Lesson 45 · The full frame - Build a Game Boy Emulator

The full frame

Today you assemble every scanline from yesterday into a full 160x144 framebuffer, producing your first complete background image at the real DMG resolution. Dump it to a PGM file and you can finally see what your emulator draws.

The goal

Assemble all 144 scanlines into a 160x144 framebuffer, then encode it as a viewable PGM image.

Start here - the target

TO DO

Scenario: Building a whole frame and saving it as an image

Giventhe tile map and tiles that made scanline 0 all shade 3

Whena full frame is rendered

Thenthe framebuffer is 160 by 144 pixels

Andthe pixel at (0,0) and the pixel at (159,143) are both shade 3

Andencoding the frame as PGM begins with the header "P5\n160 144\n255\n"

Anda shade-3 pixel becomes byte 0x00 and a shade-0 pixel becomes 0xFF (gray = 255 - shade*85)

Background

A frame is just every visible scanline stacked: loop LY from 0 to 143, render each line with yesterday’s routine, and collect them into a 160×144 framebuffer. That is the exact resolution of the real DMG screen. With this you can, for the first time, render a complete image the way the hardware does - background scroll, tiles, and palette all applied.

To actually see it, encode the buffer as a PGM grayscale image: map each shade 0–3 to a gray value and write the P5 format (a trivial header plus one byte per pixel). Open the file and there is your Game Boy background, drawn by code you wrote. Only two things stand between you and a running game now: sprites on top, and the loop that drives it all.

Make it work

func (p *PPU) RenderFrame() [144][160]uint8 {
    var fb [144][160]uint8
    for ly := 0; ly < 144; ly++ {
        fb[ly] = p.renderLine(uint8(ly))
    }
    return fb
}
// Then write a P5 (binary) PGM: the header "P5\n160 144\n255\n", followed by
// one gray byte per pixel. Map shades 0..3 (lightest..darkest) with 255 - shade*85.
func EncodePGM(fb [144][160]uint8) []byte { /* header + one byte per pixel */ }