A C64 Game - Step 34

And another refinement step:
Two new items are added, one for faster reload, another for temporary invincibility. Also, not every kill is getting you an item to collect.



The most interesting new part is the state of a sprite. For now only used for the player, but will be put to good use for enemies as well. The state of an object is used for different behaviour steps, but also to describe, if an object will react to collisions. If the highest bit is set (0x80), an object will not kill the player (or the player cannot be killed).

We'll start by defining the new constants for the items:

;item types
ITEM_FAST_RELOAD = 2
ITEM_INVINCIBLE = 3

;effect counters
PLAYER_FAST_RELOAD
          !byte 0

PLAYER_INVINCIBLE
          !byte 0

;item images
ITEM_CHAR_UL
          !byte 4,8,16,20

ITEM_COLOR_UL
          !byte 7,2,1,1

ITEM_CHAR_UR
          !byte 5,9,17,21

ITEM_COLOR_UR
          !byte 4,2,2,7

ITEM_CHAR_LL
          !byte 6,10,18,22

ITEM_COLOR_LL
          !byte 7,2,2,7

ITEM_CHAR_LR
          !byte 7,11,19,23

ITEM_COLOR_LR
          !byte 4,2,2,4

The actual collect item code is simple, we're just setting some new variables. Note the SPRITE_STATE, which is set to 0x80.

.EffectFastReload
          lda #200
          sta PLAYER_FAST_RELOAD
          jmp .RemoveItem

.EffectInvincible
          lda #200
          sta PLAYER_INVINCIBLE
          lda #128
          sta SPRITE_STATE
          jmp .RemoveItem

Thus the CheckCollision adds a check for the highest bit in SPRITE_STATE, to allow for player invincibility.

CheckCollisions
          lda SPRITE_ACTIVE
          bne .PlayerIsAlive
          rts

.PlayerIsAlive
          lda SPRITE_STATE
          cmp #128
          bmi .IsVulnerable
          rts

.IsVulnerable
          ldx #1

.CollisionLoop

To visualize the invincibility state we add this code at the start of PlayerControl. If PLAYER_INVINCIBLE is set, the players color is cycled.

          lda PLAYER_INVINCIBLE
          beq .NotInvincible

          ;count down invincibility
          inc VIC_SPRITE_COLOR
          dec PLAYER_INVINCIBLE
          bne .NotInvincible

          lda #0
          sta SPRITE_STATE
          lda #10
          sta VIC_SPRITE_COLOR

.NotInvincible
          ... previous code

In the players reload code we add the double speed reload check:

          ...
          lda PLAYER_FAST_RELOAD
          beq .NoFastReload

          dec PLAYER_FAST_RELOAD

          inc PLAYER_STAND_STILL_TIME

.NoFastReload
          ...

At the location, where we previously always spawned an item we add this:

          ;only spawn item randomly
          lda GenerateRandomNumber
          and #02
          beq .CreateItem

          jmp .ShotDone

          ...

.CreateItem
          jsr SpawnItem
          jmp .ShotDone

To get things running clean we change a few spots:

On restarting the temporary states need to be reset.

          lda #0
          sta PLAYER_FAST_RELOAD
          sta PLAYER_INVINCIBLE
          sta SPRITE_STATE

step34.zip