Sprites & Metasprites

Before we dive into the player, bullets, and enemies; how they are drawn using metasprites should be explained.

For sprites, the following library is used: https://github.com/eievui5/gb-sprobj-lib

This is a small, lightweight library meant to facilitate the rendering of sprite objects, including Shadow OAM and OAM DMA, single-entry “simple” sprite objects, and Q12.4 fixed-point position metasprite rendering.

All objects are drawn using “metasprites”, or groups of sprites that define one single object. A custom “metasprite” implementation is used in addition. Metasprite definitions should a multiple of 4 plus one additional byte for the end.

• Relative Y offset ( relative to the previous sprite, or the actual metasprite’s draw position)
• Relative X offset ( relative to the previous sprite, or the actual metasprite’s draw position)
• Tile to draw
• Tile Props (not used in this project)

The logic stops drawing when it reads 128.

An example of metasprite is the enemy ship:

enemyShipMetasprite::
    .metasprite1    db 0,0,4,0
    .metasprite2    db 0,8,6,0
    .metaspriteEnd  db 128

The Previous snippet draws two sprites. One that the object’s actual position, which uses tile 4 and 5. The second sprite is 8 pixels to the right, and uses tile 6 and 7

⚠️ NOTE: Sprites are in 8x16 mode for this project.

I can later draw such metasprite by calling the “DrawMetasprite” function that

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; call the 'DrawMetasprites function. 
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Save the x position
ld a, b
ld [wMetaspriteX],a

; Save the y position
ld a, c
ld [wMetaspriteY],a

; Actually call the 'DrawMetasprites function
call DrawMetasprites;

We previously mentioned a variable called “wLastOAMAddress”. The “DrawMetasprites” function can be found in the “src/main/utils/metasprites.asm” file:

include "src/main/utils/constants.inc"
SECTION "MetaSpriteVariables", WRAM0

wMetaspriteAddress:: dw
wMetaspriteX:: db
wMetaspriteY::db

SECTION "MetaSprites", ROM0

DrawMetasprites::


    ; get the metasprite address
    ld a, [wMetaspriteAddress+0]
    ld l, a
    ld a, [wMetaspriteAddress+1]
    ld h, a

    ; Get the y position
    ld a, [hli]
    ld b, a

    ; stop if the y position is 128 
    ld a, b
    cp 128
    ret z

    ld a, [wMetaspriteY]
    add b
    ld [wMetaspriteY], a

    ; Get the x position
    ld a, [hli]
    ld c, a

    ld a, [wMetaspriteX]
    add c
    ld [wMetaspriteX], a

    ; Get the tile position
    ld a, [hli]
    ld d, a

    ; Get the flag position
    ld a, [hli]
    ld e, a
    

    ; Get our offset address in hl
	ld a,[wLastOAMAddress+0]
    ld l, a
	ld a, HIGH(wShadowOAM)
    ld h, a

    ld a, [wMetaspriteY]
    ld [hli], a

    ld a, [wMetaspriteX]
    ld [hli], a

    ld a, d
    ld [hli], a

    ld a, e
    ld [hli], a

    call NextOAMSprite

     ; increase the wMetaspriteAddress
    ld a, [wMetaspriteAddress]
    add a, METASPRITE_BYTES_COUNT
    ld [wMetaspriteAddress], a
    ld a, [wMetaspriteAddress+1]
    adc 0
    ld [wMetaspriteAddress+1], a


    jp DrawMetasprites

When we call the “DrawMetasprites” function, the “wLastOAMAddress” variable will be advanced to point at the next available shadow OAM sprite. This is done using the “NextOAMSprite” function in “src/main/utils/sprites-utils.asm”

NextOAMSprite::

    ld a, [wSpritesUsed]
    inc a
    ld [wSpritesUsed], a

	ld a,[wLastOAMAddress]
    add sizeof_OAM_ATTRS
	ld [wLastOAMAddress], a
	ld a, HIGH(wShadowOAM)
	ld [wLastOAMAddress+1], a


    ret