Pictures on a computer display or the process of creating pictures on a computer display. The term came into use when there was still a distinction between computers that could display only text and those that could also display pictures. This distinction is lost now that almost all computers employ a GRAPHICAL USER INTERFACE.
Since a computer can ultimately deal only with binary numbers, pictures have to be DIGITIZED (reduced to lists of numbers) in order to be stored, processed or displayed. The most common graphical display devices are the CATHODE RAY TUBE inside a monitor, and the PRINTER, both of which present pictures as two-dimensional arrays of dots. The most natural representation for a picture inside the computer is therefore just a list of the colours for each dot in the output; this is called a BITMAPPED representation. Bitmapping implies that each dot in the picture corresponds to one or more bits in the computer’s video memory.
An alternative representation is to treat a picture as if it were composed of simple geometric shapes and record the relative positions of these shapes; this is called a vector representation (see more under VECTOR GRAPHICS). A typical vector representation might build up a picture from straight lines, storing only the coordinates of the endpoints of each line.
Vector and bitmapped representations have complementary strengths and weaknesses. Most modern output devices work by drawing dots, which makes it more efficient to display bitmapped images. There are a few, mostly obsolete, display technologies, such as the GRAPH PLOTTER or the VECTOR DISPLAY tube, that can draw lines directly, but it is more typical for a vector image to be first converted into bitmap (the process called RENDERING) before displaying it on a dot-oriented device. Rendering involves extra work for the computer, which is performed either by software or by special hardware called a GRAPHICS ACCELERATOR.
Vector representations are easily edited by moving, resizing or deleting individual shapes, whereas in a bitmap all that can be changed is the colour of the individual pixels. Vector images can easily be rotated and scaled to different sizes with no loss of quality, the computer simply multiplying the endpoint coordinates by a suitable factor. A bitmap, on, the other hand, can be magnified only by duplicating each pixel, which gives an unsightly jagged effect.
Vector representations are most suitable for pictures that are actually drawn on the computer (such as engineering drawings, document layouts or line illustrations), and for images that must be reproduced at various sizes (such as fonts). On the other hand, bitmaps are more suitable for manipulating photographs of real world scenes and objects that contain many continuously varying colours and ill-defined shapes – both scanners and digital cameras produce bit-mapped images as their output.