To draw a vector—or, in particular, a line segment—you need only connect two points. Since drawing a line segment underpins all other graphics instructions, this task must not be neglected. Yet a computer can draw a line only by breaking it down into short line segments called vectors. Graphics software therefore proceeds by darkening certain points on the screen. But which ones? To find out, a grid of pixels, fine enough to pass through the two points in question, is overlaid on the screen plane. A series of tests then selects the pixels to darken. In practice, if examined very closely, the resulting line segment looks like a staircase… For a complete account of the algorithms used to draw straight lines, see la Droite (The Straight Line), Bibliothèque Tangente 59, 2017.
