描述点之间的相对角度（如行车路线）

Question

I have a list of points with x, y coordinates. I know how to get the distance between points with sqrt(pow($x2 - $x1, 2) + pow($y2 - $y1, 2)) and the angle between points with atan2(y1 - y2, x1 - x2).

How can I calculate the relative angle between the points (left, right, straight)? So, if I'm at point 1, what is the relative direction to point 2, then 2 to 3, 3 to 4, etc...

Thanks for any help!

Answer 1

Call atan2($y1 - $y2, $x1 - $x2)

If it's between π/2 and 3π/2, it's straight.
If it's more than 3π/2 or less than -3π/2, it's a left turn.
If it's between -π/2 and π/2, it's a right turn.

Here is a diagram:

 {3π/2}-------{π/2}-------{π/4}
      | +-----[ +y]-----+ | 
      | |               | | 
      | |               | | 
    {π} [-x]  [0,0]  [+x] {0} 
      | |               | | 
      | |               | | 
      | +-----[ -y]-----+ |
{-3π/2}-------{-π/2}------{-π/4}