This is really starting to come together, we have an easily extensible framework for creating alien movement patterns, but they are a bit toothless at the moment as they cant fire weapons of their own, so let's remedy that!
First we'll design an Alien Missile Sprite:
I'm going to use Sprite Type 3 for the Alien Missiles, but before we start coding there's something we're going to need to consider. And that's MPAGD's collision detection - since all sprites are 16 px x 16px - but our Alien Missile only occupies 3pixels wide - this might lead to some pretty mean collisions which visually dpnt appear to be a missile colliding with the player's spaceship - but will be registered by the engine as a collision, so we'll need to include some code that will help check whether the graphical part of the sprite has actually collided with the spaceship.
Also, we need to consider how the missiles travel, do they just go straight down?, should they home in on the player? or should they veer to the left or right dependent on the Alien's direction of travel when they launched the missile.
This latter approach is my favourite, and, since all of the Alien Directions are stored in the DIRECTION variable, it should be pretty easy to code the direction of the missile, so if an alien is travelling right when they released the missile then the missile should veer to the right, and so on.
Other things we need to consider include:
When should a missile be fired?
What is the probability of a missile being fired?
Should all Aliens fire at the same rate?
Should all Aliens fire missiles or only some?
How high up the screen should an alien be before they are allowed to fire missiles? (we might want to avoid them firing if they are just a few pixels above the player as that would be unfair.
We've built quite a flexible framework so far - so I'm thinking we could extend the flexibility further to allow us to vary the missile firing capabilities of each alien attack wave, that will help us make the attack waves progressively harder.
OK, so, on with the coding, first, lets extend our Spawner code to include some new variables for controlling the Aliens weapon firing capabilities, open up your Spawner event (Sprite Type 7, and add the highlighted code:
EVENT SPRITETYPE7
; SPAWNER
; ============
IF SETTINGA = 0 ; first time we run this
LET JUMPSPEED = 0 ; reset the enemies spawned counter
LET SETTINGB = 0 ; reset the timer
RESTORE ; start from the beginning of the data table
REPEAT L ; get the data for this level
READ AIRBORNE ; get the spawner movement
READ X ; get starting x position
READ Y ; get starting y position
READ J ; get the enemy sprite type
READ K ; get the enemy image
READ E ; how many enemies in this level?
READ G ; delay between releasing each enemy
READ P ; points for a kill
READ C ; colour
READ W ; weapon
READ H ; fire weapon probability
READ Q ; min height to release weapons
ENDREPEAT
LET D = E ; how many kills to get bonus?
LET F = E ; enemies remaining
ADD 1 TO SETTINGA ; prevent this running again
ENDIF
IF SETTINGB = G ; has timer reached the delay point?
IF JUMPSPEED < E ; are there still enemies left to spawn?
IF S < 9 ; are there 8 or less enemies already on screen?
SPAWN J K ; spawn an enemy
ADD 1 TO JUMPSPEED ; increment the number spawned
SPAWNED ; switch to the enemy sprite
LET DIRECTION = 0 ; set its initial direction to 0
ENDSPRITE ; retrun to the spawner
ENDIF
ELSE ; all enemies in this level have been spawned
IF F = 0 ; all enemies are off screen
ADD 1 TO L ; get ready for next level
IF L = 7 ; have we reached the end of the levels?
LET L = 1 ; yes, start again
ENDIF
LET SETTINGA = 0 ; get data for next level
ENDIF
ENDIF
LET SETTINGB = 0 ; reset the timer
ENDIF
ADD 1 TO SETTINGB ; increment the timer
DATA 0 132 8 5 2 8 8 5 67 1 10 100
DATA 0 100 8 5 2 8 8 10 68 1 10 100
DATA 0 232 16 5 5 8 6 15 69 1 8 100
DATA 0 8 16 5 5 8 6 15 70 1 8 100
DATA 0 232 146 5 2 16 6 20 71 1 4 100
DATA 0 8 146 5 2 16 6 20 66 1 4 100All we've added here is three new variables that we will read in from the data table:
W (Weapon) - 0 will mean they can't fire missiles, 1 means they can
H(Probability of firing a weapon) the higher the value the less chance they will actually fire a missile
Q (firing height) this is the Y value below which (i.e. higher up the screen since Spectrum Y co-ordinates start at the top of the screen ) an alien must be if they are allowed to fire missiles..
I've added in some values for each of the 6 currently defined attack waves in the Data table at the bottom.
Next, we're going to need to add a couple of lines of code to our player event that will Store the players X & Y coordinate values as we're going to need to use these in the Aliens event to calculate how far, horizontally, the alien is from the player, open up your player event and add these lines at the bottom of your script:
LET M = X ; store player X in M
LET N = Y ; store player Y in NNext, we'll add some code to the Alien event (Sprite Type 5), open yours up and add/update the highlighted code:
EVENT SPRITETYPE5
; FLEXI ALIENS
; =====================
SPRITEINK C ; colour the alien with C value
IF SETTINGA = 0 ; Alien first appears
RESTORE ; Restore the data table
LET RND = L ; Store the Level number in RND
ADD 50 TO RND ; and increase it by 50
WHILE DIRECTION <> RND ; While DIRECTION doesnt = RND
READ DIRECTION ; Read value from DATA table
READ SETTINGB ; Also read second value
ENDWHILE ; Keep going until we find the value in RND
LET SETTINGA = 1 ; When we find it change SA to 1
ENDIF
IF SETTINGA = 1 ; Ready to get direction & reptitions
READ DIRECTION ; Read the next movement into DIRECTION
READ SETTINGB ; Read the repetitions into SettingB
LET SETTINGA = 2 ;
ENDIF
IF COLLISION 1 ; if hit by a torpedo
OTHER ; switch to torpedo
LET DIRECTION = 99 ; run the explosion
ENDSPRITE ; return to this Alien
LET TYPE = 6 ; Switch it to explosion type
LET IMAGE = 4 ; Explosion image
LET FRAME = 0 ; first frame
LET DIRECTION = 0 ; tell explosion its an alien exploding
ENDIF
IF COLLISION 0 ; if enemy and player collide
LET TYPE = 6 ; switch it to the explosion type
LET IMAGE = 4 ; and the explosion image
LET FRAME = 0 ; reset it to first frame
LET DIRECTION = 0 ; this is an alen exploding
OTHER
LET DIRECTION = 99 ; trigger the player explosion
ENDSPRITE
ENDIF
IF DIRECTION = 0 ; Move N
SPRITEUP
SPRITEUP
ENDIF
IF DIRECTION = 1 ; Move NNE
SPRITERIGHT
SPRITEUP
SPRITEUP
ENDIF
IF DIRECTION = 2 ; Move NE
SPRITERIGHT
SPRITERIGHT
SPRITEUP
SPRITEUP
ENDIF
IF DIRECTION = 3 ; Move ENE
SPRITERIGHT
SPRITERIGHT
SPRITEUP
ENDIF
IF DIRECTION = 4 ; Move E
SPRITERIGHT
SPRITERIGHT
ENDIF
IF DIRECTION = 5 ; Move ESE
SPRITERIGHT
SPRITERIGHT
SPRITEDOWN
ENDIF
IF DIRECTION = 6 ; MOVE SE
SPRITERIGHT
SPRITERIGHT
SPRITEDOWN
SPRITEDOWN
ENDIF
IF DIRECTION = 7 ; MOVE SSE
SPRITEDOWN
SPRITEDOWN
SPRITERIGHT
ENDIF
IF DIRECTION = 8 ; MOVE S
SPRITEDOWN
SPRITEDOWN
ENDIF
IF DIRECTION = 9 ; MOVE SSW
SPRITEDOWN
SPRITEDOWN
SPRITELEFT
ENDIF
IF DIRECTION = 10 ; MOVE SW
SPRITEDOWN
SPRITEDOWN
SPRITELEFT
SPRITELEFT
ENDIF
IF DIRECTION = 11 ; MOVE WSW
SPRITEDOWN
SPRITELEFT
SPRITELEFT
ENDIF
IF DIRECTION = 12 ; MOVE W
SPRITELEFT
SPRITELEFT
ENDIF
IF DIRECTION = 13 ; MOVE WNW
SPRITELEFT
SPRITELEFT
SPRITEUP
ENDIF
IF DIRECTION = 14 ; MOVE NW
SPRITELEFT
SPRITELEFT
SPRITEUP
SPRITEUP
ENDIF
IF DIRECTION = 15 ; MOVE NNW
SPRITELEFT
SPRITEUP
SPRITEUP
ENDIF
; BOUNDARY CHECKING
; ===================
IF X <= LEFTEDGE ; if alien reaches the left edge
REMOVE ; remove it
SUBTRACT 1 FROM S ; decrement the sprite counter
SUBTRACT 1 FROM F ; decrement the all aliens in wave counter
ENDIF
IF X >= RIGHTEDGE ; if alien reaches the right edge
REMOVE ; remove it
SUBTRACT 1 FROM S ; decrement the sprite counter
SUBTRACT 1 FROM F ; decrement the all aliens in wave counter
ENDIF
IF Y >= BOTTOMEDGE ; if alien reaches the right edge
REMOVE ; remove it
SUBTRACT 1 FROM S ; decrement the sprite counter
SUBTRACT 1 FROM F ; decrement the all aliens in wave counter
ENDIF
; SHALL WE SHOOT A MISSILE?
; ===========================
IF Y < Q ; is alien above minimum height for releasing missiles?
IF W > 0 ; yes, can alien release missiles?
IF X < M ; yes, is Alien to the right of player?
SUBTRACT X FROM M ; how far to the right of player is alien?
IF M <= 8 ; 8 or less pixels?
LET AIRBORNE = 1 ; run the chance of firing routine
ENDIF
ADD X TO M ; reset M
ELSE
IF M < X ; is alien to the left of player?
SUBTRACT M FROM X ; yes, how far to the left
IF X < = 8 ; 8 pixels or less?
LET AIRBORNE = 1; yes, run the chance of firing routine
ENDIF
ADD M TO X ; reset X
ELSE ; enemy must be directly above player
LET AIRBORNE = 1 ; run the chance of firing routine
ENDIF
ENDIF
ENDIF
ENDIF
IF AIRBORNE = 1 ; Chance of firing routine
IF S < 9 ; are there 8 or less sprites on screen?
GETRANDOM H ; yes, get a random number from 0 to the aliens missile probability #
IF RND = 1 ; is it 1?
LET RND = DIRECTION ; yes, store aliens direction in RND
SPAWN 3 6 ; spawn a missile
SPAWNED ; switch to the spawned missile
LET DIRECTION = RND ; set its direction to RND
ENDSPRITE ; switch back to this alien
ENDIF
ENDIF
LET AIRBORNE = 0 ; end the chance of firing routine
ENDIF
SUBTRACT 1 FROM SETTINGB ; decrement the repetitions
IF SETTINGB = 0 ; has alien moved all repetitions?
LET SETTINGA = 1 ; get the next direction & # of repetitions
ENDIF
DATA 51 0 8 20 7 8 6 8 5 8 4 16
DATA 52 0 8 20 9 8 10 8 11 8 12 16
DATA 53 0 12 48 11 2 10 2 9 2 8 2 7 2 6 2 5 2 4 40 5 2 6 2 7 2 8 2 9 2 10 40
DATA 54 0 4 48 5 2 6 2 7 2 8 2 9 2 10 2 11 2 12 40 11 2 10 2 9 2 8 2 7 2 6 40
DATA 55 0 13 26 14 4 15 4 0 4 1 4 2 4 3 4 4 4 5 4 6 4 7 4 8 4 9 4 10 40
DATA 56 0 3 26 1 4 0 4 15 4 14 4 13 4 12 4 11 4 10 4 9 4 8 4 7 4 6 40
Read through the comments in the highlighted code and hopefully you'll understand what we are doing:
Check to see if the ALien is with 8 pixels to the left and x pixels to the right of the player
if they are, theres a random chance that a missile will be fired (provided they are also above the minimum vertical distance as set in variable Q) AND if there arent already too many sprites on screen
The probability of a missile being fired is 1 in H (where H is defined in the attack wave data table in the spawner event.
And if all these tests are passed spawn a missle (type 3, Image 6)
Then set the DIRECTION value of the spawned missile to be the same as the DIRECTION the alien was travelling in when they fired the missile.
Now all we need to do is write the missile code, so open up Sprite Type 3 and paste in this:
EVENT SPRITETYPE3
; ENEMY MISSILE
SPRITEINK 71 ; bright white
SPRITEDOWN ; go down fast (3 times
SPRITEDOWN
SPRITEDOWN
IF DIRECTION = 0 ; was alien going down when fired?
ELSE ; if so, do nothing
IF DIRECTION < 8 ; was alien going right?
SPRITERIGHT ; make missile veer right
ELSE
IF DIRECTION = 8 ; was alien going straight up?
ELSE ; do nothing
SPRITELEFT ; alien was going left, veer missile left
ENDIF
ENDIF
ENDIF
IF COLLISION PLAYER ; has missile collided with player?
IF X > M ; is missile to the right of player?
SUBTRACT M FROM X ; find out how far to the right
IF RND <= 10 ; 10 px or less? its a hit!
OTHER ; switch to the player
LET DIRECTION = 99 ; trigger the player explosion
ENDSPRITE ; switch back to this event
ENDIF
ADD M TO X ; reset X
ELSE ; not to the right
IF X < M ; is missile to the left?
SUBTRACT X FROM M ; yes, how far to left?
IF M <= 8 ; 8 or less pixels?
OTHER ;switch to the player
LET DIRECTION = 99 ; trigger the player explosion
ENDSPRITE ; switch back to this event
ENDIF
ADD X TO M ; reset M
ELSE ; must have been a direct hit
OTHER ; switch to the player
LET DIRECTION = 99 ; trigger the player explosion
ENDSPRITE ;switch back to this event
ENDIF
ENDIF
ENDIF
IF Y >= 188 ; has missile reached bottom of the screen
REMOVE ; remove it
SUBTRACT 1 FROM S ; decrement the sprite counter
ENDIF
So, we check for collisions with the player and then calculate how much of a direct hit it is, effectively creating a quick hit box, you can adjust these values to suit your own game and graphics.
One final thing to update, notice when we register a hit on the player we send an instruction to the player sprite to set its direction to 99, so we'll need to add a clause to our player event as follows ,open up your player event and add:
IF DIRECTION = 99
LET TYPE = 6 ; switch it to the explosion type
LET IMAGE = 4 ; and the explosion image
LET FRAME = 0 ; reset it to first frame
LET DIRECTION = 1 ; this is the player exploding
ENDIF
I've done this to save memory, since there may be multiple places in the code where we explode the player (notice I replaced the type, image, frame & direction settings in the Alien/Player collision in the ALien event (sprite type 5) earlier. Every byte counts!
OK, that should be everything we need to give the Aliens their firepower...time for a test!
That looks OK to me for now, we can always refine it later as the game progresses.
Next we'll do a bit of housekeeping and tidy some stuff up.
NEXT: PART 10 - A SPOT OF HOUSEKEEPING
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