Drawing functions depend somewhat upon the equipment which will perform the actual drawing, and may be divided into roughly three categories, according to whether the primitive display elements are points, lines, or areas.
Pen and ink plotters draw lines; therefore the primitive function moves the pen from one point to another. Utilizing internal memory and specifying pen up or down, the data required by individual function calls can be limited to two real (or integer) coordinates and a boolean variable.
Even so, it is sometimes convenient to call for a long pen stroke, but to actually draw only a part of it, near the beginning or near the end. Evidently there are slight differences between physical primitives which are convenient for plotter operation, and logical primitives, which would be convenient for programming.
Drawing images on oscilloscopes can be done through either points or lines. Lines were more common in the days of oscilloscopes, but for television monitors and the associated video controllers, individual points are easier to create. That means that when lines are drawn, they must be constituted from sequences of points, or in the case of incremental plotters, sequences of increments. At the same time, using individual points reduces or eliminates questions of partial visibility.
There was a class of plotter on the market which created images by attracting ink droplets electrostatically to the paper surface, not unlike the mode of operation of contemporary laser printers. Although the droplets were strictly points, the control worked by using them to fill areas, most conveniently small rectangular patches. To use such a plotter, or such a style of image construction, the drawing of an area must be taken as the primitive operation.
Conditions have changed once again, to the extent that the PostScript language emphasizes the use of areas filled with a given color or grey tone.