Sprite (computer graphics)

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In computer graphics, a sprite is a two-dimensional bitmap that is integrated into a larger scene.

Originally sprites referred to independent objects that are composited together, by hardware, with other elements such as a background.[1] This occurs as each scan line is prepared for the video output device, such as a CRT, without involvement of the main CPU and without the need for a full-screen frame buffer.[1] Sprites can be positioned or altered by setting attributes used during the hardware composition process. Examples of systems with hardware sprites include the Atari 8-bit family, Commodore 64, Nintendo Entertainment System, Sega Genesis, and many coin-operated arcade machines of the 1980s.

Use of the term "sprite" has expanded to refer to any two-dimensional bitmap used as part of a graphics display, even if drawn into a frame buffer (by either software or a GPU) instead of being composited on-the-fly at display time.

History[edit]

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, features a hardware sprite implementation where five graphical objects can be moved independently of the game playfield. The term sprite was not in use at the time. The VCS's sprites are called movable objects in the programming manual, further identified as two players, two missiles, and one ball.[2] These each consist of a single row of pixels that are 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 single-color objects per scan line: four 8-bit wide players and four 2-bit wide missiles. Each is the full height of the display—a long, thin strip. DMA from a table in memory automatically sets the graphics pattern registers for each scan line. Hardware registers control the horizontal position of each player and missile. Vertical motion is achieved by moving the bitmap data within a player or missile's strip. The feature was called "player/missile graphics" by Atari.

The Elektor TV Games Computer 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).[3] 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.

Hardware sprites[edit]

A simple C64 game with few sprites (hardware sprites)

In early video gaming, hardware sprites were a method of compositing separate bitmaps so that they appear to be part of a single image 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.

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:

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, 4× vertical, 1, 2, 4× horizontal No Bitmap layer No Color key [4]
Atari 2600, TIA 1977 Multiplied by CPU 9 (with triplication) 51 (with triplication) 1, 8 262 1 1, 2, 4, 8× horizontal Horizontal mirroring 1 bitmap layer Yes Color key [5]
Atari 8-bit, GTIA/ANTIC 1979 Display list 8 40 2, 8 128, 256 1,3 1, 2× vertical, 1, 2, 4× horizontal No 1 tile or bitmap layer Yes Color key [6]
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 [7]
Game Boy 1989 40 10 80 8 8, 16 3 No Horizontal and vertical mirroring 1 tile layer No Color key [8]
Game Boy Advance 2001 128 128 1210 8, 16, 32, 64 8, 16, 32, 64 15, 255 Yes, affine Yes, affine 4 layers, 2 layers, and 1 affine layer, 2 affine layers No Color key, blending [9]
Gameduino 2011 256 96 1,536 16 16 255 No Yes 1 tile layer Yes Color key [10]
NES, RP2C0x 1983 64 8 64 8 8, 16 3 No Horizontal and vertical mirroring 1 tile layer Partial Color key [12]
Neo Geo 1990 384 96 1536 16 16 to 512 15 Sprite shrinking Horizontal and vertical mirroring 1 tile layer Partial Color key [13][14][15]
PC Engine, HuC6270A 1987 64 16 256 16, 32 16, 32, 64 15 No No 1 tile layer 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 [16][17]
Genesis 1988 80 20 320 8, 16, 24, 32 8, 16, 24, 32 15 Integer, up to full screen Horizontal and vertical mirroring 2 tile layers Yes Color key [18][19]
OutRun, dedicated hardware 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 [20]
Sega Saturn,
Sega ST-V
1994 16,384 555 4443 8 to 504 1 to 255 15 to 32,768 Yes Yes, rotation and distortion 3-6 tile layers and 1-4 bitmap layers Yes Alpha [21][22]
X68000 1987 128 (512 with 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 [23][24][25]
PlayStation,
Namco System 11
1994 4000 128 1024 8, 16, 256 8, 16, 256 15, 255 Yes Yes 1 bitmap layer Partial Alpha [26][27]
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 1, 2× integer No 1 tile layer Partial Color key [28]
Yamaha V9938 1986 32 8 128 8, 16 8,16 1, 3, 7, 15 per line 1, 2× integer No 1 tile or bitmap layer Partial Color key
Yamaha V9958 1988 32 8 128 8,16 8,16 1, 3, 7, 15 per line 1, 2× integer No 1 tile or bitmap layer Partial 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.[citation needed] 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.

Use in 3D rendering[edit]

2D images with alpha channels constrained to face the camera may be used in 3D graphics. They are common for rendering vegetation, to approximate distant objects, or for particle effects. These are sometimes called billboards or Z-sprites. The technique was most heavily used in sega 3d game machines in the late 1990s, prior to the era of polygon rendering. If rendered on the fly to cache an approximate view of an underlying 3D model, such sprites are called impostors. Modern hardware may have a specific mode for rendering such point sprites without needing each corner to be defined, or these may be generated by vertex shaders.

Synonyms[edit]

Some hardware makers used terms other than sprite, and other terms exist to describe various forms of software-rendering of sprites:

  • 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 NES, 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.
  • 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.

See also[edit]

References[edit]

  1. ^ a b Hague, James. "Why Do Dedicated Game Consoles Exist?". dadgum.com. 
  2. ^ Wright, Steve (December 3, 1979). "Stella Programmer's Guide" (PDF). 
  3. ^ "Karl Guttag Conference on Delphi TI Net - comp.sys.ti | Google Groups". Groups.google.com. Retrieved 2009-11-29. 
  4. ^ "Plus - CPCWiki". Cpcwiki.eu. Retrieved 2009-11-29. 
  5. ^ "Television Interface Adaptor". AtariArchives.com. Retrieved 2011-02-06. 
  6. ^ "Atari 5200 FAQ - Hardware Overview". AtariHQ.com. Retrieved 2011-02-06. 
  7. ^ The MOS 6567/6569 video controller (VIC-II) and its application in the Commodore 64 at the Wayback Machine (archived August 30, 2006)
  8. ^ "GameBoy - Spielkonsolen Online Lexikon". At-mix.de. 2004-06-22. Retrieved 2009-11-29. 
  9. ^ "Specifications". Nocash.emubase.de. Retrieved 2009-11-29. 
  10. ^ "Gameduino Specifications". excamera.com. 
  11. ^ "Specifications". Nocash.emubase.de. Retrieved 2009-11-29. 
  12. ^ "Microsoft Word - NESDoc.doc" (PDF). Retrieved 2009-11-29. 
  13. ^ http://furrtek.free.fr/noclass/neogeo/mvstech.txt
  14. ^ http://furrtek.free.fr/noclass/neogeo/NeoGeoPM.pdf
  15. ^ http://www.neo-geo.com/wiki/index.php?title=Neo-Geo_Big_List_of_Debug_Dipswitches
  16. ^ Charles MacDonald. "Sega Master System VDP documentation". Archived from the original on 2014-03-18. Retrieved 2011-07-05. 
  17. ^ http://www.smspower.org/uploads/Development/richard.txt
  18. ^ Sega Programming FAQ October 18, 1995, Sixth Edition - Final at the Wayback Machine (archived January 22, 2005)
  19. ^ http://www.polygon.com/features/2015/2/3/7952705/sega-genesis-masami-ishikawa
  20. ^ Sega OutRun references:
  21. ^ http://koti.kapsi.fi/~antime/sega/files/ST-013-R3-061694.pdf
  22. ^ http://koti.kapsi.fi/~antime/sega/files/ST-058-R2-060194.pdf
  23. ^ http://museum.ipsj.or.jp/en/computer/personal/0038.html
  24. ^ https://github.com/mamedev/mame/tree/master/src/mess/video/x68k.c
  25. ^ http://shmuplations.com/chorensha68k/
  26. ^ http://psx.rules.org/gpu.txt
  27. ^ https://archive.org/stream/nextgen-issue-001/Next_Generation_Issue_001_January_1995#page/n47/mode/2up/
  28. ^ TEXAS INSTRUMENTS 9900: TMS9918A/TMS9928AITMS9929A Video Display Processors (PDF). Retrieved 2011-07-05.