Sprite (computer graphics)
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In computer graphics, a sprite (also known by other names; see Synonyms below) is a two-dimensional image or animation (2D computer graphics) that is integrated into a larger scene. Initially including just graphical objects handled separately from the memory bitmap of a video display, this now includes various manners of graphical overlays.
Originally, sprites were a method of integrating unrelated bitmaps so that they appeared to be part of the normal bitmap on a screen, such as creating an animated character that can be moved on a screen without altering the data defining the overall screen. Such sprites can be created by either electronic circuitry or software. In circuitry, a hardware sprite is a hardware construct that employs custom DMA channels to integrate visual elements with the main screen in that it super-imposes two discrete video sources. Software can simulate this through specialized rendering methods.
More recently, CSS sprites are used in web design as a way to improve performance by combining numerous small images or icons into a larger image called a sprite sheet or tile set, and selecting which icon to show on the rendered page using Cascading Style Sheets.
As three-dimensional graphics became more prevalent, sprites came to include flat images seamlessly integrated into complicated three-dimensional scenes.
Speed Race, which ran on Taito Discrete Logic hardware in 1974, featured sprites with collision detection moving along a vertical scrolling playfield. The Fujitsu MB14241 was used to accelerate the drawing of sprite graphics for various 1970s arcade games from Taito and Midway, such as Gun Fight (1975), Sea Wolf (1976) and Space Invaders (1978). It could display up to 11 sprites per scanline. In 1976, Signetics devised a video/graphics processor capable of generating sprite graphics. The Signetics 2636 video processor was first used in the Radofin 1292 Advanced Programmable Video System, released in 1976.
In 1977, the Taito Z80 arcade system board used for Super Speed Race was capable of generating up to 16 sprites in hardware and display 4 sprites per scanline. Each sprite could have a size up to 32 pixels in width or height and display up to 15 colors.
The Atari VCS, released in 1977, features a hardware sprite implementation wherein five graphical objects can be moved independently of the game playfield. The VCS's sprites, called players and missiles, are constructed from a single row of pixels that displayed on a scan line; to produce a two-dimensional shape, the sprite's single-row bitmap is altered by software from one scanline to the next. The Atari 400 and 800 home computers of 1979 feature similar, but more elaborate circuitry, capable of moving eight Player/Missile objects per scanline - four 8-bit wide players, and four 2-bit wide missiles. This more advanced version allows operation like the VCS where the CPU modifies the graphics pattern register for each scan line, or an automatic mode where the display chip performs DMA from a table in memory populating the graphics pattern registers for each scan line. In the automatic DMA mode vertical motion is simulated by moving the sprite's up and down incrementally in memory. The hardware produces a two-dimensional bitmap several pixels wide, and as tall as the screen. The width of pixels can also vary from 1, 2, or 4 color clocks. Multiple Player objects can be merged to produce a multi-color player. The four missile objects can be grouped together as a fifth Player and colored independently from the Players.
In 1979, the Namco Galaxian arcade system used specialized graphics hardware supporting RGB color, multi-colored sprites and tilemap backgrounds. It could display up to 64 hardware sprites on screen and 16 per scanline. Each sprite could be up to 16x16 pixels in size, with 3 colors each. The Galaxian hardware was widely used during the golden age of arcade video games, by game companies such as Namco, Centuri, Gremlin, Irem, Konami, Midway, Nichibutsu, Sega and Taito. The Elektor TV Games Computer in 1979 was an early microcomputer capable of generating sprite graphics, which Signetics referred to as "objects".
The term sprite was first used in the graphic sense by one of the definers of the Texas Instruments 9918(A) video display processor (VDP), released in 1981. The term was derived from the fact that sprites, rather than being part of the bitmap data in the framebuffer, instead "floated" around on top without affecting the data in the framebuffer below, much like a ghost or "sprite". By this time, sprites had advanced to the point where complete two-dimensional shapes could be moved around the screen horizontally and vertically with minimal software overhead.
The CPU would instruct the external chips to fetch source images and integrate them into the main screen using direct memory access channels. Calling up external hardware, instead of using the processor alone, greatly improved graphics performance. Because the processor was not occupied by the simple task of transferring data from one place to another, software could run faster; and because the hardware provided certain innate abilities, programs were also smaller.
Many early graphics chips had true spriting use capabilities in which the sprite images were integrated into the screen, often with priority control with respect to the background graphics, at the time the video signal was being generated by the graphics chip.
These contrasted with software and blitter methods of 2D animation which modify a framebuffer held in RAM, which required more memory cycles to load and store the pixels, sometimes with an additional mask, and refresh backgrounds behind moving objects. These methods frequently required double buffering to avoid flickering and tearing, but placed fewer restrictions on the size and number of moving objects.
The sprite engine is a hardware implementation of scanline rendering. For each scanline the appropriate scanlines of the sprites are first copied (the number of pixels is limited by the memory bandwidth and the length of the horizontal retrace) into very fast, small, multiple (limiting the number of sprites on a line), and costly caches (the size of which limit the horizontal width) and as the pixels are sent to the screen, these caches are combined with each other and the background. It may be larger than the screen and is usually tiled, where the tile map is cached, but the tile set is not. For every pixel, every sprite unit signals its presence onto its line on a bus, so every other unit can notice a collision with it. Some sprite engines can automatically reload their "sprite units" from a display list. The sprite engine has synergy with the palette. To save registers, the height of the sprite, the location of the texture, and the zoom factors are often limited. On systems where the word size is the same as the texel there is no penalty for doing unaligned reads needed for rotation. This leads to the limitations of the known implementations:
|System(s), chip(s)||Year||Sprites on screen||Sprites on line||Max. texels on line||Texture width||Texture height||Colors||Hardware zoom||Rotation||Background||Collision detection||Transparency||Source(s)|
(Midway 8080, Taito 8080)
|1975||60||9||144||4 to 32||1 to 224||1||1, 2× vertical, 1, 2× horizontal||Screen orientation||1 bitmap layer||Yes||Color key|||
|Fairchild Channel F, F8||1976||Display list (run by CPU)||9||56||4, 8||5, 8||1||No||No||1 bitmap layer||No||Color key|||
(1292, VC 4000, Arcadia)
|1976||4||4||32||8||8||1||No||No||1 bitmap layer||No||Color key|||
|Atari 2600, TIA||1977||9 (5 multiplied by CPU)||9 (with triplication)||51 (with triplication)||1, 8||192||1||1, 2, 4, 8× horizontal||Horizontal mirroring||1 bitmap layer||Yes||Color key|||
|1977||7||7||224||32||16||15||No||Screen orientation||1 tile or bitmap layer||Yes||Color key|||
|ANTIC & CTIA/GTIA
(Atari 8-bit, 5200, XEGS)
|1979||Display list||8||40||2, 8||128, 256||1, 3||1, 2× vertical, 1, 2, 4× horizontal||No||1 bitmap layer||Yes||Color key|||
|1979||Display list||15||240||8, 16||8, 16||3||1, 2, 3× vertical, 1, 2, 3× horizontal||Horizontal and vertical mirroring||1 tile layer and 1 bitmap layer||Yes||Color key|||
(TI-99/4, ColecoVision, MSX, SG‑1000)
|1979||32||4||64||8, 16||8, 16||1||1, 2× integer||No||1 tile layer||Partial||Color key|||
|Namco Pac‑Man, NVC293||1980||8||8||128||16||16||3||No||Horizontal and vertical mirroring||2 tile layers||Yes||Color key|||
Raster Display Controller
|1981||64||32||256||8, 16||8, 16||3||Vector scaling||Screen orientation||2 tile layers||Yes||Color key|||
|Sega VCO Object,
|1981||64||16||315||8 to 20||8 to 20||3, 7||Yes, analog scaling||Background tile mirroring||1 tile layer and 1 bitmap layer||Yes||Color key|||
|C64, VIC-II||1982||Display list (run by CPU)||8||96, 192||12, 24||21||1, 3||1, 2× integer||No||1 tile or bitmap layer||Yes||Color key|||
|Namco Pole Position,
04xx & 09xx
|1982||64||32||512||16, 32||16, 32||15||Yes, anisotropic||Horizontal and vertical mirroring||1 tile layer and 1 bitmap layer||Yes||Color key|||
|Sega 315‑5011 & 315‑5012
(Sega Z80, System 1/2/16, Space Harrier)
|1982||128||100||800||8 to 256||8 to 256||15||Yes, anisotropic||Horizontal and vertical mirroring||2 tile layers and 1 bitmap layer||Yes||Alpha|||
|NES, RP2C0x||1983||64||8||64||8||8, 16||3||No||Horizontal and vertical mirroring||1 tile layer||Partial||Color key|||
|Atari 7800, MARIA||1984||100 (without background)||30 (without background)||160||4 to 160||4, 8, 16||1, 3, 8||No||No||1 bitmap layer||No||Color key|||
|Amiga (OCS), Denise||1985||Display list||8||128||16||Up to 256||3, 15||Vertical by display list||No||2 bitmap layers||Yes||Color key|||
(Master System, Game Gear)
|1985||64||8||128||8, 16||8, 16||15||1, 2× integer, 1, 2× vertical||Background tile mirroring||1 tile layer||Yes||Color key|||
(MSX2, Geneve 9640)
|1985||32||8||128||8, 16||8, 16||1, 3, 7, 15 per line||1, 2× integer||Horizontal and vertical mirroring||1 tile or bitmap layer||Yes||Color key|||
|1986||128||128||1600||8 to 512||8 to 256||15||Yes, anisotropic||Horizontal and vertical mirroring||2 tile layers and 1 bitmap layer||Yes||Alpha|||
|PC Engine, HuC6270A||1987||64||16||256||16, 32||16, 32, 64||15||No||Horizontal and vertical mirroring||1 tile layer||Yes||Color key|||
|1987||512 (128 multiplied by raster interrupt)||32||512||16||16||15||1, 2× integer||Horizontal and vertical mirroring||1-2 tile layers and 1-4 bitmap layers||Partial||Color key|||
|Sega X Board,
315-5211A & 315‑5278
|1987||256||256||3200||8 to 512||8 to 256||15||Yes, anisotropic||Yes, affine||4 tile layers and 1 bitmap layer||Yes||Alpha|||
CPS-A & CPS‑B
|1988||256||65||1048||16 to 256||16 to 256||15||No||Horizontal and vertical mirroring||3 tile layers and 2 bitmap layers||Yes||Color key|||
|Sega System 24,
315-5293 & 315‑5295
|1988||2048||512||4096||8 to 1024||8 to 1024||15 to 255||Yes, anisotropic||Horizontal and vertical mirroring||4 tile layers||Yes||Alpha|||
|Sega Y Board,
315-5196 & 315‑5305
|1988||2176||400||3200||8 to 512||8 to 512||15 to 511||Yes, anisotropic||Yes, affine||1 bitmap layer||Yes||Alpha|||
|Taito B System, TC0180VCU||1988||408||110||1768||16 to 256||16 to 256||15 to 63||Sprite shrinking||Horizontal and vertical mirroring||2 tile layers and 1 bitmap layer||Yes||Color key|||
(MSX2+, TurboR, TIM)
|1988||32||8||128||8, 16||8, 16||1, 3, 7, 15 per line||1, 2× integer||Horizontal and vertical mirroring||1 tile layer and 1 bitmap layer||Yes||Color key|||
(Mega Drive, Mega‑Tech/Play, System C)
|1988||80||20||320||8, 16, 24, 32||8, 16, 24, 32||15||1, 2× integer||Horizontal and vertical mirroring||2 tile layers||Yes||Color key|||
|Fujitsu FM Towns,
FM Towns Marty
|1989||1024||64||1024||16||16||15||1, 2× vertical, 1, 2, 3, 4, 5× horizontal||Horizontal and vertical mirroring||1-2 bitmap layers||Partial||Color key|||
|Game Boy||1989||40||10||80||8||8, 16||3||No||No||1 tile layer||No||Color key|||
|SuperGrafx, HuC6270A||1989||128||32||512||16, 32||16, 32, 64||15||No||Horizontal and vertical mirroring||2 tile layers||Yes||Color key|
|Amstrad Plus, ASIC||1990||Display list (run by CPU)||16||256||16||16||15||1, 2, 4× vertical, 1, 2, 4× horizontal||No||1 bitmap layer||No||Color key|||
|1990||384||96||1536||16||16 to 512||15||Sprite shrinking||Horizontal and vertical mirroring||1 tile layer||Partial||Color key|||
|Sega System 32,
|1990||8192||512||4096||8 to 1024||8 to 1024||15 to 512||Yes, anisotropic||Yes, affine||4 tile layers and 1 bitmap layer||Yes||Alpha|||
|SNES, PPU||1990||128||34||272||8, 16, 32, 64||8, 16, 32, 64||15||Background, affine||Background affine rotation and sprite mirroring||1-4 tile layers or 1 affine mapped tile layer||Yes||Color key, averaging|||
|Amiga (AGA), Lisa||1992||Display list||8||448||16, 32, 64||Up to 256||3, 15||Vertical by display list||No||2 bitmap layers||Yes||Color key|||
|Atari Jaguar, Tom||1993||1000||90||720||8 to 360||Up to 220||Up to 255||Yes, anisotropic||Yes, affine||1 bitmap layer||Partial||Color key|||
CPS-A & CPS‑B
|1993||900||65||1048||16 to 256||16 to 256||15||No||Horizontal and vertical mirroring||3 tile layers and 2 bitmap layers||Yes||Alpha|||
|1994||16,384||512||4096||8 to 504||1 to 255||15 to 32,768||Yes, anisotropic||Yes, affine||3-6 tile layers and 1-4 bitmap layers||Yes||Alpha|||
(PSX, System 11/10/12)
|1994||4000||128||1024||8, 16, 256||8, 16, 256||15, 255||Yes, anisotropic||Yes, affine||1 bitmap layer||Partial||Alpha|||
|Game Boy Advance||2001||128||128||1210||8, 16, 32, 64||8, 16, 32, 64||15, 255||Yes, affine||Yes, affine||2-4 layers or 1-2 affine layers||No||Color key, blending|||
|Nintendo DS||2004||256 (128 per screen)||128||1210||8, 16, 32, 64||8, 16, 32, 64||15 to 32,768||Yes affine||Yes, affine||8 layers (4 per screen); each layer is independent||No||Color key, blending|||
|Gameduino||2011||256||96||1536||16||16||255||No||Yes||1 tile layer||Yes||Color key|||
|Computer, chip||Year||Sprites on screen||Sprites on line||Max. texels on line||Texture width||Texture height||Colors||Hardware zoom||Rotation||Background||Collision detection||Transparency||Source|
Many third party graphics cards offered sprite capabilities. Sprite engines often scale badly, starting to flicker as the number of sprites increases above the number of sprite units, or uses more and more silicon as the designer of the chip implements more units and bigger caches.
Sprites by software
|This section requires expansion. (February 2012)|
Many popular home computers of the 1980s lack any support for sprites by hardware. The animated characters, bullets, pointing cursors, etc. for videogames (mainly) were rendered exclusively with the CPU by software, as part of the screen video memory in itself. Hence the term software sprites.
Mainly, two distinct techniques were used to render the sprites by software, depending on the display hardware characteristics:
- Binary image masks, mainly for systems with bitmapped video frame buffers. It employs the use of an additional binary mask for every sprite displayed to create transparent areas within a sprite.
- Transparent color, mainly for systems with indexed color displays. This method defines a particular color index (typically index '0' or index '255') with a palletted display mode as a 'transparent color' which the blitter ignores when blitting the sprite to video memory or the screen.
Sprites by CSS
|This section requires expansion. (February 2012)|
To reduce the number of requests the browser makes to the server, some web designers combine numerous small images or icons into a larger image called a sprite sheet or tile set. CSS is used to select the parts of the composite image to display at different points in the page. If a page has ten 1 kB images, they can be combined into one 10 kB image, downloaded with a single HTTP request, and then positioned with CSS. Reducing the number of HTTP requests can make a Web page load much faster. In this usage, the sprite sheet format that had been developed for use in game and animation engines is being applied to static images.
Some advantages of using CSS sprites over using separate images include:
- Performance - Fewer HTTP requests means less overhead.
- Automatic preloading - In situations where you have an image which isn't displayed until some user interaction happens (e.g. a hover state), you will often see a flicker as the browser loads the new image. It is possible to preload such images, but using CSS sprites ensures that this happens automatically.
Some disadvantages are:
- Perceived load speed - Even though CSS sprites can often be used to increase a page's overall load speed, this can come at the cost of perceived speed. The browser needs to load the entire sprite sheet before it can display any of the images within that sheet. For large pages, it may be preferable to load the images individually so that the top of the page can finish rendering before the entire page has loaded.
- Development workflow - It can be inconvenient for a development workflow to be required to regenerate a sprite sheet whenever you add an image to your project. Depending on the method used to generate and manage the sprite sheet, this can range from a minor annoyance to a significant waste of productivity.
Return of sprites in casual games and mobile devices
With mobile devices and casual gaming becoming more and more popular the classic 2D games return. The modern devices lack support for hardware sprites but come with powerful 3D hardware. On these devices sprites are simulated using textures on rectangular shapes. Perspective is disabled for these games. Since the hardware often comes with constraints - e.g. that it can only use power-of-two sized textures (that is a width or height of 64, 128, 256, ...) sprite sheets are used to reduce memory consumption. This is done by packing many sprites into one texture which, as a whole, has to meet the hardware constraints. Apart from the memory usage this technique can also be used to reduce the number of draw calls to the graphics subsystem and speed up rendering.
There are several programs that still use 2D sprites in their construction. RPG Maker VX and Wolf RPG Maker are just a couple of examples. As well as the programs that can create said indie games, there are several mainstream games as well. These famous game series like Fire Emblem, Pokémon, and other handheld system games tend to use these more for the same reasons that mobile devices such as tablets and cell phones do. It saves on memory so it allows for more content, effects, music, and story to be added into the chip or cartridge. As such, for first time indie game developers, sprites offer an easy but effective way to introduce themselves into game design as a whole.
Move to 3D
Prior to the popularizing of true 3D graphics in the mid-late 1990s, many 2D games attempted to imitate the look of three-dimensionality with a variety of sprite production methods. These included:
- Rotoscoping: The filmed performances of live actors were sometimes used for creating sprites, most famously in the case of Prince of Persia which added a relative element of realism to a platform game. The method was used in a number of other fighting games, mostly in the mid 1990s.
- Claymation or the use of posable models which were used for characters that could not be portrayed by actors. Famous early examples include Goro of Mortal Kombat and various enemies from Doom. Used to a greater extent in games like Clay Fighter and Neverhood.
- Pre-rendered CGI models: Introduced by Rise of the Robots and made famous by Donkey Kong Country, and later used to a great extent in PC real-time strategy and role-playing video game games prior to the move to real-time 3D. Since computers of the day could not run complex 3D graphics, footage of pre-rendered three-dimensional character models were often used which created a (relative) illusion of 3D.
More often sprite now refers to a partially transparent two dimensional animation that is mapped onto a special plane in a 3D scene. Unlike a texture map, the sprite plane is always perpendicular to the axis emanating from the camera. The image can be scaled to simulate perspective, rotated two dimensionally, overlapped with other objects, and be occluded, but it can only be viewed from a single angle.
Sprites create an effective illusion when
- the image inside the sprite already depicts a three dimensional object;
- the animation is constantly changing or depicts rotation;
- the sprite exists only shortly;
- the depicted object has a similar appearance from many common viewing angles (such as something spherical);
- the perspective of the object from the viewer cannot possibly change fast enough for the viewer to discern a difference from true 3D geometry, as in the case of object a long distance away from the viewer in 3D space.
- the viewer accepts that the depicted object only has one perspective (such as small plants or leaves).
When the illusion works, viewers will not notice that the sprite is flat and always faces them. Often sprites are used to depict phenomena such as fire, smoke, small objects, small plants (like blades of grass), or special symbols (like "1-Up"), or object of any size where the angle of view does not appreciably change with respect to the rectilinear projection of the object (usually from a long distance). The sprite illusion can be exposed in video games by quickly changing the position of the camera while keeping the sprite in the center of the view. Sprites are also used extensively in particle effects and commonly represented pickups in early 3D games especially.
An example of extensive usage of sprites to create the illusion is the game The Elder Scrolls IV: Oblivion, whose main graphical feature was the ability to display hundreds, if not thousands of animated trees on-screen at one time. Closer inspection of those trees reveals that the leaves are in fact sprites, and rotate along with the position of the user. The tree rendering package used by Oblivion uses sprites to create the appearance of a high number of leaves. However, this fact is only revealed when the player actually examines the trees up-close, and rotates the camera.
Sprites have also occasionally been used as a special-effects tool in movies. One such example is the fire breathing Balrog in The Lord of the Rings: The Fellowship of the Ring; the effects designers utilized sprites to simulate fire emanating from the surface of the demon. Small bursts of fire were filmed in front of a black background and made transparent using a luma key. Many bursts were then attached to the surface of the animated Balrog model and mixed with simulated smoke and heat waves to create the illusion of a monster made from fire.
The term sprite is often confused with low resolution 2D graphics drawn on a computer, also known as pixel art. However, sprite graphics (bitmaps) can be created from any imaginable source, including prerendered CGI, dynamic 3D graphics, vector art, and even text. Likewise, pixel art is created for many purposes other than as a sprite, such as video game backgrounds, textures, icons, websites, display art, comics, and t-shirts.
With the advancement in computer graphics and improved power and resolution, actual pixel art sprites are becoming increasingly infrequent outside of handheld game systems and cell phones.
Sprites are typically used for characters and other moving objects in video games. They have also been used for mouse pointers and for writing letters to the screen. For on-screen moving objects larger than one sprite's extent, sprites may sometimes be scaled and/or combined.
Billboarding is the use of sprites in a 3D environment. In the same way that a billboard is positioned to face drivers on a highway, the 3D sprite always faces the camera. There is both a performance advantage and an aesthetic advantage to using billboarding. Most 3D rendering engines can process "3D sprites" much faster than other types of 3D objects. So it is possible to gain an overall performance improvement by substituting sprites for some objects that might normally be modeled using texture mapped polygons. Aesthetically sprites are sometimes desirable because it can be difficult for polygons to realistically reproduce phenomena such as fire. In such situations, sprites provide a more attractive illusion.
Sprites are also made and used by various online digital artists, usually to train their ability to make more complicated images using different computer programs or just for the fun of it. "Sprite Artists" will either create their own "custom" sprites, or use and edit pre-existing sprites (often made by other artists or "ripped" from a video game or other media) in order to create art, comics, or animations.
Major video game companies rarely (if at all) use the term "sprite" in the general public. Some other alternatives that have been used are:
- Player-Missile Graphics was a term used by Atari, Inc. for hardware-generated sprites in the company's early coin-op games, the Atari 2600 and 5200 consoles and the Atari 8-bit computers. The term reflected the usage for both characters ("players") and other objects ("missiles"). They had restricted horizontal size (8 or 2 pixels, albeit with scalability) and vertical size equal to height of the entire screen.
- Movable Object Block, or MOB, was used in MOS Technology's graphics chip literature (data sheets, etc.) However, Commodore, the main user of MOS chips and the owner of MOS for most of the chip maker's lifetime, applied the common term "sprite", except for Amiga line of home computers, where MOB was the preferred term.
- The developer manuals for the Nintendo Entertainment System, Super Nintendo Entertainment System, and Game Boy referred to sprites as OBJs (short for "objects"), and the region of RAM used to store sprite attributes and coordinates was known as OAM (Object Attribute Memory). This still applies today on the Game Boy Advance and Nintendo DS handheld systems. However, Nintendo Power referred to them as sprites in articles about the NES architecture in the magazine's third year.
- BOB, more often BLOB or 'Blitter Object', popular name for graphics objects drawn with the dedicated graphics blitter in the Amiga series of computers, which was available in addition to its true hardware sprites.
- Software sprites were used to refer to subroutines that used bit blitting to accomplish the same goal on systems such as the Atari ST and the Apple II whose graphics hardware had no sprite capability.
- The computer programming language DarkBASIC used the term Bob (for "blitter object") to refer to its software-sprite functions, before switching to the more conventionally used "sprite" term.
- Billboard or 3D Sprite is a term often used to refer to sprites that are essentially texture mapped 3D facets that always have their surface normal facing into the camera.
- Z-Sprite is a term often used for 3D environments that contain only sprites. The Z-parameter provides a scaling effect that creates an illusion of depth. For example in adventure games such as King's Quest VI where the camera never moves, normal 2D sprites might suffice, but Z-sprites provide an extra touch.
- Impostor is a term used instead of billboard if the billboard is meant to subtly replace a real 3D object.
- 2D computer graphics
- Isometric graphics in video games and pixel art
- 3D computer graphics
- Graphics processing unit
- Video display controller
- Bill Loguidice & Matt Barton (2009), Vintage games: an insider look at the history of Grand Theft Auto, Super Mario, and the most influential games of all time, p. 197, Focal Press, ISBN 0-240-81146-1
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