Raster graphics
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.
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Colours
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.
Encoding
The quality of a raster image is determined by the total number of pixels (resolution), and the amount of information in each pixel (often called colour depth). For example, an image that stores 24 bits of colour information per pixel (the standard for all displays since around 1995) can represent smoother degrees of shading than one that only stores 16 bits per pixel, but not as smooth as one that stores 48 bits (technically; this would not be discernible by the human eye). Likewise, an image sampled at 640 x 480 pixels (therefore containing 307,200 pixels) will look rough and blocky compared to one sampled at 1280 x 1024 (1,310,720 pixels). Because it takes a large amount of data to store a high-quality image, data compression techniques are often used to reduce this size for images stored on disk. Some techniques sacrifice information, and therefore image quality, in order to achieve a smaller file size. Compression techniques that lose information are referred to as "lossy" compression.
Raster graphics cannot be scaled to a higher resolution without loss of apparent quality. This is in contrast to vector graphics, which easily scale to the quality of the device on which they are rendered. Raster graphics are more practical than vector graphics for photographs and photo-realistic images, while vector graphics are often more practical for typesetting or graphic design. Modern computer monitors typically display about 72 to 130 pixels per inch (PPI), and some modern consumer printers can resolve 2400 dots per inch (DPI) or more; determining the most appropriate image resolution for a given printer resolution can be difficult, since printed output may have a greater level of detail than can be discerned on a monitor.