Graphics Overview

The Game Boy outputs graphics to a 160×144 pixel LCD, using a quite complex mechanism to facilitate rendering.

Tiles

Similarly to other retro systems, pixels are not manipulated individually, as this would be expensive CPU-wise. Instead, pixels are grouped in 8×8 squares, called tiles (or sometimes “patterns” or “characters”), often considered as the base unit in Game Boy graphics.

A tile does not encode color information. Instead, a tile assigns a color ID to each of its pixels, ranging from 0 to 3. For this reason, Game Boy graphics are also called 2bpp (2 bits per pixel). When a tile is used in the Background or Window, these color IDs are associated with a palette. When a tile is used in an object, the IDs 1 to 3 are associated with a palette, but ID 0 means transparent.

Palettes

A palette consists of an array of colors, 4 in the Game Boy’s case. Palettes are stored differently in monochrome and color versions of the console.

Modifying palettes enables graphical effects such as quickly flashing some graphics (damage, invulnerability, thunderstorm, etc.), fading the screen, “palette swaps”, and more.

Layers

The Game Boy has three “layers”, from back to front: the Background, the Window, and the Objects. Some features and behaviors break this abstraction, but it works for the most part.

Background

The background is composed of a tilemap. A tilemap is a large grid of tiles. However, tiles aren’t directly written to tilemaps, they merely contain references to the tiles. This makes reusing tiles very cheap, both in CPU time and in required memory space, and it is the main mechanism that helps work around the paltry 8 KiB of video RAM.

The background can be made to scroll as a whole, writing to two hardware registers. This makes scrolling very cheap.

Window

The window is sort of a second background layer on top of the background. It is fairly limited: it has no transparency, it’s always a rectangle and only the position of the top-left pixel can be controlled.

Possible usage include a fixed status bar in an otherwise scrolling game (e.g. Super Mario Land 2).

Objects

The background layer is useful for elements scrolling as a whole, but it’s impractical for objects that need to move separately, such as the player.

The objects layer is designed to fill this gap: objects are made of 1 or 2 stacked tiles (8×8 or 8×16 pixels) and can be displayed anywhere on the screen.

To summarise:

• Tile, an 8×8-pixel chunk of graphics.
• Object, an entry in object attribute memory, composed of 1 or 2 tiles. Can be moved independently of the background.