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  • Writer's pictureBruce

Create a cannon that fires bouncy cannonballs

Updated: Nov 19, 2020

Why? well...why not.

This is something I first wrote for Cocoa and the Time Machine. I wanted a Cannon that would fire cannonballs that followed different trajectories...and bounced when they hit the ground, steadily losing momentum.

Here's one in action.

Now, it might look like there's some complicated maths here, but have no fear, it's really quite simple and is mainly based on taking advantage of MPAGD's JUMP command.

We might think that JUMP is only relevant to the player, but MPAGD let's you make any sprite JUMP. It also takes a parameter which will control how high the sprite can jump.

So, in effect, our cannonball is just making a series of steadily reducing jumps.

First, we're going to need a couple of sprites, one for the Cannon and one for the Cannonball (or choose your artillery of choice).

For the cannon. I'm going to create two frames, one facing left and one facing right. Dependent on where I position the canon on a screen I want it to point in the correct direction (we'll need a bit of space to work with so it's likely that the cannons will be positioned either close to the right or left edges.)

Next, we'll write some code for the Cannon, I'll keep this as basic as possible, all I want is to be able to place a Cannon on a screen and then, if there isn't already a cannonball in play, fire one.

Here's the code:

So, all we are doing here is looking to see whether there is no cannonball currently in play (the S variable), and then checking if the cannon is pointing left or right (which we can tell from the FRAME), then spawning a CANNONBALL (which is my IMAGE 3 and its event script is in TYPE 8), then we change the S variable to either 1 or 2 (which tells other scripts that there is a cannonball in play (either 1 for going left or 2 for going right)

Next, we'll create the script for the Cannonballs.

Now, if you've been looking closely at the code, you'll notice that it won't work properly. That's because it requires a value in SETTINGB that will dictate the height of the JUMP (bounce).

So we will set SETTINGB in the SPRITE INITIALISATION script...we could either have a fixed value (ie LET SETTINGB = 10) or we could randomise it....perhaps something like...

Or perhaps use a DATA statement and READ SETTINGB from it, that way you can have a sequence of heights that the cannonballs will start jumping with.

We also need to put some code in for initialising the Cannon, remember I created two frames, one pointing right, one pointing left, when we initialise a Canon, we'll look which side of the screen it is placed on, and then switch the frame to point to the other side:

So, in its simplest form, that's how it's done, obviously you'll want to write some COLLISION script that will determine what happens when the cannonball hits the player.

In addition, because of the way we structured the main cannonball script, it would be really easy to get the cannonball to rebound off a wall. All we need to do is check the path ahead to see if the cannonball is about to hit a wall block, and if it is, change the S variable. We can do this like this (within the IF S = 1...i.e when it is going left):

This just checks 1 pixel to the left of the sprite (remember the x value is the left hand side of the sprite) to see if its a wall block, and if it is change S to 2 which will send it right.

Then, in the IF S = 2 ....ENDIF we'll add:

Note that we have to add 16 to X to check what is to the right of the sprite

We're not really changing the value of X of the sprite - which is why we always return X to its original value as soon as we've done the GETBLOCK....else your sprite will move incorrectly.

So now our code should look like:

Which should end up looking a bit like this:

You could easily adapt this script for your own games, remember, I kept it simple and only allow one cannonball in play at any one time, this then allows me to have other sprites on screen (you need to be careful not to exceed the maximum number per screen else weird things will happen).

Hope this helps!

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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 including 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 ...or better still - buy or download my games :)

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

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