2.5D

2.5D (two-and-a-half dimensional, alternatively pseudo-3D or three-quarter) perspective refers to one of two things:
By contrast, games, spaces or perspectives not restricted in these ways are said to be "true 3D".
Common in video games, 2.5D projections have also been useful in geographic visualization (GVIS) to help understand visual-cognitive spatial representations or 3D visualization.
The terms "three-quarter perspective" and "three-quarter view" trace their origins to the three-quarter profile in portraiture and facial recognition, which depicts a person's face that is partway between a frontal view and a side view.
Computer graphics.
Axonometric and oblique projection.
In axonometric projection and oblique projection, two forms of parallel projection, the viewpoint is rotated slightly to reveal other facets of the environment than what are visible in a top-down perspective or side view, thereby producing a three-dimensional effect. 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." Lines perpendicular to the plane become points, lines parallel to the plane have true length, and lines inclined to the plane are foreshortened.
They are popular camera perspectives among 2D video games, most commonly those released for 16-bit or earlier and handheld consoles, as well as in later strategy and role-playing video games. The advantage of these perspectives is that they combine the visibility and mobility of a top-down game with the character recognizability of a side-scrolling game. Thus the player can be presented an overview of the game world in the ability to see it from above, more or less, and with additional details in artwork made possible by using an angle: Instead of showing a humanoid in top-down perspective, as a head and shoulders seen from above, the entire body can be drawn when using a slanted angle; Turning a character around would reveal how it looks from the sides, the front and the back, while the top-down perspective will display the same head and shoulders regardless.
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 problems of anti-aliasing and square pixels found on most computer monitors.
In oblique projection typically all three axes are shown without foreshortening. 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 face pointed toward the camera retains its right angles with respect to the image plane.
Two examples of oblique projection are "" and "Paperboy". Examples of axonometric projection include "SimCity 2000", and the role-playing games "Diablo" and "Baldur's Gate".
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 technique was commonly used in early 1990s video games when consoles did not have the hardware power to render fully 3D objects. This is also known as a backdrop. 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 objects such as particles (smoke, sparks, rain) and low-detail vegetation. It has since become mainstream, and is found in many games such as ', where it is exploited to simultaneously display thousands of individual soldiers on a battlefield. Early examples include early first-person shooters like "Marathon Trilogy", "Wolfenstein 3D", "Doom", ' and "Duke Nukem 3D" as well as racing games like "Carmageddon" and "Super Mario Kart".
Skyboxes and skydomes.
Skyboxes and skydomes are methods used to easily create a background to make a game level look bigger than it really is. If the level is enclosed in a cube, the sky, distant mountains, distant buildings, and other unreachable objects are rendered onto the cube's faces using a technique called cube mapping, thus creating the illusion of distant three-dimensional surroundings. A "skydome" employs the same concept but uses a sphere or hemisphere instead of a cube.
As a viewer moves through a 3D scene, it is common for the skybox or skydome to remain stationary with respect to the viewer. This technique gives the skybox the illusion of being very far away since other objects in the scene appear to move, while the skybox does not. This imitates real life, where distant objects such as clouds, stars and even mountains appear to be stationary when the viewpoint is displaced by relatively small distances. Effectively, everything in a skybox will always appear to be infinitely distant from the viewer. This consequence of skyboxes dictates that designers should be careful not to carelessly include images of discrete objects in the textures of a skybox since the viewer may be able to perceive the inconsistencies of those objects' sizes as the scene is traversed.
Scaling along the Z axis.
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. Sega's 1986 video game "Out Run", which runs on the Sega OutRun arcade system board, is a good example of this technique.
In "Out Run", 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 video 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".
Some mobile games that were released on the Java ME platform, such as the mobile version of Asphalt: Urban GT and , used this method for rendering the scenery. While the technique is similar to some of Sega's arcade games, such as Thunder Blade and Cool Riders and the 32-bit version of Road Rash, it uses polygons instead of sprite scaling for buildings and certain objects though it looks flat shaded. Later mobile games (mainly from Gameloft), such as and the mobile version of Iron Man 2, uses a mix of sprite scaling and texture mapping for some buildings and objects.
Parallax scrolling.
Parallaxing refers to when a collection of 2D sprites or layers of sprites are made to move independently of each other and/or the background to create a sense of added depth. This depth cue is created by relative motion of layers. The technique grew out of the multiplane camera technique used in traditional animation since the 1940s. This type of graphical effect was first used in the 1982 arcade game "Moon Patrol".
Examples include the skies in "Rise of the Triad", the arcade version of "Rygar", "Sonic the Hedgehog", "Street Fighter II", "Shadow of the Beast" and "", as well as "Super Mario World".
Mode 7.
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 the SNES.
Ray casting.
Ray casting is a technique in which a ray for every vertical slice of the screen is sent from the position of the camera. These rays shoot out until they hit an object or wall, and that part of the wall is rendered in that vertical screen slice. 
Early first-person shooters used ray casting as a technique to create a 3D effect from a 2D world. While the world appears 3D, the player cannot look up or down, nor actually move on the "y"-axis, because the playing field is 2D.
Bump, normal and parallax mapping.
Bump mapping, normal mapping and parallax mapping are techniques applied to textures in 3D rendering applications such as video games to simulate bumps and wrinkles on the surface of an object without using more polygons. To the end user, this means that textures such as stone walls will have more apparent depth and thus greater realism with less of an influence on the performance of the simulation.
Bump mapping is achieved by perturbing the surface normals of an object and using a grayscale image and the perturbed normal during illumination calculations. The result is an apparently bumpy surface rather than a perfectly smooth surface although the surface of the underlying object is not actually changed. Bump mapping was introduced by Blinn in 1978.
In normal mapping, the unit vector from the shading point to the light source is dotted with the unit vector normal to that surface, and the dot product is the intensity of the light on that surface. Imagine a polygonal model of a sphere—you can only approximate the shape of the surface. By using a 3-channel bitmapped image textured across the model, more detailed normal vector information can be encoded. Each channel in the bitmap corresponds to a spatial dimension ("x", "y" and "z"). These spatial dimensions are relative to a constant coordinate system for object-space normal maps, or to a smoothly varying coordinate system (based on the derivatives of position with respect to texture coordinates) in the case of tangent-space normal maps. This adds much more detail to the surface of a model, especially in conjunction with advanced lighting techniques.
Parallax mapping (also called offset mapping or virtual displacement mapping) is an enhancement of the bump mapping and normal mapping techniques implemented by displacing the texture coordinates at a point on the rendered polygon by a function of the view angle in tangent space (the angle relative to the surface normal) and the value of the height map at that point. At steeper view-angles, the texture coordinates are displaced more, giving the illusion of depth due to parallax effects as the view changes.
Film and animation techniques.
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", an adaptation of film producer Robert Evans's memoir, 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).
On a larger scale, the 2018 movie "In Saturn's Rings" used over 7.5 million separate two-dimensional images, captured in space or by telescopes, which were composited and moved using multi-plane animation techniques.
Graphic design.
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 graphic design 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.
History.
The first video games that used pseudo-3D were primarily arcade games, the earliest known examples dating back to the mid-1970s, when they began using microprocessors. In 1975, Taito released "Interceptor", an early first-person shooter and combat flight simulator that involved piloting a jet fighter, using an eight-way joystick to aim with a crosshair and shoot at enemy aircraft that move in formations of two and increase/decrease in size depending on their distance to the player. In 1976, Sega released "Moto-Cross", an early black-and-white motorbike racing video game, based on the motocross competition, that was most notable for introducing an early three-dimensional third-person perspective. Later that year, Sega-Gremlin re-branded the game as "Fonz", as a tie-in for the popular sitcom, "Happy Days". Both versions of the game displayed a constantly changing forward-scrolling road and the player's bike in a third-person perspective where objects nearer to the player are larger than those nearer to the horizon, and the aim was to steer the vehicle across the road, racing against the clock, while avoiding any on-coming motorcycles or driving off the road. That same year also saw the release of two arcade games that extended the car driving subgenre into three dimensions with a first-person perspective: Sega's "Road Race", which displayed a constantly changing forward-scrolling S-shaped road with two obstacle race cars moving along the road that the player must avoid crashing while racing against the clock, and Atari's "Night Driver", which presented a series of posts by the edge of the road though there was no view of the road or the player's car. Games using vector graphics had an advantage in creating pseudo-3D effects. 1979's "Speed Freak" recreated the perspective of "Night Driver" in greater detail.
In 1979, Nintendo debuted "Radar Scope", a shoot 'em up that introduced a three-dimensional third-person perspective to the genre, imitated years later by shooters such as Konami's "Juno First" and Activision's "Beamrider". In 1980, Atari's "Battlezone" was a breakthrough for pseudo-3D gaming, 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.
Sega's arcade shooter "Space Tactics", released in 1980, allowed players to take aim using crosshairs and shoot lasers into the screen at enemies coming towards them, creating an early 3D effect. It was followed by other arcade shooters with a first-person perspective during the early 1980s, including Taito's 1981 release "Space Seeker", and Sega's "Star Trek" in 1982. Sega's "SubRoc-3D" in 1982 also featured a first-person perspective and introduced the use of stereoscopic 3-D through a special eyepiece. Sega's "Astron Belt" in 1983 was the first laserdisc video game, using full-motion video to display the graphics from a first-person perspective. Third-person rail shooters were also released in arcades at the time, including Sega's "Tac/Scan" in 1982, Nippon's "Ambush" in 1983, Nichibutsu's "Tube Panic" in 1983, and Sega's 1982 release "", notable for its fast pseudo-3D scaling and detailed sprites.
In 1981, Sega's "Turbo" was the first racing game to feature a third-person perspective, rear view format. It was also the first racing game to use sprite scaling with full-colour graphics. "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.
"Zaxxon", a shooter introduced by Sega in 1982, was the first game to use isometric axonometric projection, from which its name is derived. Though Zaxxon's playing field is semantically 3D, the game has many constraints which classify it as 2.5D: a fixed point of view, scene composition from sprites, and movements such as bullet shots restricted to straight lines along the axes. It was also one of the first video games to display shadows. The following year, Sega released the first pseudo-3D isometric platformer, "Congo Bongo". Another early pseudo-3D platform game released that year was Konami's "Antarctic Adventure", where the player controls a penguin in a forward-scrolling third-person perspective while having to jump over pits and obstacles. It was one of the earliest pseudo-3D games available on a computer, released for the MSX in 1983. That same year, Irem's "Moon Patrol" was a side-scrolling run & gun platform-shooter that introduced the use of layered parallax scrolling to give a pseudo-3D effect. In 1985, "Space Harrier" introduced Sega's "Super Scaler" technology that allowed pseudo-3D sprite-scaling at high frame rates, with the ability to scale 32,000 sprites and fill a moving landscape with them.
The first original home console 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. In 1984, Sega ported several pseudo-3D arcade games to the Sega SG-1000 console, including a smooth conversion of the third-person pseudo-3D rail shooter "Buck Rogers: Planet of Zoom".
By 1989, 2.5D representations were surfaces drawn with depth cues and a part of graphic libraries like GINO. 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. 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. 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.
The specific term "two-and-a-half-D" was used as early as 1994 by Warren Spector in an interview in the North American premiere issue of PC Gamer magazine. At the time, the term was understood to refer specifically to first-person shooters like Wolfenstein 3D and Doom, to distinguish them from System Shock's "true" 3D engine.
With the advent of consoles and computer systems that were able to handle several thousand polygons (the most basic element of "3D computer graphics") per second and the usage of 3D specialized graphics processing units, 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. 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 several decades old.
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. 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. 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. 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.
Animators who use 2.5D animation have the ability to blend the most appealing techniques of 2D and 3D animation together to form a hybrid technique that is a game changer in the animation field today.  This amazing hybrid animation style has expanded the job market for animators like never before. 2.5D animators use this hybrid style to create computer generated 3D objects and place them into a 2D background, essentially making the object the very center of attention and creating a more engaging visual for viewers. This 2.5D style is fast becoming the go-to option for animators and businesses alike due to its cost-effective marketing appeal and attention-grabbing look (2.5D Animation). An animated artist typically works with film, T.V., websites, and marketing companies and is in charge of the development, pre-production, production and post-production of an animation project as part of a team or within a freelance position. The main goal for an animator is to infuse your own unique creative style into helping to develop main characters, draw storyboards and create cutting-edge computer-generated backgrounds that marketers, viewers, and consumers will want and ultimately buy. 
Technical aspects and generalizations.
The reason for using pseudo-3D instead of "real" 3D computer graphics is that the system that has to simulate a 3D-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 bitmaps. 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".
Marketability of 2.5D and 3D Animation.
The art of animation plays an important and vital role in the business world today and is a fast growing marketing option. Alyssa Maio with "StudioBinder" has said that, “Animation is the method of photographing successive drawings and positions of models and inanimate objects to create the illusion of movement within a sequence”. By using computer generated imagery, animators can essentially breathe life into any shape and make it mimic the real world. Animation has become an international sensation and has the power to transcend language, ethnic, and location barriers, by touching the global masses in a way that mere writing and live action cannot. The appeal of animation in the realm of digital marketing, video games, graphic design and mobile technology, has the ability to catch the attention of both adults and children alike and inspire marketers to pursue its usage and consumers to pursue their products. Businesses love the idea of using animation in their marketing because of how cost effective it is, has a larger viewer appeal and increases their brand awareness. The field of animation is continuously growing and choosing a career within animation is one that has proven to be a satisfying and lucrative profession.
Animation videos have taken the internet by storm and are growing in appeal every year for businesses who are looking to expand their audience reach. It is estimated that this year, 2021, 80% of the videos found on the internet are marketing related and experts believe that those businesses who do not chose to include animation videos as part of their marketing plan, are at risk of losing thousands of potential consumer dollars. According to recent studies by "Forrester", companies who use animation video marketing as part of their email marketing plan, they, on average, see a 200% - 300% increase rate in click-through hits and "ComScore" noted that 45.4% of internet users viewed at least one email marketing video within a months time. And based on studies performed by the "University of Dartmouth", 86% of colleges and universities have a YouTube presence online.  Businesses who incorporate Explainer videos, which highlight a business's positive attributes and gives consumers reasons to purchase their products/services, into their business websites, find them to be a simple and more effective way to reach target markets. Studies show that 81% of people tend to buy a product/service after viewing the Explainer video on the businesses website. The implementation of 2.5D animation for marketing purposes had proven to be a fairly inexpensive and more effective way for businesses of today to market their goods and services and it is only going to grow as the technology grows.
A Career in 2.5D Animation.
2.5D hybrid animation is fast becoming the most appealing style technique within animation today. Its use is appealing to animators because of how it blends the best of what 2D and 3D animation have to offer, making it the most versatile option in animation that is available. The use of 2.5D animation as a multidimensional technique is the most unique and more lucrative career choice in the field of animation. Pursuing a career in the 2.5D animation field has more advantages than disadvantages. Any career within the field of animation, requires that an artist have excellent drawing skills and a solid understanding of color, light and texture as well as an art related degree. A desirable animation employee will also have at least a bachelor’s degree in either animation, computer animation, computer graphics or another fine art related area. Employers often look for individuals who have a good portfolio and strong technical skills, as well as strong team building and time-management skills. Computer, communication and presentation skills are an absolute plus within the creative realm of animation and having a firm grasp of all the latest imagery software is crucial to land an animation job that has an entry-level pay that is in the higher range.
Companies that utilize 2.5D animators often expect long hours to be worked and animators in general are often asked to create within nail-biting deadlines which inflict those longer than normal hours of work. A seasoned animator could potentially expect to carry over into the weekend and making total weekly work hours of 50 or more. 
Currently, the median annual wage for multimedia artists and animators is $75,270. However, individual salaries can vary greatly based on a number of factors ranging from industry experience to location. Keeping current with the latest animation techniques is a great way for a would-be 2.5D animator to increase marketability in today's animation field. It is vital for an animator to stay in the know of what the latest animation software is and how to use it within their job in order to remain relevant and noteworthy. The employment of multimedia artists and animators is projected to grow by about 4% by 2029, which is an increased stability outlook that is positive for anyone looking to become a part of this growing and promising career. 