Parallax scrolling
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Parallax scrolling is a special scrolling technique in computer graphics, seen first in the 1982 arcade game Moon Patrol. In this pseudo-3D technique, background images move by the "camera" slower than foreground images, creating an illusion of depth in a 2D video game and adding to the immersion. The technique grew out of the multiplane camera technique used in traditional animation since the 1940s.
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[edit] Example
By moving layer 2 twice as fast as layer 1, and layer 3 three times as fast as layer 1, a suggestion of perspective is achieved.
These are the layers:
 Background layer: a sky with moon. |
 Layer 1: a chain of mountains. |
 Layer 2: background vegetation. |
 Layer 3: foreground vegetation and path. |
[edit] Methods
There are four main methods of parallax scrolling used in titles for video game console systems.
[edit] The layer method
Some display systems support multiple background layers that can be scrolled independently in horizontal and vertical directions and composited on one another. On such a display system, a game can produce parallax by simply changing each layer's position by a different amount in the same direction. Layers that move more quickly are perceived to be closer to the virtual camera. However, placing too much in front of the playfield, the layer containing the objects with which the player interacts, obscures the action of the game and may distract the player.
[edit] The sprite method
If there is much in the way of 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 the Super NES used this technique for the layer immediately in front of the main playfield.
[edit] The repeating pattern/animation method
Scrolling displays built up of individual tiles can be made to 'float' over a repeating background layer by animating the individual tiles' bitmaps in order to portray the parallax effect. This software effect gave the illusion of another (hardware) layer. Many games used this technique for a scrolling star-field, but sometimes a more intricate or multi-directional effect was achieved, such as in the game Parallax by Sensible Software.
[edit] The raster method
In raster graphics, the lines of pixels in an image are typically composited and refreshed in top-to-bottom order, and there is a slight delay, called horizontal blank, between drawing one line and drawing the next line. Games designed for older graphical chipsets, such as those of the third and fourth generations of video game consoles, those of dedicated TV games, or those of similar handheld systems, take advantage of the raster characteristics to create the illusion of more layers.
Some display systems have only one layer. These include most of the classic 8-bit systems (such as the Nintendo Entertainment System, the original Game Boy, and the PC Engine). The more sophisticated games on such systems generally divide the layer into horizontal strips, each with a different position and rate of scrolling. Typically, strips higher up the screen will represent things farther away from the virtual camera, or one strip will be held stationary to display status information. The program will then wait for horizontal blank and change the layer's scroll position 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, Mega Drive/Genesis, Game Boy Advance, Game Boy, Nintendo DS) provide a horizontal blank interrupt for automatically setting the registers independently of the rest of the program; others, such as the NES, require the use of cycle-timed code, which is specially written to take exactly as long to execute as the video chip takes to draw one scanline, or timers inside game cartridges that generate interrupts after a given number of scanlines have been drawn. Many NES games 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 (16-bit), Sonic The Hedgehog 2, ActRaiser, and Street Fighter II used this effect well. If each scanline has its own layer, the Pole Position effect is produced, which creates a pseudo-3D road (or in the case of NBA Jam, a pseudo-3D ball court) on a 2D system. If the display system supports rotation and scaling in addition to scrolling, an effect popularly known as Mode 7, changing the rotation and scaling factors can draw a projection of a plane (F-Zero, Super Mario Kart) or can warp the playfield to create an extra challenge factor (Lockjaw: The Overdose).
