Sprite (computer graphics)

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

Sprites were originally invented as a method of quickly compositing several images together in two-dimensional video games using special hardware. As computer performance improved, this optimization became unnecessary and the term evolved to refer specifically to the two dimensional images themselves that were integrated into a scene. That is, figures generated by either custom hardware or by software alone were all referred to as sprites. 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.

SpriteExampleRevealed.jpg|346px|thumb|right| A closer look at the graphic can help reveal imperfections of this rendering technique. Take a look at the puffs of smoke about the character and to the left. Notice that one sprite at the bottom of the largest puff of smoke is cutting into the ground, revealing its actual geometry is not an amorphous puff but a flat plane. Also notice that the grass near his feet is interacting with the flower near his feet in a strange way. The base of the grass is closer to the camera than the flower yet the blades appear to be behind. For this to be a correct perspective the grass would have to be leaning toward the ground away from the camera at an obtuse angle. Even though the original image that created the grass sprite was clearly created using textured polygons, it is being added to this scene as a sprite. These imperfections escape the notice of most viewers because they are rare, and often do not last very long as they and the camera move.]]

More often sprite now refers to a partially transparent two dimensional animation that is mapped onto a special plane in a three dimensional 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, it can be rotated two dimensionally, it can overlap other objects and be occluded, but it can only ever be viewed from the same 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 for a short period of time
the depicted object has a similar appearance from many common viewing angles (such as something spherical)
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"). 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 have also occasionally been used as a special effects tool in movies. Most notably, the creators of the fire breathing Balrog in The Fellowship of the Ring (film) used 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 can be applied more loosely to mean any 2D graphic drawn on a computer, also known as Pixel Art.

Billboarding

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.

Rationale

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 might be desirable because polygons might never be able to realistically reproduce phenomena such as fire. Sprite images of fire might provide a more attractive illusion.

Alternative terms

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 Heir Today, Gone Tomorrow 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.

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.

The Blitter is a hardware implentation of the Painter's algorithm. For each frame the sprites are first bit blited (short for "bit block transfer") into the fast, large, double, and costly frame buffer and then the frame buffer is sent to the screen. The Blitter was renamed to graphics accelerators as more complicated rendering algorithms are used. The Blitter has a high initial cost for simple scenes.

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 texels 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 a special sprite: The background. It is larger than the screen and tiled, were the tile map is cached, but the tile set is not. For every pixel every sprite unit signals it 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 were the word size is the same as the texel there is no penality of doing unaligned reads needed for rotation. This leads to the limitations of the known implementations:

Computer using Chip	sprites on screen	sprites on line	max texels on line	texture width	texture height	colors	anisotropic zoom	rotation	background	collision detection	transparency	source
GBA	128	32	256	8	8		yes	yes	affine mapped tile layer		alpha
SNES	128	32	256	8	8		yes	yes	affine mapped tiles		alpha
Out Run using system16	128	32		8	8		yes	no	3 tile layers		alpha	[http://www.system16.com/hardware.php?id=697
Amiga using Denise		8		16	arbitrary	3,15	no	no	bitmap		color key
PC Engine using HuC6270A	64	8		16,32	16,32,64	15	no	no			color key
Sega Genesis	80	20	320	8,16,24,32	8,16,24,32	15	no	no	2 tile layers		color key	^*
C64 using VIC-II		8		12,24	21	1,3	1,2	no	1 tile layer	yes	color key	^*
Atari using ANTIC				2,8	128, 256	1,3					color key	^*
Sega Master System Sega Game Gear	64	8		8	8,16						color key	^*
NES	64	8		8	8,16	15	-2, -1, +1, +2				color key	^*
Game Boy	40	32	256	8	16		no	no			color key	^*

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.

History

In the Mid 70's Texas Instruments devised the first video/graphics chips or processors capable of generating 'sprite graphics'

The Atari 400 and Atari 800 systems were the first PC's capable of generating 'sprite graphics' or PMG's to use Atari's name

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

The central processor can 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 is not occupied by the simple task of transferring data from one place to another, software can run faster; and because the hardware provided certain innate abilities, programs that use CISC or BIOS were also smaller.

Separate locations in memory were used to hold the main display and the sprites.

There is a technique called "dirty rectangles", useful when the Blitter was not sufficiently powerful or software sprites were being used. There is only one frame buffer that wraps around the edges, but only those parts that have changed since the last repaint are updated either with the background or the sprites in their updated positions.

Some sprite engines could only store 8 positions in their registers and the unchallenged CPU was programmed to update them several times per frame. Software blitting was complicated by some very strange addressing modes into video ram.

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.

Alternative terms

For on-screen moving objects larger than one sprite's extent, sprites may sometimes be scaled and/or combined.

Player-Missile Graphics was used on the Atari 400/800 and Early Atari Coin Op games to refer to hardware-generated sprites. The term reflected the usage for both characters ("players") and other objects ("missiles"). They had restricted horizontal resolution (8 or 2 pixels, albeit with scalability, and a potential 192 lines of vertical resolution), limiting their use somewhat.
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".
On the Nintendo Entertainment System, Super Nintendo Entertainment System, and Game Boy, sprites were referred to 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.
BLOB's or 'Blitter Objects', the sprite system used in the Amiga series of computers.
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 Dark Basic used the term Bob (for "blitter object") to refer to its software-sprite functions, before switching to the more conventionally-used "sprite" term.

Sprite culture

More recently, sprites are edited or created by fans in a process called "spriting". "Spriters" mostly use them to become sprite comic artists, for the purpose of creating a video game-themed comic. It has been continued by Macromedia Flash animators who create sprite cartoons. In these communities, spriting has been made into small sections; recoloring, edits, customs, etc. Sprites can be alternated by using techniques such as the one's above. By doing this, Spriters can create their very own "Sprite character" to use in "Sprite sheets" to show that the sheet was made by that spriter but the spriter must put a "sprite tag" on the sheet saying somthing like "Please do not steal" or "give credit" or "If you wish to put this on your site, do not remove this tag",etc. Sprites can be edited from any game where sprites are available. Making pictures with sprites is called a "Hoax" which is the sprites in a group or doing certain actions but a "Hoax" is not a true image from a game.

External links

Sprite archives

Computer graphics | Computer game design

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Billboarding

Rationale

Alternative terms

Hardware sprites

History

Application

Alternative terms

Sprite culture

External links

Sprite archives