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< C A T E R P I L L A R    F A Q    B Y :    D R A K U L >
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                  v e r s i o n    1 . 0 


Note: This FAQ requires prior knowledge of RM2K and it's
      event scripting language. I will be using specific
      elements including variables, fork conditions, and
      cycles. Please be sure you know the basic concepts
      of events and event commands before you attempt to
      follow my examples.


What is this FAQ about?
-----------------------
This FAQ will explain how to create a group of NPCs events
that will play follow the leader with your main character.
These NPCs can be used to visually represent other members
of your party, or just about whatever you want.


Why did you write this?
-----------------------
Well that's a simple one. I liked the way RPG Maker 95 had
the members of your party following the lead character. In
RPG Maker 2000 when a character joined your party, well he
kinda just disappeared, like he jumped into the main heros
pocket or something. There is a simple way to add just one
following character, but it doesn't work well when you add
a third or fourth NPC. So I had to figure out a better way
and this is that way.


Getting the basic idea.
-----------------------
To better understand the techniques I will be showing you,
you should first take a look at these five steps. They are
the basic method for the caterpillar scheme:

1. Determine Leader's Current X,Y Location
2. Determine NPC's Current X,Y Location
3. Determine Leader's Relative Location to the NPC
4. Move the NPC Left, Right, Up, or Down according
   to the Leader's relative location.
5. Repeat



We need some variables!
-----------------------
The first thing we are going to want to do is create a set
of variables that will hold the leader's cartesian values.
(i.e. X and Y coordinates). For this example the main hero
will be our leader. Create two variables HERO-X and HERO-Y
that will hold his X and Y coordinates respectively. We'll
start out with a single NPC follower, so lets create a set
for him too, call them NPC1-X and NPC1-Y.


Adding the first event.
-----------------------
OK, now that we have our variables all set its time to put
them to use. After you have set your hero's start position
on the map, create an event on one of the adjacent squares
and name it NPC1. Leave all the Event Conditions blank and
set the character graphic to whatever graphic you wish. In
the Movement Type box select Stay Still (all movement will
be determined through a cycle in the Event Commands window
but we'll get to that). Make sure that you set Event Start
Condition to Parallel Process and set the Priority Type to
Common Char Above. The Movement Speed must match the speed
your leader is set to, if you have not changed the default
setting for your hero this will be Fast(Standard). For now
you can set the Animation Type to Common/Without Stepping,
but it can be set to whatever best suits your NPC needs. 


Creating the first NPC.
-----------------------
OK, now we get to the hard part. I will try to step though
the code slowly and explain each step. First thing we need
to do is create a loop and throttle it, otherwise the loop
will just run as fast as it possibly can, and that will be
hell on your processor. So add a cycle event to the window
and inside of the cycle event put a wait event of .1 which
will look something like this:

<>Cycle
  <>Wait 0.1s.
  <>
:End Cycle
<>

Now that we have our loop setup, we need to get our hero's
X,Y coordinates and store them in the proper variables. To
do this we will use the Change Variable event. Insert this
event directly below the wait event then set the following
on the Variable Management Dialog. Create Variable must be
set to One and 0001:HERO-X, Operation should be set to Set
, and Operand should use the Charac option with the fields
Hero and X Coordinate. When you click OK you should end up
with something like this:

<>Cycle
  <>Wait 0.1s.
  <>Variable Op:[0001:HERO-X] Set, Hero X Pos
  <>
:End Cycle
<>

Now create another Change Variable event, and set it up to
record the hero's Y coordinate. And then create two events
to record NPC1's X and Y coordinates. After you finish you
should see this:

<>Cycle
  <>Wait 0.1s.
  <>Variable Op:[0001:HERO-X] Set, Hero X Pos
  <>Variable Op:[0002:HERO-Y] Set, Hero Y Pos
  <>Variable Op:[0003:NPC1-X] Set, NPC1 X Pos
  <>Variable Op:[0004:NPC1-Y] Set, NPC1 Y Pos
  <>
:End Cycle
<>

At this point we have the basic update position code done.
This loop will track the positions of the party leader and
this one NPC follower. We will now need to determine where
our leader is in relation to this NPC. So that we can move
this NPC in the correct direction. Let's start out looking
to the north of the NPC. To do this we will have to create
a Fork Condition (which is basically an IF statement). Add
a Fork Condition below the last Variable Change and set it
to a variable condition of 0002:Hero-Y. We now want to use
the variable NPC1-Y as a comparison value. To do this, set
the check value to variable and 0004:NPC1-Y. Then select a
comparison value of smaller. You'll end up with this:

<>Cycle
  <>Wait 0.1s.
  <>Variable Op:[0001:HERO-X] Set, Hero X Pos
  <>Variable Op:[0002:HERO-Y] Set, Hero Y Pos
  <>Variable Op:[0003:NPC1-X] Set, NPC1 X Pos
  <>Variable Op:[0004:NPC1-Y] Set, NPC1 Y Pos
  <>Fork Optn:Varbl[0002:HERO-Y]-V[0004]Small than
    <>
   :Excepting Case
    <>
   :End Case
  <>
:End Cycle
<>

OK that's great so far, but we need to set an action based
on this condition. In this case the action will be to move
this NPC in the correct direction. To do this create a Set
Chara Movement event in between the initial Fork Condition
and it's Excepting Case. Set the Object Character to NPC1,
set the Movement Frequency to 8, and insert "Step Up" into
the Movement Route. Make sure that Repeat Action is set to
OFF by unselecting it's check box. You should now see code
similar to this:

<>Cycle
  <>Wait 0.1s.
  <>Variable Op:[0001:HERO-X] Set, Hero X Pos
  <>Variable Op:[0002:HERO-Y] Set, Hero Y Pos
  <>Variable Op:[0003:NPC1-X] Set, NPC1 X Pos
  <>Variable Op:[0004:NPC1-Y] Set, NPC1 Y Pos
  <>Fork Optn:Varbl[0002:HERO-Y]-V[0004]Small than
    <> Set Chara's Movement: NPC1, Up
    <>
   :Excepting Case
    <>
   :End Case
  <>
:End Cycle
<>

Now if this event determines that the main hero is north of
NPC1, NPC1 will continue moving north until either the case
becomes FALSE or it cannot move north any longer. Of course
this event only covers one of four directions, but I'm sure
by now you've gotten the hang of things and will be able to
handle the rest of it from here on in. Just in case I wrote
out the complete event listing for one NPC. 


Complete event listing.
-----------------------
Below is a listing of what your code will look like in the
Event Commands window. It may vary slightly depending upon
the variable and event names you chose to use, but overall
it should look very similar. I have added some comments in
to clear up what's going on. They are denoted by <>Note:

<>Cycle
  <>Note:
  <>Note: THROTTLE PAUSE
  <>Wait 0.1s.
  <>Note: 
  <>Note: GET COORDINATES
  <>Variable Op:[0001:HERO-X] Set, Hero X Pos
  <>Variable Op:[0002:HERO-Y] Set, Hero Y Pos
  <>Variable Op:[0003:NPC1-X] Set, NPC1 X Pos
  <>Variable Op:[0004:NPC1-Y] Set, NPC1 Y Pos
  <>Note:
  <>Note: HANDLE CASE (NORTH)
  <>Fork Optn:Varbl[0002:HERO-Y]-V[0004]Small than
    <> Set Chara's Movement: NPC1, Up
    <>
   :Excepting Case
    <>
   :End Case
  <>Note:
  <>Note: HANDLE CASE (SOUTH)
  <>Fork Optn:Varbl[0002:HERO-Y]-V[0004]Large than
    <> Set Chara's Movement: NPC1, Down
    <>
   :Excepting Case
    <>
   :End Case
  <>Note:
  <>Note: HANDLE CASE (EAST)
  <>Fork Optn:Varbl[0001:HERO-X]-V[0003]Large than
    <> Set Chara's Movement: NPC1, Right
    <>
   :Excepting Case
    <>
   :End Case
  <>Note:
  <>Note: HANDLE CASE (WEST)
  <>Fork Optn:Varbl[0001:HERO-X]-V[0003]Small than
    <> Set Chara's Movement: NPC1, Left
    <>
   :Excepting Case
    <>
   :End Case
  <>
:End Cycle
<>




Adding sequential NPCs.
-----------------------
If you were able to follow the above steps, this should be
quite simple. All you need to do is create a new event for
each NPC and reference the coordinates of the NPC in front
of it, rather than the hero. For instance, NPC1 referenced
the leader's X and Y position, well just treat NPC1 as the
leader for NPC2, and NPC2 as the leader for NPC3 and so on
until you finish. Keep in mind that each new NPC will also
need a new set of variables to track it's own position.  


The Slip Through event.
-----------------------
As you may have noticed the initial script for creating an
NPC follower has some limitations. You can't back track in
certain narrow areas and it is possible for your main hero
to become entirely trapped in other areas. This is where a
Slip Through event comes in handy. Simply adding two extra
events in each Set Chara Movement can prevent such errors.
Just enclose the movements with a Start Slip Through and a
End Slip Through event, like this:

Start Slip Through
Step Up
End Slip Through

it's just that easy. Do it for each Set Chara Movement and
you will gain 100% freedom of movement. The only draw back
is that when you stop moving all the NPC followers overlap
or rather "underlap" onto your main hero's tile. This will
leave your hero visible on top, below him will be the NPC1
then NPC2 and so on. When you start to move they will line
up behind you and follow as they normally would. I'll work
on a more visually pleasing Slip Through, but for now this
method seems to handle lot of the little errors that we've
seen pop up without the Slip Through added in. 


Things to keep in mind. 
-----------------------
1. This code is in no way perfect. In fact there are still
   a few glitches which still need to be addressed. If you
   need any help or come across any problems please let me
   know. I'll try my best to help out and correct them.

2. When moving to a new map, you will need to recreate all
   the NPC followers that were on the previous map. Simply
   copy and paste them into the new map. However make sure
   you paste them adjacent to the teleport event. That way
   they will continue to follow directly behind you. Don't
   forget you'll have to use switches to place the NPCs at
   various locations if you have more than one teleport on
   map.

3. If you plan to use this technique to implement a follow
   scheme for party members that may or may not be in your
   party at the time you enter the map, it might be useful
   to set the "Hero Need" event condition for each member.

4. The leader does not need to be human controlled. It can
   very well be another NPC leading the group. Very useful
   when you want to split your party up or to create a NPC
   with random movements that has something following him.
   A boy and his pet dog for instance.




             ^v^

                    ^v^
        ^v^ 


      Drakul@MyEZMail.com