You are given two line segments. Do they cross?
Consider one segment as (x1,y1) to (x2,y2), the other segment as (x3,y3) to (x4,y4). You are given a = [x1 y1; x2 y2]; b = [x3 y3; x4 y4]. Return tf=true if a and b intersect or tf=false if a and b do not touch.
When lines do intersect they will do so cleanly at exactly one non-endpoint. That is, they will not nest, overlap, or "kiss" at the endpoints.
a = [0,0; 1,1]; b = [0,1; 1,0]; tf = true
a = [0,0; 1,0]; b = [0,1; 1,1]; tf = false