How do I make an NPC have a "line of sight" or an area in which it can detect a hero and how do I make it act upon it?

From OHRRPGCE-Wiki

Sometimes you want an NPC to be activated from a distance away, for example, a pacing guard who sees your heroes if they step in front of him. There is no built-in "line of sight" or "distance" feature for triggering NPC's, but you can fake the effect with plotscripting.

For this example we will write a script that makes a guard NPC do something if you stand in front of him within 8 spaces.

This script must run as a continual loop. Normally you would make it the "map autorun" script for the map the guard is on. This script assumes that the guard is NPC 0

include, plotscr.hsd

plotscript, vigilant guard, begin
  variable(guard, this map)

  guard := NPC reference (0)
  this map := current map

  while (this map == current map) do, begin
    variable (n1, n2, dist)

    n1 := hero X(me)
    n2 := NPC X(guard)
    dist := n2 -- n1
    if(n1 == n2, and, dist <= 8, and, dist >= -8) then (activate NPC (guard))

    n1 := hero Y(me)
    n2 := NPC Y(guard)
    dist := n2 -- n1
    if(n1 == n2, and, dist <= 8, and, dist >= -8) then (activate NPC (guard))

    wait(1)
  end

end

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This script hasn't actually been tested in a real RPG yet, so I can't consider this article finished until it has been.

Alternative method: You can place a certain number of "step-on" invisible NPCs in front of the guard, but this only works if he is standing still.