Lesson 42 · Decoding a tile - Build a Game Boy Emulator

Decoding a tile

Today you decode the Game Boy's packed 2bpp tile format, combining a low and high bit-plane byte into eight pixel color indices. Getting this decoding right is the key to turning raw video RAM bytes into anything resembling a picture.

The goal

Decode one row of a tile from its two bit-plane bytes into eight pixel color indices.

Start here - the target

TO DO

Scenario: Combining two bit planes into pixels

Givena tile row whose low byte is 0x80 and high byte is 0x00

Whenthe row is decoded into eight pixels (leftmost first)

Thenpixel 0 is color index 1 and pixels 1 through 7 are index 0

Andif both bytes are 0xFF instead, every pixel is index 3

Background

Graphics are built from tiles: 8×8 pixel squares stored in video RAM. Each pixel is 2 bits, giving four possible color indices (0–3), and the encoding is the part that surprises everyone. A tile is 16 bytes - 2 per row - stored as two bit planes. For each pixel, one bit comes from the low byte and one from the high byte; combine them as high<<1 | low to get the index.

The leftmost pixel is bit 7, not bit 0, so you walk the bits from high to low. Low byte 0x80 alone lights bit 7 with index 1; both bytes 0xFF give every pixel index 3. This packed format is why copying graphics is byte-efficient and why naively reading tile memory looks like garbage. Decode one row correctly and a whole tile is just eight rows of the same.

Make it work

// Decode two bytes into 8 pixels. Two gotchas to get right:
//   - bit 7 is the LEFTMOST pixel (pixel i reads bit 7-i)
//   - each pixel's 2-bit id combines both planes: the LOW byte supplies the
//     low bit, the HIGH byte the high bit -> id = highBit<<1 | lowBit (0..3)
func decodeRow(lo, hi uint8) [8]uint8 {
    // ...loop i = 0..7, pull bit (7-i) from each byte, combine
}