2.5D

2.5D (two-and-a-half dimensional) is an informal term used to describe visual phenomena which are considered "between" 2D and 3D. This is often also called pseudo-3D.

The term is usually used with computer graphics, especially video games, where a computer system uses 2D computer graphics to visually simulate 3D computer graphics. One such method is where a 2D image has an added "depth" channel or Z-buffer which may act like a height map. The term is also used to describe 3D scenes built completely or partially from a composite of flat 2D images; and also where gameplay is restricted to a 2D plane while the display uses 3D graphics and 3D models.

While a 2D game usually allows movement only vertically and horizontally, a pseudo-3D game most often gives depth as a third dimension of acting. It was a big step in gaming history from abstract graphics towards realistic graphics. The passage to texture mapping is fluent. Technically all graphics displayed on a screen are 2D, which makes Pseudo-3D impossible to concisely define. For example, '3D' games such as Quake are subject to gimbal lock which imposes certain movement constraints on the player, despite the common perception of a true 3D environment.

While the term is largely restricted to computer graphics, especially video games, it has also been used to describe visual perception, especially stereoscopic vision, which may be considered 2.5D because the 3D environment of the observer is projected onto the 2D planes of the retinas, which, while effectively 2D, still allow for depth perception.

The concept is unrelated to modern mathematical ideas of non-integer dimension.

Computer graphics and image generation

2.5D imagery

2.5D modelling, techniques or scenes. These are scenes in three dimensions but where objects at different distances are represented stylistically by flat surfaces. Rather like the use of flat targets that use imagery (pictures of soldiers, tanks etc) at different distances on a weapon range. Also the flat modelling techniques for objects such as trees and houses, used in some Image Generation systems. Sometimes also called `billboards' because the object concerned is presented as if it was drawn on a 2-D billboard which always points towards the eye-point so that no visual anomalies of perspective are seen

Enhanced paint programs

2.5D is used as a descriptive and marketing term in some specialised 3D computer graphics software, such as Pixologic's Zbrush. The idea is that the program's canvas represents a normal 2D painting surface, but that the data structure that holds the pixel information is also able to store information regarding z-index (depth) as well as other information such as material settings, specularity, etc. With this data it is thus possible to simulate lighting, shadows, and so on.

Animation technique

The term is also used to describe an animation effect commonly used in music videos and, more frequently, title sequences. Brought to wide attention by the motion picture The Kid Stays in the Picture based on the book by film producer Robert Evans, it involves the layering and animating of two-dimensional pictures in three dimensional space. Earlier examples of this technique include Liz Phair's music video "Down" directed by Rodney Ascher and "A Special Tree" directed by musician Giorgio Moroder and starring actor Adam Baldwin.

Axonometric projection

The term has also been applied to games using an isometric projection (or, more recently dimetric or trimetric projections), and those which use genuine 3D graphics but whose gameplay largely takes place on a 2D plane.

First-person shooters

In early first-person shooters like Doom, one use of the term is to describe the use of 2D character and object sprites in a 3D world. This technology was also implemented with the Virtual Boy game system by Nintendo. It is also used to describe the lack of true so-called "room-over-room" situations. In other words, overhanging elements are not possible. This is because the game levels are generated from a 2D blueprint, which in fact reduces the gameplay mostly to a 2D plane, making height less important.

Image-based rendering

In a polygonal 3D world, geometry that is sufficiently distant can be seamlessly replaced with a 2D sprite for a significant performance boost. A pioneering use of this technique was in the game Jurassic Park: Trespasser; it has now become mainstream, and is found in many games such as Rome: Total War, where it is exploited to simultaneously display thousands of individual soldiers on a battlefield.

Platforming games

2.5 platformers are sidescrolling platforming games that use polygons to render the world and characters (3D graphics) but the gameplay is restricted to a 2D plane. Examples include Pandemonium, Klonoa: Door to Phantomile, Nights into Dreams..., Viewtiful Joe, Kirby 64: The Crystal Shards, New Super Mario Bros, Wario World, Yoshi's Story, Tomba!, The Simpsons Game (DS) and Sonic Rivals. The Crash Bandicoot series is sometimes referred to as 2.5D because although characters and scenery are rendered in 3D, it is not free-roaming like 'true' 3D platformers.
Some fighting games such as the the Super Smash Bros. series and Street Fighter IV also utilize 2.5D to limit the action to a 2D plane. The area of gameplay can be described as a two-dimensional surface twisting and bending in a three-dimensional world. Inside this surface, the character and physics behave like in a traditional 2D platformer. There is however a number of twists that aren't possible with normal 2D platformers: It is common in such games to let the two-dimensional plane cross itself or other planes on certain points, thus creating "track switches" in the course. Players can explore different areas of the 3D world that way or can be brought back to previous points seamlessy. Interactions with the "background" (non-accessible points in the 3D landscape) are also used extensively.

Sprites

One use of the term is to describe a style of graphics in video games with 2D gameplay, but with a limited 3D appearance, popular in the late 1980s and early 1990s. A collection of 2D sprites moves independently of each other, and/or the background, using the theory of parallaxing to create a sense of depth. A number of games also uses the closely related concept of parallax scrolling, which creates a sense of depth between the actual interactive game elements, and the background. Because of this method, the effect only works when the game character is in motion. One good example of a game using this style is F-Zero.

Virtual light source

The term also refers to the slight 3D illusion created by the virtual presence of a light source to the left (in some cases right) side, and above a computer monitor. The light source itself is always invisible but its effects are seen in the lighter colors for the top and left side, simulating reflection, and the darker colours to the right and below of such objects, simulating shadow. This technique is often used in the design of icons and entire windows in graphical user interfaces or GUIs.

Examples

Out Run's title screen

1986's Out Run is a good example of a classic pseudo-3D racing game. As one can see in the given picture, the player has to drive his Ferrari in the depth of the game window. The palms on the left and right side of the street are basically the same bitmap, but have been scaled to different sizes, creating the illusion that some are closer than others (this technique is also used in more recent games, such as Far Cry, to create the illusion of dense foliage). The angles of movement are left and right and into the depth (while still capable of doing so technically, this game didn't allow to make a U-turn or go into reverse, therefore moving out of the depth, as this did not make sense to the high-speed game play and tense time limit). Notice the view is comparable to that which a driver would have in reality when driving a car. The position and size of any billboard is generated by a (complete 3d) perspective transformation as are the vertices of the poly-line representing the center of the street. Often the center of the street is stored as a spline and sampled in a way that on straight streets every sampling point corresponds to one scan-line on the screen. Hills and curves lead to multiple points on one line and one has to be chosen. Or one line is without any point and has to be interpolated lineary from the adjacent lines. Very memory intensive billboards are used in OutRun to draw corn-fields and water waves which are wider than the screen even at the largest viewing distance and also in Test Drive to draw trees and cliffs.

Screenshot of the original Sonic game

Sonic the Hedgehog for the Sega Mega Drive (A.K.A. Sega Genesis in U.S.) uses parallax scrolling for aesthetic reasons. Parallax scrolling can be considered a form of pseudo-3D, as it uses 2D graphics that moves corresponding to the rules of three dimensional geometry.

The Street Fighter II games used parallax scrolling on the ground of each stage, for a good pseudo-3D effect.

The same effect was used in the first Real time strategy game to use pseudo-3D or 3D graphics, Stronghold (1993). The game was described as Dungeons and Dragons meets SimCity and displayed a pseudo-3D city with different structures built by humans, dwarves, elves etc. spread across a hilly terrain.

Examples of pseudo 3-D with true 3D graphics and effects but 2-D restricted gameplay are Strider 2, R-Type Delta, R-Type Final and Contra: Shattered Soldier.

In Sonic Rush, Sonic and Blaze are both 3-D while running through a side-scrolling world, like the original Sonic the Hedgehog.

History

The first computer games that used pseudo-3D were primarily arcade games. Atari's 1976 racing game Night Driver was the first driving game to use a pseudo-3D first person perspective. Games using vector graphics had an advantage in creating pseudo 3D effects. 1978's Speed Freak recreated the perspective of Night Driver in far greater detail. The following year, a major breakthrough for pseudo-3D gaming came in the form of Atari's Battlezone, recreating a 3D perspective with unprecedented realism(though the gameplay was still planar). It was followed up that same year by Red Baron, which used scaling vector images to create a forward scrolling rail shooter.

Turbo by Sega (1982), pioneered the trailing camera racing game that is now so familiar in true 3D games, and introduced the linescroll road effect, similar to the ones that would be used in racing games through the remainder of the 2D era.

The first home video game to use pseudo-3D, and also the first to use multiple camera angles mirrored on television sports broadcasts, was Intellivision World Series Baseball (1983) by Don Daglow and Eddie Dombrower, published by Mattel. Its television sports style of display was later adopted by 3D sports games and is now used by virtually all major team sports titles.

As this era of gaming opened, there was a strong need for games that added believed new game play and intensive, cool visual effects. After Pole Position, Space Harrier, After Burner II, Out Run, and Hang On are among the most popular arcade games of that time.

The first game to use pseudo-3D to create optical illusions for play may be Realm of Impossibility by Mike Edwards, which was published by EA in 1984.

With the advent of computer systems that were able to handle several thousands of polygons (the most basic element of 3D computer graphics) per second and the usage of 3D specialized graphics processing unit, pseudo 3D became obsolete. But even today, there are computer systems in production, such as cellphones, which are not powerful enough to display true 3D graphics, and therefore use pseudo-3D for that purpose. Interestingly, many games from the 1980s' pseudo-3D arcade era and 16-bit console era are ported to these systems, giving the manufactures the possibility to earn revenues from games that are now nearly twenty years old.

Technical aspects

The reason for using pseudo-3D instead of "real" 3D computer graphics is that the system that has to simulate a three dimensional looking graphic is not powerful enough to handle the calculation intensive routines of 3D computer graphics, yet is capable of using tricks of modifying 2D graphics like bitmap. One of these tricks is to stretch a bitmap more and more, therefore making it larger with each step, as to give the effect of an object coming closer and closer towards the player.

Generalization

Even simple shading and size of an image could be considered pseudo-3D, as shading makes it look more realistic. If the light in a 2D game were 2D, it would only be visible on the outline, and because outlines are often dark, they would not be very clearly visible. However, any visible shading would indicate the usage of pseudo-3D lighting and that the image uses pseudo-3D graphics. Changing the size of an image can cause the image to appear to be moving closer or further away, which could be considered simulating a third dimension.

See also

• 2.5D (machining)

External links

• Is SimCity 4 3D? - describes usage of the term in application to SimCity 4's graphics engine.ja:2.5次元

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