![]() ![]() ![]() If each scanline has its own layer, the Pole Position effect is produced, which creates a pseudo-3D road (or a pseudo-3D ball court as in NBA Jam) on a 2D system. A system can achieve a very effective depth of field if layers with rasters are combined Sonic the Hedgehog, Sonic The Hedgehog 2, ActRaiser, Lionheartand Street Fighter II used this effect well. More advanced raster techniques can produce interesting effects. 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. 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. Some platforms (e.g., SNES, 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. This is called a "raster effect" and is also useful for changing the system palette to provide a gradient background. 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. Typically, strips higher on the screen will represent things farther away from the virtual camera or one strip will be held stationary to display status information. The more sophisticated games on such systems generally divide the layer into horizontal strips, each with a different position and rate of scrolling. These include most of the classic 8-bit systems (such as the Nintendo Entertainment System, the original Game Boy, and the TurboGrafx-16). Some display systems have only one layer. 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. In raster graphics, the lines of pixels in an image are typically composited and refreshed in top-to-bottom order with a slight delay (called the horizontal blanking interval) between drawing one line and drawing the next line. Many games used this technique for a scrolling star-field, but sometimes a more intricate or multi-directional effect is achieved, such as in the game Parallax by Sensible Software. This software effect gives the illusion of another (hardware) layer. Color cycling can be used to animate tiles quickly on the whole screen. ![]() 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. For instance Star Force, an overhead-view vertically scrolling shooter for NES, used this for its starfield, and Final Fight for the SNES used this technique for the layer immediately in front of the main playfield. Programmers may also make pseudo-layers of sprites-individually controllable moving objects drawn by hardware on top of or behind the layers-if they are available on the display system. There are three main methods to simulate parallax scrolling in home computer or video game console systems that to not support multiple background layers. Many arcade system boards, especially from Sega, Capcom, and Irem, as well as Sega's Mega Drive and Saturn consoles, support the scrolling of individual rows and/or columns of a tilemap ( ref) ( ref) This allows the hardware to either produce parallax scrolling effects from a single tilemap, or add more layers of scrolling to increase the depth of parallax scrolling. Layers can be placed in front of the playfield-the layer containing the objects with which the player interacts-for various reasons such as to provide increased dimension, obscure some of the action of the game, or distract the player. Layers that move more quickly are perceived to be closer to the virtual camera. 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. Some display systems support multiple background layers that can be scrolled independently in horizontal and vertical directions and composited on one another, simulating a multiplane camera. ![]()
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