Games - Sprite

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	Games - Sprite

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.

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 Lord of the Rings: The Fellowship of the Ring 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 Kings Quest VI 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 the 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:

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Games - Sprite

Billboarding

Rationale

Alternative terms

Hardware sprites

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