RGB fade engine (#14)

engine/gfx/rgb_fade.asm

@@ -0,0 +1,250 @@
+; at normal speed:
+; each color takes around 3.2 scanlines to fade
+; up to 10 (11?) colors can be faded per frame
+
+FadeStepColorsToBlack:
+.loop
+	push de
+	push bc
+	call FadeStepColorToBlack
+	pop bc
+	pop de
+	inc de
+	inc de
+	dec c
+	jr nz, .loop
+	ret
+
+FadeStepColorsToDarker:
+; decrease rgb channels of a color by 2 points each
+; input
+;	 c: number of consecutive colors to fade
+;   de: pointer to c long array of 2-byte rgb colors to fade
+;   hl: pointer to c long array of 2-byte rgb colors with cap values for each channel
+.loop
+	push hl
+	push de
+	push bc
+	call FadeStepColorToDarker
+	pop bc
+	pop de
+	pop hl
+	inc de
+	inc de
+	inc hl
+	inc hl
+	dec c
+	jr nz, .loop
+	ret
+
+FadeStepColorToBlack:
+	ld hl, BlackRGB
+	; fallthrough
+
+FadeStepColorToDarker:
+; decrease rgb channels of a color by 2 points each
+; input
+;   de: pointer to 2-byte rgb color to fade
+;   hl: pointer to 2-byte rgb color with cap values for each channel
+	push de
+
+; convert source and cap colors to channels
+	push hl
+	ld hl, hRGBFadeSourceChannels
+	call RGBColorToChannels
+	pop de
+	ld hl, hRGBFadeCapChannels
+	call RGBColorToChannels
+
+; apply fading to source channels accounting for caps
+	ldh a, [hRGBFadeCapRChannel]
+	ld b, a
+	ldh a, [hRGBFadeSourceRChannel]
+	sub 2
+	jr c, .nok1
+	cp b
+	jr nc, .ok1
+.nok1
+	ld a, b
+.ok1
+	ldh [hRGBFadeSourceRChannel], a
+	ldh a, [hRGBFadeCapGChannel]
+	ld b, a
+	ldh a, [hRGBFadeSourceGChannel]
+	sub 2
+	jr c, .nok2
+	cp b
+	jr nc, .ok2
+.nok2
+	ld a, b
+.ok2
+	ldh [hRGBFadeSourceGChannel], a
+	ldh a, [hRGBFadeCapBChannel]
+	ld b, a
+	ldh a, [hRGBFadeSourceBChannel]
+	sub 2
+	jr c, .nok3
+	cp b
+	jr nc, .ok3
+.nok3
+	ld a, b
+.ok3
+	ldh [hRGBFadeSourceBChannel], a
+
+; convert faded source channels to color
+	pop de
+
+	ld hl, hRGBFadeSourceChannels
+	call RGBChannelsToColor
+	ret
+
+FadeStepColorsToWhite:
+	ld hl, WhiteRGB
+.loop
+	push de
+	push bc
+	call FadeStepColorToWhite
+	pop bc
+	pop de
+	inc de
+	inc de
+	dec c
+	jr nz, .loop
+	ret
+
+FadeStepColorsToLighter:
+; increase rgb channels of a color by 2 points each
+; input
+;	 c: number of consecutive colors to fade
+;   de: pointer to c long array of 2-byte rgb colors to fade
+;   hl: pointer to c long array of 2-byte rgb colors with cap values for each channel
+.loop
+	push hl
+	push de
+	push bc
+	call FadeStepColorToLighter
+	pop bc
+	pop de
+	pop hl
+	inc de
+	inc de
+	inc hl
+	inc hl
+	dec c
+	jr nz, .loop
+	ret
+
+FadeStepColorToWhite:
+	ld hl, WhiteRGB
+	; fallthrough
+
+FadeStepColorToLighter:
+; increase rgb channels of a color by 2 points each
+; input
+;   de: pointer to 2-byte rgb color to fade
+;   hl: pointer to 2-byte rgb color with cap values for each channel
+	push de
+
+; convert source and cap colors to channels
+	push hl
+	ld hl, hRGBFadeSourceChannels
+	call RGBColorToChannels
+	pop de
+	ld hl, hRGBFadeCapChannels
+	call RGBColorToChannels
+
+; apply fading to source channels accounting for caps
+	ldh a, [hRGBFadeCapRChannel]
+	ld b, a
+	ldh a, [hRGBFadeSourceRChannel]
+	add 2
+	cp b
+	jr c, .ok1
+	ld a, b
+.ok1
+	ldh [hRGBFadeSourceRChannel], a
+	ldh a, [hRGBFadeCapGChannel]
+	ld b, a
+	ldh a, [hRGBFadeSourceGChannel]
+	add 2
+	cp b
+	jr c, .ok2
+	ld a, b
+.ok2
+	ldh [hRGBFadeSourceGChannel], a
+	ldh a, [hRGBFadeCapBChannel]
+	ld b, a
+	ldh a, [hRGBFadeSourceBChannel]
+	add 2
+	cp b
+	jr c, .ok3
+	ld a, b
+.ok3
+	ldh [hRGBFadeSourceBChannel], a
+
+; convert faded source channels to color
+	pop de
+
+	ld hl, hRGBFadeSourceChannels
+	call RGBChannelsToColor
+	ret
+
+RGBColorToChannels:
+; convert 2-byte rgb color at de to rgb channels into hl
+; red channel
+	ld a, [de]
+	ld c, a
+	and %00011111
+	ld [hli], a
+; green channel
+	inc de
+	ld a, [de]
+	and %00000011
+	swap a
+	srl a
+	ld b, a ; 000gg000
+	ld a, c
+	and %11100000
+	swap a
+	srl a ; 00000ggg
+	add b
+	ld [hli], a
+; blue channel
+	ld a, [de]
+	and %01111100
+	srl a
+	srl a
+	ld [hl], a
+	ret
+
+RGBChannelsToColor:
+; convert rgb channels at hl to 2-byte rgb color into de
+; first byte: gggrrrrr
+	ld a, [hli]
+	ld c, a
+	ld a, [hl]
+	and %00000111
+	swap a
+	sla a
+	add c
+	ld [de], a
+; second byte: 0bbbbbgg
+	inc de
+	ld a, [hli]
+	and %00011000
+	srl a
+	srl a
+	srl a
+	ld c, a
+	ld a, [hl]
+	sla a
+	sla a
+	add c
+	ld [de], a
+	ret
+
+BlackRGB:
+	RGB 00, 00, 00
+
+WhiteRGB:
+	RGB 31, 31, 31

ram/hram.asm

@@ -65,6 +65,18 @@ hMathBuffer:: ds 5
 NEXTU
 ; PrintNum scratch space
 hPrintNumBuffer:: ds 10
+
+NEXTU
+; engine/gfx/rgb_fade.asm
+hRGBFadeSourceChannels::
+hRGBFadeSourceRChannel:: db
+hRGBFadeSourceGChannel:: db
+hRGBFadeSourceBChannel:: db
+hRGBFadeCapChannels::
+hRGBFadeCapRChannel:: db
+hRGBFadeCapGChannel:: db
+hRGBFadeCapBChannel:: db
+
 ENDU
 
 UNION