longer tracked) at some point may be used later to refer to another entity that 'appears' (becomes tracked), and that other entity may not necessarily be **a person.**
Unlike the specification of person**, it is not clear from the CAVIAR data whether a tracked**
entity is an object, and for this reason we do not have a fluent explicitly representing that an entity is an object. person(P) = false does not necessarily imply that P **is an object; it**
may be that P is not tracked, or that P **is an inactive person. Note finally that axiom (9)**
incorporates a (reasonable) simplifying assumption, that a person **entity will never exhibit** 'inactive' activity at the moment it first 'appears' (is tracked). If an entity is 'inactive' at the moment it 'appears' it can be assumed to be an object, as in the first two conditions of axiom (9). (This assumption is adequate for CAVIAR. Removing it raises further issues we do not have space to discuss fully here.)
The lack of explicit information that a tracked entity is an inanimate object may compromise recognition accuracy in certain conditions. A discussion about the effects of the limitations of CAVIAR's dictionary on recognition accuracy will be presented in the next section.

Axiom (10) expresses the conditions in which a leaving object activity ceases to be recognised. In brief, leaving object **is terminated when the object in question is picked up. An**
object that is picked up by someone is no longer tracked - it 'disappears' - terminating leaving **object**.

The long-term activity immobile **was defined in order to signify that a person is resting**
in a chair or on the floor, or has fallen on the floor (eg, fainted). Note that there is no short-term activity in the CAVIAR annotation for the motion of leaning towards the floor or a chair. The absence of such a short-term activity substantially complicates the definition of immobile**, and, as discussed in the next section, sometimes reduces the accuracy of**
recognising immobile. Below is one of the axioms of the immobile **definition:**

 $\mathsf{W}$ is one of the axioms of the *imimitable* element  $\mathsf{ initiatedAt}(immobile(P)=\mathsf{true},\,\,\,T)\leftarrow$   $\mathsf{happensFor}(inactive(P),\,\,\,Intervals),$   $(\,T,\,\,T_I)\,\in\,\,Intervals,\,\,T_I>T+54,$   $\mathsf{holdsAt}(person(P)=\mathsf{true},\,\,\,T),$   $findall(S,\,\,\,shop(S),\,\,\,Shops),$   $\mathsf{holdsAt}(farS(P,\,\,\,Shops,\,\,\,24)=\mathsf{true},\,\,\,T)$    if the following conditions are satisfied, First 

$$(11)$$
immobile(P) is recognised if the following conditions are satisfied. First, P **stays inactive**
for more than 54 **frames (see lines 2–3 of axiom (11)). We chose this number of frames,**
like all other numerical constraints of the definitions, based on empirical analysis of the CAVIAR dataset. Second, P **is a person (see line 4 of axiom (11)). With the use of this**
constraint we distinguish between an inanimate object, which is inactive since it is first tracked, from an immobile person. Third, P **is not 'close' to a shop (see lines 5–6 of axiom** (11)). If P were 'close' to a shop then he would have to stay inactive much longer **than** 54 frames before immobile **could be recognised. (Those conditions are specified in other axioms**
defining immobile **not shown here.) In this way we avoid classifying the activity of browsing**
a shop as immobile. farS(A, List, D**) is** true when A is more than D **pixel positions away**
from every element of the **List**.

immobile(P) is terminated when P **starts walking, running or 'disappears', ie he is no**
longer tracked by the video cameras. The relevant axioms for terminatedAt **are straightforward**
and are not shown here.

meeting (of two persons P1 and P2**) is recognised when two people 'interact': at least**
one of them is active or inactive, the other is neither running nor moves abruptly, and the distance between them is at most 25 **pixel positions. In the CAVIAR annotations, this**