Table of Contents## Elliptical Arc

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Lines are simple; two points on a path uniquely determine the line segment between them. Since an infinite number of curves can be drawn between two points, you must give additional information to draw a curved path between them. The simplest of the curves we will examine is the elliptical arc — that is, drawing a section of an ellipse that connects two points.

Although arcs are visually the simplest curves, specifying a
unique arc requires the *most* information. The first
pieces of information you need to specify are the
*x*- and *y*-radii of the ellipse
on which the points lie. This narrows it down to two possible ellipses,
as you can see in section (a) of Figure 6-4. The two points divide
the two ellipses into four arcs. Two of them, (b) and (c), are arcs that
measure less than 180 degrees. The other two, (d) and (e) are greater
than 180 degrees. If you look at (b) and (c), you will notice that they
are differentiated by their direction; (b) is drawn in the direction of
increasing negative angle, and (c) in the direction of increasing
positive angle. The same relationship holds true between (d) and
(e).