Palette fader for GBC

pal_fade.asm

INCLUDE "defines.asm"

SECTION "Palette buffer fading", ROM0

; Applies fade to both palette buffers
;
FadePaletteBuffers::
	ld hl, wFadeSteps
	dec [hl]
	inc hl
	assert wFadeSteps + 1 == wFadeStep

	ld a, [hli]
	ld e, a
	assert wFadeStep + 1 == wFadeAmount
	add a, [hl]
	jr z, .clamp ; 0 is an illegal value
	ld d, a
	rra ; Get carry into bit 7
	xor e ; Expect bit 7 of offset to match carry
	add a, a
	ld a, d
	jr nc, .noOverflow
.clamp
	; If we got an overflow, clamp depending on bit 7 of offset
	; If the offset is positive, clamp at $FF; otherwise, at 1
	sla e ; Bit 7 (sign) into carry
	sbc a, a ; 0   or $FF
	and 2    ; 0   or 2
	dec a    ; $FF or 1
.noOverflow
	ld [hli], a
	assert wFadeAmount + 1 == wBGPaletteMask
	add a, a ; Test sign bit
	ld c, LOW(rBCPS)
	jr nc, .fadeToBlack

	ld d, a
	; ld a, [hConsoleType]
	; and a
	; jr nz, FadeDMGToWhite
.fadeBufferToWhite
	ld a, $80
	ldh [c], a
	inc c
	ld a, [hli] ; Read palette mask
	scf
	adc a, a
.fadePaletteToWhite
	ldh [hPaletteMask], a
	jr nc, .noWhiteFade
	ld b, 4
	ld a, c
	cp LOW(rOCPD)
	jr nz, .fadeColorToWhite
	dec b
	; Do two dummy writes to advance index
	; The index is increased even if the writes fail
	ldh [c], a
	ldh [c], a
.fadeColorToWhite
	ld a, [hli] ; Read green
	add a, d
	jr nc, .notFullGreen
	sbc a, a ; ld a, $FF
.notFullGreen
	rlca
	rlca
	ld e, a
	wait_vram
	ld a, [hli] ; Read red
	add a, d
	jr nc, .notFullRed
	sbc a, a ; ld a, $FF
.notFullRed
	rra
	rra
	rra
	xor e
	and $1F
	xor e
	ldh [c], a
	wait_vram
	ld a, [hli] ; Read blue
	add a, d
	jr nc, .notFullBlue
	sbc a, a ; ld a, $FF
.notFullBlue
	rra
	xor e
	and $FC ; $7C works just as well, but $FC gets bit 7 always cleared
	xor e
	ldh [c], a
	dec b
	jr nz, .fadeColorToWhite
.fadedPaletteWhite
	ldh a, [hPaletteMask]
	add a, a
	jr nz, .fadePaletteToWhite
	inc c
	ld a, c
	cp LOW(rOCPS)
	jr z, .fadeBufferToWhite
	ret

.noWhiteFade
	dec c
	ldh a, [c]
	add a, 4 * 2
	ldh [c], a
	inc c
	ld a, l
	add a, 4 * 3
	ld l, a
	adc a, h
	sub l
	ld h, a
	jr .fadedPaletteWhite

.fadeToBlack
	cpl
	inc a
	ld d, a
	; ldh a, [hConsoleType]
	; and a
	; jr nz, .fadeDMGToBlack
.fadeBufferToBlack
	ld a, $80
	ldh [c], a
	inc c
	ld a, [hli] ; Read palette mask
	scf
	adc a, a
.fadePaletteToBlack
	ldh [hPaletteMask], a
	jr nc, .noBlackFade
	ld b, 4
	; OBJ palettes only have 3 colors
	ld a, c
	cp LOW(rOCPD)
	jr nz, .fadeColorToBlack
	dec b
	; Do two dummy writes to advance index
	; The index is increased even if the writes fail
	ldh [c], a
	ldh [c], a
.fadeColorToBlack
	ld a, [hli] ; Read green
	sub d
	jr nc, .stillSomeGreen
	xor a
.stillSomeGreen
	rlca
	rlca
	ld e, a
	wait_vram
	ld a, [hli] ; Read red
	sub d
	jr nc, .stillSomeRed
	xor a
.stillSomeRed
	rra
	rra
	rra
	xor e
	and $1F
	xor e
	ldh [c], a
	wait_vram
	ld a, [hli] ; Read blue
	sub d
	jr nc, .stillSomeBlue
	xor a
.stillSomeBlue
	rra
	xor e
	and $FC ; $7C works just as well, but $FC gets bit 7 always cleared
	xor e
	ldh [c], a
	dec b
	jr nz, .fadeColorToBlack
.fadedPaletteBlack
	ldh a, [hPaletteMask]
	add a, a
	jr nz, .fadePaletteToBlack
	inc c
	ld a, c
	cp LOW(rOCPS)
	jr z, .fadeBufferToBlack
	ret

.noBlackFade
	dec c
	ldh a, [c]
	add a, 4 * 2
	ldh [c], a
	inc c
	ld a, l
	add a, 4 * 3
	ld l, a
	adc a, h
	sub l
	ld h, a
	jr .fadedPaletteBlack


SECTION UNION "Scratch buffer", HRAM

hPaletteMask: db

SECTION "Fade state memory", WRAM0

wFadeSteps:: db ; Number of fade steps to take
wFadeStep:: db ; Value to add to wFadeAmount on each step

; 00    = bugged equivalent of 00, do not use
; 01-7F = 01 is fully black, 7F is barely faded
; 80    = not faded
; 81-FF = FF is fully white, 81 is barely faded
wFadeAmount:: db

wBGPaletteMask:: db ; Mask of which palettes to fade (01234567)
wBGPaletteBuffer:: ; 24-bit GRB, in this order
	ds 8 * 4 * 3 ; 8 palettes, 4 colors, 3 bytes
wOBJPaletteMask:: db
wOBJPaletteBuffer:: ; Same as above
	ds 8 * 3 * 3 ; Same, but only 3 colors