Sprite (computer graphics)

The term sprite is used in computer graphics to refer to 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. The term originally referred to a specific shortcut for rendering overlapping bitmaps onto a display device. As computer performance improved this optimization became unneccessary and the term evolved to refer specifically to the two dimensional images themselves that were integrated into a scene. 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.

More often a sprite now refers 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 eminating 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 refered to as billboarding.

Sprites create an effective illusion when:

• the image inside the sprite already depicts a three dimension 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 phenomenon 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 Lord of the Rings: The Fellowship of the Ring used sprites to simulate fire eminating from the surface of the demon. Small bursts of fire where filmed infront 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.

Hardware Sprites

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

Rationale

During the 1980s, hardware speed was measured in tens of megahertz and memory was limited to kilobytes. With these constraints video game programmers resorted to extreme measures to speed up the process of writing bitmaps onto the display. A sprite engine is hardwired into a computer or videogame system's architecture. The central processor can instruct the engine to fetch source images and integrate them into the main screen using direct memory access channels. (This is related to what a genlock does with video sources and to a playfield ). Calling up sprite hardware, instead of the using processor alone greatly improved graphics performance. Because the processor is not occupied by the simple task of transfering data from one place to another software can run faster and because the hardware provided certain innate abilities programs that used sprites were also smaller.

Sprites are rare in most video hardware today. More commonly, a brute-force method called bit block transfer operations is employed or more complicated rendering algorythms are used. For extreme graphics performance graphics accelerators now have a similar role.

Capabilities

Sprite engines were varied in their capabiliies. The various parameters included:

• collision detection (see Atari Lynx)
• scaling images (The first game that used this technique to give the illusion of perspective was The Secret of Monkey Island.)
• rotating images
• sprite image size
  • fixed
  • arbitrary (see Amiga)
• transparency

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 the term used by the Atari 400/800 to refer to sprites. The term reflected the usage for both characters ("players") and other objects ("missiles") These graphics were very narrow and of limited use.
• 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).
• Software sprites were used to refer to subroutines that accomplished the same goal on systems such as the Atari ST and the AppleII that had no hardware sprites. (This was of course oxymoronic)

Notable Implementations

• Many third party graphics cards offered sprite capabilities.
• MOS Technology VIC-II and the Atari ANTIC were well-featured sprite-handling chips of the 8-bit era
• The Amiga Agnus and Denise custom ASICs carried the torch on to the 16 and 32 bit systems.

External links

• Game Sprite Archives