A C64 Game - Step 10

So you found out the enemies couldn't hurt you? Well, we're working towards that goal in this step. We add collision checks. Since I'm not completely sure about later changes we are NOT relying on the VICs collision checks but roll our own. Remember the object size contraints from step #6? We apply those to the object collision checks as well.

We add a new subroutine CheckCollisions which in turn uses IsEnemyCollidingWithPlayer. We do not check for collisions between enemies. The check is not completely exact (behaves more like 9 pixel * 16 pixel), but that's good enough. To test the function a collision is signalled by setting the border color to white.



The routine CheckCollisions is simply added to the main game loop:

GameLoop
          jsr WaitFrame
          jsr ObjectControl
          jsr CheckCollisions
          jmp GameLoop

The function CheckCollision just loops through the active object list and calls IsEnemyCollidingWithPlayer for every active entry:

;------------------------------------------------------------
;check object collisions (enemy vs. player etc.)
;x
;------------------------------------------------------------
CheckCollisions
          ldx #1
.CollisionLoop
          lda SPRITE_ACTIVE,x
          bne .CheckObject

.NextObject
          inx
          cpx #8
          bne .CollisionLoop

          lda #0
          sta VIC_BORDER_COLOR
          rts

.CheckObject
          stx PARAM2
          jsr IsEnemyCollidingWithPlayer
          bne .PlayerCollidedWithEnemy

          ldx PARAM2
          jmp .NextObject

.PlayerCollidedWithEnemy
          lda #1
          sta VIC_BORDER_COLOR
          ;ldx #0
          ;jsr RemoveObject
          rts

IsEnemyCollidingWithPlayer employs a few tricks to ease the calculation.
First we do the Y coordinate check to weed out. For the X coordinate: Since the actual X position is 9 bits we half the value (half the X coordinate and add 128 if the extended X bit is set). Now the comparation is easy.
The routine then returns 1 if a collision occurs and 0 if not.

;------------------------------------------------------------
;check object collision with player (object 0)
;x = enemy index
;return a = 1 when colliding, a = 0 when not
;------------------------------------------------------------
!zone IsEnemyCollidingWithPlayer

.CalculateSimpleXPos
          ;Returns a with simple x pos (x halved + 128 if > 256)
          ;modifies y
          lda BIT_TABLE,x
          and SPRITE_POS_X_EXTEND
          beq .NoXBit

          lda SPRITE_POS_X,x
          lsr
          clc
          adc #128
          rts

.NoXBit
          lda SPRITE_POS_X,x
          lsr
          rts

IsEnemyCollidingWithPlayer
          ;modifies X
          ;check y pos
          lda SPRITE_POS_Y,x
          sec
          sbc #( OBJECT_HEIGHT )

          ;offset to bottom
          cmp SPRITE_POS_Y
          bcs .NotTouching
          clc
          adc #( OBJECT_HEIGHT + OBJECT_HEIGHT - 1 )
          cmp SPRITE_POS_Y
          bcc .NotTouching

          ;X = Index in enemy-table
          jsr .CalculateSimpleXPos
          sta PARAM1

          ldx #0
          jsr .CalculateSimpleXPos
          sec
          sbc #4

          ;position X-Anfang Player - 12 Pixel
          cmp PARAM1
          bcs .NotTouching
          adc #8
          cmp PARAM1
          bcc .NotTouching

          lda #1
          rts

.NotTouching
          lda #0
          rts

step10.zip