Backported the code of four hookscripts from 4.x.

* FINDTARGET, BARTERPRICE, WITHINPERCEPTION, and INVENTORYMOVE. For
unifying the behavior of common hooks between 3.8 and 4.x.

Added the example script of WITHINPERCEPTION from 4.x.
Updated gl_highlighting_lite.

artifacts/example_mods/ItemHighlight_Lite/gl_highlighting_lite.ssl

@@ -100,6 +100,11 @@ procedure KeyPressHandler begin
    end
 end
 
+procedure InventoryMoveHandler begin
+   // remove item outline when player picks up the item
+   if (isHighlight and get_sfall_arg == 7) then set_outline(get_sfall_arg, 0);
+end
+
 procedure start begin
    if game_loaded and (sfall_ver_major < 4) then begin
       call InitConfigs;
@@ -116,5 +121,6 @@ procedure start begin
       highlightFailMsg2 := Translate("HighlightFail2", "Your motion sensor is out of charge.");
 
       register_hook_proc(HOOK_KEYPRESS, KeyPressHandler);
+      register_hook_proc(HOOK_INVENTORYMOVE, InventoryMoveHandler);
    end
 end

artifacts/example_mods/ItemHighlight_Lite/readme.txt

@@ -17,5 +17,5 @@ Also, you should disable the built-in function in ddraw.ini (ToggleItemHighlight
 
 Note that due to the lack of newer game hooks in sfall 3.8.x, there are some minor visual glitches with the lite version:
 - items will be kept highlighted when entering combat while holding the highlight key.
-- when you pick up items while holding the highlight key, they will be kept highlighted if you drop them on the ground.
+- when you pick up items while holding the highlight key, they will be kept highlighted if you drop them on the ground (fixed for 3.8.18+).
 Both glitches can bo solved by pressing and releasing the highlight key again.

artifacts/example_mods/WithinPerception/gl_withinperception.ssl

@@ -0,0 +1,60 @@
+/*
+   Example implementation of the algorithm of how the game engine checks if one critter sees another critter.
+
+   NOTE: the AllowUnsafeScripting option must be enabled.
+*/
+
+#include "..\headers\define.h"
+#include "..\headers\command.h"
+#include "..\headers\sfall\sfall.h"
+#include "..\headers\sfall\define_extra.h"
+
+#define can_see_(source, target)    call_offset_r2(0x412BEC, source, target)
+#define obj_dist_(target, source)   call_offset_r2(0x48BBD4, target, source)
+
+procedure start;
+
+procedure start begin
+   if game_loaded then begin
+      register_hook(HOOK_WITHINPERCEPTION);
+   end else begin
+      variable
+         source := get_sfall_arg,
+         target := get_sfall_arg,
+         original := get_sfall_arg,
+         hookType := get_sfall_arg, /* new arg */
+         result := 0,
+         distance;
+
+      if target then begin
+         distance := get_critter_stat(source, STAT_pe);
+
+         if can_see_(source, target) then begin
+            distance *= 5;
+            if (get_flags(target) bwand FLAG_TRANSGLASS) then distance /= 2;
+         end else if combat_is_initialized then begin
+            distance *= 2;
+         end
+
+         if (target == dude_obj) then begin
+            if sneak_success then begin
+               distance /= 4;
+               if has_skill(target, SKILL_SNEAK) > 120 then distance -= 1;
+            end else if dude_is_sneaking then begin
+               distance := distance * 2 / 3;
+            end
+         end
+
+         if obj_dist_(target, source) <= distance then result := 1;
+
+         // example
+         if (result) then begin
+            display_msg("hs_withinperception: " + obj_name(source) + " sees " + obj_name(target) + " [original: " + original + " script: " + result + "]");
+         end else begin
+            display_msg("hs_withinperception: " + obj_name(source) + " does not see " + obj_name(target) + " [original: " + original + " script: " + result + "]");
+         end
+      end
+
+      //set_sfall_return(result);
+   end
+end