Abstract:
A system for detecting and counting individuals in a stationary or moving crowd based on a digital or digitized image captured from a single camera. Initial information is assumed based on a foot-to-head plane homology, where a geometric construct is developed to best enclose image features with a high probability of being an individual within a crowd. These geometric constructs are then subjected to further probabilistic analysis to determine individuals. The vector track of each individual is determined to validate the determination of a group of features as individuals, thereby compensating for occlusion of an individual within any given frame image. A virtual gate is then employed to count the individuals moving past the virtual gate. Also, by weighting portions of the individual, events near the gate can be predicted.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. provisional application No. 60/695,632 filed Jun. 30, 2005, which is incorporated herein in its entirety by reference. 
     
    
     BACKGROUND  
       [0002]     The invention relates generally to a system and method for identifying discrete objects within a crowded environment, and more particularly to a system of imaging devices and computer-related equipment for ascertaining the location of individuals within a crowded environment.  
         [0003]     There is a need for the ability to segment crowded environments into individual objects. For example, the deployment of video surveillance systems is becoming ubiquitous. Digital video is useful for efficiently providing lengthy, continuous surveillance. One prerequisite for such deployment, especially in large spaces such as train stations and airports, is the ability to segment crowds into individuals. The segmentation of crowds into individuals is known. Conventional methods of segmenting crowds into individuals utilize a model-based object detection methodology that is dependent upon learned appearance models.  
         [0004]     A number of surveillance applications require the detection and tracking of people to ensure security, safety, and site management. Examples include the estimation of queue length in retail outlets, the monitoring of entry points, bus terminals, or train stations.  
         [0005]     Also, automatic monitoring of mass experimentation on cells involves the high throughput screening of hundreds of samples. An image of each of the samples is taken, and a review of each image region is performed. Often, this automatic monitoring of mass experimentation relates to the injection of various experimental drugs into each sample, and a review of each sample to ascertain which of the experimental drugs has given the desired effect.  
         [0006]     Substantial progress has been made in detecting individuals in constrained settings, i.e., frames of reference in which the size and/or shape of the individuals are assumed. To achieve this, it is often assumed that the individuals in the frame of reference are well separated and that identifying foreground objects is possible using a statistical background model. Certain actions can only be detected if the location of all individuals in the frame of reference is known. However, in all of the scenarios just mentioned, the individuals actually appear in groups. Additionally, the number of individuals present in the frame of reference may also be desired to be known.  
         [0007]     Various techniques have been applied to construct fast and reliable individual detectors, for example, for surveillance applications. Classification techniques can be applied to decide if a given image region contains a person. The use of Support Vector Machines is one way to approach this problem. Another method for solving this problem is using a tree based classification to represent possible shapes of individuals within a group. Yet another way to determine if a region contains a person is to use dynamic point distribution models.  
         [0008]     An alternative to modeling the appearance of an entire individual is to design detectors for specific parts of the individual, such as a head or a foot of a human or the neck of a bottle, and combine the result of the detection of those specific parts. The idea of learning part detectors using Ada-Boost and a set of weak classifiers is one approach to this problem. A learning approach is then being used to combine the set of weak classifiers to body part detectors, which are further combined using a probabilistic person model. All these approaches require a fair amount of training data to learn the parameters of the underlying model. Although these classifiers are robust to limited occlusions, they are not suitable to segment a group into individuals, especially when the individuals within the group have freedom of movement, such as crowds of animals or people.  
         [0009]     One way in which segmenting a group into individuals has been achieved is to use the information available from several frames of reference, for example, various views from multiple cameras. For example, the M2-tracker explicitly assigns the pixels in each camera view to a particular person using color histograms. A. Mittal and L. S. Davis,  M 2 tracker: A multi - view approach to segmenting and tracking people in a cluttered scene using region - based stereo , P ROC.  7 TH  E UROPEAN  C ONF . C OMPUTER  V ISION , Kopenhagen, Denmark, vol. X, pp. 18-33 (2002)  
         [0010]     Another way of resolving multiple camera views is by using the camera calibration to locate possible head locations using a head detector. The locations of all individuals in a scene are estimated by maximizing an observation likelihood using Markov Chain Monte Carlo. In this situation, it is extremely helpful to know the location of the ground plane and the camera parameters, as the head detector is based on edge information. However, under certain imaging conditions, extracting clean edge maps can be challenging. Additionally, installing multiple cameras is expensive, and managing the acquisition and analysis of the images from all of the cameras is complex and expensive.  
         [0011]     A traditional non-video way of counting individuals is to use turnstiles. However, the installation of the turnstiles can be costly, both in initial expense and in the loss of floor space and flexibility of entrance/egress.  
         [0012]     As such, a need exists for a way to analyze a frame of reference to segment a group into individuals for counting and tracking without the need for multiple frames of reference. A further need exists for using this information to determine likely action scenarios of individuals within a group.  
       SUMMARY  
       [0013]     Therefore, according to one aspect of the invention, a system for segmenting a crowd into individual objects includes an image capturing system and a computing system connected to the image capturing system. The computing system utilizes a foot-to-head plane technique to segment the crowd.  
         [0014]     According to another aspect of the invention, a method for counting at least one individual in a crowd comprises the steps of (i) calibrating an image capturing device and an image processing device; (ii) capturing a frame of an image with the image capturing device; (iii) drawing a virtual gate through the image; (iv) applying a foreground estimation module to the image; (v) applying a tracking module to the image; (vi) applying a crowd segmentation module to the image to separate the individual from the crowd; and (vii) counting the individual.  
         [0015]     According to another aspect of the invention, a method for detecting an event comprises the steps of: (i) calibrating an image capturing device and an image processing device; (ii) capturing a frame of an image with the image capturing device; (iii) drawing a boundary of a zone of interest in the image; (iv) applying a foreground estimation module to the image; (v) applying a crowd segmentation module to the image to separate the individual from the crowd; (vii) identifying a point on the individual; and (viii) determining the location of the point with respect to the boundary. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     These and other features, aspects, and advantages of the invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:  
         [0017]      FIG. 1  is a schematic view of a system for counting objects and detecting events according to an embodiment of the invention;  
         [0018]      FIG. 2  is a block diagram showing the functionality of the system of  FIG. 1 ;  
         [0019]      FIG. 3A  is an image captured by the system of  FIG. 1 , where the image features have been labeled;  
         [0020]      FIG. 3B  is the image of  FIG. 3A  showing defined clique sets;  
         [0021]      FIG. 3C  is the image of  FIG. 3A  showing segmentation of the image into individuals;  
         [0022]      FIG. 4  is a flowchart showing an embodiment of an algorithm used by the system of  FIG. 1 ;  
         [0023]      FIG. 5  is a graph reflecting experimental data collected by an implementation of the system of  FIG. 1 ;  
         [0024]      FIGS. 6A-6D  are experimental frame images corresponding to points a-d shown in  FIG. 5 ; and  
         [0025]      FIGS. 7A-7D  are segmented frame images of the system of  FIG. 1  which reflect event detection capabilities thereof. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0026]     As illustrated in the accompanying drawings and discussed in detail below, an exemplary embodiment of the invention is directed to a system to detect individuals within a crowd for counting and other purposes. Applications for embodiments of the invention are described above and below and include use in security and quality assurance applications. It should be appreciated, however, that the embodiments of the invention are not limited to the discussed applications.  
         [0027]      FIG. 1  illustrates a system  10  for segmenting and counting a crowd of individuals  1  within an area of interest  20 . The individuals  1  may be any type of object or animal, including humans, livestock, bottles, cans, consumer products, or the like. The individuals  1  may be either stationary or ambulatory. For example, the individuals  1  may be a group of people, people walking, roaming livestock, or objects moved on a conveyor belt. Similarly, the area of interest  20  may be any fixed location, such as a building entrance, an airport security gate, a livestock pen, a section of a production line, or the like.  
         [0028]     The system  10  includes an image capturing device  12 . Preferably, only one fixed-location image capturing device  12  is used, although certain applications may require additional image capturing devices  12  from additional locations. The image capturing device  12  is preferably a video or single-image camera, such as a CCD camera, which captures frame-by-frame images in either color or black and white. The image capturing device  12  may be either analog or digital. If an analog device is used, the image is digitized using any commercially available or custom-made frame gabber known in the art. While any number of frames per second may be taken by the image capturing device  12 , preferably the number of frames per minute does not drop below five (5) frames per second. Preferably, the resolution of the image(s) taken by the image capturing device  12  is such that the objects of interest in the image are about twenty (20) pixels by thirty (30) pixels. Higher resolution may undesirably slow the processing of the image, while too low a resolution may make the processing of the image unreliable. If a higher resolution image capturing device  12  is used, the resultant image(s) may be down-sampled to obtain a more optimal image resolution.  
         [0029]     A field of view  18  of the image capturing device  12  is focused on the individuals  1  contained within the area of interest  20 . Preferably, the field of view  18  is trained on a counting area  24  which surrounds a virtual gate  22 , both of which will be discussed in greater detail below.  
         [0030]     The image capturing device  12  is connected to an image processor  14  via a link  16 . Link  16  may be any image-transferring capable link known in the art, such as a video cable or a wireless transmission link. One end of link  16  is connected to the image capturing device  12  while the other end of link  16  is connected to the image processor  14 .  
         [0031]     The image processor  14  may be any type of processor capable of executing an algorithm for interpreting the image(s) taken by the image capturing device  12 . For example, the image processor  14  may be a PC, a laptop, a processing chip integrated into an instrument panel or station, or the like. Optionally, the image processor  14  is connected to peripheral equipment such as recording devices, external communication links, and power sources.  
         [0032]     As shown in  FIG. 2 , the image processor  14  is configured or programmed to execute an image processing algorithm which includes three main components: a low-level foreground detection module  30 , a tracking module  32 , and a crowd segmentation module  34 . All three modules  30 ,  32 ,  34  are combined into a tightly coupled framework. In basic operation, once the image processor  14  has received a digital frame image from the image capturing device  12 , the foreground detection module  30  isolates the crowd elements from the background elements of the image. The tracking module  32  then notes the groups of individuals  1  and generates a probabilistic foreground mask to determine the direction in which the groups of individuals  1  are moving based on historical data from the preceding frame images. The segmentation module  34  then divides the group into discrete individuals  1 . The tracking module  32  and the segmentation module  34  operate together in an iterative fashion to produce likely counting and other determinative scenarios. The algorithms and other preferred methodologies used by each of these modules  30 ,  32 ,  34  is discussed in greater detail below.  
         [0033]     The foreground detection module  30  may be any such module known in the art. Preferably, the foreground detection module  30  is a low level foreground detection module. Such a low level foreground detection module is described in Chris Stauffer and W. E. L. Grimson,  Adaptive Background Mixture Models for Real - time Tracking , CVPR99, vol. II, pp. 246-252, which is incorporated herein in its entirety by reference.  
         [0034]     Once the foreground detection module  30  has roughly identified the crowds and the background scene, the tracking module  32  preferably uses an adaptive appearance-based approach to note the direction of movement of the individuals  1  in the crowd from frame to frame.  
         [0035]     The tracking module  32  includes two parts: a multiple hypothesis tracker as described above, and a data association module. Given enhanced segmentation results from neighboring frames developed iteratively with the segmentation module described in greater detail below, an algorithm is used to find the optimal data association. The algorithm used may be any algorithm known in the art such as the well-known Hungarian algorithm, more fully described in Paul E. Black,  Hungarian Algorithm , D ICTIONARY OF  A LGORITHMS AND  D ATA  S TRUCTURES , National Institute of Standards and Technology (NIST), Paul E. Black, ed. accessible at http://www.nist.gov/dads/HTML/HungarianAlgorithm.html, last accessed on Nov. 17, 2005. The two-dimensional distance between a pair of segmented individuals from the neighboring frames is used in computing the cost matrix for the Hungarian algorithm. Once the data association is performed for every neighboring pair of frames, the track  3  of each segmented individual  1  over time can be determined.  
         [0036]     The tracking module  32  can follow individuals  1  within a group as well as other components of the frame of reference to develop a track  3  or history of the movement of the group or individual  1  over a series of frames, i.e., over time. In other words, track  3  is the vector for the individual  1 . For example, in  FIG. 1 , if the individuals  1  are moving in the direction of the arrow, then tracks  3  can be developed by comparing the image from a frame image with its adjacent frame images. Various algorithms may be used within the tracking module  32  for initiating, merging, splitting and deleting tracks  3 . For example, each track  3  may be modeled by a signature such as a color signature, an appearance template, and a probabilistic target mask. The foreground mask used in the tracking module  32  is preferably an autoregressive estimate of the foreground information as obtained in the previous stage.  
         [0037]     The tracking module  32  may use one of various methods for overcoming occlusion problems, i.e., the obscuring of one individual  1  in the field of view  18  from the image capturing device  12  by another individual  1 . For example, the tracking module  32  handles short term occlusion between isolated tracks  3 , but groups closely spaced targets together into group tracks  3 . Only foreground regions which are large enough to contain a number of people and image regions that contain closely spaced tracks  3  are forwarded to the segmentation module  34  for further analysis. In addition, an improved foreground region image is composed based on the information maintained by the tracking module  32  and also supplied to segmentation module  34 . The motivation for this is so that the properties of the target masks compare favorably to the direct estimate of the foreground. First, an autoregressive process that is preferably used to maintain the target masks suppresses high frequency variations and noise in the foreground image. Second, since the target masks are estimated from the foreground image relative to the moving tracks  3 , foreground region information is effectively integrated across multiple images along the motion paths of targets, thereby resulting in more accurate overall estimates.  
         [0038]     Tracking according to the above description generally utilizes knowledge of the geometry of the area of interest  20  and the parameters of the image capturing device  12  so that a connection between the image taken and the world measurements may be established. This knowledge can constrain the analysis such that solutions to the relevant equations are more readily determined. However, geometric information is rarely readily available for a given area of interest  20  and may be difficult to obtain after a system such as system  10  is installed. Therefore, system  10  includes an autocalibration approach that utilizes information from the observed scene.  
         [0039]     One such approach to autocalibration is based on vanishing points and vanishing lines that can be obtained from tracking the individual targets in the image. Preferably, however, system  10  includes a methodology for autocalibration that estimates the foot-to-head plane homology, which obtains the internal and external calibration parameters of the camera from head and foot location measurements. The estimation of the homology may be performed using any method known in the art, for example using a Bayesian approach which can perform the estimation while handling measurement uncertainties, outliers as well as prior information. The full posterior distribution of calibration parameters given the measurements is estimated, which allows making statements about the accuracy of both the calibration parameter and the measurements involving the calibration parameters.  
         [0040]     The foot-to-head plane homology is further described below, including the assumptions involved. When observing people, each foot location on the ground plane corresponds to exactly one location in the so-called head plane which is located at a height h parallel to the observed ground plane. It is assumed for the purposes of simplifying the analysis that all observed individuals  1  have the same average height h. In other words, the image processor  32  assumes that each individual  1  has the same geometric shape with respect to the ground plane. For example, when processing an image of a group of people, it can be assumed that each person is a rectangle, where the long legs of the rectangle represent the height of the person and where all of the rectangles have long legs of equal lengths. This concept is shown in  FIGS. 3B and 3C , where rectangles  42  and  44  represent individuals.  
         [0041]     It can be shown that the homography that maps the images of ground planes to the images of the corresponding points in the head plane is in fact a homology H given by the following equation, Eq. 1:  
             H   =     ∏       -     h   z       ⁢             v   ~     ∞     ⁡     (       1   ~     ∞     )       T           (       v   ~     ∞     )     T     ⁢       1   ~     ∞                     Eq   .           ⁢   1             
 
 where z is the height of the camera above the origin of the ground plane, {tilde over (v)} ∞  is the vanishing point and {tilde over (1)} ∞  is the horizon line. It can be shown that the horizon line and the vanishing point are given by the following equations, Eq. 2 and Eq. 3, respectively:  
                 1   ~     ∞     =     [       sin   ⁡     (   ρ   )       -       cos   ⁡     (   ρ   )       ⁢     f     tan   ⁡     (   θ   )             ]             Eq   .           ⁢   2                   v   ~     ∞     =     [       f   ⁢           ⁢     sin   ⁡     (   ρ   )       ⁢     sin   ⁡     (   θ   )         -     f   ⁢           ⁢     cos   ⁡     (   ρ   )       ⁢     sin   ⁡     (   θ   )       ⁢     cos   ⁡     (   θ   )           ]             Eq   .           ⁢   3             
 
 where ρ is the roll angle of a camera, θ is the tilt of the camera towards the ground plane and f is the camera focal length. Using standard assumptions about the remaining parameters of the camera, this foot-to-head plane homology provides a complete metric calibration of the camera with respect to the ground plane. 
 
         [0042]     The preferred autocalibration approach including a description of the standard assumptions are discussed in Nils Krahnstoever, Paulo R. S. Mendonca,  Bayesian Autocalibration for Surveillance , Tenth IEEE International Conference on Computer Vision (ICCV&#39;05), vol. 2, October 2005, which is incorporated herein by reference in its entirety. A summary of the method is included here. Given a sufficient number of isolated people observations, including head and foot image location measurements with associated measurement uncertainties, an initial foot to head homography is estimated using a standard Direct Linear Transformation approach, as is discussed in R. Hartley and A. Zisserman,  Multiple view geometry computer vision , Cambridge University Press, pp. 71-76 (2000), which is incorporated herein by reference in its entirety. The eigenvalue structure of the targeted homography is exploited to obtain the closest foot-to-head homology consistent with the data. Finally, the initial homology estimate is refined in a Bayesian framework, taking the noise and other nuisance variables into consideration, to estimate the posterior distribution of the camera parameters given the measurements.  
         [0043]     Segmentation module  34  preferably uses an algorithm that processes all regions in the image that the tracking module  32  has judged to contain groups of individuals. The resulting segmentation observation Ŝ t  at frame t contains information about the detected number of people and their location in the image given by the following equation, Eq. 4: 
 
{circumflex over (X)} t ={{circumflex over (n)} t ,({circumflex over (x)} i   t ,ŷ i   t ),i=0, . . . ,{circumflex over (n)} t }  Eq. 4 
 
 As discussed above, noise in the feature extraction process as well as inherent ambiguities leads the estimate {circumflex over (X)} t  to deviate from the true state Ŝ t . To reduce the error in the resulting segmentation, the estimated values are processed by a simplified multiple hypothesis tracker, such as the one described in S. Blackman and R. Popoli, D ESIGN AND  A NALYSIS OF  M ODERN  T RACKING  S YSTEMS , Artech House, Norwood Mass., pp. 360-369 (1999), which is incorporated herein in its entirety by reference. Within each group, individual tracks  3  may be smoothed using a constant velocity Kalman filter. 
 
         [0044]     Although any method known in the art can be used, a model based approach to crowd segmentation is preferred. Description of the steps involved in an exemplary process is given with reference to  FIGS. 3A, 3B  and  3 C. Given a foreground segmentation, a set of low level image features  40 , Z={z i }, are extracted. In addition, an exhaustive set of feature groups or cliques  42  C={c i } is hypothesized. Each grouping corresponds to a potential individual. These groupings are constrained by a geometric shape model which is parameterized by X={x i }; here, the geometric shape model is a rectangle. Each feature is assigned to a single grouping, and the shape parameters of each grouping is then estimated. An assignment vector V={v i } establishes the feature assignments. A likelihood function P(Z, V; X) is defined based on pair-wise and single assignments of features to groupings with shape parameters X. The goal, therefore, is to determine maximum likelihood estimates of both V and X to establish discrete segmentations  44 , such as shown in  FIG. 3C  for an estimation of X* and one sample of V.  
         [0045]     A formulation based on Expectation Maximization (EM) is used, where V is viewed as a hidden variable. EM provides a method to estimate a distribution {tilde over (P)}(V) as well as an estimate of X. Once this has been achieved, likely values of V can be selected by sampling {tilde over (P)}(V). Estimates of {tilde over (P)}(V) and X are found by maximizing the free energy equation, given by: 
 
 F ( {tilde over (P)}, X )= E   {tilde over (P)} [log  P ( V, Z; X )]+ H ( P )  Eq. 5 
 
 In order to regularize the optimization process, a temperature term T is introduced: 
 
 F ( {tilde over (P)}, X )= E   {tilde over (P)} [log  P ( V, Z; X )]+ TH ( {tilde over (P)} )  Eq. 6 
 
 For example, initially, T is set to a very large value that favors the entropy term. As a result, the initial estimate of {tilde over (P)}(V) can be set to a uniform distribution. An annealing process may then be performed by iteratively decreasing T. At each iteration, both an Expectation or E-step and a Maximization or M-step may be performed. In the E-step, X is fixed to its current value and the free energy is optimized with respect to {tilde over (P)}(V). In the M-step, {tilde over (P)}(V) is fixed and optimization is performed with respect to X. The application of the mean field approximation to {tilde over (P)}(V) allows for gradient accent in the E-step. The use of a simplistic shape model allows for the use of an exhaustive search in the M-step. As T approaches 0, the estimate of {tilde over (P)}(V) converges to a delta function centered on a local maxima of the likelihood function P(Z, V; X). This form of optimization is similar to soft assign. 
 
         [0046]     Given a video sequence of interest, the segmentation module  34  in the previous section classifies the crowd in each frame into multiple individuals  1 . However, in order to determine the number of individuals  1  and their movements, the trajectories of each individual  1  is determined over time. As such, the tracking function and crowd segmentation function described above are integrated. The crowd segmentation results may also, then, be stabilized and enhanced. Over a long sequence, the crowd segmentation might not obtain the correct number of individuals  1  in certain frames, due to occlusion. Therefore, the information from the tracking module is used to smooth the segmentation result such that the segmentation result and the tracking result align.  
         [0047]     In order to count individuals  1  within the group effectively, the virtual gate  22  is imposed on the image. Given a scene captured by the image capturing device  12 , a user and/or the image processor  14  draws a line at any location in the field of view. This line serves as the virtual gate  12 . The algorithm then continuously counts how many individuals  1  cross this arbitrarily selected line. The virtual gate  22  may have any shape, including a straight horizontal line, a straight vertical line, a straight line having an angle relative to the horizontal and/or vertical plane, a curved line, a closed polygonal shape, or the like.  
         [0048]     Tracking an individual  1  if no occlusion occurs can be relatively simple. However, groups of individuals  1 , such as people in a crowd, tend to move together. When this occurs at the virtual gate  22 , the segmentation module  34  for each individual frame might not determine the same number of individuals  1 .  
         [0049]     Once a segmented individual  1  enters the counting area  24 , the counting module begins to function. The counting module performs two basic tasks: retrieve the trajectory of the individual  1  and determine the moving direction of the individual  1  with respect to the virtual gate  22 , i.e., whether the individual  1  is entering or exiting the virtual gate  22 . Many different methods may be employed to determine the direction of track  3 , any of which is suitable for use in embodiments of the invention. For example, comparing two instances of an individual&#39;s  1  location along the direction perpendicular to the virtual gate  22  may be used to determine the direction of track  3 . Alternatively, an identification number ID may be added to a stored list of individuals  1  entering the virtual gate (Li) or exiting the virtual gate (Lo), depending upon the desired (arbitrary) direction. The purpose of these two lists is to avoid multiple counting when an individual  1  remains inside the counting area  24  in future frames. Furthermore, if the individuals  1  are associated with the ListIn Li and ListOut Lo lists based on the appearance information, the duration of stay of the individual  1  within the area of interest  20 .  FIG. 4  shows a flowchart of this iterative process.  
         [0050]     To test the performance of system  10  as a people counter, a single camera (the image capturing device  12 ) was mounted on the second floor of a building and pointed at the entrance of the building (the area of interest  20 ). A number of individuals  1  were asked to walk along the entrance in different combinations of groups. The combinations were set up so that both easy and difficult scenarios for people counting were presented. An easy scenario is one in which, for example, a single individual walks through the counting area  24 . A difficult scenario is one in which, for example, a plurality of people walk together through the counting area  24  in a cluster, or when small groups of people walk toward each other.  
         [0051]     A 10-minute video clip is captured and analyzed, where people pass the selected virtual gate  22  times during the testing sequence. At each instance of passing the virtual gate  22 , a true number of people is plotted as histogram bar  50 , and the count from system  10  is plotted as histogram bar  52 . The test results are shown in  FIG. 5 , with positive numbers referring to people going “in” the virtual gate  22  and negative numbers refer to people going “out” of the gate  22 .  FIGS. 6A-6D  are individual video frames that correspond to points a-d on the graph in  FIG. 5 .  
         [0052]     The data collected reflects the accuracy of system  10 . In most of the instances, system  10  detects the correct number of individuals, i.e., histogram bar  50  equals histogram bar  52 . In case a, three individuals are present in the counting area  24  and are moving out of virtual gate  22 , as is shown by histogram bar  50  at point a in  FIG. 5  and shown in  FIG. 6A . System  10  successfully counts all three individuals  44   1 ,  44   2  and  44   3 , as is shown by histogram bar  52  at point a in  FIG. 5 , in spite of some occlusion while passing the virtual gate  22 . In case b, five individuals are present in the counting area  24  and are moving out of virtual gate  22 , as is shown by histogram bar  50  at point a in  FIG. 5  and shown in  FIG. 6B . However, due to severe occlusion, system  10  successfully counts only four individuals  44   1 ,  44   2 ,  44   3 ,  44   4 , as is shown by histogram bar  52  at point b in  FIG. 5 . In case c, five individuals are present in the counting area  24  and are moving into virtual gate  22 , as is shown by histogram bar  50  at point c in  FIG. 5  and shown in  FIG. 6C . However, due to severe occlusion, system  10  successfully counts only four individuals  44   1 ,  44   2 ,  44   3 ,  44   4 , as is shown by histogram bar  52  at point c in  FIG. 5 . In case d, five individuals are present in the counting area  24 , two of which are moving out of virtual gate  22  and three of which are moving into virtual gate  22 . System  10  successfully counts all five individuals  44   1 ,  44   2 ,  44   3 ,  44   4 ,  44   5 , as is shown by histogram bar  52  at point d in  FIG. 5 , in spite of some occlusion while passing the virtual gate  22 . In cases such as cases b and c, the use of multiple cameras could provide views in which the nearly completely occluded individuals are not occluded.  
         [0053]     Referring now to  FIGS. 7A-7F , system  10  may also be used to detect behavioral events. For example, it may be of interest to detect automatically dangerous behavior, such as the entry of an individual into a restricted area, such as a person leaning over the edge of a subway platform or illicitly entering a security area. System  10  can make these detections by detecting the individuals, as described above with respect to  FIG. 1 , detecting the boundary  23  of the zone of interest  25 , e.g., the edge of a subway platform, and detecting when a portion of an individual  1  extends into the zone of interest  25 .  
         [0054]     The boundary  23  of a zone of interest  25  can generally be defined by a simple line. However, as most image capturing devices view a scene from an angle, the determination of whether or not an individual has crossed into a zone of interest  25  is more complex than merely determining whether or not an individual has crossed the line defining the boundary of the zone of interest  25 . For example, the top individual  64  in  FIG. 7A  apparently crosses the boundary  23 , but is not actually near the zone of interest  25 . In other words, system  10  should account for depth perception while still utilizing a single frame of reference.  
         [0055]     The assumption is made, therefore, that proximity to the boundary  23  is only determined when an individual&#39;s foot is near the boundary  23 . For the subway scenario, the foot location can be modeled as the lower-left corner  65  of a person segmentation as shown in  FIG. 7B . For a given image location, a “foot strength” factor can be determined as a response to a corner-detection template. To detect proximity to the boundary  23 , foot strength is measured along the boundary  23  line. Non-maximal suppression and thresholding then yield foot locations. Detecting a leaning-over event is then noted by determining if a part of the person segmentation falls to the right of a line extending upward from the defined foot location  65 , as happens in  FIG. 7B  with individual  64 , and in  FIG. 7C  and  FIG. 7F  (which is a crowd segmented version of  FIG. 7C ) with individuals  62 ,  64 .  
         [0056]     An additional step is taken to assure accuracy in cases of occluding segmented individuals, i.e., when one individual blocks another in the field of view. The crowd segmentation module makes a determination of the location of the individuals within the crowd.  FIG. 7C  shows a situation where the first person partially obscures the second person, so that the foot location, and, therefore, the foot strength, of the second person cannot be determined. After applying crowd segmentation however, it can be determined that the foot of the second person is also at the platform edge, and that his head is beginning to cross into the dangerous area, as shown in  FIG. 7F .  
         [0057]     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.