Halo Example

breed [runners runner]
breed [halos halo]

to setup
  clear-all
  ;; the "thin ring" shape was made using the shapes editor;
  ;; it's a simple, unfilled circle
  set-default-shape halos "thin ring"
  create-runners 250
    [ setxy random-xcor random-ycor ]
  reset-ticks
end 

to go
  ask runners [ fd 0.1 ]
  tick
end 

to make-halo  ;; runner procedure
  ;; when you use HATCH, the new turtle inherits the
  ;; characteristics of the parent.  so the halo will
  ;; be the same color as the turtle it encircles (unless
  ;; you add code to change it
  hatch-halos 1
  [ set size 5
    ;; Use an RGB color to make halo three fourths transparent
    set color lput 64 extract-rgb color
    ;; set thickness of halo to half a patch
    __set-line-thickness 0.5
    ;; We create an invisible directed link from the runner
    ;; to the halo.  Using tie means that whenever the
    ;; runner moves, the halo moves with it.
    create-link-from myself
    [ tie
      hide-link ] ]
end 


; Public Domain:
; To the extent possible under law, Uri Wilensky has waived all
; copyright and related or neighboring rights to this model.