;+
; NAME:
;       NBOX
;
; PURPOSE:
;       This function computes the number of tracked particles
;       in each frame of the movie. 
;
; CALLING SEQUENCE:
;       Result = NBOX(t)
;
; INPUTS:
;       t:      The result of TRACK.PRO
;
; KEYWORD PARAMETERS:
;       
;       - None -
;
; OUTPUTS:
;
; Result[0,*]        Time-ordered number of tracked particle in each frame  
;
; EXAMPLE:
;       The array t is the output of tracking
;        nb = NBOX(t)
;       Calculate STDDEV(nb)/MEAN(nb) to check you perform a constant density
;       tracking.
;
; REFERENCE:
;       - None -
;
; MODIFICATION HISTORY:
;       Written by:  Thierry Savin @ Doyle Group @ MIT - June 2006
;-

FUNCTION NBOX, tr

Dim_t = N_ELEMENTS(tr[*,0])
Ind_Time = Dim_t - 2

t = tr[Ind_Time,*]
t = t[SORT(t)]
u = UNIQ(t)

res = u - SHIFT(u,1)
res[0] = u[0] + 1

RETURN,res

END