Lesson 46 · Sprites - Build a Game Boy Emulator

Sprites

Today you add sprites, the moving objects like the player and enemies, compositing them over the background from OAM data while treating color index 0 as transparent. With sprites in place every visible element of a game is finally on screen.

The goal

Draw an 8x8 sprite from OAM over the background, treating color index 0 as transparent.

Start here - the target

TO DO

Scenario: Drawing a sprite over the background

GivenOBP0 is 0xE4, OAM entry 0 has Y=16, X=8, tile=1, and tile 1's top-left pixel is index 2 with the rest index 0

Whensprites are composited onto a background of shade 0

Thenscreen pixel (0,0) is shade 2 (index 2 mapped through OBP0=0xE4)

Andthe sprite's index-0 pixels stay transparent, leaving the background shade 0 beneath them

Background

Sprites (the hardware calls them objects) are the moving pieces - the player, enemies, the falling blocks. Their positions live in OAM at 0xFE00, 40 entries of four bytes each: Y, X, tile number, and attributes. The positions carry a fixed offset so sprites can slide partly off-screen: a sprite at Y=16, X=8 draws at screen (0, 0), so you subtract 16 and 8 to find where it lands.

The rule that makes sprites look right is transparency: color index 0 is not drawn at all, letting the background show through. So you composite in two passes

background first, then sprites on top, skipping their index-0 pixels. Sprites use their own palettes (OBP0/OBP1) rather than BGP. Add these and every visible element of a game is on screen; all that is left is to make it run.

Make it work

// OAM lives at 0xFE00: 40 entries of 4 bytes (Y, X, tile, attributes). Read an
// entry with mem.Read, just like tileMapEntry reads the map. A sprite at OAM
// Y=16, X=8 lands at screen (0,0): screenY = Y-16, screenX = X-8.
// Reuse tilePixel (the same unsigned 0x8000 tile addressing as the background):
idx := p.tilePixel(s.tile, px, py)
if idx != 0 { // color index 0 is transparent for sprites
    fb[sy][sx] = shade(p.obp0, idx)
}