• Bruce

Let's create an MPAGD game - Part 14: Editing the Player Script

Now that we've added a ladder to our game, created a sprite for our character when he goes up and down the ladder, and set the keys for Up and Down, we need to write the code that will control him moving up and down the ladders.


To do that, let's open up the Player Event, it should look like this:



EVENT PLAYER

IF KEY LEFT
    LET DIRECTION = LEFT
    LET IMAGE = 1
    IF X <= LEFTEDGE
        SCREENLEFT
        LET X = RIGHTEDGE
        EXIT
    ELSE
        IF CANGOLEFT
            SPRITELEFT
            ANIMATE
        ENDIF
    ENDIF
ENDIF

IF KEY RIGHT
    LET DIRECTION = RIGHT
    LET IMAGE = 0
    IF X >= RIGHTEDGE
        SCREENRIGHT
        LET X = LEFTEDGE
        EXIT
    ELSE
        IF CANGORIGHT
            SPRITERIGHT
            ANIMATE
        ENDIF
    ENDIF
ENDIF

IF KEY FIRE
    JUMP 7
ENDIF

FALL

IF DEADLY
    KILL
ENDIF

GETBLOCKS

We'll cover this script in detail in the next Part, but for now just note that the script is broken into a number of clauses:


  • What to do if the left key is being pressed

  • What to do if the right key is being pressed

  • What to do if the fire key is being pressed

  • Make the player FALL if there is no block beneath them to stand on

  • What to do if the player touches a deadly block

  • and our GETBLOCKS command that we added early when creating our collectible Bones


So there is nothing here that will control what should happen if the player presses UP or Down to ascend/descend a ladder.


Let's add some code for UP, cut and paste this code directly before the IF KEY FIRE clause...



IF KEY UP
   ANIMATE SLOW
   IF LADDERABOVE
      IF CANGOUP 
         LET IMAGE = 3
    	 SPRITEUP
         LET DIRECTION = UP
      ELSE
         ADD 1 TO Y
      ENDIF
   ELSE
      SPRITELEFT
      IF LADDERABOVE
      ELSE
         SPRITERIGHT 
         SPRITERIGHT
         IF LADDERABOVE
      	 ELSE
            SPRITELEFT
         ENDIF
      ENDIF
   ENDIF
ENDIF
    
    

The code for going down the ladder is fairly similar...



IF KEY DOWN
   ANIMATE SLOW
   IF LADDERBELOW
      LET IMAGE = 3
      SPRITEDOWN
      LET DIRECTION = DOWN
   ELSE
      SPRITELEFT
      IF LADDERBELOW
         ANIMATE
      ELSE
         SPRITERIGHT
         SPRITERIGHT
         IF LADDERBELOW
         ELSE
            SPRITELEFT
         ENDIF
      ENDIF
   ENDIF
ENDIF

Again, paste this code directly before the IF KEY FIRE clause. Save the script, then Build and test your game. The dog can now go up and down the ladders.






In MPAGD each Sprite has a number of parameters, we can take advantage of these in our code. These are:


  • X - The coordinate on the x-axis of the screen of the top left pixel of the sprite

  • Y - The coordinate on the y-axis of the screen of the top left pixel of the sprite

  • IMAGE - The image number of the sprite

  • FRAME - The Frame number of the IMAGE of the sprite

  • TYPE - The event type number that the sprites code is in (The player is TYPE = 0)


In addition each Sprite has three, user-definable parameters, we can use these for whatever purpose we like, these are:


  • DIRECTION - usually used to store the direction that the player is travelling, although you can use it for a different purpose if you need to

  • SETTINGA

  • SETTINGB


There are a couple of other sprite parameters that hold information about the sprite if it is in the air (jumping or falling for example) ...we'll look at those another time.




It's always a good idea to comment our code - by putting an explanation next to each line, explaining what we think its doing...when you start to experiment and try new things, commenting becomes invaluable. Remember, we are trying to squeeze as much as possible into just 40K or so of memory, if you are anything like me you will soon start trying to write your own code to do things beyond the capabilities of MPAGDs stock scripts...if you don't comment your code you'll soon forget what you *think* it is trying to do, making debugging even more difficult!.


We comment our code by adding a semicolon and then adding a comment. The MPAGD engine knows to ignore your comments when compiling your code.


Now is probably a good time to comment our player script, here's the whole script as it exists so far, with my comments:




EVENT PLAYER

IF KEY LEFT                    ; Is the left key pressed?
    LET DIRECTION = LEFT       ; if so, remember I am going LEFT
    LET IMAGE = 1              ; display the left facing image
    IF X <= LEFTEDGE           ; are we at the left edge of the screen?
        SCREENLEFT             ; if so, go to the screen to left on the map
        LET X = RIGHTEDGE      ; and put me at the right edge of that screen
        EXIT                   ; and stop processing this event
    ELSE                       ; if I'm not at the left edge
        IF CANGOLEFT           ; is there space to the left of me?
            SPRITELEFT         ; if so move me left
            ANIMATE            ; and animate me
        ENDIF
    ENDIF
ENDIF

IF KEY RIGHT                   ; Is the right key pressed?
    LET DIRECTION = RIGHT      ; if so, remember I am going RIGHT
    LET IMAGE = 0              ; display the right facing image
    IF X >= RIGHTEDGE          ; Am I at the right edge of the screen?
        SCREENRIGHT            ; if so go to the screen to right on the map
        LET X = LEFTEDGE       ; and put me at the left edge
        EXIT                   ; and stop processing this event
    ELSE                       ; if I'm not at the right edge
        IF CANGORIGHT          ; is there space to the right of me?
            SPRITERIGHT        ; if so, move me right
            ANIMATE            ; and animate me?
        ENDIF
    ENDIF
ENDIF

IF KEY UP                      ; is the UP key pressed?
   ANIMATE SLOW                ; if so, animate me slowly
   IF LADDERABOVE              ; is there a ladder above me?
      IF CANGOUP               ; can I go up?
         LET IMAGE = 3         ; display the image of me climbing
    	 SPRITEUP              ; move me up
         LET DIRECTION = UP    ; remember I am going up
      ELSE                     ; if there is a ladder above me, but I cant go up
         ADD 1 TO Y            ; nudge me 1 pixel to the right
      ENDIF
   ELSE                        ; if there isn't a ladder above me
      SPRITELEFT               ; move me left
      IF LADDERABOVE           ; now is there a ladder above me?
      ELSE                     ; if not
         SPRITERIGHT           ; move me back where I was
         SPRITERIGHT           ; then a bit more to the right
         IF LADDERABOVE        ; now is there a ladder above me?
      	 ELSE                  ; if not
            SPRITELEFT         ; move me back where I was
         ENDIF
      ENDIF
   ENDIF
ENDIF


IF KEY DOWN                    ; is the DOWN key pressed?
   ANIMATE SLOW                ; if so, animate me slowly
   IF LADDERBELOW              ; is there a ladder below me?
      LET IMAGE = 3            ; if so, set my image to the climbing one
      SPRITEDOWN               ; move me down
      LET DIRECTION = DOWN     ; remember I am going down
   ELSE                        ; if there isn't a ladder below me
      SPRITELEFT               ; move me left
      IF LADDERBELOW           ; is there a ladder below me now?   
      ELSE                     ; if not
         SPRITERIGHT           ; move me back where I was
         SPRITERIGHT           ; then a bit more to the right
         IF LADDERBELOW        ; now is there a ladder below me?
         ELSE                  ; if not
            SPRITELEFT         ; move me back where I was
         ENDIF
      ENDIF
   ENDIF
ENDIF


IF KEY FIRE                    ; is the Fire key being pressed?
    JUMP 7                     ; if so, make me Jump to height 7
ENDIF

FALL                           ; make me fall if there is nothing below me

IF DEADLY                      ; am I touching something deadly?
    KILL                       ; if so, kill me
ENDIF

GETBLOCKS                      ; if I touch any collectible blocks, run the Collected Block script


So the important things to understand in this script are:


The LET DIRECTION = statements are just a way of remembering which direction we are going in case we need to refer back to this elsewhere in our code (we will do later)


The LADDER code looks a bit convoluted, with a bunch of additional IF statements - all these are doing are making it easier to go up a ladder - in other words the player doesn't need to be 100% below the ladder to go up it, the code shifts him a couple of pixels to help get him into position.


Hopefully, by reading the comments, you'll get a good understanding of how basic movement and controls are handled in MPAGD script.




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aboutME

Hello, I'm Bruce and I write games for old 8bit computers using Jonathan Cauldwell's excellent Multi-Platform Arcade Games Designer (MPAGD)

I've written a few successful* games for the Sinclair ZX Spectrum and MSX platforms that have been (largely) well received with my most recent game - Twenty Four Hour Parsley People scoring a 10 out of 10 on Planeta Sinclair.

In my blog I am sharing lots of the code that I wrote for my games, in a way that you can use in your own games.   I've commented it so that you'll learn some of the techniques I use to create interesting new mechanics and help your games stand out from the pack.

MPAGD includes lots of standard scripts, they're great to get you started, but if you're new (or just rusty) when it comes to writing code, hopefully my tutorials will help you get started and  turn your imagination into awesome 8 bit games!

All my code is free to use and do with as you please, but if you find them useful please feel free to buy me a coffee :)

*successful is a very relative term in 8bit computer games

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