Scrolling Background

Scrolling the background is an easy task. However, for a SMOOTH slow scrolling background: scaled integers¹ will be used.

⚠️ Scaled Integers¹ are a way to provide smooth “sub-pixel” movement. They are slightly more difficult to understand & implement than implementing a counter, but they provide smoother motion.

Initializing the Background

At the start of the gameplay game state we called the initialize background function. This function shows the star field background, and resets our background scroll variables:

Just like with our title screen graphic, because our text font tiles are at the beginning of VRAM: we offset the tilemap values by 52

INCLUDE "src/main/utils/hardware.inc"
INCLUDE "src/main/utils/macros/text-macros.inc"

SECTION "BackgroundVariables", WRAM0

mBackgroundScroll:: dw

SECTION "GameplayBackgroundSection", ROM0

starFieldMap: INCBIN "src/generated/backgrounds/star-field.tilemap"
starFieldMapEnd:
 
starFieldTileData: INCBIN "src/generated/backgrounds/star-field.2bpp"
starFieldTileDataEnd:

InitializeBackground::

	; Copy the tile data
	ld de, starFieldTileData ; de contains the address where data will be copied from;
	ld hl, $9340 ; hl contains the address where data will be copied to;
	ld bc, starFieldTileDataEnd - starFieldTileData ; bc contains how many bytes we have to copy.
    call CopyDEintoMemoryAtHL

	; Copy the tilemap
	ld de, starFieldMap
	ld hl, $9800
	ld bc, starFieldMapEnd - starFieldMap
    call CopyDEintoMemoryAtHL_With52Offset

	xor a
	ld [mBackgroundScroll], a
	ld [mBackgroundScroll+1], a
	ret

To scroll the background in a gameboy game, we simply need to gradually change the SCX or SCX registers. Our code is a tiny bit more complicated because of scaled integer usage. Our background’s scroll position is stored in a 16-bit integer called mBackgroundScroll. We’l increase that 16-bit integer by a set amount.

; This is called during gameplay state on every frame
UpdateBackground::

	; Increase our scaled integer by 5
	; Get our true (non-scaled) value, and save it for later usage in bc
	ld a, [mBackgroundScroll]
	add a, 5
    ld b, a
	ld [mBackgroundScroll], a
	ld a, [mBackgroundScroll+1]
	adc 0
    ld c, a
	ld [mBackgroundScroll+1], a

We won’t directly draw the background using this value. De-scaling a scaled integer simulates having a (more precise and useful for smooth movement) floating-point number. The value we draw our background at will be the de-scaled version of that 16-bit integer. To get that non-scaled version, we’ll simply shift all of it’s bit rightward 4 places. The final result will saved for when we update our background’s y position.

    ; Descale our scaled integer 
    ; shift bits to the right 4 spaces
    srl c
    rr b
    srl c
    rr b
    srl c
    rr b
    srl c
    rr b

    ; Use the de-scaled low byte as the backgrounds position
    ld a, b
	ld [rSCY], a
	ret