Orthoptic (directrix) of a parabola
The orthoptic curve of a curve C is the geometric locus of points through which passes a pair of perpendicular tangents to C.
If C is a parabola, its orthoptic curve is a line perpendicular to the axis to the parabola, called its directrix.
Check the parabola and its directrix when changing the value of the parameter p of the parabola.