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doing some tank-pong like logic for ball/missile. help!


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Am currently trying to work through some logic to do TANK-PONG like logic to a ball, for my Dodgeball game.

 

P0Velocity is arranged as thus: RLDU 0000 where the lower nibble is a delay in frames.

The rest should be pretty straightforward, (and the S variables are being persisted outside this routine), am trying to do something like this description;

 

 

; Immediately after a collision, it tries a vertical reflection,
; jiggering the result so that it won't be exactly vertical or
; exactly horizontal.
;
; If this is the next frame (MxPFcount=$01) that failed, so
; we reverse direction 180 degrees to turn it into a horizontal
; reflection.
;
; On MxPfcount=$02 we take no action, since the missile may need
; the cycle to re-emerge from a wall.
;
; On MxPFcount=$03 or higher, we retrieve the original heading and
; turn it 180 degrees, assuming a corner reflection.  And we keep
; applying this same bearing until it's out of the #*%@ wall.
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Using RLDU means you're limited to just 8 directions, and a number of them are duplicates (if R and L are both 0 or both 1 then there's no left/right movement). Using 4 bits you can encode 16 directions instead of 8.

 

In Medieval Mayhem I use a single byte to denote speed and direction, SSSDDDDD. Direction value of 0 is up, 8 is right, etc:

     0
     ^	
     |
24<--+-->8
     |
     v
    16

To prevent the fireballs from getting stuck in a loop I would "add some english" by randomly adding (or perhaps that was subtracting) 1 from the D value during a bounce.

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In Medieval Mayhem I use a single byte to denote speed and direction, SSSDDDDD. Direction value of 0 is up, 8 is right, etc:

 

 

To clarify: it counts clockwise around the directions? ie you then have 7 diagonal up-and-right values?

 

It seems like this nicely encodes direction, but requires more code to translate the direction byte into actual movement? (which is a reasonable tradeoff, just making sure I understand)

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To clarify: it counts clockwise around the directions? ie you then have 7 diagonal up-and-right values?

Yep - there's 32 different directions the fireballs can travel, much better than the 4 diagonals in Warlords. Yes in Warlords you can release a horizontal or vertical shot, but it'll change to one of the 4 diagonals after it hits something.

 

It seems like this nicely encodes direction, but requires more code to translate the direction byte into actual movement? (which is a reasonable tradeoff, just making sure I understand)

No need to decode DirSpeed, it's used to index values in movement tables. The code is basically Y = Y + MoveTableY[DirSpeed] and X = X + MoveTableX[DirSpeed] for 16 bit values, or this in 6507 code:

MoveFireball:
  clc
  ldy FireballDirSpeed  ; the SSSDDDDD value
  lda FireballYPosLow
  adc MoveTableYlow,y
  sta FireballYPosLow
  lda FireballYPosHigh
  adc MoveTableYhigh,y
  sta FireballYPosHigh

  clc
  lda FireballXPosLow
  adc MoveTableXlow,y
  sta FireballXPosLow
  lda FireballXPosHigh
  adc MoveTableXhigh,y
  sta FireballXPosHigh


plus some additional checks for boundaries and adding english to bounces.

 

Due to the use of 16 bit values there's 4 tables with 256 bytes in each table, so 1K of data in a 32K game.

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