2.5D

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Part of a series on:
Video Game Graphics
Types 2.5D & 3/4 perspective First person (video games) Fixed 3D Full motion video based game Graphical adventure game Graphical video game Side-scrolling video game Text-based game Third-person view Tile-based video game Top-down perspective Vector game Video games with isometric graphics
Topics Computer graphics 2D computer graphics 3D computer graphics 3D rendering Animation Cel-shaded animation Digitizing Rotoscoping Graphics engine First-person shooter engine Tile engine Parallax scrolling Pixel art Skybox Sprite Tile set Virtual camera system Voxel
Lists List of FMV-based games List of stereoscopic video games List of text-based computer games
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2.5D ("two-and-a-half-dimensional"), also called pseudo-3D, is an informal term used to describe either:

• graphical projections and techniques which cause a series of images or scenes to fake or appear to be three-dimensional (3D) when in fact they are not, or
• gameplay in an otherwise 3D game that is restricted to a two-dimensional plane.

In the former case the term is sometimes used interchangeably with "3/4 perspective", in which case it describes games using some form of graphical projection (usually parallel projection) taken from a fixed perspective that reveals multiple facets of an object and offers a wide field of view.

These projections have also been useful in geographic visualization (GVIS) to help understand visual-cognitive spatial representations or 3D visualization.^[1]

[edit] Pseudo 3D graphics

[edit] 3/4 perspective & parallel projection

Lincity tiles 2D axonometric graphical elements to create a 2.5D game environment.

In portraiture and facial recognition, the terms 3/4 perspective or 3/4 view are used to describe a graphical projection which is halfway between a frontal view and a side view.^[2]

In video games however the term refers to any point of reference that is rotated slightly to reveal other facets of the game environment than what is visible in a top-down perspective or side view, sometimes intentionally producing a three-dimensional effect. It is a popular camera perspective in 2D video games, most commonly those released for 16-bit or earlier and handheld consoles, as well as later strategy and role-playing video games. The advantage of this perspective is that it combines the visibility and mobility of a top-down game with the character recognizability of a side-scrolling game.^{[clarification needed]} Both axonometric and oblique video games are often drawn in 3/4 perspective.

In axonometric projection, a form of parallel projection, an object is "considered to be in an inclined position resulting in foreshortening of all three axes", and the image is a "representation on a single plane (as a drawing surface) of a three-dimensional object placed at an angle to the plane of projection."^[who?] Lines perpendicular to the plane become points, lines parallel to the plane have true length, and lines inclined to the plane are foreshortened.

There are three main divisions of axonometric projection: isometric (equal measure), dimetric (symmetrical and unsymmetrical), and trimetric (single-view or only two sides). The most common of these drawing types in engineering drawing is isometric projection. This projection is tilted so that all three axes create equal angles at intervals of 120 degrees. The result is that all three axes are equally foreshortened. In video games, a form of dimetric projection with a 2:1 pixel ratio is more common due to the problem of anti-aliasing and square pixels found on most computer monitors.

In oblique projection typically all three axes are shown unforeshortened. All lines parallel to the axes are drawn to scale, and diagonals and curved lines are distorted. One tell-tale sign of oblique projection is that the image or sprite still fits inside a square tile.

Two of the most consistent examples of 3/4 perspective and oblique projection are the The Legend of Zelda series of games for the Game Boy, Game Boy Color, and Game Boy Advance; and the Pokémon series for Game Boy Color, Game Boy Advance, and Nintendo DS. Examples of axonometric projection include the latter games in the SimCity series, and role-playing games such as Diablo and Baldur's Gate.

[edit] Billboarding

In three-dimensional scenes, the term billboarding is applied to a technique in which objects are sometimes represented by two-dimensional images applied to a single polygon which is typically kept perpendicular to the line of sight. The name refers to the fact that objects are seen as if drawn on a billboard. This can be used to good effect for a significant performance boost when the geometry is sufficiently distant that it can be seamlessly replaced with a 2D sprite. In games, this technique is most frequently applied to object such as particles (smoke, sparks, rain) and low-detail vegetation.

[edit] Skybox/skydome

A skybox is a method used to easily create a background to make a game level look bigger than it really is. The level is enclosed in a cube; the sky, distant mountains, distant buildings, and other unreachable objects are rendered to the cube's faces, thus creating the illusion of distant three-dimensional surroundings. A skydome employs the same concept but uses either a sphere or a hemisphere instead of a cube.

[edit] Virtual light source

The term also refers to an often-used effect in the design of icons and graphical user interfaces (GUIs), where a slight 3D illusion is created by the presence of a virtual light source to the left (or in some cases right) side, and above a person's 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.

An advanced version of this technique can be found 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 with respect to a z-index, as well material settings, specularity, etc. Again, with this data it is thus possible to simulate lighting, shadows, and so forth.

[edit] Scaling along the Z axis

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In some games, sprites are scaled larger or smaller depending on its distance to the player, producing the illusion of motion along the Z (forward) axis.

The 1986 video game Out Run is a good example of this technique, though the earlier Pole Position is contender for the first. In the game, the player drives a Ferrari into depth of the game window. The palms on the left and right side of the street are the same bitmap, but have been scaled to different sizes, creating the illusion that some are closer than others. The angles of movement are left and right and into the depth (while still capable of doing so technically, this game did not allow making a U-turn or going 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 Out Run 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.

Drakkhen was notable for being among the first role-playing games to feature a three-dimensional playing field. However, it did not employ a conventional 3D game engine, instead emulating one using character-scaling algorithms. The player's party travels overland on a flat terrain made up of vectors, on which 2D objects are zoomed. Drakkhen features an animated day-night cycle, and the ability to wander freely about the game world, both rarities for a game of its era. This type of engine was later used in the game Eternam.

[edit] Parallax scrolling

An example of parallax scrolling.

Parallaxing refers to when a collection of 2D sprites are made to move independently of each other and/or the background to create a sense of added depth. This type of graphical effect was first used in the 1982 arcade game Moon Patrol.^[3] Mode 7, a display system effect that included rotation and scaling, allowed for a 3D effect while moving in any direction without any actual 3D models, and was used to simulate 3D graphics on systems such as the SNES. Examples include Sonic the Hedgehog, Street Fighter II and Dracula X Chronicles.

[edit] Bump mapping

This section requires expansion.

[edit] Parallax mapping

This section requires expansion.

[edit] Film and 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.

[edit] 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.

Pole Position by Namco is one of the first racing games to use the trailing camera effect that is now so familiar. In this particular example, the effect was produced by linescroll—the practice of scrolling each line independently in order to warp an image. In this case, the warping would simulate curves and steering. To make the road appear to move towards the player, per-line color changes were used, though many console versions opted for palette animation instead.

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.

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 often 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.

By 1989, 2.5D representations were surfaces drawn with depth cues and apart of graphic libraries like GINO.^[4] 2.5D was also used in terrain modeling with software packages such as ISM from Dynamic Graphics, GEOPAK from Uniras and the Intergraph DTM system.^[4] 2.5D surface techniques gained popularity within the geography community because of its ability to visualize the normal thickness to area ratio used in many geographic models; this ratio was very small and reflected the thinness of the object in relation to its width, which made it the object realistic in a specific plane.^[4] These representations were axiomatic in that the entire subsurface domain was not used or the entire domain could not be reconstructed; therefore, it used only a surface and a surface is one aspect not the full 3D identity.^[4]

The resurgence of 2.5D or visual analysis, in natural and earth science, has increased the role of computer systems in the creation of spatial information in mapping.^[1] GVIS has made real the search for unknowns, real-time interaction with spatial data, and control over map display and has paid particular attention to three-dimensional representations.^[1] Efforts in GVIS have attempted to expand higher dimensions and make them more visible; most efforts have focused on "tricking" vision into seeing three dimensions in a 2D plane.^[1] Much like 2.5D displays where the surface of a three dimensional object is represented but locations within the solid are distorted or not accessible.^[1]

[edit] Technical aspects & generalizations

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.

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.

Dimensions are the variables of the data and can be mapped to specific locations in space; 2D data can be given 3D volume by adding a value to the x, y, or z plane. "Assigning height to 2D regions of a topographic map" associating every 2D location with a height/elevation value creates a 2.5D projection; this is not considered a "true 3D representation", however is used like 3D visual representation to "simplify visual processing of imagery and the resulting spatial cognition".

[edit] 3D games with a two-dimensional playing field

The term "2.5D" is also applied to 3D games that use polygonal graphics to render the world and/or characters, but whose gameplay is restricted to a 2D plane. Examples include Pandemonium, Klonoa: Door to Phantomile, Nights into Dreams..., Viewtiful Joe, Strider 2, Kirby 64: The Crystal Shards, New Super Mario Bros., Wario World, Duke Nukem: Manhattan Project, Yoshi's Story, Tomba!, The Simpsons Game (DS), Sonic Rivals, & New Super Mario Bros. Wii. The Crash Bandicoot series is sometimes referred to as 2.5D because although the characters and scenery are rendered in 3D, it is not as free-roaming like 'true' 3D platformers. Other examples include R-Type Delta, Sonic Rush, Contra: Shattered Soldier and Castlevania: The Dracula X Chronicles.

Some fighting games such as the Super Smash Bros. series, Marvel Vs. Capcom 2, Street Fighter IV, and BlazBlue also utilize 2.5D to showcase 3D backdrops and/or characters while limiting the action to a 2D plane.

In some games, the area of gameplay can be described as a two-dimensional surface twisting and bending in a three-dimensional space. Inside this surface, the character and physics behave like in a traditional 2D platformer. There are 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.

[edit] See also

• Bas-relief
• Trompe-l'œil

[edit] External links

• Lou's Pseudo 3d Page - a study of 2.5D racing game graphics engines
• Is SimCity 4 3D? - describes usage of the term in application to SimCity 4's graphics engine.

[edit] References

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