Friday, 5 June 2015

Game # 6 : Jetpac Complete

Jetpac is now complete and is in the repository and can be run here.

As you can see it does look like the original, unsurprisingly :) It also has a title screen and a fake Spectrum loading screen (and no, you cannot select "Kempston Joystick")

I've split the events up into sections in this one, so I'll comment on them one at a time.


This controls general flow and set up. Event 1 resets score and lives if the level has been set to -1, and calls "SetupLevel". This function (Event 6 and 7) sets the rocket number (there are four), the enemy type (there are eight) and the game state which is either -3 (about to land) or -2 (construction). You can see -2 above, it's where the rocket needs assembling. -3 animates the rocket landing.

The game state is "How far you are through the process" -2 and -1 are rocket building, 0 to 5 adding fuel. When it reaches 6 the rocket takes off. The state is changed by dropping the correct objects (fuel bars and rocket bits) on the rocket.

Function 8 simply updates the scores.

Function 9 controls the set up of the screen. It selects the animation in the rocket object (there are four, one for each rocket) and stops it, because the rocket is not animated, it uses animation frames to display fuel states (the gradual colouring of the rocket). It creates two "components" - the two rocket bits on the platforms, positions them, selects the appropriate animation, and sets their carryID. This is compared to the game status, e.g. when game status is -1 this means you get the top of the rocket, and the top of this rocket has a carryID of -1 - this is how it knows you've picked up the right bit. If yo've already done the middle bit event 11 moves it onto the rocket and turns its physics off.

Event 12 deals with the fuel levels. Event 13 triggers a rocket landing (flight direction = 1 = down), which is handled by events 3 to 5, which moves the rocket until it is off the top or bottom, depending on which way it is going, only if this flight direction is set. Event 14 corresponds to game state 6 (the rocket is full of fuel) and makes it take off the other way (e.g. flight direction = -1)


Controllers just modularises (is this a word ?) the controls. Doing it this way means it is easy to convert for different types of control. Different instance variables in the player maintain values depending on left/right/thrust and fire.


Controls the enemies. Event 1 is a testing hack. Events 2 and 3 keep the numbers up to 4 by creating a new one every second if there aren't four. Event 4 handles the set up of the enemy colour, position and type, and Event 5 tweaks that if it is going right to left.

Objects are update every 0.25 seconds by calling "controlEnemy" with the enemy's UID. This allows us to control their behaviour. Event 7 does this, allowing the rate of change of that behaviour to be modified by "adjustRate". Events 9-11 handle platform collisions, enemies either die or bounce depending on an instance variable. Event 11 handles being shot by a laser, and event 12 destroys consequent explosions.

Event 14 is where the behaviour starts. It picks the correct instance and then varies the behaviour depending on the switch statement. I borrowed this description of how enemies move and this is roughly the same as that.

Events 19 through 23 are more complex - an enemy can either be stuck at the edge in which case it moves up and down (20-23) but it can be released to charge across 1 time in 10 (event 22 and 23).


Carriables are things the player carries or collects. The current carried object is identified by an instance variable carriedUID in the player, which is cleared in Event 1. Event 2 stops dropping objects from accelerating.

Event 3 is the main object pick up - if the player hits a carriable object, and its the right one (game State = carryID) and its not carrying anything (carriedUID = 0) then pick it up. We make it stick in Event 7, and make it fall in Event 8 if it is approximately over the rocket. It also sets a "yTerminal" value for the carried object, this fires event 11 when it is dropped to a particular height and sets the game to the next status.

Events 12 and 13 launch fuel bars from the top of the screen when required (after the rocket is built if it needs building, e.g. gameState > 0, if it hasn't finished, and only one at a time. Event 14 similarly launches collectables which are objects that are just extra points, and are randomly coloured, these are picked up and scored by Event 15.

Player Movement

Handles player moving and firing, and is all conditional on the Player visibility. The player is driven using physics, so Events 2-4 apply forces for left, right and thrust. Events 5,6 and 7 set the animation depending on what the player is doing (note event 6 also reduces the velocity when walking).

Event 8 detects firing and creates a laser of quasi random colours, event 9 flips it if the player is facing left, and 10 and 11 change the colours.

Event 12 deals with player death (there is a cheat mode here), the number of collisions is reduced to 1 by "Pick Enemy Instance 0", and it reduces lives and spawns an explosion where the player was.

Event 14 handles the consequences of this. When that explosion finishes, it either restarts the layout (if there are lives left) or returns to the title page.

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