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6. Paths > Elliptical Arc

Elliptical Arc

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).


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