A:   A raster graphics image, digital image, or bitmap, is a data file or structure representing a generally rectangular grid of pixels, or points of colour, on a computer monitor, paper, or other display medium.
   A bitmap or a raster image corresponds bit for bit with an image displayed on a screen, probably in the same format as it would be stored in the display's video memory or maybe as a device independent bitmap. A bitmap is characterised by the width and height of the image in pixels and the number of bits per pixel, which determines the number of colours it can represent.
   In the printing and prepress industries raster graphics are known as contones (from "continuous tones") whereas vector graphics are known as line work.
   The colour of each pixel is individually defined; images in the RGB colour space, for instance, often consist of coloured pixels defined by three bytes — one byte each for red, green and blue. Less colourful images require less information per pixel; for example, an image with only black and white pixels requires only a single bit for each pixel. Raster graphics are distinguished from vector graphics in that vector graphics represent an image through the use of geometric objects such as curves and polygons.
   A coloured 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 colour encodings are also used, such as four- or eight-bit indexed representations that use vector quantisation 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 in this component.