Perpendicular Line to a given line forming a T-shape

Given any two points, two other points can be constructed forming a T shape

Given any two points, this function calculates two other points that lie along a line which is perpendicular to the line joining the given points. If these two lines are made to intersect at the first given point, we get a T-shape. This is a simple trivial function and the title picture illustrates everything. The perpendicular line is found by elementary mathematics (m1m2 = -1).