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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:

{{#include ../../galactic-armada/src/main/states/gameplay/objects/enemies.asm:enemy-metasprites}}

MetaspriteDIagram.png

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

{{#include ../../galactic-armada/src/main/states/gameplay/objects/enemies.asm:draw-enemy-metasprites}}

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

{{#include ../../galactic-armada/src/main/utils/metasprites.asm}}

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"

{{#include ../../galactic-armada/src/main/utils/sprites-utils.asm:next-oam-sprite}}