Running Scripts On Arrows
One of the limitations of the Construction Set is that it won't allow you to attach scripts to arrows. This is an obstacle for many mod ideas. A partial workaround is to add a Script Effect enchantment to the arrow; the script will run when the arrow hits an actor. But there are times when you want to detect or control the behavior of an arrow while it is in flight, or trigger some action before the arrow hits an actor. This tutorial will show you how to do that.
Getting a reference to an arrow
The first step is to get a reference to the arrow. This is done by placing a trigger zone at the player's location. When an arrow collides with the trigger zone, an OnTriggerMob block will run. The reference to the triggering object can then be returned with GetActionRef.
We'll create a new trigger zone object using the TrigZone01.nif and place a reference to it somewhere in the gameworld. Make sure it is a persistent reference, and has a unique reference ID. We also want to scale it to 0.5 scale. We'll call the reference acTrigZoneREF.
Moving the trigger zone
Now, how do we keep the trigger zone glued to the player's location? SetPos and MoveTo will be useful here. We will use a quest script to handle the MoveTo calls for when the trigger zone is not in the same cell as the player, and update its position in the current cell with SetPos.
scriptName acQuestSCR ; attached to the control quest ; moves the trigger zone to the player when he changes cells ; updates trigger zone's position within the player's cell float xp float yp ; player coordinates float zp float fQuestDelayTime ; controls processing speed of this script short relocate ; set this to 1 when we need to call moveTo begin gameMode if ( fQuestDelayTime != 0.001 ) set fQuestDelayTime to 0.001 ; process every frame endif if ( relocate == 1 ) acTrigZoneREF.moveTo player set relocate to 2 return elseif ( relocate == 2 ) acTrigZoneRef.disable set relocate to 3 return elseif ( relocate == 3 ) acTrigZoneRef.enable set relocate to 4 return elseif ( relocate == 4 ) set xp to player.getPos x acTrigZoneREF.setpos x xp set relocate to 0 endif if ( player.getDistance taTrigNoisemakerRef < 200 ) ; trigZone is in same cell as player set xp to player.getPos x set yp to player.getPos y ; store player's coordinates set zp to player.getPos z acTrigZoneRef.setpos x xp acTrigZoneRef.setpos y yp ; and move trigZone to those coordinates acTrigZoneRef.setpos z zp else ; player has changed cells set relocate to 1 endif end
Notice that the use of the relocate variable splits the movement from one cell to the next into four stages. This is to make sure that the trigger zone retains its collision properties after it is moved.
Adjusting the vertical position
That script works...sort of. The problem is that the trigger zone is always at the player's feet. We need to adjust the Z position of the trigger zone so that it will always be at the correct height to collide with an arrow fired by the player. Three factors are involved here: the player's height, which is defined by his race; the player's vertical rotation (X angle); and whether or not the player is sneaking. We'll add a section to the script to calculate the necessary adjustment and apply it to the trigger zone's coordinates:
scriptName acQuestSCR
; attached to the control quest
; moves the trigger zone to the player when he changes cells
; updates trigger zone's position within the player's cell
float xp
float yp ; player coordinates
float zp
float pxRot ; player's vertical rotation
float zOffset ; adjustment to zPos
float fQuestDelayTime ; controls processing speed of this script
short relocate ; set this to 1 when we need to call moveTo
begin gameMode
if ( fQuestDelayTime != 0.001 )
set fQuestDelayTime to 0.001 ; process every frame
endif
if ( relocate == 1 )
acTrigZoneREF.moveTo player
set relocate to 2
return
elseif ( relocate == 2 )
acTrigZoneRef.disable
set relocate to 3
return
elseif ( relocate == 3 )
acTrigZoneRef.enable
set relocate to 4
return
elseif ( relocate == 4 )
set xp to player.getPos x
acTrigZoneREF.setpos x xp
set relocate to 0
endif
set zOffset to player.getScale ; get player's height
set zOffset to ( zOffset * ( 115 - ( player.IsSneaking * 20 ) ) )
set pxRot to player.getAngle x ; get player's vertical facing
set pxRot to ( pxRot / -1.5 )
set zOffset to ( zOffset + pxRot )
if ( player.getDistance taTrigNoisemakerRef < 200 ) ; trigZone is in same cell as player
set xp to player.getPos x
set yp to player.getPos y ; store player's coordinates
set zp to player.getPos z
set zp to ( zp + zOffset ) ; adjust for zOffset
acTrigZoneRef.setpos x xp
acTrigZoneRef.setpos y yp ; and move trigZone to those coordinates
acTrigZoneRef.setpos z zp
else ; player has changed cells
set relocate to 1
endif
end
Here, GetScale returns the player's height as a proportion of the default racial height of 128. For the default height, we want the trigger zone placed 115 units above the player's feet when standing, and 95 units above when sneaking. Then we modify the offset to account for the player's vertical facing. Note that it would be more efficient to do some of the scale calculations only once, when the game is first loaded, since that information is not likely to change.
The fun part
Great, now we have a trigger zone glued to the player's location, capable of intercepting any arrows he fires. We can attach a demo script to the trigger zone itself to show how this works:
scriptName acDemoTrigSCR
; Causes any projectile fired by the player to disappear
ref trigRef ; reference to the triggering object
begin onTriggerMob ; a mobile object has collided with the trigger zone
set trigRef to getActionRef
if ( trigRef.IsActor == 0 ) ; it's not an NPC
trigRef.disable ; so make it disappear
endif
end
That's fun, but not entirely useful. Also, a serious problem arises: the onTriggerMob block will run when any projectile collides with the trigger zone, including arrows and spells fired at the player. What we need is a method to determine whether the projectile is an arrow, and whether it was fired by the player.
Validating the reference
If we know the ID of the arrows we want to run scripts on, this is easy. Just check GetIsID on the triggering reference:
scriptName acTrigZoneSCR
ref trigRef
begin onTriggerMob
set trigRef to getActionRef
if ( trigRef.getIsID "myArrowID" )
trigRef.disable
endif
end
This assumes that only the player has this type of arrow, so we know it wasn't fired at the player by an NPC.
Detecting any arrow is more complicated. We will rely on OBSE functions for this:
scriptName acTrigZoneSCR
ref trigRef
begin onTriggerMob
set trigRef to getActionRef
if ( trigRef.IsAmmo ) ; OBSE, returns 1 if ref is an arrow
trigRef.disable
endif
end
More fun with OBSE
Another more useful example results in a bottomless quiver of arrows:
scriptName acTrigZoneInfiniteArrowsSCR
ref trigRef
ref baseObject
short triggered ; flag to avoid repeated triggering by the same object
begin onTriggerMob
if ( triggered )
if ( getActionRef == trigRef ) ; same reference as last frame
return
else
set triggered to 0
endif
endif
set trigRef to getActionRef
if ( trigRef.IsAmmo )
set baseObject to trigRef.getBaseObject
player.addItem baseObject 1
else
set trigRef to 0
endif
end
We still haven't eliminated the possibility that the arrow was fired by an NPC. We'll tackle that later.
Tracking the arrow's flight
At this point, we have a means of obtaining a reference to an arrow, and we can run scripts on that reference. However, it would be useful to know what the arrow is doing. We can figure this out by tracking its position and rotation over the course of its flight:
- If the arrow's rotation changes and it continues moving, it has bounced off of a surface;
- If its rotation changes and then it stops moving, it is lying on a flat surface, or on the ground;
- If it stops moving, but its rotation remains the same, and its coordinates do not return 0, it has stuck into a surface, such as a wooden door;
- If its coordinates all return 0, it has hit an actor and no longer exists in the gameworld.
Using these guidelines, we can write a slightly more complicated script. This script simply reports on the state of the arrow over the course of its flight. We use a doOnce variable triggered to make sure we only track one arrow at a time:
scriptName acTrigZoneSummonSCR
ref trigRef
float xp
float yp ; the arrow's current coordinates
float zp
float ox
float oy ; coordinates from previous frame
float oz
float ax
float ay ; arrow's starting angles
float az
short triggered ; set to 1 while an arrow is being tracked
begin onTriggerMob
if ( triggered == 0 )
set trigRef to getActionRef
if ( trigRef.IsAmmo )
set triggered to 1
set ax to trigRef.getAngle x
set ay to trigRef.getAngle y
set az to trigRef.getAngle z
message "Arrow fired."
endif
endif
end
begin gameMode
if ( triggered ) ; currently tracking an arrow
set xp to trigRef.getPos x
set yp to trigRef.getPos y ; get current coordinates of arrow
set zp to trigRef.getPos z
if ( ( xp + yp + zp ) == 0 ) ; arrow is not in gameworld
message "Arrow has hit an actor!"
set triggered to 0
return
elseif ( trigRef.getAngle x != ax || trigRef.getAngle y != ay || trigRef.getAngle z != az )
message "Arrow has bounced off of something!"
set triggered to -1 ; remember that the arrow bounced
set ax to trigRef.getAngle x
set ay to trigRef.getAngle y
set az to trigRef.getAngle z
elseif ( xp == ox && yp == oy && zp == oz ) ; arrow stopped moving
if ( triggered == 1 ) ; arrow never bounced
message "Arrow stuck in a surface!"
else ; arrow bounced
message "Arrow is lying on the ground!"
endif
set triggered to 0
else
set ox to xp
set oy to yp ; set 'last frame' coordinates to check against next frame
set oz to zp
endif
endif
end
Admittedly, that script isn't terribly useful in itself, but it should give you a starting point in setting up your own scripts on arrows.
Determining who fired the arrow
Remember that issue I said we'd tackle later? A problem may arise if an NPC fires an arrow at the player - if it collides with our trigger zone, the script has no way of recognizing that it wasn't fired by the player. One way to fix that would be to track the arrow's flight over the course of a few frames to determine where it is heading. But there may not be enough time for that, especially in low frame-rate situations.
The GetAngle function provides a more useful method. At the moment the player fires an arrow, the arrow's angles will match the player's precisely. So a bit of code like this:
begin onTriggerMob
set trigRef to getActionRef
if ( trigRef.isAmmo && trigRef.getAngle z == player.getAngle z && trigRef.getAngle x == player.getAngle x )
message "The player fired the arrow."
endif
end
...would likely do what we want. However, the player might move the mouse before the trigger zone detects the arrow, which would cause the angles to differ. So to be safe, you'd want to compare the arrow's angles to the player's within a reasonable margin of error.
Tying it all together
Bear in mind that if two scripts attempt to use this approach at the same time, they will conflict with each other. Therefore, it's a good idea to conditionalize things as specifically as possible. For instance, if you only need to detect a certain type of arrow, then you would enable the trigZone only while the player has that type of arrow equipped.
Also, you'll probably realize that the usefulness of trigger zones isn't limited to detecting arrows. For instance, by keeping a trigZone glued in front of the player at all times, you could detect what item or actor is in the crosshairs at any given moment, and perform actions on the target. I hope to find time to post some more tutorials on the uses of trigger zones, but in the meantime, have fun experimenting.