Patent Publication Number: US-9432620-B2

Title: Determining a synchronization relationship

Description:
CROSS-REFERENCE TO RELATED U.S. APPLICATIONS 
     This Application is a Continuation of U.S. patent application Ser. No. 12/883,150, filed Sep. 15, 2010, entitled “Determining a Synchronization Relationship”, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Visual-collaborative systems provide a variety of benefits to users. Such benefits include natural interactivity between users, correct eye contact and gaze direction and media sharing, including gaze awareness (knowing where someone is looking) with respect to shared media. However, visual-collaborative systems are often afflicted by video cross-talk. 
     Video cross-talk is when content displayed for viewing by a local user is also captured by a camera for delivery to a remote user. Moreover, media streams that are not synchronized can make video cross-talk more complex. 
     Numerous attempts have been made to reduce video cross-talk, such as, various forms of multiplexing (e.g., temporal, wavelength (color), and polarization). However, these attempts often have performance and cost limitations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-B  illustrate examples of a visual collaborative system, in accordance with embodiments of the present invention. 
         FIG. 2  illustrates an example of a video cross-talk reducer, in accordance with an embodiment of the present invention. 
         FIG. 3  illustrates an example of a visual collaborative system, in accordance with an embodiment of the present invention. 
         FIG. 4  illustrates an example of synchronization relationship, in accordance with an embodiment of the present invention. 
         FIG. 5  illustrates an example of a flow chart of a method for determining a synchronization relationship, in accordance with an embodiment of the present invention. 
     
    
    
     The drawings referred to in this description should be understood as not being drawn to scale except if specifically noted. 
     DESCRIPTION OF EMBODIMENTS 
     Reference will now be made in detail to embodiments of the present technology, examples of which are illustrated in the accompanying drawings. While the technology will be described in conjunction with various embodiment(s), it will be understood that they are not intended to limit the present technology to these embodiments. On the contrary, the present technology is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the various embodiments as defined by the appended claims. 
     Furthermore, in the following description of embodiments, numerous specific details are set forth in order to provide a thorough understanding of the present technology. However, the present technology may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present embodiments. 
     Embodiments of the present invention are directed to visual-collaborative systems. Visual-collaborative systems enable groups or individuals to engage in interactive collaborative video communication. These visual-collaborative systems attempt to recreate the best aspects of actual face-to-face communications. It should be appreciated that a visual-collaborative system includes: a capture device such as a camera disposed at any location that is capable of capturing images and/or audio (e.g. images/audio of a user and/or associated objects) to be transmitted to a corresponding remote visual-collaborative system and a display screen configured to display images captured at a remote location and transmitted to a local visual-collaborative system. 
     Moreover, it is to be understood that a remote location can refer to a distant location, such as, another city. However, a remote location can also be a local location. For example, an adjacent room or even different locations within the same room. In this later case, there may be two or more people within the same room who use two or more visual-collaborative systems to collaborate together. This may be for work or entertainment, e.g., computer games. 
     Specific examples will follow in which visual-collaborative systems include a camera that captures images through a display screen. It will be understood that other visual-collaborative systems can include cameras at any location (as described above). However, for the purposes of brevity and clarity, examples of visual-collaborative systems will be used that capture images through a display screen. 
     Similarly, specific examples will follow in which visual-collaborative systems include a projector that projects images onto a back side of a display screen. It will be understood that other visual-collaborative systems can include a projector on the front side (e.g., same side as the user). Moreover, visual-collaborative systems may not include a projector and images are displayed solely by a partially transmissive display, such as a partially transparent organic light emitting diode (OLED) display screen. However, for the purposes of brevity and clarity, examples of visual-collaborative systems will be used that project images onto and through a display screen. 
     A discussion regarding embodiments of a visual-collaborative system is provided below. First, the discussion will describe the structure or components of various embodiments of visual-collaborative systems. Then the discussion will describe the operational description of the visual-collaborative system. 
       FIG. 1A  depicts an embodiment of visual-collaborative system  100 A, in accordance to an embodiment of the present invention. Visual-collaborative system  100 A includes display screen  110 A, projector  120 , capture device  130  and video cross-talk reducer  140 . 
     Projector  120  is configured to project images, via projected display signal  125 , captured at a remote location onto display screen  110 A. In one embodiment, input visual information signal  122  is received by projector  120  and subsequently projected through projector  120  as images onto display screen  110 A. 
     Capture device  130  is configured to capture images for transmission to a remote location. For example, the captured images are transmitted to a corresponding visual-collaborative system at a remote location. 
     Display screen  110 A is configured to display images captured at a remote location. In one embodiment, display screen  110 A is a rear projection display screen comprising a holographic screen material that diffuses light striking a surface from specific angles corresponding to the projector direction. 
     In one embodiment, display screen  110 A is a fog film. For example, viewers are able to view images projected onto a thin curtain of fog. In another embodiment, display screen  110 A is a spinning mirror system. For example, multiple simultaneous viewers around the spinning mirror system are able to view images projected onto the mirrors. 
     Video cross-talk reducer  140  is configured to estimate video cross-talk that occurs on display screen  110 A and captured by capture device  130 . Video cross-talk reducer  140  is also configured to reduce the estimated video cross-talk from captured images by capture device  130 , which will be described in detail below. 
     During use and operation of visual-collaborative system  100 A, input visual information signal  122  (e.g., video signal transmitted from a remote location) is received by projector  120 . Display signal  125  is projected by projector  120  onto first side  111  of display screen  110 A. For example, the video signal received from the remote location is projected onto display screen  110 A. Display signal  125  is transmitted through display screen  110 A and viewed by user  105  on second side  112  of display screen  110 A. 
     Capture device  130  captures light  131 . In one embodiment, capture signal  133  is captured images (e.g., video signals). For example, capture signal  133  comprises, but is not limited to, user  105 , any markings on display screen  110 A and/or any objects in the vicinity of user  105 . 
     Capture device  130  also captures backscattered light  127 . Backscattered light  127  is reflected light of display signal  125  off of display screen  110 A. As a result, video cross-talk is generated. As described above, video cross-talk is when back scattered light  127  is also captured by capture device  130  for delivery to a remote user. 
     Capture device  130  captures light  131  and backscattered light  127  and generates capture signal  133 . 
     It is to be understood that display signal  125  exists in several forms: (1) internal to projector  120 , as projected light; (2) as a component of backscattered light  127  (backscatter light  127  could also be display signal  125 ); and (3) as a component of capture signal  133 . 
     In various embodiments, visual-collaborative system  100 A employs optics and/or hardware to separate and remove the cross-talk signals. For example, visual-collaborative system  100 A employs (1) time multiplexing, (2) polarization and (3) wavelength (color) division multiplexing. 
     In one embodiment, time multiplexing uses synchronized temporal multiplexing of video signals. In particular, projector  120  and capture device  130  are synchronized such that when projector  120  projects display signal  125 , capture device  130  does not receive light  131  or  127 . Similarly, when capture device  130  receives light  131 , projector  120  does not project display signal  125 . As a result, capture device  130  does not receive backscattered light  127  and video cross-talk is reduced. In one embodiment, time multiplexing is performed by generator lock (genlock) hardware. 
     However, this method is not incorporated with consumer-grade devices (e.g., off-the-shelf or legacy projectors and cameras). Even if professional-grade projectors and cameras provide the ability to be controlled by a synchronization signal, it would lead to very expensive hardware setups. 
     In various embodiments, visual-collaborative system  100 A includes filters  170  and  180  to reduce video cross-talk. In one embodiment, filters  170  and  180  are orthogonal polarizing filters. For example, filter  170  filters out horizontally propagating light and filter  180  filters out vertically propagating light, or vice versa. As a result, video cross-talk is reduced. 
     However, approximately one half of display signal  125  is filtered out and not projected on display screen  110 A. Similarly, approximately one half of light  131  is filtered out and not received by capture device  130 . Other problems also occur, such as, it is very difficult to get perfect polarization-based alignment, and there is generally some amount of light leakage which results in cross talk. Accordingly, performance of visual-collaborative system  100 A is diminished. 
     In another embodiment, filters  170  and  180  are multiple pass band optical filters that separate the video signals by their light wavelength. For example, filter  170  filters out the lower half of the color spectrum and filter  180  filters out the upper half of the color spectrum, or vice versa. As a result, video cross-talk is reduced. 
     However, similar to the polarizing filters, approximately one half of display signal  125  is filtered out and not projected on display screen  110 A. Similarly, approximately one half of light  131  is filtered out and not received by capture device  130 . Accordingly, performance of visual-collaborative system  100 A is diminished. 
     In one embodiment, video cross-talk reducer  140  is configured to reduce video cross-talk without requiring optical hardware and/or synchronization hardware methods. For example, video cross-talk reducer  140  is able to reduce video cross-talk based on signals rather than using hardware optical elements (e.g., polarization and wavelength multiplexing) and synchronization (e.g., genlock) hardware. In another embodiment, video cross-talk is reduced by the combination of (1) video cross-talk reducer  140  and (2) optical hardware and/or synchronization hardware methods. 
     It should be appreciated that projector  120  and capture device  130  may be simple off-the-shelf or legacy play back and image capture devices. Moreover, projector  120  and capture device  130  do not require any revision of hardware and/or software in order to facilitate in video cross-talk reduction as described herein. 
     Projector  120  (or other display device) may consist of a cascade of modules that subject their respective inputs to algorithmic transformation before being passed to the next module, and ultimately to a final display module. For example, a single input frame may be converted into several frames via color separation. It should be appreciated that input visual information signal  122  may be the input to projector  120 , or it may be the input to any intermediate module, for example the final display module. 
     In general, video cross-talk reducer  140  is able to reduce video cross-talk by forward modeling input visual information signal  122  propagating through visual-collaborative system  100 A. For example, video cross-talk reducer  140  forward models (e.g., estimates) how input visual information signal  122  is propagated through projector  120  and captured as video cross-talk at capture device  130 . Once video cross-talk is properly forward modeled, it is then reduced (e.g., subtracted) from the actual video cross-talk captured at capture device  130 . Then output  155  is generated and subsequently transmitted to a remote location. 
     In particular, the following is accounted for to properly forward model the video cross-talk: (1) mapping from digital input (e.g., input visual information signal  122 ) to projector  120  to projected display signal  125 ; (2) mapping from display signal  125  through visual-collaborative system  100 A to light  127  hitting a camera sensor (accounting for color correction, grayscale correction, geometric correction, spatially-varying black-level offset and gain; and spatial-varying blur); and (3) mapping from light received (e.g., back scattered light  127 ) by capture device  130  to capture signal  133 . 
     In one embodiment, the forward model is implemented as a closed loop model that maps directly from input visual information signal  122  to capture signal  133 . 
     In one embodiment, shown in  FIG. 1A , video cross-talk reducer  140  is able to reduce video cross-talk based on signals because of the following principles regarding light. The linearity of light means that the radiance emerging from first surface  111  (e.g., the back of the display screen  110 A) is,
 
 s ( x,y,t )= s   p ( x,y,t )+ s   d ( x,y,t ),  (1)
 
     where continuous signal s(x, y, t) represents radiance, composed of two terms: (1) s p (x, y, t) (e.g., back scattered light  127 ) from the video of the remote participant displayed by projector  120 , and resulting in the cross-talk signal in capture device  130 , and (2) from the desired light s d (x, y, t) (e.g., light  131 ) coming from a room, containing user  105  on side  112  of display screen  110 A. In one embodiment, at capture device  130 , because of polarizing filters  170  and  180 , the two signals are attenuated differently, but linearity continues to hold if capture device  130  is controlled and its linearity enforced by setting the camera gamma to the identity. 
     The resulting video frames at capture device  130  are given by,
 
 c ( n   1   ,n   2   ,k )= c   p ( n   1   ,n   2   ,k )+ c   d ( n   1   ,n   2   ,k ),  (2)
 
     where the functions c( ), c p ( ) and c d ( ) are 3-color functions of discrete spatial indices n 1  and n 2  and discrete temporal index k (color index not indicated for simplicity). 
     In one embodiment, input visual information signal  122  comprises a sequence of input frames p(m 1 , m 2 , k). The inputs to video cross-talk reducer  140  are the corrupted signal c(n 1 , n 2 , k) (e.g., capture signal  133 ) and a short sequence of input frames p(m 1 , m 2 , l) for lε[l min (k), l max (k)]. The output (e.g., output  155 ) is an estimate of the desired c d (n 1 , n 2 , k). 
     Linearity allows the solving of the signal subtraction problem for any arbitrary interfering cross-talk signal. In one embodiment, the entire view of capture device  130  contains a desired signal as well as cross-talk and it is not possible to segment the cross-talk artifacts for removal. 
     In one embodiment, the forward model f( ) is a transformation from input signals p(m 1 , m 2 , l) to camera cross-talk signal c p (n 1 , n 2 , k), which is used to subtract the estimated signal ĉ p (n 1 , n 2 , k)=f(p(m 1 , m 2 , l)) from Equation 2. In another embodiment, forward model f( ) is a transformation from a single input frame (e.g., input visual information signal  122 ) to capture signal  133 . 
     To obtain desired video cross-talk reduction, photometric, geometric and optical factors that comprise f( ) are characterized. In addition to forward modeling, video cross-talk reduction (e.g., subtraction) is desired to provide the cross-talk reduced signals to the remote participants. 
     In contrast, conventional technology in projector-camera modeling has developed inverse models to modify multiple projector input signals to result in uniform and well-blended signals on a screen. The camera is used incidentally to characterize the inverse model but the camera is not used during operation. 
       FIG. 1B  depicts an embodiment of visual collaborative system  100 B. Visual collaborative system  100 B operates similarly to visual collaborative system  100 A, however, visual collaborative system  100 B includes display screen  100 B. Unlike display screen  110 A, display screen  110 B does not require a projector for images to be displayed to user  105 . This display screen can directly display content on the screen. This display screen also has the ability to be partially transmissive. For example, this may be a partially transparent organic light emitting diode (OLED) display screen. 
     During use and operation of visual-collaborative system  100 B, capture device  130  captures light  131  and light  160  that is displayed on display screen  110 B. As a result of capturing light  131  and light  160  that is displayed on partially transparent display screen  110 B, video cross-talk is generated. 
     In general, video cross-talk reducer  140  is able to reduce video cross-talk by forward modeling input visual information signal  122  propagating through visual-collaborative system  100 B. For example, video cross-talk reducer  140  forward models (e.g., estimates) how input visual information signal  122  is propagated through partially transparent display screen  110 B and captured as video cross-talk at capture device  130 . Once video cross-talk is properly forward modeled, it is then reduced (e.g., subtracted) from the actual video cross-talk captured at capture device  130 . Then output  155  is generated and subsequently transmitted to a remote location. 
       FIG. 2  depicts video cross-talk reducer  250  in accordance to an embodiment of the present invention. Video cross-talk reducer  250  forward models (e.g., estimates) how input visual information signal  122  is propagated through projector  120  and captured as video cross-talk at capture device  130 . Once video cross-talk is properly forward modeled, it is then reduced (e.g., subtracted) from the actual video cross-talk captured at capture device  130 , as presented above. In one embodiment, video cross-talk reducer  250  uses static (time-invariant) characterizations of: 1) color transformation; 2) geometric transformation; 3) space-varying color gain and offset, and 4) space-varying blur. In a more general case, the characterization may be performed periodically, or continuously, depending on the time-varying nature of the characteristics. 
     In particular, video cross-talk reducer  250  includes color corrector  210 , geometric corrector  220 , space-varying offset and gain corrector  230  and space-varying corrector  240  to estimate the different parameters of f( ). In various embodiments, video tests are sent to projector  120  to estimate the different parameters of f( ). In one such embodiment, the test patterns include, but are not limited to, color patches, grid patterns, horizontal and vertical stripes, and uniform white, black and gray level signals. In another such embodiment, the video test patterns are sent while the room is dark. This calibration may also be performed during manufacture and before shipping. 
     Space-varying offset and gain corrector  240  is configured to account for and correct space-varying black level and space-varying brightness. For example, by averaging captured uniform white video frames and black video frames, (spatially-varying) white response, W(n 1 , n 2 ), and the black response, B(n 1 , n 2 ), of visual-collaborative system  100 A is determined. For input c l (n 1 , n 2 , k), (normalized to be in the range [0,1]) the output is given by,
 
 c   o ( n   1   ,n   2   ,k )= c   l ( n   1   ,n   2   ,k )[ W ( n   1   ,n   2 )− B ( n   1   ,n   2 )]+ B ( n   1   ,n   2 ).  (3)
 
     Color corrector  210  is configured to provide color transformation. For example, in one embodiment, given the gain offset transformation (as described above), a global color transformation is determined by fitting between measured colors and color values c l ( ) generated using the inverse of Equation 3. 
     Measured average color values for gray input patches are used to determine 1D lookup tables applied to the input color components, and measured average color values for primary R, G, B inputs are used to determine a color mixing matrix using the known digital input color values. Determining the fits using the spatially renormalized colors allows the color model to fit the data with a small number of parameters. 
     Geometric corrector  220  is configured to provide geometric transformation. In one embodiment, geometric transformation is determined using a traditional multidimensional polynomial transformation model. 
     Space-varying blur corrector  240  is configured to account for and correct space-varying blur. In one embodiment, space-varying blur corrector  240  is utilized to obtain good results at edges in the cross-talk signal. If space-varying blur  240  is not utilized, objectionable halo artifacts remain visible in output  155 . 
     The parameters of the space-varying blur are determined by estimating separable blur kernels in the horizontal and vertical directions. Captured horizontal and vertical step edges at different locations in the frames are fit using scaled erf error functions. The standard deviations a of best fit are also the parameters for the space-varying Gaussian blurs that are applied. In one embodiment, the range of values found are σε[1, 4]. In another embodiment, the sparsely sampled blur estimates, 50 points each for horizontal and vertical estimates, are interpolated to a spatially dense set of horizontal and vertical blur parameters, σ h (n 1 , n 2 ) and σ v (n 1 , n 2 ). 
     Direct implementation of space-varying blur, 
     
       
         
           
             
               
                 
                   
                     
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     Thus, the linear (but shift variant) operation of Equation 4 is approximated by, 
     
       
         
           
             
               
                 
                   
                     
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     In one embodiment, i=4, so that four separable convolutions are followed by pixel-wise linear combination with weights α i (n 1 , n 2 ) that are predetermined for efficiency. 
       FIG. 3  depicts an embodiment of visual-collaborative system  300 , in accordance to an embodiment of the present invention. Visual-collaborative system  300  operates similarly to visual-collaborative system  100 A, however, visual-collaborative system  300  includes a whiteboard  310  (e.g. non-transparent display screen) rather than a see-through display screen  110 A. 
     Accordingly, during use, projector  120  projects display signal  125  onto the front side  311  of whiteboard  310 . Capture device  130  captures light  131  (e.g., written text on whiteboard  310 ) and backscattered light  127  (e.g., reflectance of whiteboard  310 ) and generates capture signal  133 . 
     As described above, in various embodiments, video cross-talk reducer  250  (or  150 ) generates output  155  by reducing an estimated video cross-talk from capture signal  133 . For example, the additive model of the portions of projected light of the remote scene and the light from the locally captured scene shown in the embodiment of  FIG. 3  can be described as:
 
 c ( x,t )= p ( x,t )+ l ( x,t ),  (6)
 
     where c(x,t) is the camera signal, p(x,t) is portion of light projected by the projector and l(x,t) is the portion of light due to the local scene (e.g., light reflected from user). After a forward model of p(x,t) is determined, it is reduced or subtracted out of c(x,t). 
     In another embodiment, video cross-talk reducer  250  generates output  155  by reducing an estimated video cross-talk generated from projector  120  projecting display signal  125  onto whiteboard  310 . Accordingly, an “idealized” model of the projected light and light of the whiteboard writing is:
 
 c ( x,t )= p ( x,t ) w ( x,t ),  (7)
 
     where c(x,t) is the camera signal, p(x,t) is portion of light projected by the projector and w(x,t) is the reflectance of the white board, including written text on the white board, for example. In other words, camera signal c(x,t) is light of the projector p(x,t) multiplied by the reflectance of the whiteboard w(x,t). After a forward model of p(x,t) is determined, it is reduced or subtracted out of c(x,t). 
     In another embodiment, video cross-talk reducer  250  generates output  155  by reducing an estimated video cross-talk generated from projector  120  projecting display signal  125  onto whiteboard  310  in a dark room. Accordingly, an “idealized” model of the projected light and light of the whiteboard writing is:
 
log  c ( x,t )=log  p ( x,t )+log  w ( x,t ),  (8)
 
     where c(x,t) is the camera signal, p(x,t) is portion of light projected by the projector and w(x,t) is the white board writing. After a forward model of log p(x,t) is determined, it is reduced or subtracted out of log c(x,t). 
     In a further embodiment, video cross-talk reducer  250  generates output  155  by reducing an estimated video cross-talk generated from projector  120  projecting display signal  125  onto whiteboard  310  in a room with ambient light that does not change. Accordingly, a model of the projected light and light of the whiteboard writing is:
 
 c ( x,t )= w ( x,t )[ p ( x,t )+ a ( x )],  (9)
 
     where c(x,t) is the camera signal, p(x,t) is portion of light projected by the projector, w(x,t) is the white board writing, and a(x) is ambient light. After a forward model of p(x,t) is determined, it is reduced or subtracted out of c(x,t). 
     In yet another embodiment, video cross-talk reducer  250  generates output  155  by reducing an estimated video cross-talk generated from projector  120  projecting display signal  125  onto whiteboard  310  in a room with ambient light that does change. Accordingly, a model of the projected light and light of the whiteboard writing is:
 
 c ( x,t )= w ( x,t )[ p ( x,t )+ a ( x,t )],  (10)
 
     where c(x,t) is the camera signal, p(x,t) is portion of light projected by the projector, w(x,t) is the white board writing, and a(x,t) is changing ambient light. After a forward model of p(x,t) is determined, it is reduced or subtracted out of c(x,t). 
     Synchronization Relationship Determination 
     Video cross-talk can be further reduced by determining a synchronization relationship between input visual information signal  122  and capture signal  133 . Typically, there is an offset (forward or backwards) between when projector  120  receives input visual information signal  122  and when capture device  130  generates capture signal  133  that contains the cross-talk from input visual information signal  122 . By taking into account the offset, the reduction in actual video cross-talk is further enhanced. 
       FIG. 4  depicts a synchronization relationship between display signal  125  generated by projector  120  and capture signal  133  generated by capture device  130 , in accordance to an embodiment of the present invention. Projector  120  and capture device  130  operate on different clocks. Accordingly, a synchronization relationship such as an offset can occur between visual information signal  122  input to projector  120  and capture signal  133 . 
     In one embodiment, projector  120  receives input visual information signal  122  and transforms it into display signal  125  at a projector frame rate. For example, projector  120  projects frame j at time T0, frame j+1 at T1, frame j+2 at T2, frame j+3 at T3 and so on. It should be appreciated that display signal  125  may involve a time sequential presentation of different colors in input visual information signal  122  and may use a different color space such as RGBW physically implemented using a color wheel, for example. If visual information signal  122  is divided into sequential frames, the projector frame rate may be different from the input frame rate. For example, projector  120  may produce 6 display frames per input frame, or it might produce 24 frames for every 30 input frames. It should also be appreciated that there may be a gap in time between each projected frame, and also that projected frames may be repeated with or without intervening projected frames. 
     Capture device  130  views display signal  125  according to its own clock. The viewing of display signal  125  by capture device  130  is then generated into capture signal  133 . 
     In particular, a shutter of capture device  130  is open at pulses  422 - 428  based on the camera frame rate. For example, at pulse  422 , the shutter is open and captures frame j. At pulse  424 , the shutter is open and captures a portion of frame j and frame j+1. At pulse  426 , the shutter is open and captures a portion of frame j+1 and j+2. At pulse  428 , the shutter is open and captures frame j+3. It should be appreciated that in other cases capture device  130  may capture portions of three or more sequential frames. 
     The frame rate of capture device  130 , as depicted, is an arbitrary frame rate. Moreover, regardless of the frame rate of capture device  130 , the duration the shutter is open can vary as a function of time based on the amount of light. 
     As such, display signal  125  and capture signal  133  may be unsynchronized due to, but not limited to, a different frame rate between projector  120  and capture device  130  and the duration the shutter is open as a function of time. 
     Referring to  FIG. 2 , video cross-talk reducer  250  also includes synchronization relationship determiner (SRD)  260 . In one embodiment, video cross-talk reducer  250  does not include SRD  260 . For example, SRD  260  is separate from video cross-talk reducer  250 . 
     SRD  260  includes (1) capture signal receiver  261  configured to receive capture signal  133 , (2) input visual information signal receiver  262  configured to receive input visual information signal  122 , (3) time offset determiner  264  configured to determine a time offset between input visual information signal  122  and capture signal  133 , (4) mapping generator  266  configured to generate a mapping between time intervals of input visual information signal  122  and capture signal  133 , (5) best match determiner  268  configured to determine a best match between a frame of input visual information signal  122  with a frame of capture signal  133 , and (6) best blend determiner  269  configured to determine a best blend for a sequential set of adjacent frames of input visual information signal  122 . Optionally, best blend determiner  269  is configured to determine a different best blend for a sequential color presented by display signal  125 . 
     SRD  260  is configured to determine a synchronization relationship between input visual information signal  122  and capture signal  133 . The synchronization relationship determination is signal based. 
     In general, given a combination of a desired signal (e.g., light  131 ) and an undesired signal (e.g., backscattered light  127 ), SRD  260  facilitates in extracting the desired signal. 
     For example, a discrete time signal s(k)=d(k)+O(u(k−l(k))) at discrete time k contains a desired signal d(k) at time k and an undesired signal given by O(u(k−l(k))). The goal is to extract the desired signal d(k) from s(k), given knowledge of u(k). In the equation, shift-invariant operator O is applied to sequence u(k−l(k)) with kε[−∞,∞] and the lack of synchronization between desired and undesired signal is reflected by the use of the time-varying time shift l(k). The goal is, given u(k), is to recover the shift l(k) as a function of k. This model encompasses cases where there is time smearing in the undesired signal, as well as capturing deterministic transformations (in O) such as color and geometric transformations in projector-camera systems. 
     In one embodiment, O is memory-less and corresponds to color and geometric transformations in a projector-camera system. In this embodiment, the solution consists of vector projection of the signal s(k) onto the output of the estimated operator O. Accordingly, l(k) is successfully found and the synchronization information is subsequently used to reduce video cross talk. 
     In various embodiments, input visual information signal  122  includes audio information. Also, the visual-collaborative system includes a microphone to capture the audio information. For example, capture device  130  includes a microphone. As such, SRD  260  may use the captured audio information independently or in conjunction with input visual information signal  122  to facilitate in determining a synchronization relationship. 
     SRD  260  facilitates in finding the approximate synchronization offset between input frames p(m 1 , m 2 , l) and captured camera video c(n 1 , n 2 , k). For a given camera frame with cross-talk the index {circumflex over (l)} is estimated for the best match input frame. In other words, SRD  260  facilitates in determining which frame from input visual information signal  122  is best able to account for the video cross-talk in capture signal  133 . In one embodiment, best match determiner  268  determines a best match between a frame of input visual information signal  122  with a frame of capture signal  133 . 
     The forward model is applied to input visual information signal  122  in a buffer of frames to generate the estimate for cross talk signal ĉ p (n 1 , n 2 , k) and generate the projection 
     
       
         
           
             
               
                 
                   
                     
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     where d( ) is a bandpass filter that detrends and mean-subtracts its input signals, without which spurious matches may occur. Equation 11 is similar to a bank of matched filters, where the filters are the estimates d(ĉ p (n 1 ,n 2 ,l)) for different l values. The value {circumflex over (l)} identifies one of the interfering frames in the unsynchronized system. 
     In one embodiment, time offset determiner  264  determines a time offset between input visual information signal  122  and capture signal  133 . The time offset can be fixed for all frames or may vary from frame to frame. 
     Mapping generator  266  generates a mapping between time intervals of input visual information signal  122  and capture signal  133 . The time intervals may be specified relative to input visual information signal  122  or to display signal  125 . For example, the mapping may indicate that a particular frame of capture signal  133  is associated with an interval containing the last 20% of frame j of display signal  125 , all of frame j+1 of display signal  125 , and 55% of frame j+2 of display signal  125 . The mapping may also be stochastic, such as the mean and variance for the frame offsets, or probability distribution for the frame offsets. 
     SRD  260  may be further improved by taking into account the detailed manner in which video cross-talk arises from at least two sequential input frames. For example, assume that two input frames p(m 1 , m 2 , {circumflex over (l)}) and p(m 1 , m 2 , {circumflex over (l)}+1) produce video cross-talk. The forward model of the video cross-talk,
 
 ĉ   p ( n   1   ,n   2   k ,α)=α f ( p ( m   1   ,m   2   ,{circumflex over (l)} ))+(1−α) f ( p ( m   1   ,m   2   ,{circumflex over (l)}+ 1)),  (12)
 
     corresponds to the physical assumption that projector  120  displays {circumflex over (l)} for a proportion α of the camera capture time, and for the remaining camera capture time, 1−α, projector  120  displays {circumflex over (l)}+1. 
     To estimate α, a total variation measure is used, elsewhere applied for image denoising and restoration. The total variations of a differentiable image l(x,y) is defined by: 
     
       
         
           
             
               
                 
                   
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     Signal c(n 1 , n 2 ,k) in equation 2 has edges from desired signal c d (n 1 ,n 2 ,k) and spatially uncorrelated edges from video cross-talk signal c p (n 1 ,n 2 ,k). Minimizing the total variation finds the signal that leaves only the edges of the desired signal. Finding {circumflex over (α)} uses a line search of a function generated using simple image differencing operations. In various embodiments, actual implementation involves picking the minimum from two line searches because it is not known whether the frames corresponding to times {circumflex over (l)} and {circumflex over (l)}+1, or {circumflex over (l)} and {circumflex over (l)}−1. 
     In one embodiment, to reduce video cross-talk, a line search is performed for α for each candidate input frame j. After finding α j , the model gives an improved version of video cross-talk in the camera image which is made up of some blend of input frames j and j+1. When trying to decide which input frame matches the camera image best (instead of using input frames alone), interpolated input frames are used. That is, for interval (j, j+1) the best blend of frame j and j+1 is found. For interval (j+1, j+2), the best blend of frame j+1 and frame j+2 is found, and so on. The best overall winner from the intervals is picked and declared to be the time offset for that interval, thereby providing a more accurate synchronization. 
     In one embodiment, best blend determiner  269  determines a best blend for a sequential set of adjacent frames of input visual information signal  122 , wherein the sequential set of adjacent frames corresponds to a sequential set of adjacent frame intervals. Subsequently, best match determiner  268  determines a best fit or match between the best blend for the sequential set of adjacent frames with a frame of captured signal  133 . 
       FIG. 5  depicts method  500  for determining a synchronization relationship, in accordance with an embodiment of the present invention. In various embodiments, method  500  is carried out by processors and electrical components under the control of computer readable and computer executable instructions. The computer readable and computer executable instructions reside, for example, in a data storage medium such as computer usable volatile and non-volatile memory. However, the computer readable and computer executable instructions may reside in any type of computer readable storage medium. In some embodiments, method  500  is performed at least by visual collaborative system  100 A as described in  FIG. 1A . 
     At  510  of method  500 , input visual information signal  122  is received, wherein display signal  125  is based on input visual information signal  122 . At  520 , capture signal  133  is generated by capture device  130 , wherein capture signal  133  comprises display signal  125 . 
     At  530  of method  500 , a synchronization relationship is determined between input visual information signal  122  and capture signal  133 , wherein the determining the synchronization relationship is signal based. In one embodiment, at  531 , a synchronization relationship is determined between an audio signal of input visual information signal  122  and an audio signal of capture signal  133 . In another embodiment, at  532 , a time offset is determined between input visual information signal  122  and capture signal  133 . In a further embodiment, at  533 , a best fit is determined between a frame of input visual information signal  122  with a frame of capture signal  133 . 
     In one embodiment, at  534 , a mapping is generated between time intervals of input visual information signal  122  and the capture signal  133 . In another embodiment, at  535 , a best blend is determined for a sequential set of adjacent frames of input visual information signal  122 , wherein the sequential set of the adjacent frames corresponds to a sequential set of adjacent frame intervals; and a best fit is determined between the best blend for the sequential set of adjacent frames with a frame of captured signal  122 . 
     Various embodiments of the present invention are thus described. While the present invention has been described in particular embodiments, it should be appreciated that the present invention should not be construed as limited by such embodiments, but rather construed according to the following claims.