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 that is integrated into a larger scene. Initially used to describe graphical objects handled separately of the memory bitmap of a video display, the term has since been applied more loosely to refer to various manner 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 circuitry or software. In circuitry, a 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.

As three-dimensional graphics became more prevalent, the term was used to describe a technique whereby flat images are seamlessly integrated into complicated three-dimensional scenes.

History

2D coordinates of an isometric sprite.

3D coordinates of an isometric sprite.

The use of read-only memory in arcade games from around 1974 allowed the widespread use of sprites. Taito released some of the earliest known video games with sprites that year, including Basketball, a sports game that represented four basketball players and two baskets as sprite images,^[1] and Speed Race, a racing video game that represented cars as sprite images,^[2] which could collide with each other and vertically scroll across a race track.^[3] The following year, they released Western Gun, a run and gun multi-directional shooter that was the earliest known video game to visually represent game characters as sprites,^[4] as well as the first to depict a gun on screen.^[5]

In the mid-1970s, Signetics devised the first video/graphics processors capable of generating sprite graphics. The Signetics 2636 video processors were first used in the 1976 Radofin 1292 Advanced Programmable Video System.

The Atari VCS, released in 1977, featured a hardware sprite implementation wherein five graphical objects could be moved independently of the game playfield. The VCS's sprites, called players and missiles, were 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 was altered from by software from one scanline to the next.

The Atari 400 and 800 home computers of 1979 featured similar, but more elaborate circuitry, capable of moving eight Player/Missile objects. This more advanced version allowed a two-dimensional bitmap several pixels wide, and as tall as the screen. To simulate vertical motion, the sprite's bitmap had to be moved up and down incrementally in memory.

The Elektor TV Games Computer was an early microcomputer capable of generating sprite graphics, which Signetics referred to as "objects".

The term "sprite", a Greek fairy, was coined by one of the definers of the Texas Instruments 9918(A) video display processor (VDP).^[6] 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.

During most of the 1980s, hardware speed was in the low, single-digit megahertz and memory was measured in kilobytes. Beside CISC processors, all chips are hardwired. Sprites are rare in most video hardware today.

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.

Hardware sprites

In early video gaming, sprites were a method of integrating unrelated bitmaps so that they appear to be part of a single bitmap on a screen.

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. This improved performance greatly since the sprite data did not need to be copied into the video memory in order to appear on the screen, and further since this spared the programmer of the task of having to save and restore the underlying graphics, something which otherwise was needed if the programmer chose to progressively update.

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 # 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:

Sprite Hardware Features

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
Amiga, Denise	1985	display list	8	?	16	arbitrary	3, 15	vertical by display list	No	2 bitmap layers	Yes	color key
Amiga (AGA), Lisa	1992	display list	8	?	16, 32, 64	arbitrary	3, 15	vertical by display list	No	2 bitmap layers	Yes	color key
Amstrad Plus, Asic	1990	display list run by CPU	16 min	?	16	16	15	1, 2, 4x vertical, 1, 2, 4x horizontal	No	bitmap layer	No	color key	[7]
Atari 2600, TIA	1977	multiplied by CPU	9 (with triplication)	51 (with triplication)	1, 8	262	1	1, 2, 4, 8x horizontal	horizontal mirroring	1 bitmap layer	Yes	color key	[8]
Atari 8-bit, GTIA/ANTIC	1979	display list	8	40	2, 8	128, 256	1,3	1, 2x vertical, 1, 2, 4x horizontal	No	1 tile or bitmap layer	Yes	color key	[9]
C64, VIC-II	1982	display list run by CPU	8	96,192	12, 24	21	1, 3	1x 2x integer	No	1 tile or bitmap layer	Yes	color key	[10]
Game Boy	1989	40	10	80	8	8, 16	3	No	No	1 tile layer	No	color key	[11]
GBA	2001	128	128	1210	8, 16, 32, 64	8, 16, 32, 64	15, 255	Yes Affine	Yes Affine	4 layers, 2 layers + 1 affine layer, 2 affine layers	No	color key, blending	[12]
Gameduino	2011	256	96	1536	16	16	255	No	Yes	1 tile layer	Yes	color key	[13]
NES, RP2C0x	1983	64	8	64	8	8,16	3	No	No	1 tile layer	Partial	color key	[15]
Neo Geo	1990	384	96	1536	16	Up to 512	15	sprite shrinking	No	No	No	color key	.
Out Run, dedicated hardware	1986	128	32	?	8	8	?	Yes Anisotropic	No	3 tile layers	?	alpha	,[16][17]
PC Engine, HuC6270A	1987	64	16	256	16, 32	16, 32, 64	15	No	No	1 tile layer	Yes	color key
Sega Master System Sega Game Gear	1985	64	8	64	8	8,16	15	No	No	1 tile layer	Yes	color key	[18]
Mega Drive	1988	80	20	320	8,16,24,32	8,16,24,32	15	No	No	2 tile layers	Yes	color key	[19]
Sharp X68000	1987	128	32	?	16	16	15	No	No	?	?	color key
SNES	1990	128	34	272	8, 16, 32, 64	8, 16, 32, 64	15	background only	background only	3 tile layers or 1 affine mapped tile layer	Yes	color key, averaging
Texas Instruments TMS9918	1979	32	4	64	8, 16	8, 16	1	1x 2x integer	No	1 tile layer	Partial	color key	[20]
Yamaha V9938	1986	32	8	128	32	32	16	No	No	1 tile layer	Yes	color key
Yamaha V9958	1988	32	8	128	?	?	16	?	?	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

An animated sprite of a fencer unit from Wesnoth

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

Example of a sprite sheet

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.^[21] CSS is used to select the parts of the composite image to display at different points in the page. If a page has 10 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.^[22][23]

Move to 3D

Prior to the popularizing of true 3D graphics in the 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.
• 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 true 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. This rendering method is also referred to as billboarding.

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.

Application

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 one term used to describe 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 & edit pre-existing sprites (Usually made by other artists or "ripped" from a video game or other media) in order to create art, comics, or animations.

Synonyms

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's, more often BLOB's or 'Blitter Objects', 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.
• 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.

See also

• 2.5D
• Video games with isometric graphics

References

1. ^ Basketball at the Killer List of Videogames
2. ^ Speed Race at the Killer List of Videogames
3. ^ 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 0240811461
4. ^ Chris Kohler (2005), Power-up: how Japanese video games gave the world an extra life, p. 19, BradyGames, ISBN 0744004241
5. ^ Stephen Totilo, In Search Of The First Video Game Gun, Kotaku
6. ^ "Karl Guttag Conference on Delphi TI Net - comp.sys.ti | Google Groups". Groups.google.com. http://groups.google.com/group/comp.sys.ti/msg/73e2451bcae4d91a. Retrieved 2009-11-29. 
7. ^ "Plus - CPCWiki". Cpcwiki.eu. http://cpcwiki.eu/index.php/Plus. Retrieved 2009-11-29. 
8. ^ "Television Interface Adaptor". AtariArchives.com. http://www.atariarchives.org/dev/tia/description.php. Retrieved 2011-02-06. 
9. ^ "Atari 5200 FAQ - Hardware Overview". AtariHQ.com. http://www.atarihq.com/5200/5200faq/04_02.html. Retrieved 2011-02-06. 
10. ^ The MOS 6567/6569 video controller (VIC-II) and its application in the Commodore 64 at the Wayback Machine (archived August 30, 2006)
11. ^ "GameBoy - Spielkonsolen Online Lexikon". At-mix.de. 2004-06-22. http://www.at-mix.de/gameboy.htm. Retrieved 2009-11-29. 
12. ^ "Specifications". Nocash.emubase.de. http://nocash.emubase.de/gbatek.htm. Retrieved 2009-11-29. 
13. ^ "Gameduino Specifications". excamera.com. http://gameduino.com/. 
14. ^ "Specifications". Nocash.emubase.de. http://nocash.emubase.de/gbatek.htm#dstechnicaldata. Retrieved 2009-11-29. 
15. ^ "Microsoft Word - NESDoc.doc" (PDF). http://nesdev.parodius.com/NESDoc.pdf. Retrieved 2009-11-29. 
16. ^ http://emustatus.rainemu.com/games/outrun.htm^{[dead link]}
17. ^ "Out Run Hardware (Sega)". System 16. http://www.system16.com/hardware.php?id=697. Retrieved 2009-11-29. 
18. ^ Charles MacDonald. "Sega Master System VDP documentation". http://cgfm2.emuviews.com/txt/msvdp.txt. Retrieved 2011-07-05. 
19. ^ Sega Programming FAQ October 18, 1995, Sixth Edition - Final at the Wayback Machine (archived January 22, 2005)
20. ^ TEXAS INSTRUMENTS 9900: TMS9918A/TMS9928AITMS9929A Video Display Processors. http://emu-docs.org/VDP%20TMS9918/Datasheets/TMS9918.pdf. Retrieved 2011-07-05. 
21. ^ Shea, Dave (2004-03-05). "Articles: CSS Sprites: Image Slicing’s Kiss of Death". A List Apart. http://www.alistapart.com/articles/sprites. Retrieved 2009-11-29. 
22. ^ "Performance Research, Part 1: What the 80/20 Rule Tells Us about Reducing HTTP Requests » Yahoo! User Interface Blog". YUIBlog. http://yuiblog.com/blog/2006/11/28/performance-research-part-1/. Retrieved 2009-11-29. 
23. ^ Brock, Matt. "CSS Sprites: How Yahoo.com and AOL.com Improve Web Performance". Websiteoptimization.com. http://www.websiteoptimization.com/speed/tweak/css-sprites/. Retrieved 2009-11-29. 

External links

• SpriteMe
• CSS Sprites Demo
• Sprite Tutorial
• More Sprite Tutorials