Method for counting the number of people crossing an entry barrier

A method for counting a number of objects passing through an entry threshold, comprising the steps of disposing a sensor (11) having a matrix of sensing points in an area of the entry threshold, identifying successive complete footprints of the same object on the sensor, and distinguishing between different footprints using clustering. The method is used in association with a pressure mat disposed at the entry threshold and tracks the progress of the object across the mat. Specifically, when a footprint vanishes from the mat, retroactive processing is effected in respect of the body associated with that footprint so that other footprints belonging to the same body may be ignored. The number of footprints thus associated with the body and which are not ignored serves as a counter of the number of bodies crossing the entry threshold.

FIELD OF THE INVENTION
 This invention relates to a method and system for estimating an number of
 objects effecting dynamic contact with a confined area of floor space. In
 particular, the invention finds application for estimating the number of
 occupants in a confined area such as a store, public transport and the
 like.
 BACKGROUND OF THE INVENTION
 Various prior art proposals exist for estimating a number of occupants in a
 confined spaces such as, for example, an elevator, store and so on. For
 example, in International Publication No. WO 97/02474, there is proposed
 an improved method based on the use of a matrix of pressure contacts for
 estimating an actual occupied area of floor in a confined space. Pressure
 contact points are clustered so as to form composite areas of floor which
 are occupied and a boundary is provided around each of these areas in
 order to allow for minimum breathing space between adjacent passengers. On
 this basis, the total occupied area may be estimated. However, such a
 method still does not give an accurate estimation of the actual number of
 passengers in the elevator because the footprints of two passengers
 standing in very close proximity will be so close that they are clustered
 together and, to all intents and purposes, are treated as a single
 footprint. This, of course, does not matter when only occupied or free
 area in a confined space is of interest. However, it is critical when an
 actual number of occupants is to be estimated.
 This having been said, there is a fundamental difference between, on the
 one hand, the situation where people are static in a confined space and,
 on the other hand, when people are moving across a confined space so as to
 effect dynamic contact therewith. In the static case, for example in an
 elevator car, as more people are confined into the limited space thereof,
 their feet inevitably are brought into ever closer proximity. Eventually,
 it becomes impossible to determine whether adjacent pressure contact
 points belong to the same footprint or to the adjacent footprints of two
 people standing almost on top of one another. As noted, this does not
 matter where an estimation of occupied floor space is all that is
 required, but it clearly militates against an accurate estimation of the
 number of occupants.
 However, in the second, dynamic case, where people are constantly on the
 move and where they effect only transient contact with the sensor, the
 likelihood that two different people in close proximity will exert
 pressure on the sensor simultaneously is so slim as to be safely
 negligible. This means that, contrary to the static case discussed above,
 each distinct cluster of pressure points may be identified with a unique
 instance of a person making his way across the sensor.
 SUMMARY OF THE INVENTION
 It is an object of the invention to provide a method and system for
 estimating a number of objects effecting transient contact with a floor
 area.
 According to a broad aspect of the invention there is provided a method for
 counting a number of objects passing through an entry threshold, the
 method comprising:
 (a) disposing a sensor having a matrix of sensing points in an area of the
 entry threshold,
 (b) identifying successive complete footprints of the same object on said
 sensor, and
 (c) distinguishing between different footprints using clustering.
 Thus, in accordance with the invention a sensor is employed for registering
 objects entering and leaving an entry threshold it being assumed, as noted
 above, that two or more people will not be so close on the trail of one
 another that their respective footprints make virtually simultaneous and
 contiguous contact with the sensor.
 The method according to the invention also allows for wheeled objects such
 as baby carriages and wheel chairs to be counted as well as objects
 dragged across the sensor such as, for instance, a piece of luggage and
 the like.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 FIG. 1 is a schematic view of a system 10 according to the invention
 comprising a sensor 11 coupled to a processing unit 12 for counting a
 number of people 13 or other objects traversing the sensor 11. The sensor
 11 is of similar form to that described in International Publication No.
 WO 97/02474, comprising a matrix of pressure contacts. The length of the
 sensor 11 must be such as to guarantee that a person walking across a
 floor area on which the sensor is located must inevitably tread fully on
 the sensor. On the other hand, in the interest of economy there is nothing
 to be gained by making the sensor 11 unnecessarily long.
 The process of generating pressure contact with the floor area is dynamic.
 Initially a person brings his heel into pressure contact with the floor
 and the tip of the heel then serves as a hinge axis for subsequent
 rotation of the shoe until fill contact is achieved. Full contact is
 maintained for a short time, whereafter the sole serves as a new hinge
 axis for subsequent rotation of the shoe. During such movement, only
 partial pressure contact of the shoe is achieved until the foot is lifted
 altogether from the floor and no pressure contact therewith is produced.
 Each footprint is characterized by its corresponding contact points
 creating a so-called "cluster".
 FIGS. 2a to 2h show subsequent stages during the evolution of a complete
 footprint. In FIG. 2a only the tip of the heel is in contact with the
 floor. In FIG. 2b, still only the heel is in partial, albeit greater,
 contact with the floor. In FIG. 2c the heel is in complete contact with
 the floor and part of the sole has started to make contact therewith. In
 FIGS. 2d and 2e, the sole continues to effect greater contact until, in
 FIG. 2e, the footprint effects complete contact with the floor. FIGS. 2f
 to 2h show subsequent stages during which the heel is lifted and the sole
 makes progressively less contact with the floor until it disappears
 altogether.
 FIGS. 3a and 3b show the dynamic evolution of the pressure profiles
 generated by contact of the heel and sole from creation to disappearance
 of a footprint. It is shown that the creation of a new footprint is
 heralded by the heel starting to make contact whilst pressure contact with
 the sole only follows later. An overlapping time period of approximately
 130 ms corresponds to full pressure contact of the complete footprint.
 FIGS. 4a and 4b illustrate how it is thus possible to determine the
 direction of passage across the floor area. In FIG. 4a, at a time t.sub.1
 four pressure contact points are detected corresponding to initial contact
 with the heel. At a subsequent time t.sub.2, further pressure contact
 points are detected corresponding to an increased area of pressure
 contact. Thus, evolution of the pressure profile occurs in the direction
 of arrow A which is thus the direction of passage across the sensor. In
 FIG. 4b, the opposite is the case, evolution of the pressure profile
 occurring in the direction of arrow B which is thus the direction of
 passage across the sensor.
 Contact points are sampled continuously at a fixed sampling rate, typically
 in the order of 25 ms so as to generate successive samples S.sub.i each
 being a bitmap of contact points. From each such sample S.sub.i those
 contact points which are also present in the previous sample S.sub.i- are
 disregarded. This is done by logical ANDing the respective bit-maps
 corresponding to successive samples S.sub.i and S.sub.i- and subtracting
 the result from the present sample S.sub.i. Thus for each sample S.sub.i
 the fresh contact points F.sub.i which are sensed for the first time are
 given by F.sub.i =S.sub.i-(S.sub.i {character pullout}S.sub.i-1). A time
 of creation t.sub.fi is associated with each fresh contact point F.sub.i
 so as to enable its direction of evolution to be determined as explained
 above with reference to FIG. 4. The fresh contact point F.sub.i may belong
 to an existing footprint or may be the first occurrence of a new
 footprint. In practice, footprints themselves are not detected: rather
 pressure contact points are sensed, each corresponding to a respective
 footprint, and the pressure contact points are then analyzed in order to
 associate them with different footprints. This analysis is called
 "clustering" since different clusters of contact points are associated
 with respective footprints. A cluster representing a footprint in complete
 contact with the sensor is termed a "maximum contact cluster".
 It is to be noted that throughout this specification and claims, the term
 "footprint" is to be understood in its most general definition of a
 contour produced by contact of an object with the sensor. If the object is
 a person, then the footprint corresponds to the outline of the person's
 foot or foot apparel. However, in the general case, where other objects
 are concerned, the footprint is merely the outline which it impresses on
 the sensor.
 When a footprint is first detected, it is not possible with foresight to
 know whether it corresponds to a left foot or to a right foot.
 Consequently, the next footprint of the same person could be either to the
 left or to the right of the first footprint. FIG. 5 shows schematically
 the need to define a boundary extending a distance D either side of an
 existing footprint within which a successive footprint of the same person
 may reasonably be expected to fall. In fact, although a person crossing an
 entry threshold may be expected to walk in the general direction towards
 or away from the entry threshold, in practice some angular deviation from
 a direct passage across the sensor 11 must be allowed for. FIG. 6 shows an
 allowed deviation of an angle a from the direct passage giving rise to a
 boundary having a generally W-shaped contour within which a successive
 footprint of the same person may legitimately fall. The extent L of the
 W-contour must be sufficiently long relative to a person's stride so as to
 accommodate the person's successive footprint; but also sufficiently short
 so as to ensure that a successive footprint matching that which gave rise
 to the W-contour may be assumed to be a successive instance of the same
 person.
 Clearly, the object of the invention is to count objects or people not
 footprints. To this end, for every legitimate footprint, a decision must
 be made as to whether the footprint is a successive footprint of an
 already known footprint of a person or, to the contrary, is the first
 footprint of a person crossing the sensor. At the rudimentary decision
 level, a legitimate footprint which falls within the W-contour can be
 considered as a successive instance of the footprint which gave rise to
 the W-contour. Although a footprint which falls within the W-contour has a
 high probability of being a successive instance of the footprint which
 gave rise to the W-contour, it need not be so.
 Therefore, in order to reduce the likelihood of a false decision, the
 direction of passage of a footprint falling within the W-contour is
 compared to the direction of passage of the footprint which gave rise to
 the same W-contour. Only if the direction of passage of both footprints is
 identical will they be associated with the same person.
 In order to enhance even further the reliability of the decision,
 differentiation between footprints is effected by comparing their contact
 areas. Thus, a footprint which falls within a W-contour will be considered
 a successive footprint only if its contact area is substantially equal to
 the contact area of the footprint which gave rise to the W-contour. The
 number of sensor points in each footprint is indicative of contact area
 and thus those footprints which are within a reasonable span of one
 another and have essentially the same number of pressure contact points
 may be taken to belong to the same object.
 Such an approach requires that only full contact footprints be compared
 with each other and, specifically, that partial footprints as shown in
 FIG. 7 caused when a person first steps on the sensor, for example, be
 discarded. Were this not done, then such a partial footprint would have no
 match with any other full contact footprint of the same person and would
 thus be counted as a different person. This would therefore give rise to a
 estimated number of objects or people higher than the correct value.
 FIG. 8 shows how partial footprints may be discarded. At opposite
 extremities 15 and 16 of the sensor 11, there are defined "dead" areas
 each having a width d within imaginary margins 17 and 18. Any footprint
 which is wholly or partially within only one of the dead areas is ignored.
 Likewise, any discrete footprints whose contact area does not correspond to
 a person but does not correspond to an object which is rolled or dragged
 along the sensor are disregarded. By such means, a person crossing the
 floor area with a dog on a lead will be counted; but the dog itself will
 be ignored.
 It is apparent that not all footprints are continuous. FIG. 9a shows
 pictorially a man's shoe 20 having a heel portion 21 and a sole portion 22
 both of which make pressure contact with the sensor; and a central bridge
 portion 23 which makes no contact with the sensor. FIG. 9b shows
 pictorially a flat sports shoe 24 which makes complete contact with the
 sensor. FIG. 9c shows pictorially a woman's stiletto shoe 25 having a
 narrow heel portion 26 and a sole portion 27 both of which make pressure
 contact with the sensor; and a central bridge portion 28 which makes no
 contact with the sensor. Thus, it is necessary to cluster contact points
 such that all the contact points in the footprints shown in FIGS. 9a and
 9c are nevertheless associated with the same footprint in spite of the
 discontinuities represented by the respective bridge portions 23 and 28.
 FIG. 10 shows how clustering is effected. A specific contact point will be
 associated with a specific cluster if even a single contact point
 belonging to the cluster falls within an imaginary contour surrounding the
 specific contact point. In the first sample, all contact points are also
 fresh contact points F.sub.i. The first contact point F.sub.1 is
 surrounded with an imaginary contour as will be described below. Any of
 the contact points F.sub.2, F.sub.3, F.sub.4 etc. which fall within the
 imaginary contour of the contact point F.sub.1 are associated with the
 cluster C.sub.1. Otherwise, a new cluster C.sub.2 is created in respect of
 the first contact point which cannot be associated with the first cluster
 C.sub.1 and this contact point also is surrounded by an imaginary contour.
 Any remaining unclustered fresh contact points which fall within the
 imaginary contour surrounding F.sub.2 will be associated with the cluster
 C.sub.2. The same procedure is repeated until all contact points F.sub.i
 in the current sample are associated with clusters.
 In each of the subsequent samples, the first fresh contact point F.sub.1 is
 surrounded with the imaginary contour and first checked as to whether it
 can be associated with an already existing cluster. If not, then it is
 assigned a new cluster. Likewise, any of the remaining fresh contact
 points are checked as to whether they can be associated with already
 existing clusters and, if not, they are assigned new clusters.
 With firer regard to FIG. 10, it will be appreciated that the shape and
 dimensions of the imaginary contour are selected so that adjacent contacts
 points which are separated from one another owing to the design of a
 person's shoe are nevertheless associated with the same cluster. On the
 other hand, adjacent contacts points which are separated from one another
 but emanate from two different footprints near one another must be
 associated with different clusters. To this end, there may be exploited
 the fact that a person's foot is substantially elliptical in shape having
 respective major and minor axes. Specifically, the length of a person's
 foot is significantly greater than the width thereof. Consequently, two
 adjacent contact points belonging to the same footprint may be expected to
 lie within an elliptical contour having a major axis parallel to the major
 axis of the footprint and being centered on one of the contact points.
 Such an elliptical contour will include two displaced contact points along
 the length of the footprint but will exclude two contact points displaced
 by the same distance along the width of the footprint. By such means, the
 contact points A and B are associated with the same footprint; whilst the
 contact points A and C, which are displaced by the same distance r are
 associated with the different footprints.
 As noted above with reference to FIG. 6 of the drawings, there must be
 defined a boundary extending either side of an existing footprint within
 which a successive footprint of the same person may reasonably be expected
 to fall. Such a boundary defines a path on either side of a center point
 of the existing footprint within which a successive footprint may be
 associated with the same body.
 FIG. 11 shows how the center point of the footprint is derived. Once a
 maximum contact cluster has been derived, the number of constituent
 contact points along its major and minor axes are determined. The
 mid-point along each of these axes corresponds to the center point of the
 footprint.
 The invention has been described so far with particular reference to
 people's footprints which are characterized by discrete imprints each
 representing a successive instance of the person. However, the method
 according to the invention is equally well applicable to the counting of
 objects which do not have associated therewith discrete footprints
 representative of successive instances of the object. Such objects leave a
 continuous track across the complete length of the sensor commensurate,
 for example, with being rolled or dragged along the sensor.
 FIG. 12 shows schematically two pairs of parallel tracks 30 and 31
 corresponding to two objects which are dragged or wheeled across the
 sensor. Such objects may be identified by the fact that their respective
 maximum contact clusters extend across a whole length of the sensor
 including respective dead areas at opposite extremities thereof.
 Referring to FIGS. 13a and 13b there will now be summarized the principal
 method steps associated with the invention. All the sensor points are
 sampled and "fresh" contact points F.sub.i are determined. The fresh
 contact points F.sub.i are then grouped into clusters C.sub.j each
 corresponding to a respective instance of an object. A list of clusters is
 maintained which is updated whenever a fresh contact point F.sub.i cannot
 be associated with an existing cluster thus requiring the creation of a
 new cluster which must be added to the cluster list.
 Likewise, whenever no fresh contact points or any other contact points can
 be associated with an existing cluster in the list thus indicating that
 the footprint corresponding to the cluster has now vanished, the now
 vanished cluster is processed as follows.
 First, a check is performed as to whether the cluster extends into only one
 of the "dead" areas at opposite extremities of the sensor. If so, the
 cluster constitutes a "partial footprint" and is deleted from the cluster
 list as an illegitimate footprint and the next cluster in the list is
 processed. Otherwise, a check is performed as to whether the cluster
 extends across both of the "dead" areas at opposite extremities of the
 sensor. If so, the cluster constitutes an object which has been dragged
 across the sensor or to the track of a wheeled object such as a
 wheelchair, perambulator, trolley etc. A separate counter is maintained of
 such wheeled objects (referred to generically by the term "wheelchair" in
 FIG. 13a ) and this counter is thus incremented. Otherwise, a check is
 performed as to whether the cluster size is out of human range (e.g.
 corresponds to an animal such as a pet dog). If so, then here also the
 cluster is deleted from the cluster list. After either deleting an
 "illegal" cluster from the cluster list or incrementing the "wheelchair"
 counter, the next cluster in the list is processed.
 Otherwise, the currently processed vanished cluster corresponds to a
 person's complete footprint whose center is determined as described above
 with reference to FIG. 11. The W-contour is created and added to the
 W-contour list and the time history of the cluster's contact points is
 scanned in order to determine the direction of evolution of the footprint
 across the sensor. A check is now performed as to whether the W-contour of
 an earlier cluster contains the vanished cluster. If so, this means that
 the cluster whose W-contour contains the vanished cluster might,
 legitimately be a previous instance of the same object corresponding to
 the currently processed vanished cluster. The likelihood of the two
 footprints deriving from the same object is now checked by confirming that
 the direction of travel of both footprints is the same and that their
 cluster sizes are equal. If so, then it is assumed that the two footprints
 do indeed derive from the same object. In this case, the object count
 should not be incremented. The previous cluster is deleted from the
 cluster list and its W-contour is deleted from the W-contour list. It is
 also to be noted that the length of the sensor is such that a person
 walking normally will walk from one end thereof to the other within only
 several seconds. Thus, bearing in mind that the method according to the
 invention assumes continuous dynamic motion of people across the sensor,
 any legal footprint which has not already been deleted is, in any case,
 deleted together with its corresponding W-contour after such a time
 interval.
 If none of the above checks is affirmative, then having established that
 the vanished cluster is representative of a valid footprint which has no
 previous instance in the cluster list, the object count may now be
 incremented by one in the corresponding direction of passage across the
 sensor.
 If there are more vanished clusters, then each of these is processed in
 like manner; otherwise, the value of i is incremented and the next sample
 S.sub.i is created.
 It will be appreciated that whilst, in accordance with the preferred
 embodiment, illegal footprints are disregarded in order to enhance the
 accuracy of the result there may be occasions when a sufficiently accurate
 result may be obtained without such elimination or with only partial
 elimination.
 It is further to be noted that the invention effectively counts only a
 first instance of an object's footprint. Thus, in order that an object
 should not be counted more than once, successive footprints are not
 counted. It will be noted that the elimination of successive footprints
 requires the definition of various criteria as to which maximum contact
 clusters constitute successive footprints of an object. In accordance with
 these criteria it is required that a successive footprint lie within the
 boundary of a preceding footprint, that both footprints have the same
 number of contact points and are iso-directional.
 However, there may be occasions when a sufficiently accurate result may be
 obtained without applying all of these criteria. For example, there may
 exist situations wherein the likelihood of a non-successive footprint
 falling within the W-contour of a first footprint is considered so low
 that the size and direction criteria can safely be dispensed with.
 Alternatively, there may exist situations wherein the likelihood of a
 non-successive footprint falling within the W-contour of a first footprint
 and being of both equal area and direction is considered so low that one
 or both of the size and direction criteria can safely be dispensed with.
 Other variations to the specific algorithm described within will be
 apparent to those skilled in the art without departing from the spirit of
 the invention as defined in the appended claims.
 In the method claims which follow, alphabetic characters used to designate
 claim steps are provided for convenience only and do not imply any
 particular order of performing the steps.