Parallax scrolling: Difference between revisions

Revision as of 09:42, 29 June 2005

Revision as of 01:14, 10 July 2005

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There are three main methods of [[parallax]] scrolling used in titles for [[video game console]] systems. Parallax scrolling was first used in the [[1982]] [[arcade game]] [[Moon Patrol]]. Parallax scrolling, in short, simply means the background moves by slower than the foreground, creating an illusion of depth. This technique grew out of the [[multiplane camera]] technique used in [[traditional animation]] since the 1940s.

An interesting variation is to have the background and foreground move in ''opposite'' directions. This creates an effect of rotation. An early example is the scene in ''[[Snow White and the Seven Dwarfs]]'' where the evil Queen drinks her potion, and her surroundings appear to spin around her.

== The layer way ==

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Some display systems have only one layer. These include most of the classic 8-bit systems (such as the [[Nintendo Entertainment System]] and the original [[Game Boy]]). 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 Nintendo Entertainment System|Super NES]], [[Sega Genesis]], [[Game Boy Advance]], [[Game Boy]]) 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. 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 (Megadrive)|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 the case of ''[[NBA Jam]]'', a pseudo-3D ball court) on a 2D system.

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== The sprite way ==

If there is much in the way of the [[sprite (computer graphics)|sprite]]s (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 [[SNES]]Super NES used this technique for the layer immediately in front of the main playfield.