Aliens perform a movement pattern which are a combination of directional movements, Direction 0-15, each one representing a point on a 16 point compass, and a number of repetitions.
Patterns are stored in a DATA table at the bottom of the event. Each row of the data table represents a pattern unique to the level the player has progressed to.
In the demo game, aliens in an attack wave are removed when they are shot by the player or exit the screen, but you could easily change this for different gameplay styles.
The number of aliens in an attack wave and other parameters are defined separately in the Spawner Event.
A default alien sprite is included in Sprite Image 2, it has 16 frames, each one corresponding to a direction value, this makes it easy to ensure the correct frame is shown based on the aliens current direction of travel.
Aliens can also shoot missiles at the player, how and when they do this is fully customisable and is based on the settings defined in the Spawner event for each specific attack wave, so you can set how high up the screen they have to be before firing, their probability of firing and how far to the left and right of the player will allow a potential missile launch.
EVENT SPRITETYPE5 ;======= ENEMIES ============ ;==================================== ; ; VARIABLES ; ======================================================= ; DIRECTION - CURRENT DIRECTION OF TRAVEL,( 254= OUT OF BOUNDS, 255= EXPLODE) ; SETTINGA - STATUS (0= INITIALISE, 1=GET DIRECTION & REPS, 2=MOVE) ; SETTINGB - # REPETITIONS OF CURRENT DIRECTION ; AIRBORNE - CHANCE OF FIRING WEAPON (0 = NO, 1 = YES) ; JUMPSPEED - availabe for use in your game ; ======================================================== 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 ; ready to do the movement IF IMAGE 2 ; is this image 2? LET FRAME = DIRECTION ; yes, set the frame to the same as direction ENDIF ENDIF ; COLLISIONS ; ============================== IF COLLISION 1 ; if hit by a player torpedo OTHER ; switch to torpedo LET DIRECTION = 99 ; run the explosion ENDSPRITE ; return to this Alien LET DIRECTION = 255 ; request alien explosion ENDIF IF COLLISION 0 ; if enemy and player collide OTHER ; switch to the player LET DIRECTION = 99 ; trigger the player explosion ENDSPRITE ; return to this sprite LET DIRECTION = 255 ; request alien explosion ENDIF ; EXPLODE THE ALIEN ; ======================= IF DIRECTION = 255 ; Alien explosion requested 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 ENDIF ; MOVEMENT ; ==================== 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 LET DIRECTION = 254 ; enemy is out of bounds ENDIF IF X >= RIGHTEDGE ; if alien reaches the right edge LET DIRECTION = 254 ; enemy is out of bounds ENDIF IF Y >= BOTTOMEDGE ; if alien reaches the right edge LET DIRECTION = 254 ; enemy is out of bounds ENDIF IF DIRECTION =254 ; is enemy out of bounds? REMOVE ; remove it SUBTRACT 1 FROM S ; decrement the sprite counter SUBTRACT 1 FROM F ; decrement the alien counter ENDIF ; CHECK IF ALIEN CAN RELEASE WEAPON ; ================================= 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 ; yes, how far to the right of player? IF M <= I ; I 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 < = I ; I 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 ; CHANCE OF FIRING ; ==================== 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 ; REPETITIONS CHECK ; ====================== SUBTRACT 1 FROM SETTINGB ; decrement repetitions remaining IF SETTINGB = 0 ; has alien moved all repetitions? LET SETTINGA = 1 ; get the next direction & # of repetitions ENDIF ; MOVEMENT PATTERNS ; ======================== 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 DATA 57 0 8 16 7 4 6 4 5 4 4 4 3 4 2 4 1 4 0 4 15 4 14 4 13 4 12 4 11 4 10 4 9 4 8 4 7 4 6 4 5 4 4 60 DATA 58 0 8 16 9 4 10 4 11 4 12 4 13 4 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 4 11 2 12 60 DATA 59 0 8 16 9 4 10 4 11 4 12 4 13 4 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 4 11 2 12 60 DATA 60 0 4 20 13 4 14 4 15 4 0 8 1 4 2 4 3 4 4 4 8 8 7 2 6 2 5 2 4 40 DATA 61 0 5 2 6 2 7 2 8 2 9 2 10 2 11 2 12 2 11 3 10 3 9 3 8 3 7 3 6 3 5 4 6 4 7 4 8 4 9 4 10 4 DATA 62 0 8 3 4 16 9 24 4 40
DATA 59 0 8 16 9 4 10 4 11 4 12 4 13 4 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 4 11 2 12 60
DATA 60 0 4 20 13 4 14 4 15 4 0 8 1 4 2 4 3 4 4 4 8 8 7 2 6 2 5 2 4 40
DATA 61 0 5 2 6 2 7 2 8 2 9 2 10 2 11 2 12 2 11 3 10 3 9 3 8 3 7 3 6 3 5 4 6 4 7 4 8 4 9 4 10 4
DATA 62 0 8 3 4 16 9 24 4 40