Vector Graphics
In the prepress and printing industries, raster images are known as continuous tone images (“contones”), while vector graphics are known as “line work”. These nicknames are descriptive of the differences that exist between the two types of graphic storage methods. Whereas contones rely on pixel-dependent bitmaps, vector graphics use geometric primitives to store image data. The geometric primitives used by vector formats include, amongst others, points, lines, curves, shapes and polygons.
Vector images are graphics that are constructed from various vectors/geometrical primitives (also known as paths or strokes) leading through control points on a “work plan”. The work plan is the plane that arises in the space created by the establishing of X and Y axes. Each control point can be allocated a specific and definite set of (x,y) co-ordinates, and can be understood as an element from which other information is extracted. This additional information includes the direction of the vector which defines the direction of the track, and each track can have a colour, shape, thickness and fill assigned to it.
The vector file format describes how a graphic is drawn rather than create a bitmap of the image. The immediate implication of this is that whereas the possible maximum size of a bitmap image is dependent on its resolution (pixels per inch) the vector graphic remains the same quality no matter its size. When a bitmapped graphic containing a set number of pixels (meaning that the file is of a set memory size) is scaled up, it begins to pixelate, which in turn reduces the quality of the image. The vector image can be scaled up infinitely and not risk losing image quality because the file isn’t dependent on pixels for its generation. This, however, doesn’t necessarily mean that the vector format is superior to the bitmap format: each format is appropriate in different circumstances. Vector images are best suited to contexts in which graphics aren’t in need of photo-realism. This includes line drawings, technical documents, and graphic designs. Contone images are used in images that it would be impractical to convert into line drawings. Both types of file format have programmes that allow for the editing and manipulation of graphics.
Vector drawings have to be converted into raster images to be viewed on pc screen (screens use pixels) or to be printed on non-plotter type printers like inkjets and laser jets. It is possible, if the quality of the raster image needs to be high, to have a large raster file generated from the conversion of a relatively small vector file. This is due to the fact that the size of a vector graphic depends on the number of graphic elements contained in the graphic itself: the vector file should simply be understood as the list of descriptions that defines the drawing. The standard file used for vector drawings is the Scalable Vector Graphics file (SVG). However useful the SVG might be, the file format isn’t well supported by internet browsers.
Primitives that vector file formats support are as follows:
• Lines, polylines and polygons
• Bezier curves and bezigons
• Circles and ellipses
• Text
• colour gradient
• splines and Catmull-Rom splines
• NURBS
• iterated function systems
• super-ellipses and super-ellipsoids