Thank you, Jon.
User will put the arrow-mark on the line, so Arrow-mark and line are parallel.
Like below, there are two cases to check.
case 1 : Same direction arrow-mark with line (StartPoint -- EndPoint)
case 2 : Opposite direction arrow-mark with line(EndPoint -- StartPoint)
I like to know the condition of the arrow-mark placement by user
How can I know case1 or case 2 ?
