). The greyscale image is then thresholded (step  202 ). During thresholding, pixels having values above 245 and below 220 are set to black and pixels having values in the range of 220 to 245 representing a disparity (i.e. an object in front of the projection screen  16 ) are set to white. A region of interest (ROI) within the thresholded image that encompasses the majority of the white pixels is then selected (step  204 ). A morphological open operation is then performed on the thresholded image to remove noise (step  208 ) and a flood fill operation is performed (step  208 ) to remove white pixel clusters that are smaller than a threshold size, in this example smaller than 2% of the projection screen area in the captured images. In this manner, the remaining white pixel clusters representing the presenter are isolated. The center (x,y) of the presenter is calculated based on the white pixel clusters in the region of interest (ROI). A principle component analysis is then performed on the white pixel clusters in the ROI (step  210 ) to detect the major axis of the presenter, the minor axis of the presenter and the orientation of the major axis with respect to the vertical. 
         [0066]    With the disparity image preprocessing completed, the principle component analysis results are used to calculate the centerline of the presenter&#39;s head and top of the presenter along the center line (step  214  in  FIG. 7   b ). The top outline of the presenter&#39;s head is then calculated and possible head positions are determined (step  216 ). Following this, the right and left side outlines of the presenter are calculated and the actual head position is determined (step  218 ). With the head position determined, the center of the presenter&#39;s head is calculated (step  220 ). During processing of the outlines, to enhance detection of the presenter&#39;s head, one or two mathematical profiles referred to as rotational profiles may be fitted to the outline to locate the presenter&#39;s head. 
         [0067]    At step  110 , during tracking, after the center position of the presenter&#39;s head has been calculated at step  220 , the center position is stored in memory (step  300  in  FIG. 8 ). A check is then made to determine if a threshold number of center positions are stored in the memory (step  302 ). If not, the process reverts back to step  300  awaiting the next calculated head center position. If a threshold number of center positions exist, a historical center position data group and a historical velocity data group are formed (steps  304  and  306 ). A median filter is then applied to the historical center position data group (step  308 ) and an average center position (X ay , Y ax ) is calculated (step  310 ) A median filter is also applied to the historical velocity data group (step  312 ) and an average velocity (V ax , V ay ) is calculated (step  314 ). The average center position and average velocity is then used to predict a position (P x , P y ) representing the center of the presenter&#39;s head (step  316 ) according to: 
         [0000]    
       
      
       P 
       x 
       =X 
       ay 
       +V 
       ax 
       Δt  
      
     
         [0000]    
       
      
       P 
       y 
       =Y 
       ax 
       +V 
       ay 
       Δt  
      
     
         [0068]    This center position (P x , P y ) is used to calculate a circular mask encompassing the presenter&#39;s head (step  312 ). The pixels of the video data stream falling within the circular mask are then dimmed as described above so that when the video data stream is input to the projector  12  and a resultant image is projected on the projection screen  16 , the area of darkness  24  encompasses the presenter&#39;s head inhibiting the presenter from being blinded by projected light. 
         [0069]    As will be appreciated, the projection system  10  tracks a presenter p moving in front of the projection screen  16  and masks the video data stream input to the projector  12  to inhibit the presenter from being blinded. 
         [0070]    Depending on the projection system setup, the cameras  18  may or may not be located close to the projector  12 . If the cameras  18  are not close to the projector  12 , the viewpoints of the cameras  18  as compared to the projector  12  may be significantly different. In order to ensure that the masked region of the video data stream accurately tracks the presenter&#39;s head and does not extend appreciably beyond the intended region to be darkened, a correction to compensate for this difference in viewpoints is desired. For example,  FIGS. 11 and 12  show an alternative projection system geometry in the horizontal direction. In this geometry, the left camera  18  is spaced further from the projection screen  16  than the projector  12 . From  FIG. 12 , it can be seen that the following relationship exists: 
         [0000]        r   2   =s   2 +( t+v ) 2   (3) 
         [0000]    Solving Equation (3) for the projection screen to projector distance yields: 
         [0000]        v=t± √{square root over ( r   2   −s   2 )} 
         [0000]    The angle ø of the left camera&#39;s field of view can be expressed as: 
         [0000]    
       
         
           
             
               tan 
                
               
                 ( 
                 θ 
                 ) 
               
             
             = 
             
               s 
               
                 
                   
                     r 
                     2 
                   
                   - 
                   
                     s 
                     2 
                   
                 
               
             
           
         
       
     
         [0071]    The offsets s and t of the camera  12  with respect to the projector  12  can be measured. The camera to projection screen distance dSB is known from the projection system set up. Because the cameras  18  are located at a distance from the projector  12 , the projected size of the presenter&#39;s head is different from the viewpoints of the cameras  18  and projector  12 . Simply projecting the presenter&#39;s head as seen by the cameras  18  onto the reference frame of the projector  12  may result in inaccuracies. Let w′ denote the projected size of the presenter&#39;s head at the cameras  18  and w′ denote the projected size of the presenter&#39;s head at the projector  12 . The projected size of the presenter&#39;s head at the cameras  18  is expressed by the perspective projection as: 
         [0000]    
       
         
           
             w 
             = 
             
               fW 
               d 
             
           
         
       
     
         [0000]    where: 
         [0072]    f is the focal length of the left camera  18 ; and 
         [0073]    W is the diameter of the presenter&#39;s head. 
         [0074]    A similar expression exists for the projector. The projected size w′ of the presenter&#39;s head at the projector  12  is related to the projected size w of the presenter&#39;s head at the camera  18  by: 
         [0000]        w′=w  cos(φ) 
         [0075]    The angle ø is computed on the basis of the measured offsets s and t. With the angle ø computed, the projected size of the presenter&#39;s head as seen by the cameras  18  can be converted into a corresponding projected size as seen by the projector  12 . In other words, the projected size of the presenter&#39;s head is converted to a size as would be seen by the cameras if the cameras were positioned at the projector  12 . The converted projected size of the presenter&#39;s head as seen by the projector  12  is then scaled to take into account the different resolutions of the cameras  18  and projector  12 . The position of the presenter&#39;s head can then be accurately mapped from the reference frame of the left camera  18  to the reference frame of the projector  12  by computing the homography relating the camera and projector focal planes assuming that the keystone correction of the projector  12  orients the optical axis of the projector such that it is orthogonal to the plane of the projection screen  16 . In this manner, the size of the circular mask that is used to dim pixels of the video data stream can be properly selected so that the area of darkness encompasses at least the presenter&#39;s eyes but does not extend appreciably beyond the presenter&#39;s head irrespective of the orientation of the cameras  18  with respect to the projector  12 . 
         [0076]    If desired, the distance d of the presenter P from the projection screen  16  can be determined from the captured right and left camera images. In order to calculate the projection screen to presenter distance d, the computer  14  selects a rectangular sample region or search template, in this embodiment a 24×12 pixel region, from the right camera image that is positioned on the presenter&#39;s face. The search template is then scanned horizontally across the left camera image until a matching region in the left camera image is determined. During comparing of the search template to pixel regions of the left camera image, matching of horizontal edges is emphasized. Once the matching region in the left camera image is determined, the difference between the coordinates of the search template in the right camera image and the matching region in the left camera image is calculated. With the coordinate differences known, based on the stereo disparity geometry shown in  FIG. 14 , the disparity can be calculated according to: 
         [0000]    
       
         
           
             
               Disparity 
                
               
                   
               
                
               
                 ( 
                 cm 
                 ) 
               
             
             = 
             
               Disparity 
                
               
                   
               
                
               
                 ( 
                 pixels 
                 ) 
               
                
               
                 
                   image 
                    
                   
                       
                   
                    
                   width 
                    
                   
                       
                   
                    
                   
                     ( 
                     pixels 
                     ) 
                   
                 
                 
                   W 
                    
                   
                       
                   
                    
                   
                     ( 
                     cm 
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    With the disparity calculated, the projection screen to presenter distance or depth d can be calculated according to: 
         [0000]    
       
         
           
             
               Depth 
                
               
                   
               
                
               
                 ( 
                 cm 
                 ) 
               
             
             = 
             
               
                 Disparity 
                  
                 
                     
                 
                  
                 
                   ( 
                   cm 
                   ) 
                 
                  
                 
                     
                 
                  
                 dSB 
                  
                 
                     
                 
                  
                 
                   ( 
                   cm 
                   ) 
                 
               
               
                 
                   Basline 
                    
                   
                       
                   
                    
                   
                     ( 
                     cm 
                     ) 
                   
                 
                 + 
                 
                   Disparity 
                    
                   
                       
                   
                    
                   
                     ( 
                     cm 
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    where: 
         [0077]    Baseline is the distance between the focal points of the cameras  18 . Knowing the distance or depth d of the presenter P from the projection screen  16  allows the size of the video data stream mask to be accurately determined. 
         [0078]    Although the above embodiment describes the cameras  18  as being fixed in position on opposite sides of the projector  12 , if desired, the cameras  18  can be mounted on rails or tracks and moveable manually or by drive mechanisms along the tracks to allow the camera to projector spacings to be adjusted. In this latter case, calculation of the projection screen to presenter distance d can be used as feedback for the drive mechanisms so that the drive mechanisms can be operated to adjust the camera to projector spacing dCP as the presenter P moves towards and away from the projection screen  16 . In this manner, the cameras  18  can be positioned to ensure presenter P remains in the fields of view of the cameras. 
         [0079]    Although embodiments have been described with particular reference to the drawings, 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.