In computer animation, the term “raster graphics” refers to animation frames made of pixels rather than scalable components, such as vertices, edges, nodes, paths or vectors. Storing images as pixels rather than vectors or vertices enables much deeper and more realistic lighting and color because the computer doesn't have to render each frame in real time as it does in a 3-D video game. However, because a fast PC can take 10 to 20 minutes to render one frame, rendering an entire animation usually requires a network of render nodes.
Bitmaps and Scalable Vector Graphics
Raster animation doesn't only refer to 3-D graphics, although demand for 2-D animation in movies, TV, video games and commercials has decreased since processing power has become affordable enough to render 3-D animations on a small budget. A raster image is simply another word for a bitmap, or pixel-based image. In comparison, a vector image is a 2-D picture created in a scalable vector graphics editor such as Adobe Illustrator or open source Inkscape. SVG files take up less disk space than bitmaps because they only store the paths that delineate the shapes in a picture, whereas bitmaps store data for every pixel. Bitmaps store all the depth and subtlety of light that the image resolution allows, while SVGs have simple, cartoonlike colors.
The term “raster image” refers to the way the image is stored rather than how it's displayed. When your video card renders a frame of a video game, you see the same pixels you'd see if you pre-rendered the frame using the same settings. The file read by the game stores the image as an enormous array of vertices, and the video game contains software routines that move the vertices based on events in the game. Video games sacrifice realism for smoothness during game play, but they often contain pre-rendered movies with fully realized graphics. These scenes, stored as MPEG or a similar format, usually cause modern game sizes to exceed 1GB.
Traditional Raster Animation
Before 3-D animation became affordable, animated films and TV shows were mostly hand-painted, but video games used low-detail raster animation to store graphics on a cartridge or disc. Video game artists in the 1980s and 1990s animated these character bitmaps using sprite sheets, which enabled them to separate all the moving objects in the game. The game's software routines played the frames in each object's sprite sheet independently of one another so that the game could react to the player's actions.
Modern Raster Animation
Many modern cartoons use raster animation to add color to hand-drawn animation cels. Each animation frame is either scanned into a computer or sketched on a graphics tablet, and the entire animation is stored as a digital movie. Programming languages such as Flash, HTML and Java include animation libraries that generate 2-D animations based on user input events, such as mouse clicks or keystrokes. Like vector graphics, these generated animations can be scaled to fit any window, whereas pre-rendered raster graphics have a predetermined resolution and become pixelated when scaled up.