Lesson 57 · Sprite attributes - Build a Game Boy Emulator

Sprite attributes

Lesson 46 composited sprites but ignored each one's attribute byte, the fourth OAM byte that flips the sprite and chooses its palette. Games flip a sprite to make a character face the other way and swap its palette to recolor it, all without new tile data. Today you honor that byte, and draw the composited sprites into the finished frame.

The goal

Honor the OAM attribute byte — X/Y flip and OBP0/OBP1 palette select — and composite the sprites into the full frame.

Start here - the target

TO DO

Scenario: The attribute byte flips a sprite and picks its palette

Givensprites are composited into the frame, OBP0 (0xFF48) is 0xE4 and OBP1 (0xFF49) is 0x1B, and a sprite at screen (0,0) whose top tile row is pixel 0 = index 3 and pixels 1–7 = index 0

Whenthe sprite's attribute byte is 0x20 (X-flip, bit 5)

Thenthe sprite is mirrored — screen (7,0) is shade 3 and screen (0,0) shows the background (shade 0)

Andwith attribute byte 0x10 instead (OBP1 select, bit 4, no flip), the index-3 pixel at (0,0) maps through OBP1 = 0x1B to shade 0

Andwith no on-screen sprites the frame is byte-identical to the background-only frame

Background

Lesson 46 drew sprites over the background but read only three of an OAM entry’s four bytes. The fourth, the attribute byte, is how one tile does many jobs. Bit 5 mirrors the sprite left-to-right and bit 6 top-to-bottom, so a character walking left is the walking-right tile flipped, no extra art. Bit 4 picks between two sprite palettes, OBP0 (0xFF48) and OBP1 (0xFF49), so the same tile can be two colors.

Applying a flip is just sampling the tile at the mirrored coordinate: for X-flip read column 7-px, for Y-flip row 7-py. Palette select is a choice of which register to look up. While you are here, make the full-frame render actually put the sprites on top of the background, so everything you have built for sprites finally appears in the frame the emulator hands back, not just in a helper nobody calls. (One more attribute bit, priority, decides when a sprite hides behind the background; that one is left for the scope notes.)

Make it work

// The attribute byte is OAM byte 3 of the entry. Bits that matter today:
//   bit 4 = palette   (0 → OBP0 0xFF48, 1 → OBP1 0xFF49)
//   bit 5 = X flip     (read tile column 7-px instead of px)
//   bit 6 = Y flip     (read tile row 7-py instead of py)
// When sampling the sprite's tile pixel, apply the flips to px/py; map a
// non-zero index through the SELECTED palette, read LIVE from its register
// (0xFF48/0xFF49) exactly as lesson 56 reads BGP. Also make the full-frame render
// actually composite the sprites (draw background, then sprites on top), so all
// of this shows up in the frame the emulator returns.