Bruce
Apr 2, 20215 min
Updated: Oct 30, 2021
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.