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A Directrix is a line outside the elliptical curve that is perpendicular to the major axis. This line, or directrix, is used to define the ellipse in this way: The illiptical curve is created such that any point maintains a ratio of two segments; The first segment is a line from the point on the curve to the directrix where it is perpendicular to the directrix, and the second segment is a line form the point on the curve to the nearest focus (F1). As shown in Figure 1 below, the first segment is designated L1 and the second segment is designated L2.
Based on this definition, the following is true: L1 / L2 = a constant for any point on the curve. Note that this ratio can be any value greater than zero and less than 1. The example shown in Figure 1 below has a ratio of 0.7. Note that an ellipse has two directrixes based on the two focal points.
This ratio is also known as the eccentricity of the curve. If this ratio is greater than one, or less than or equal to zero, then a different conic curve is generated as shown in Figure 2.
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