Palette rotation

Palette rotation is a non-NeoGeo-specific video effect produced by rotating (shifting in a loop) a range of color entries inside a palette.

Like the auto animation feature, this effect was frequently used for cyclic animations (waterfalls, neon signs...) without having to use multiple tiles, saving up graphics memory and VRAM access time.

• 

  League Bowling's title screen uses palette rotation as a mean to provoke eye cancer.

• 

  Fatal Fury 2 uses palette rotations to animate water efficiently.

Example code

The easiest way is to simply cycle through a table of pre-baked palettes. It can also be done procedurally, for example:

; Untested ! Doesn't care about transparent index.
; Executes in about 720 cycles

  lea     COLOR_LIST,a0   ; Pointer to list of 32 colors
  lea     PALETTE,a1      ; Pointer to destination palette in palette RAM
  lea     64(a0),a2       ; End of list (32 words)
  clr.l   d0
  move.b  ROT_INDEX,d0    ; Offset variable
  andi.b  #31,d0          ; Warp on 32
  add.w   d0,d0           ; Colors are words
  adda.l  d0,a0
  move.b  #16,d7          ; Number of entries in a palette
-:
  move.w  (a0)+,(a1)+     ; Copy
  subq.b  #1,d7
  beq     +               ; Are we done ?
  cmp.l   a0,a2
  bne     -               ; Warp ?
  lea     COLOR_LIST,a0   ; Warp !
  bra     -
+:

  addq.b  #1,ROT_INDEX    ; Animate