This article may be too technical for most readers to understand. |
Parallax scrolling is a special scrolling technique in computer graphics. It gives a 2D video game a greater sense of depth and immersion by creating the illusion of a third dimension. There are three main methods of parallax scrolling used in titles for video game console systems. The first game to feature parallax scrolling was an arcade game called Moon Patrol.
The layer way
If the display system supports multiple background layers, it can scroll them by simply changing each layer's horizontal offset by a different amount. However, placing too much in front of the layer containing the objects with which the player interacts obscures the action of the game and may be distractive to some users.
The raster way
To parallax scroll within one layer (necessary on classic 8-bit systems such as Nintendo Entertainment System and the original Game Boy), the systems generally divide the layer into sub-layers and change the layer's horizontal scroll value just before the display system begins to draw each scanline. This is called a "raster effect" and is also useful for changing the system palette to provide a gradient background.
Some platforms (Super NES, Sega Genesis, Game Boy Advance, Game Boy) provide horizontal-blank interrupts for automatically setting the registers; others, such as the NES require the use of cycle-timed code. Many NES games such as the classic Super Mario Bros. use this technique to draw their status bars, and Teenage Mutant Ninja Turtles II: The Arcade Game and Vice Project Doom for NES use it to scroll background layers at different rates.
More advanced raster techniques can produce interesting effects. A system can achieve breathtaking depth of field if layers with rasters are combined; Sonic the Hedgehog 2, ActRaiser, and Street Fighter II used this effect well. If each scanline has it's own layer, the Pole Position effect is produced, which creates a pseudo-3D road (or in NBA Jam's case, a pseudo-3D ball court) on a 2D system. If the display system supports rotation and scaling in addition to scrolling, changing the rotation and scaling factors can draw a projection of a plane (F-Zero, Super Mario Kart) or can warp the image to create an extra challenge factor (Tetanus On Drugs).
The sprite way
If there is much in the way of the sprites (individually controllable moving objects drawn by hardware on top of or behind the layers) available on the display system, the programmer may want to make a pseudo-layer out of sprites. Star Force, an overhead-view vertically-scrolling shooter for NES, used this for its starfield, and Final Fight for Super NES used this technique for the layer immediately in front of the main playfield.