[b]TrimLine3Pts[A,B,C, m1, m2] [/b] gives the segment shown.

A, B, C may fall in any order; the segment adjusts to follow.[br][br]I find this helps produce clear diagrams, and stays partial loss of sanity.[br][br][b]Notes:[/b][br][math]\;\;[/math]I prefer not to redefine the endpoints when C crosses A or B:[br][math]\;\;\;\;[/math] Segment[A + Min[0, λ] a - m1 u, A + Max[1, λ] a + m2 u][br][math]\;\;[/math]Set the margins to 0 for an ordinary segment.