2.5D

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2.5D, short for two-and-a-half dimensional, although technically incorrect (the concept is unrelated to modern mathematical ideas of non-integer dimension), is a term that has become popular to describe a number of different phenomena, most of which are related to computing and especially computer and video games.

Computing

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 eachother, 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 game using this style is NiGHTS Into Dreams.

First-person shooters

In early first-person shooters like Doom and its clones, 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. (In more recent first-person shooters the limitation with overhanging elements has returned with the use of 2D graphics to generate terrain with height differences depending on the pixel colors.)

Up to 1996, "Doom clone" was the most widely used description of such games. 2.5D was never widely used as a term until "full 3D" was achieved with the release of Quake that year. All past games before Quake were then retroactively labelled 2.5D.

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 similar to the first-person shooters mentioned above.

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.

See also

• Pseudo-3D

Machining

In machining the term 2.5D refers to a surface which is a projection of a plane into 3rd dimension - although the object is 3-dimensional, there are no overhanging elements possible. Objects of this type are often represented as a contour map that gives the height (thickness, depth, etc) of the object at each point.

2.5D objects are often greatly preferred for machining, as it is easy to generate g-code for them in an efficient, often close to optimal fashion, while optimal cutting tool paths for true 3-dimensional objects can be NP-complete, although many algorithms exist.

2.5D objects can be machined on a 3-axis milling machine, and do not require any the features of a higher-axis machine to produce.

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