image, digital image, or bitmap, is a data structure representing a generally rectangular grid of pixels, or points of color, viewable via a computer monitor, paper, or other display medium. Raster images are stored in image files with varying formats.
A bitmap corresponds bit-for-bit with an image displayed on a screen, generally in the same format used for storage in the display's video memory, or maybe as a device-independent bitmap. Bitmap is technically characterized by the width and height of the image in pixels and by the number of bits per pixel (a color depth, which determines the number of colors it can represent).
The printing and prepress industries know raster graphics as contones (from "continuous tones") and refer to vector graphics as "line work".
The word "raster" has its origins in Latin rastrum (a rake) and Latin radere (to scrape), and recalls metaphorically the systematic sampling of a grid-pattern of individual pixel-spaces with a view to representing an overall image.
Each pixel has an individually defined color; images in the RGB color space, for instance, often consist of colored pixels defined by three bytes — one byte each for red, green and blue. Less colorful images require less information per pixel; for example, an image with only black and white pixels requires only a single bit for each pixel. One can distinguish raster graphics from vector graphics in that vector graphics represent an image through the use of geometric objects such as curves and polygons.
A colored raster image (or pixmap) will usually have pixels with between one and eight bits for each of the red, green, and blue components, though other color-encodings also occur, such as four- or eight-bit indexed representations that use vector quantization on the (R, G, B) vectors. The green component sometimes has more bits than the other two to allow for the human eye's greater discrimination of this component.
