Lesson 44 · Rendering a scanline - Build a Game Boy Emulator

Rendering a scanline

Today you bring together the tile map, tile-data addressing, decoding, and palette lookup to render one full 160-pixel background scanline (with no scroll yet). Get one line right and a full frame is just this same routine run 144 times.

The goal

Render one background scanline of 160 pixels at scroll 0, combining the tile-map lookup, tile-data addressing, decoding, and palette.

Start here - the target

TO DO

Scenario: Drawing one line of the background

GivenSCX and SCY are 0, the tile map at 0x9800 selects tile 1 everywhere, and tile 1 is filled with color index 3

Whenbackground scanline LY=0 is rendered with BGP=0xE4

Thenthe scanline is 160 pixels wide

Andevery pixel on the line has shade 3

Background

Now the pieces converge into an actual picture. The background is a 32×32 grid of tile numbers - the tile map at 0x9800 - describing which tile sits where. A tile number then has to become tile data: in the standard unsigned mode, tile n’s bytes live at 0x8000 + n*16, with 2 bytes per row (so row r starts at + r*2). That tile-data addressing is the new idea today; the map lookup, the decode from lesson 42, and the palette from lesson 43 you already have.

To render scanline LY, walk x from 0 to 159: find the tile-map cell, read the tile number, address its data, decode the pixel, and run the index through BGP - 160 finished shades. Keep SCX/SCY at 0 for now; those scroll offsets (which slide a world larger than the screen past the window) are an easy extension once the un-scrolled line is right. Get one line right and the full frame is just this same routine run 144 times.

Make it work

func (p *PPU) renderLine(ly uint8) [160]uint8 {
    var line [160]uint8
    for x := 0; x < 160; x++ { // no scroll yet - SCX = SCY = 0
        // tile map: a 32x32 grid of tile numbers at 0x9800
        tile := p.mem.Read(0x9800 + uint16(ly/8)*32 + uint16(x/8))
        // tile DATA (unsigned mode): 16 bytes per tile, 2 bytes per row
        base := 0x8000 + uint16(tile)*16 + uint16(ly%8)*2
        row := decodeRow(p.mem.Read(base), p.mem.Read(base+1))
        line[x] = shade(p.bgp, row[x%8])
    }
    return line
}