Parallax Scrolling

Overview

Parallax scrolling exploits a simple perceptual truth: distant objects appear to move more slowly than near ones. By scrolling multiple background layers at different speeds, games created convincing depth on hardware that could only display flat 2D graphics. Moon Patrol pioneered the technique in 1982; by the 16-bit era, games like Sonic the Hedgehog used it as standard.

The principle

Real-world parallax

When you look out of a moving car:

Nearby trees rush past quickly
Distant hills move slowly
The moon appears stationary

Game implementation

Layer	Scroll speed	Typical content
Foreground	1:1 with player	Platforms, obstacles
Middle	1:2	Trees, buildings
Background	1:4	Mountains, clouds
Far background	1:8 or static	Sky, horizon

Implementation approaches

Software scrolling

Redraw each layer at its speed:

update_parallax:
    scroll_layer_0(player_x)
    scroll_layer_1(player_x / 2)
    scroll_layer_2(player_x / 4)

Hardware scrolling

Some systems supported multiple scroll registers:

SNES: Modes 0/1 give 3-4 background layers, each with its own scroll registers — the standard way to do parallax on SNES. Mode 7 is a separate, single rotated/scaled background; not multi-layer in itself, but can be combined with one extra normal layer.
Mega Drive: Two scroll planes (A, B) plus a fixed Window plane for HUDs.
Amiga: Dual playfield mode renders two independent layers from the bitplanes, each with its own scroll value via BPLCON1.

Colour cycling tricks

On limited hardware, animate palette entries to simulate motion without actually scrolling data.

Platform specifics

Commodore 64

Single hardware scroll register
Software-based layer rendering
VSP (Variable Screen Position) tricks
Character-based scrolling efficient

Amiga

Dual playfield mode enables two independent layers
Copper can change scroll values mid-screen
Blitter enables fast layer compositing

NES

Single background layer (nametable)
Parallax via CHR bank switching
Sprite-based parallax for extra layers
Scanline tricks for fake layers

Mega Drive

Two background planes (A and B)
Per-line scroll values via the H-scroll table in VRAM (different scroll per scanline = arbitrary layer counts)
Window plane for HUD

Per-platform parallax tricks

Sega Master System (Sonic 1, 1991): the SMS has a single nametable. Sonic gets parallax by swapping CHR pattern table banks mid-frame using the VDP's H-counter interrupt — different tile data appears on different scanlines, faking layer separation.
Amiga (Shadow of the Beast, 1989): uses the Copper to rewrite BPLxPT (bitplane pointers) every scanline, effectively giving each scanline its own scroll position. With careful palette and bitplane assignment this produced the famous 12+ layers of parallax — far beyond what the dual-playfield hardware alone supports.
NES (Battletoads, Mega Man series): uses MMC3 scanline IRQ to reload $2005 scroll values mid-frame, giving 2-3 effective layers from a single nametable.
C64 (Mayhem in Monsterland): uses VSP ($D016 mid-line writes) plus careful character-cell design to give multi-speed scroll without redrawing data.

Performance considerations

Approach	CPU cost	Memory cost
Single layer	Low	Low
Software multi-layer	High	Medium
Hardware multi-layer	Low	High
Colour cycling	Very low	Very low

Notable examples

Game	Year	Layers
Moon Patrol	1982	3
Shadow of the Beast	1989	13
Sonic the Hedgehog	1991	4+
Castlevania: Rondo	1993	Multiple

Modern use

Parallax remains common:

2D indie games
Website design (CSS parallax)
Mobile games
Retro-styled titles

Depth through motion